Assessment of balance control in humans David A
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Assessment of balance control in humans David A

Winter Aftab E Patla 8 James S Frank Dcpamc of Kincriology Univcnify of Wacrloo Waterloo Onmrio nada Key wordr balancc posturc biomechanics standing gait Summary Balance and posture of the body is essential to most human locomotion Because humans

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Assessment of balance control in humans David A




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Assessment of balance control in humans David A. Winter, Aftab E. Patla 8 James S. Frank Dcpamc~ of Kincriology, Univcnify of Wacrloo, Waterloo, Onmrio, &nada Key wordr: balancc, posturc, biomechanics, standing, gait Summary Balance and posture of the body is essential to most human locomotion. Because humans arc bipeds with about Z3 of their mass located 213 of body hcight from the ground the control system is critical. In the elderly balancc control degenerates. Falls represent a major health problem and the fear of falls is the major deterrenr to daily mobility. Many

measures have evolved to assess balance, varying from crude balance tasks to sophisticated perturbations. This paper summarizes the balance control task as it relates to standing and walkingand detailscurrent assessment techniques and equipment. Additional information is providrd by the authors to demonstrate from an electromyographical and biomechanical perspective the mechanisms and characteristics of the postural control system in both standing and walking. The maintenance of a stable posture is important for all animals. For humans it is particularly chal- lenging by virtue of our

s'acturc. Approximately hvo thirds of our body mass includingsornc delicate organs arc precariously balanccd some distance from the ground (about Z3 of our height) over two spindly structures, the legs, which provide a narrow . base of support. This imposes critical demands on the postural and balancc control system. Balance is integral to the safe execution of most movements. When this important control system deteriorates with age for example, the results can be dcvas- tating. Falls in the elderly have been identified as a major health problem [2]. The focus of this article is on reviewing

our current knowledge on balance control in humans and identifying the problems and the use of technology in the assessment of balance. The subsystems that make up the postural con- .trol system (Fig. 1) include the following: the scn- sory system made up of the vestibular. visual and proprioccptive systems; the central nervous system (04s); the muscirlo-skcletiil system. The proprib ccptive system which consists of muscle, joint rr.d cutaneous reupton provide us with irtf~rrnaticn about the state of the cfiector system (LC. lengl;; pnd force output oi the mcsciu, relaii~e orier,- tation of

body segments), and ir~foimation ahui our environment (e.g. temperature, contact rcrr- face condition, prcssur; distribution, presence of any noxious stimuli). The vestibular system pro- vides us with information about our body oricx- tation in the inertial frame of reference and axclcr- ations of the body. The visual system has also been catcgorised as a propriocxptivc system because it not only provides information about our environ- ment but also about the orientation and movement of our body, and because of this it is referred to as exproprioception [27]. It is clear that useful in-

formation is obtained for maintenance of posture from a number of sources. The redundancy present not only allows for compensation when one of the systcm deteriorates but also enables the verifica- tion of the inputs (sometimes conflicting) by com- paring them before an action is taken. The rich array of inputs coming into the system have therc-
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External forcer Fig. I. A conceptual schema~ic diagram of thc postural mnuol sysccm. 1 fore to be evaluated and integrated, and an appro- priate plan of action has to be decided upon by the central nervous system. This plan of action

is exe- cuted by the musculo-skeletal system to regulate body posture and movement. Because the response of the postural control sys- tem is so vital for our daily activities, it stands to reason that the action plans to cope with various dcstabilising situations must be programmed ratb- er than organised on a need basis. Researchers have tried to identify these action plans by mirnick- ing a variety of destabilising conditions. These plans' manifest themselves in the specific spatic;- temporal muscle activation patterns as revealed by electromyography; thcse have been termed postu- ral

synergies. The muscle activation produces forces which act to correct the imbalance. This aetkmcm be ~wr&~~irrgfdr~cxert-~ ed by the body (using a forceplate, for example), calculating the joint moments (through inverse dy- namics), and by recording the movement kinemat- ics. In this review, the balance responses under four major classes of experimcnis, representing incrcas- ing degiea of postural instability, are discussed. It wiH be shown that the scnsori-motor organization of balance control is a complex phenomenon and is not easily amenable to characterization by a single assessment tat.

The four classes of experiments include the following: maintenan- of a static un- perturbed posture; static posture mntroi under the presence of pe~urbations; balance control during the voluntary execution of a movement; and bal- ance control during movement in the presence of perturbations. To focus the discussion, he postdre selected is standing and the movement is locomo- tie" because of their importance to our daiiyfmc -A- tlon. ~eperturbarioTsd&ribedre~r~nt boih self-initiated, and thus expected, and unexpected. Body Palure k Movement C Tuk Coal I Paturrl Synergia - 6 Sensory Integralion

- + Ll -p - - - - - - I" ------- ! Sensory ---- Receptors: Viru.1,
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11. Bnlanct during unperturbed sfanding - To assess this ubiquitous posture, researchers have tried to analyte the movement of centre of gravity (C of G) and the center of prcssure (C of P) [25,26, 481. Thut two variables are not the same. The C of P is a time-varying signal that is readily available from the fore plate and is too often erroneously referred to as sway or C of G. The centre of pres- sure during quiet standing times the ground reac- tion force is equal to the moment-of-fore generat- ed by the

ankle muxlcs (e-g., a 600 N subject with the C of P lOcm anterior of ankle joint mn because there is a 60 N - m plantar-flexor moment). Thus the C of P reflects the net motor pattern at the ankle and thus the response of the CNS to correct the imbalance of the C of G. The centre of maul gravity of movements can be calculated by time- integrating the horizontal accelerations (measured by the force plate) twice. Unfortunately, this calcu- lation is prone to considerable error due to a lack of knowledge of the initial conditions and integration errors due to biases in the force plate signal.

erron that accumulate over time. The C of G of course can be determined from the anthropometry and kinematics of body segments or approximated by monitoring the movement of the hip joint for ex- ample with the help of a potentiometer [a]. The difference between C of G and C of P is shown in Fig. 2. Here we see a subject swaying back and forth while standing erect on a force plate. Each figure shows the chznging situation at five different points ovir time. Time 1 has the body's C of G ahead of the C of P and the angular velocity, w. is assumed to be clockwise. Body weight. W, is equal and

opposite to the vertical reaction force, R. and these 'parallelogram of forces' act at a distance, g and p, respectively, from the ankle joint. Both Wand R will remain constant during quiet standing. Assuming the body to be an inverted pendulum, pivoting about the ankle. a counter-clockwise moment equal to Rp and a clockwise moment equal to Wg will be acting. If Wg>Rp, thus the body will be experiencing a clockw'ise angular acceleration, a. In order to cor- rect this fonvard 'sway' the subject will increase his or her c of P (by increasing plantarflexion activa- tion) such that at Xmc 2 the C

of P will be anterior lo the C of G. Now Rp>Wg, thus a will revene ?nd will start to decrease w, until at Time 3 the time-intrgral of a will result in a reversal of w. Now both o and a are counter-clockwise and the body is experiencing a backward sway. When the CSS senw that this posterior shift of the C of G needs correcting the C of P deceases (by decreased ?Ian- farflexor activation) until it lies posterior to the C of G. Thus a will reverse to become clock~ise again at Time 4, and after a period of time w will again denease and revene, and the body will re- turn to the original conditions,

as seen for Time 5. From this sequence of C of G and Cof P conditions it can be seen that the plantarflcxors/dorsiflexors in controlling the net ankle moment an regulate the body's C of G. However, it is apparent the dynamic range of C of P m'ust be somewhat greater than the C of G: C of P must be continuously moving ante- riorly and posteriorly with respect to the C of G. Thus if the C of G were allowed to move within a few crns of the toes it is possible that a corrective movement of the C of P to the extremes of the toes would not be adequate to reverse W. Here the sub- ject would have to

move a limb forward to arrest the forward fall. Gntre of pressure has been used more frequent- ly than centre of gravity to examine postural con- trol during unperturbed standing. However, mea- sures of CofP have offered very limited insight into the control of standing posture. Measures recorded include the mean amplitude, range, variability, ar- ea, dominant direction and frequency spectrum of the excunions over a fixed duration (10-20s). Lucy and Hayes (291 reported that the average varia- bility (root mean square ofthe amplitude) of C of P exertions for normal young adults standing on two

feet with the eyes open n 2.61 2 1-01 rnrn for the an~erior-posterior direction and 1.79 5 0.41 rnm for the lateral direction. These values were found to increase with age; by is90 yean of ase averaee variability scores for the anterior-posterior and (at- era1 directions were 3.9821.37mm and 3.04 i 1.34 mm, respectively. For normal young adults these variability scores increase upon closine the eyes. However, for older adults this difference diminishes with age demonstrating p&r visual sta-.
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.- - s C of G vs C of P During Standing Posture Fig. 2. Vhcriatiom ol the antre of

gravity (C ol G) vcms thc pressure (C ol P) during standing shoving how Ac hcnkk nnruiu control (he C of P md rhw con~inuously regulate the body's C of G. . b. bilization by older adults. A Rhomberg quotient based on the ratio of eyes openleyes closed per- formance b often reported to demonstrate the role of vision in postural control. Soames and Atha [SO] reported that in normal young adults %95% of the energy in the power spectrum is below ZHz with the peak power at 0.30-0.45 Hz. This distribu- tion characterizes both anterior-posterior and lat- eral excunions of the C of P. An increase in

energy at certain frequencies has been associated with dii- ferent sensory and motor deficits. An increase in energy at all frequencies below 1 Hz has been asso- _datedwith poor visual stabilization [lo, 111. An increase in energy below 1 Hz accompanied by good visual stabilization, i.c., a high Rhomberg quoticnr on other Cof P measures, has been associ- ated with a peripheral sensory neuropathy (301 and Friedreich's ataxia (101. An increase in energy at 2-3 Hz, a predominant anterior-posterior instabil- ity and good visual stabilization has txcn associ- ated with atrophy of the anterior lobe

of the ccrc- bellum [lo. 291.' 111. Balance control during perturbed standing Perturbations of the centre of gravity while stand- ing can arise from forces which an be anticipated. often of internal origin, as well as unexpected forces exerted by our environment. The ability to execute skill movement and move about with safe- ------- - ty depends on exccutrng appropnate posturxaz justments to counteract both of these typcs of per- turbations. In the discussion below, the anticipato- ry nature and organization of postural synergies which serve to counteract expected perturbations
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are stressed. Discussion of unexpeacd pcrturba- tions highlights the contribution of various sensory inputs which detect destabilization and the orga- nization of postural synergies triggered by an un- cxpccted perturbation. which tend to disturb postural equilibrium. By re- cording the acceleration of limb segmcnb, they revealed an anticipatory upward, forward,-and lat- eral (i.~., away from the mobilized limb) accelcr- ation of the body centre of gravity, as well as, a resultant moment about the vertical axis oriented towards the contralateral side. I. Erpecred pcrrurbation of !he

ccnrcr of gravity While numerous investigations have reported anticipatory postural activity whcn raising or low- Many of the perturbations cxpcricnccd during dai- ering the arm [4,6,7, 18,24,28,53] and pulling or ly aclivity can be anticipated and compensated for pushing on a stiff handle [8, 9, 20, 21, 591 the prior to the loss of upright stability. Such is thc case reported dclay between postural and f&l muscle during voluntary movements of our limbs and activation vanes greatly. For example, whcn rais- trunk. Destabilizing forces arise from the mass and ing the arm forward. Belenkii ct al.

(41 reported incrtia of the body segment moved, as well as, that biceps fcmoris prcceded anterior deltoid acti- objects acted on. For example, when grasping the vation by &SO ms while Lee [28] rcported a dc!ay handle of a door to pull it open, bath the forward of 0-5 ms. By examining the onset of acceleration position of the arm and the incrtia of the door will of the shank and the wrist during bilateral and move the centre of gravity forward. This usually is unilateral arm flexion. Bouit~et and Zattara (61 compensated for by an initial activation of postcri- suggest that this dclay may dcyrld

on the degree of or muscles of the lower limbs and trunk which postural compensation required. Bilateral flexion limits forward sway of the body. Hence, the per- requires ampensation in the anterior-posterior formancc of coordinated voluntary movement dc- and vertical planes, while unilateral flexion also has pcnds on the precise interaction between the con- medial-later31 and rotational components. The de- trot of posture and movement. lay bctwcen shank and wrist acceleration was Expected perturbations of the centre of gravity shonut during bilaterai flexion (27.4 2 4.0rns). havc been

examined most frequently by asking increased for unilaleral flexion (38.4 k 4.1 rns), subjects to raise or lower their arms in the saggital and further increased with the addition of I kg plane or push or pull on a stiff handle. The central load ' to the wrist during unilateral flexion issues in this research have been (1) the temporal (50.9 -C 5.0mi). - coordination of postural and fccal, i.e., task-specif- Postural synergies refer to the discrete rtmjxrsl ic, muscle activation and (2) definition of postural and spatial patterns of leg and trunk muscle con- synergies regulating upright

stance. Although it tractions used to preserve standing posture. Nasltn- has been known for some time that postural ad- er and colleagues [32-411 have devoted much rc- justmcnts must accompany voluntary movement search to defining postural synergies triggcrcd by [22], it was not known until the work of Belenkii et unexpected perturbatio~s ol the base of suppcrt. al. [4] whether thcsc compensations begin prior to, However. anticipatory posrural adjustments offer during, or after the beginning of movement itsclf. the opportunity to define and assess postural syn- Belenkii et al. [4] demonstrated

that when sabjccts ergies without the need of an elabor~te system to are asked to raise their arm, activation of anterior deliver external perturbations. Furthermore, an- deltoid (a focal muscle) is preceded by activation of ticipatory postural adjustments bypass sensory sys- the ipsilateral biceps femoris and contralateral sac- tems and. therefore. permit an isolated assessment rolumbar muscles, as well as, silencing of any tonic of motor processes responsible for the organization activity in the contralateral biceps femoris muscle. of postural synergies. During btlateral arm flexion Bouisset

and Zattara [7] have demonstratcd that the postural synergy invoked involvcs bilateral ac- this anticipatory activity serves to counterbalance tivation of biceps fcmoris followed by erector spi- the incrtia of forces due to movement of the arm, nae [6,53!. The postural synergy is more compicx
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during unilateral arm flexion involving activation of the ipsilateral biccps'femoris and silencing of the contralateral biceps femoris followed by activation of the contralateral erector spinac (41. Pulling and pushing on a stiff handle limits perturbations of the centre of gravity to the

anterior-posterior plane. Recordings from lateral gastrocnemius. tibialis an- terior, biceps femoris, rcaus femoris, erector spi- nae and rcctus abdominus reveal that posterior musclcs are activated in a distal-to-proximal order whcn pulling on the handlc while anterior muscles are activated in a distal-to-proximal order whcn pushing on the handle (8. 9, 20, 241. These latter perturbations of the ccntre of gravity are of partic- ular interest since the postural synergies invoked appear similar to those triggered by unexpected pcrturbations of the handlc (Patla, unpublished observations) and

translations of the base of sup- port [9]. This suggests that for perturbations in the anterior-postcrir~r plane, the same set of postural synergies subser\c anticipatory and externally (rig- gercd postural adjustments. This greatly simplifies the task of controlling upright stance under a varie- ty of situations. The regulation of upright stance during expected perturbations of the centre of gravity can be com- promised in a number of ways. Rogers and Kukuk- la [47] havc demonstrated that Parkinson's patients often fail to' display anticipatory postural adjust- ments when performing rapid arm

flexion; hence, the basal ganglia may be important to the cwr- dination of postural and foal commands. We also have observed this in some elderly subjects [20]. An inappropriate or lcss efficient postural synergy also might be triggered. In elderly subjects we oc- casionally observed a proximal-to-distal activation of posterior musclcs of the leg when 7ulling on a stiff handle. Also arnons elderly subjects. we often have observed tonic cocontraction of Iawer limb muscles prior to pullins or pushing on a stiff han- dle. While this synergy may reduce the destabiliz- ing forces of the

perturbation, it is lcss efficient and fails to adequately compensate for large destabiliz- ing forces. 2. Unupccrtd pcmubation of the ccntcr of graviry Unexpected perturbations of the centre of gravity prescnt the most obvious threat to standing pos- ture. Such pcrturbations are experienced whcn a bus or elevator makcs a sudden start or :top. whcn pushed in a crowd, whcn tripping over an obstacle or when losing footing on a slippery or uneven surface. Unexpected pcrturbations also present the most serious threat to standing posture; postural adjustments follow destabilization and hence may

arrive tw late to recover balancc. Postural rcac- tions to unexpected perturbations wn be used to examine the integrity of xnsory and motor pro- cesses involved in the regulation of standing ps- ture. Unlike expected pcrturbations, postural rcac- tions to unexpected pcrturbations are triggered by sensory input which detect deviations from the dc- sired posture. In laboratory settings, unexpected pcrturbations of the ccntre of gravity have been applied by movement of the support surface on which the subject stands or translation of a handle held by the subject. The central issues of [hi$ re-

search have been [I] to identify the contribution of the various sensory systems which detect destabil- ization and [2] to define the postural synergies reg- ulating balance. Deviations from upright stance can be detected by proprioccptive visual and vcstibular recepton. Proprioccptive receptors signal oricntation and movement of the body parts relative to each other; vision signals orientation and movement of tbe body relative to the surrounding environment; and vestibular receptors signal orientation and rnovc- mcnt of the body relative to inertia and gravity. Nashner and collea~ues (32. 34,

37, 381 havc cm- ployed a movable platiorm (3Ocm for ZCOrnsj :o examine the relative contribution of these sensory inputs to the regulation of standing posture. A backward translation of the platform on which [he subject stands generates a forward body sway. ic- sition of the ccntre of gravity k recovered by activa- tion of posterior muscles oi the lower limbs and trunk with a late~lcy of 90-110rns following plat- form movement. The contribution of proprioccg tive, visual and vestibular inputs to the triggeringof this postural adjustment was examined by manip-
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ulating the

orientation of the platform and the vi- sual surround. By coupling a downward rotation of the platform to backward translation muscle recep- tor input derived from stretch of the posterior an- kle muscles was reduced. Under this condition, postural adjustments were delayed until 185- 250 ms after platform displacement. Visual contri- butions were assessed by coupling a forward dis- placement of the visual sunound to backward translation of the platform; this serves to stabilize visual inputs as the body sways forward. Under this condition, postural adjustments were triggered at a latency of

90-110 ms following platform movement but the magnitude of muscle activation was atten- uated. These findings suggest that muscle receptors act as a 'first-alert'system for the triggering of rapid postural adjustments (W110ms) to preserve standing posture. Vision, on the other hand, trig- gen slow compensations (185250 ms), but also acts to attenuate rapid postural adjustments when conflicting sensory input is present. Vestibular in- . puts also appear to trigger slow compensations (18S-250ms). but more importantly provide an ab- solute orientation reference for comparison with other sensory

inputs. Muscle receptor and visual inputs can provide false information leading to un- necessary postural adjustments which destabilize standing posture. Such inappropriate postural ad- justments are observed during movements of a vi- sual surround (271 and rotations of the support surface on which subjects stand [32]. However, subjects are able to attenuate these responses after several exposures by comparing misleading senso- ry inputs to vestibular inputs. The moving platform studies of Nashner and colleagues [31,35,40] suggest that standing posture is regulated by a limited set of postural

synergies. Nashner has defined these as: an ankle sway syn- ergy, a hip sway synergy. and a suspensory synergy. The ankle sway synergy is triggered by forward and backward displacement of the centre of gravity. This postural synergy involves activation of poste- rior muscles of the legs and trunk to compensate for forward sway and anterior muscles of the legs and trunk to compensate for backward sway. In both cases. muscles are activated in a distal-to-proximal order. Torque generated about the ankle is primar- ily responsible for restoring balance. ?his ankle sway synergy has been

demonstrated most fre- quently . by translations of the base of suppon. Nashner has employed a movable platform (32,331 while others have used a trolley moved by a pulley and weight system [I] or accelerations of a tread- mill [13]. Another method of inducing forward and backward destabilization is to displace a handle held by the subject [9,51]. Recently, we have k- gun to employ this approach since it affords direct comparison of postural synergies evoked by ex- pected (voluntary hand:e displacement) and un- expected (externally triggered handle displact- ment) perturbations of the centre of

gravity. Our system involvcs a handle whose position can be controlled by a stepping motor. The motor can set a background force which the subject must resist. as well as, displace the handle forward or backward. This system is shown in Fig. 3. Unpublished data by Patla (Fig. 4) demonstrates that a forward handie perturbation (200ms) triggers an activation of pos- terior leg and trunk muscles in a distal-to-proximal order. An interesting feature of perturbations in- duced by handle displacement is that postural ad- justments appear to be triggered by propricxeptive input from the arms. Traub

et al. [Sl] noted that the soleus muscle is activated following handle dis- placement but prior to a change in ankle joint ankle. Hence, postural synergies can be triggered by more remote proprioccpton which signal an impending destabilization. An exception to the rule that forward and back- ward destabilization is compensated by an ankle sway synergy is the finding of Wolfc (581. He in- duced forward destabilization by applying a brief impulse to the trunk while subjects were standing. A cable was attached to the subject at the level of the sternum. By releasing a stretched spring at- tached

to the able, a brief forward impulse (I5 m) was delivered to the trunk. A breakable wire link placed in series with the cable and subject regu- lated the duration of the impulse. Under this condi- tion. posterior leg muscles were activated as ear!y as 46ms after the onset of the perturbation: this was foilowed 40 ms later by acivation of anterior leg muscles. Contrary to what has been observed for the ankle sway synergy, muscles were activated
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fig. 3. A xhcmatic figurc of thc cxpcrirncn~al apparatus to provide cxpcctcd and uncxpcctcd perturbation to thc trunk via the handlc

hcld by thc subjcct during standing and rhythmic movcmcntc. Subjcct stands's~cpron thc force platform (I) whilc holding thc handlc vith strain gag= mounrcd on it (2). Thc DC stcpping motor (3) locatcd on the adjustable platform (4) is wd to apply form on thc handlc. in a proximal-to-distal ordcr; the medial and lateral hamstring musclcs were activated before the more distal medial gastrocnemius and solcus musclcs. An examination of the overall pattern of lower limb moments revealed a general flexion response at the ankle (doniflexion), knee, and hip beginning 120ms aftc; the onset of

perturbation. This would tend to lowcr the centre of gravity perhaps as a protection against injury when falling. Hence, it appears that perturbations applied at high levels of the trunk and/or of rapid onset trigger postural adjustments at a shorter latency and with a proxi- mal-to-distal progression. Though less extensively investigated, Nashner and collesgues [cf. 401 also have identified a hip sway synergy and a suspensory synergy. When the support surface is too narrow to allow the ankle to exert sufficient force to restore balance. a hip sway synergy is employed. To demonstrate this.

subjects were required to stand on a narrow beam during fonuard and backward translations of the platform. During forward sway, anterior lower trunk and thigh muscles were activa'ted in a proximal to distal order. During backward sway posterior muscles were activated in a proximal to distal order. For- ward and backward movements of the hips arc primarily responsible for restoring balance. Verti- cal displaamcnts of the platform trigger a suspcn- sory synergy. During elevation of the platform. posterior muscles of the calf and anterior muscles of the thigh were activated in a distal to

proximal ordcr. During lowering of the platform, anterior muscles of the calf and posterior muscles of the thigh were activated in a distal to proximal ordcr. These responses serve to resist load changes on the lowcr limbs and hence maintain vertical height of the body. The regulation of standing posture during un- , expected perturbations of the centre of gravity can be comprised in a number of ways. Traub ct al. (511 demonstrated that Parkinson's paticnb and pa- tients with cerebellar trunkal ataxia display weak or absent postural adjustments when attempting to resist displacement of a

handle whilc standing. While an appropriate muxle synergy might be acti- vated, the onset latency might increase thereby permitting the ccntre of gravity to move further from the base of support. Shunway-Cook and Woollacott [49] have reporred that children with Down's syndrome display the ux of appropriate postural synergies to transiations of the support surface; however onset latencies are increased. The meall onset latency for backward translations of the platform was 162 2 40 ms for cliildicn with Down's syndrome and 112 ms for normal children. Another disturbance to the regulation of

standing posture is a change in the normal organization of the postural synergy. Woollacott. Shumway-Cook and Nashner (591 have reported that older adults occasionally display a proximal-to-distal activation of posterior leg muscles following backward trans- lations of the platform. This pattern of muscle acti- vation also has been observed in children with spas- tic hemiplcgic type cerebral palsy [39] atld is a reversal of the sequence of muscle activation nor- mally associated with the ankle sway synergy. Fi- nally, subjects may display the use of appropriate postural synergies but fail to

adapt these under conditions of sensory conflict. For example, a toe- up rotation of the support surface will activate pos- terior muscles of the leg in a distal-to-proximal order. However, this proves to be an inappropriate
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fig. 4. A wrnplc of mpmc to arm pull (KC fig. 3 for Ihe apparatus) while the subjed was ,landing on the force plate. The rcrponx is the average over ten pcnurbatjonr.The abbrena~iom arc: BB - Biqx Brachii; ES- Erector Spinae: BF- Biapr Femoris: RF- Rcaus Fcmoris; SO - Soleus; HD - Handle Dbplaccmenl; HF - Handle Force; Fx - anterior-portcrior grwnd

reaction force; Fy - rcnical ground rcaction forcc.The raponw may fa11 below the normal van'abili~y a1 tima rflcr the initial onxt.Theonx~ofchangcs in last lour channels arc shown by the wnial line. The horizonla1 linc in each window on rc;nucntr the zero Icvcl. response since rotation of the support surface d0.u not move the centre of gravity away from the base of support. Over several trials, normal subjects will attenuate this response to prevent destabilization [32]. A number of investigations have shown that children with Down's syndrome [49], patients with vestibular deficits [38] and

patients with cerebellar ataxia [32] fail to attenuate inappropriate postural adjustments. These groups appear to be unable to integrate the various sensory inputs to determine the appropriateness of the postural response trig- gered. 111. Dynamic balance during gait The task of balancing the human body during walk- ing must first be examined before looking at how the human motor control system accomplishes that task. Such a clarification is newsary because the vast majority of the literature deals with standing posture, in which the findings relate to one very specific tasic: to keep the C of

G safely within the area of the feet. In walkingthe C of G may never pass within the area of the foot. In double support it ties somewhere between the two feet. A1 the start of single support the C of G lies posterior and medial of the stance heel. With the forward mo- mentum of the body the C of G moves forward. but may not pass within the area of the foot. Shimba [48] has shown that C of G moved fcrward just outside the medial border of the foot. Then during the push-off phase the C of G moves well ahead of the foot as the plantarflexors gerlerate the major energy to propel the body's C of M

upwards and forwards. Thus it can be seen that the task of bal- ance and posture in gait does not require 1he.C of G to ever lie within the foot. It can therefore be said that the body, in walking, is in a continuous state of imbalance, and the only way that we prevent falling
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is to position our swinging foot ahead of and lateral to the fonvard-moving C of G. Failure to accom- plish this is the greatest fear of amputees, who compensate with an excessive ground clearance. Superimposed on the abvc foot placement task is the problem of regulating the large mass above the hip

joints: head, arms and trunk (H.A.T.). This mass represents a very large inertial load. Figure 5 shows the values of the moment of inertia of H.A.T. of a typical adult as seen about four differ- ent centres of rotation. About its own centre of mass, I, is 3.2 kg. m2, about the hip, I, is 7.0 kg - m2, about the knee, I, is 20 kg- m2 and about the anklc I, is 55 kg. m2. Thus the task of regulating this large inertial load and keeping it upright becomes appar- ent. Because I, is about eight times I,, the ankle muscles would need about eight times the moment- of-force to accomplish the same

angular acccler- ation of H.A.T. as would the hip muscles. The human CNS recognizes this major difference and it will now be documented that the dynamic balance of H.A.T. is the prime responsibility of the hip muscles, with virtually no involvement of the ankle muxlcs during gait. Such regulating control of bal- ance is totally opposite to that in standing balance 1 where the anklc musclcr arc all imponant. The hip flexors and extensors acting during stance control the angular acceleration of H.A.T. [57j. However, these hip muscles arc also involved with a motor synergy known as support moment

(541. A review of the invariant and variant mo- ment-of-force patterns at the ankle, knee and hip is nectssary to see these two independent synergies. Figure 6 plots the moment patterns for an individu- al subject who was asked to walk her natural ca- dence over nine different days while a full bio- mechanical snalysis of one stride was completed [54j. The solid lines represent the average moment- of-force over the nine strides, the dotted line was one standard deviation at each 296 of the stride period. With extensor moments as being positive the support moment. MI, was calculated as: The

interpretation ofthesignificance of M,is that it rcprcscn:s the total extensor thrust of the lower . - DYNAMIC BALANCE OF H.A.T. DURING GAIT Fig. 5. Thc morncnu of inertia of the head, arms and trunk (H.A.T.) a alalatcd about diffcrcnt rotation poin~s: 1, - about the antrc oi mau o( H.A.T.; 1,- about rhc hip joint: I, - about thc kncc joint: I. - about the rnkk joint. Incrcrsins incnial had, from proximal to distal joints demonstrate the morc c[rkcn~ control of H.A.T. by the hip muxk ~han by the anklc, which is dcmonstratcd by thc kinetic pattcmduring gait. limb during stance. An anklc

extensor moment, for example, will accelerate all body segments above the ankle in an upward direction, the knee will do the same for all segments above the knee joint, and so on up to the hip. Thus it cannot escape notice that M, docs reflect the shape of the vertical ground .reaction force, which represents the net vertical acrcleration of the body's C of M. All the subjects and patients assessed at the Gait Laboratory at the
Page 11
rXmCN1 of fORCC (natural cadence, n.9) 1 -_1- I 0 0 0 0 0 * * w :. 0 - % of STRIDE: mechanics of stance has shown that the kinematia of the thigh, leg

and foot can remain virtually the same with different combinations of moments at the th;ee joints [56]. However, if we examine the moment curves separately (Fig. 6) we note that the greatest variability is at the knee and hip. If the motor patterns at these joints varied randomly with respect to each other then the sum of the curves would have even higher variability. But the varia- bility of M, is seen to k greatly reduced. Thus the stride-to-stride moments did not vary in a random manner, rather there is some considerable cancel- lation of variability taking place in this M, summa- tion.

This is seen in the summation of hip + kne: where the coefficient of variation (CV) drops from the &70% range to 21%. Therefore some signif- icant coupling exists between the motor patterns at thex joints. fig. 6. Moment-of-form profila lor ninc rcpt ralkingtriahof .If the motor patterns at any two joints are .Corn- the same subject done da~ span. Mcan (rotid line) and stan- pletely independent of each other their covariance + dard deviation (dolled line) of ninc trials arc plotled over the would be zero. if there is some coupling it can be stridc pcricd which is nonnalizcd to 100%. Sce text

for details. quantified with a covariance gar the University of Waterloo (in excess of 200) had a positive'M, during stance in spite of considerable variability at individual joints, especially at the hip . and knee. The consistency of M, is what causes the angular kinematics of the ankle, knee and hip joints to k quite invariant [56], and documents lhe CNS motor control over the collapse of the kne= . above-mentioned subject along with that from s second subject who was assessed over I0 repeat trials, but whose data were collected minutes apart rather than days apan the covariances are

present- ed in Fig. 7. Thc calculation of these mean varia- nc:s and covariances is based on the following quations: with all units in (N - m)': joint during stance. gL = a: + o: - oi,, If we look at the support moment we see that for the tint half of stance (0-30%) all three joints where: contribute to the net extensor pattern. This means that all three motor patterns contribute to the mn- trol of the amount of knee flexion: the ankle plan- tarflexon by controlling the forward rotation of the Ies over the foot; the knee extenson by direct control; the hip extensors by controlling collapse of

the thigh from the proxhnal end. , The variability of these motor patterns is also extremely irnprtant in ascertaining the consisten- cy of this synergy. An examination of the bio- ~ and of are the mean variances over stancc at the hip and knee $,,, is the mean variance of the sum of hip i knee moment profile & is the covariance between hip and knee mo- ment pattenn 4, can be expressed as a percent of the maximum possible covariance, which would be 1@0% if
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fig. 7. Covariance ktwecn the moment patlcrru seen at he hip. knce and ankle of two group 01 repeat walking trials: klt

plot - nine repeat trials dap apan, right plot - Icn rcpeat [rials dw rninutcs apart. High variances wcrc wcn at thc hip and knce. but high covariance wu cvidcnt ktwun the hip and knee patterns and modcntely high between the ankle and knce pto- files. Scc text lor dctailcd interprc~xtion. d,, = 0, and would mean that the variability in the knce moment was completely opposite with that at the hip. Thus, the percent covariance, COV, is given by: In the first set of repeat trials the COV between the hip and knee was 89/0. A similar calculation can be made at the knee and ankle, and this yields a

COV of 76%. Thcse high covarianccs are extremely strong evidence of tight coupling between the mo- " tor patterns at these adjacent joints and are not surprising when we consider the opposite and can- celling functions of the hamstrings and rectus femo- ris muscles at the hip and knee, and of the gastroca nemius muscle at the knee and ankle. Therefore this tight coupling is partially due to the anatomy of these biarticuiate muscles. Anatomically, the po- tential for force generation can be considered to be approximately proportional to the physiological cross-sectional area (PCA) of each

muscle. How- ever, the single joint musclcs constitute 2/3 of the PCA of all the musclcs crossing the hip and knce joints [52]. Thus, the magnitude of COV is well in excess of that pdssible anatomically, and therefore must be part of a neural control pattern. The lower COV's from the second subject's triab (7396 be- tween the hip and knee and 49% between the knee and ankle) appears to be due to the lower varia- bility (adaptability) seen over these repeat trials that took place minutes apart. Thus it appean on a day-today basis we adopt patterns which have larger differences than over shorter

pcriods of time. It is important to recognize that these day-to- day and minute-to-minute alterations are largcly very deterministic and reflect the plasticity of the motor control system. It is now important to xe the nature of thuc stride-to-stride trade-offs and it will be seen that they arc related to a xcond motor synergy, that of balance. 2. Stridc-@-stridc and subjccf-10-su bjccr diffcrcnccs figure 8 is presented to identify the nature of these stride-to-stride differences [56]. Two of the nine trials for this subject were selected along with the mean of the nine strides (solid line).

WM22D is shown by the long dashed line, WMZU by the short daslied. During the J trial the subject had a dom- inant hip extensor pattern and a knee flexor pat- tern, while D trial was biased in the revene direc- tion: hip flexor and knee extensor. Thus the dom- inant musde pattern for the J trial was hamstrings . (posterior muscles) vs. the D trial which was pri- marily rectus femoris (anterior muscles). The net extensor pattern acting on the thigh for bth of the strides was about the same but in one case it was being accomplished by anterior muscles, in the other case control was in the hands

of posierior muscles. Thus this anterior1post:rior trade-off did not influence the net extensor (support pattern). In a similar manner we can see far more dramatic differences when we compare two subjects..Fi yre 9 is a similar set of moment patterns for two sub- jects walking their natural cadence. Both subjects had similar lower limb kinematics and as can be seen the support moments were almost identical.
Page 13
Fig. 8. Two trials xlcacd from the nine rcpt trials (Fig. 6) to show the nature of the nrialioru at the kncc and hip. Sec text for dctails. WfUB had a dominant hip

extensor p~ttern, knee flexor pattern and a higher plantarflexor Go- ment, while WMZOA was biased to a hip flexor pattern, knee extensor and a lower plantarflexor profile. The major difference between these two subjects was a bias of WM24B towards greater activity in his posterior muscle group and WMZOA towards his anterior muscle groups (or in the case of plantadexon. a much lower level of activity). In spite of these considerable.differenccs at all three joints the total extensor synergy, as indicated by the suppon mom&t. was essentially thesame. This comparison is further evidence that

contradicts the generalization of Pierrot-Deseilligny, Bergcgo and Mazieres [45] that the 'quadriups contraction is of paramount impnancc during stance phase of hu- man gait'. In fact, we have noted patients with knee pathologies who walk fairly normally but compcn- sate without their use of quadriceps during all of stance (551. The question remains. are these changes seen across al; subjects or across many strides of the same subject? Figure 10 is a plot of the mean knee Fig. 9. Analpa of two subjas wk hcr limb kimrtiu vrr rpproximately the wme and yet vho gcncratcd to(a1ly dilfcrcnt motor

pattcrns at an (hrcc joints. See lcxt lor dctails. moment vs. the mean hip moment during stance for the nine repeat day-to-day trials of subject WM22. As can be seen the slope of the linear regression is effectively minus 1, meaning that there is a cne-for-. one trade-off for each individual walking trial. Trials plotted within quadrant two documented a dominant hip extensor and knee flexor pattern; within quadrant four there was a dominant knee extensor and hip flexor pattern. To see if this trend extended over a large number of subjccts walking a range of cadences a plot was made of the same

variables (Fig. 11). The regression for thcx 54 separate subject trials was also effectively -I. This plot documents two major synergies. one of which has already been discussed. The one-for-one trade- off of moments between the knee and hip shows that there is a constant net extensor moment acting on the thigh during stance, and this will further explain why the support moment (which is the sum of this thigh moment plus ankle moment) is so consistent. in spite of major individual differences
Page 14
HERN HIP MOMENT vs HERN KNEE HOHENT DURING STRNCE P FLEXOR .J .2 . .I . 0 <, -.I .

-.Z . -.3 KNEE tlOHENT/BODYHASS (N.M/Kg) .. . . . . . fi. 10. Mean hip mornen1 wnm man knce moment during stam for the nine rqul trials of thc wmcmbjd (Fgurc 6). Rcgrdn rho- a slope ol -I which poinu to a one-for-onc rradcdff ktwcen the hip and knee. h trials in quadrant Zrcpresrnt r dominance of thc ptcrior rnudcs (lip cxtcnson. knee flcxon); thox plotted in quadrant 4 rcprcxnt r dorninancr of anterior mda (knc: extenion, hip flcxon). Thae stride-~estridc tradcofrs in Lhc antcriorlpostcrior direction arc an indiation of a balancc syncrgy to amtrol H.A.T. WM22 INTRASUBJECT: DRY-TO-DRY Mh -

0.002 - 1.01Mk (R - -0.90, P <.001) EXTENSOR ANTERIOR . L - v at the knee and hip. The second major synergy is ourselves that H.A.T. represents 2A of the body related to the anteriorfposterior trade-off and has mass, these upper body accelerations represent a been shown to be highly correlated with the bal- large ballistic load which must be controlled by the ancc of H.A.T. [57]. lowcr limb muscles. Presumably, this control must be exerted during stance. To decelerate the trunk or pull it more upright the gluteus maximus and 3. ~alcnce synergy hamstrings would have to be more active. Con-

venely, to accelerate the trunk foward or to cor- On a stride-to-stride basis there is consistency in rect its posture in a forward direction the iliopsoas, the kinematics of the ankle and knu joints (CV's rectus femoris and vasti muscles would have to be of 9% and 10%) for a'n individual subject. On these more active during stance. Thus balance and pos- =me repeat analyses the head horizontal acccler- ture control u an antcriorfposterior regulating task ation and trunk angular acceleration had CV's of that changes from stride-to-stride, and these 106% and 124%, respecti-~ely. Whcn we remind

changes are evident in the random kleaion of
Page 15
MERN HIP MOMENT vs MERN KNEE MOMENT n I r FLEXOR -.4 -.3 -.2 -.I 8 -1 -2 -3 -4 .S -6 -7 .B HEAN KNEE HOflENT/BODY~RSS DURING STANCE (N.H/Kg) fib. 11. A similar plot ol the mun hip wnm man khee rnorncnt for 54 wbjear -king fut (I). natural (n) and dow(s) cadenas. SJopc of mgrarion h effcaivrly - 1 demonrtrating the one-for-me trade-off across subjcdt. Thue ample strides from many individuals are 'p(orted to demo'mtratc a population trend. and points to J single 'operating' tine along which all cubjm nq. strides that were measured

and plotted in Figs. 10, and 11. However, in the procas of correcting bal- ance there was no significant change in the net extensor (support) pattern. Thus, the control of milapse of the lower limb appears to be quite indc- pendent of the control of posture and balance. From a biomechanical perspective (Fig. 2) the easiest way to control H.A.T. would be by the hip flexor and extensor muscles and this control would take place during stance, primarily during single support. This has, in fact, been shown to be true (571. For the nine repeat trials on subject WM22 it was shown that the correlation

between the hip moment and the angular accelera!ion of the trunk was 0.93 duringsingle support, and the slope of this regression was within 10% of a text-book calcula- tion of the moment of inertia of H.A.T. This corre- lation dropped as the correlation period was ex- panded to include double support, which is not surprising when one considers that both ipsilateral and contralateral hip muscles can now mntrol H.A.T. Only when a simultaneous bilateral analy- .$is is done can the balance of H.A.T. be document- ed. However, based on our initial analyses it would be easy to hypothesize that the

control of H.A.T. is transferred from one limb to the other during each double support period. Thus the primary role of the hip muscles is to control posture and balance of H.A.T. The balancc synergy we see here is closely linked to the support synergy such that the knee motor pattern makes up the difference to supply a sufficient extensor pat-
Page 16
tern to control, along with the ankle plantarflexon, knee flexion. Because the balanu control system must continuously alter the anterior/posterior mo- tor patterns on a stride-to-stride basis there will k considerable variability in

the hip and knee motor patterns. But because of the one-to-one trade-off between the hip and knee the total extensor contri- bution to the support process is almost constant. IV. Balance control during perturbed locomotion In the previous sections the postural control system responses during standing and normal locomotion havc ken discussed. During normal locomotion as discussed, the hip joint moment conrrols the trunk stability on a stride-to-stride basis. This implies specific roles for various limb musculature during this important and inherently unstable task. The question that is

addressed in this section is: How does the postural control system respond to pertur- bations, both unexpected and anticipated, applied during normal locomotion? ?he answer to this question is important for our understanding of how the normal locomotor synergy is adapted to the needs of the environment. Some of the common perturbations that are encountered during daily activities are: stepping on an unstable surface or a sharp object; king jostled in a crowd; unexpected- ly stepping off a curb; hitting an obstacle during the swing phase of locomotion; and adjusting our step length to avoid

obstacles. Various researchers have tried to mimic these perturbationsin a laboratory environment and studied the responscs [5. 13. 36. 421. These are discussed next. To displace the body enter of gravity, researchers have manipulated the support surface or move- ment of a body segment during locomotion. Nash- ner 1361 used a movable force platform to apply an unexpected perturbation as subjects were walking. Because of the setup, the perturbations can be applied only dunng the stance phase of locorno- tion. He used three types of disturbances: rotation (+I-5 deg), horizontal (+I-8m) and

vertical (+I-Scm) displacement of the platform over 125 ms. These were applied during four functional phases of locomotion. The responscs were charac- terised by the EMG activities of tibialis anterior and gastrocnemius muscles, ground reaction forces and kinematics of the lower limb. The results dcm- onstrated perturbation specific responses that were similar to those obtained with the same perturba- tions applied during standing. For example, low- ering the platform elicited a response in the gas- trocnemius muxle and represents a part of the suspension synergy identified dunng standing.

This observation is limited since with two muscles it is difficult to compare these results with the ones obtained from standing where four muscles were monitored. However, the important finding of the study was the phase dependency of the response. The magnitude of the muxle response was mod- ulated. This was the lint study that examined the balance adjustments during walking. Despite some - limitations. it does demonstrate the adaptive na- ture of the postural system which makes it possible to successlully complete the task even in the pres- . cnce of perturbations. An ingenious way of

manipulating the support surface while the subject was walking is the one used by Figura et al. 1191 and by Deitzet at. [IS, 16, 171 and Berger et al. (51. These researchen aucler- atcd (4 to 9 kmhr in 70ms) or decelerated (4 to -1.5 kmihr in 60ms) the treadmill belt while the subjects were walking. The penurbation.impulse was applied during various times in the stance phase. The basic results obtained were in agree- ment with those found by Nashncr (361: the rc- sponses were phase dependent and perturbation specific such that falling was prevented. Berger et al. [5] monitored ankle muscles

and the kinematia while Figura et at. [19] measured responscs from trunk and various leg muscles and leg and head kinematics. To impede the limb movement during the swing phase of walking, researchers havc used a torque motor to hold and release the limb [I41 and sudden clamping of a wire attached to the ankle [Uj. In the latter study the duration and the magnitude of the resistance was not specified, while Deitz ct al. [I41
Page 17
used a peak force of approximately 200N over varying duration (20 to lams). Once again the functional nature of the response suited to the phase of

locomotion was borne out. For example, when the holding impulse was applied during the early part of the swing phase the major response was observed in the contralateral limb in mntact with the ground while during later part of the swing phase the same stimulus elicited a major response in the same limb to provide an earlier touchdown. . Both of these responses provide body stability in the presence of the external perturbation. Another common way of simulating a penurba- tion is by electrically stimulating a body segment. Recently we have attempted to use such a pcrtur- bation to study the

adaptive nature of the response [3,43j. The stimulus used was a train of pulses with the intensity set at a noxious level. The stimulus was applied to the second toe (with the ground on the top of the foot) during various phases while the subjects were performing various rhythmic move- ments ranging from treatmill walking, bicycling and overground hopping: As expected the phase dependency of the response was seen in all tasks. To elaborate further on how the nature of the task and phase affected the response we will discuss the results from overground hopping (431. The re- sponse for.the two

phases, while the subject was in air and on the ground, are summarized in Table 1. Inhibition of the soleus in the stance phase facil- itates the tibialis anterior in the withdrawal of the foot, while in the swing phase it probably allows for , the re-orientation of the foot segment for proper landing following the withdrawal of the foot by the earlier enhanced tibialis anterior response. The bi- ctps femoris response provides another insight into the functional specific strategy. Because the veni- cal impulse was not altered even though the stance duration was reduced and soleus was

inhibited. the enhancement of the biceps fenoris during stance phase is interpreted as an extensor response. In the swing phase, the same muscle response produces flexion at. the knee join! and extension at the hip joint, thus withdrawing the limb from the stimulus. This interpretation is confirmed by the increase ir. the swing phase duration as measured by the foot- switch signal. The contralateral limb lands at the regular time as measured from the form channel. It is dear from these studies that the response to unerp&ed perturbation generated by the system is functionally appropriate to

maintain stability and allow the subject to continue with the movement. The responses discussed occur within a hundred milli~conds of the stimulus and usually last for about IGO-150ms. Most of the researches have noted that the subject returns to their normal walk- ing pattern by the next step IS]. The return to a normal pattern quickly may bc a consequence of the use of the treadmill which affords little psibil- ity for deviations without compromising the sub- jects afety. In a recent expcrimcnt when the sub- jects' upper body was perturbed while they were riding a stationary bicycle or

hopping, speeding up following a perturbation was observed. This agrecs with the aftereffect of a stumbling reaction expcri- enccd by most of us at one time or another. The involvement of ankle musculature in maintenance of stability that the other researchers have found when the support surface is manipulated is in con- trast to the role of these muscles discussed in nor- mal walking in the previous section. This difference may k attributed to the site of perturbation.' How these responses are generated has ken a Table I. The response to stimulation during ho Qhrca ol overground bopping

(Pitla & Eelanger, 198i). A ' repracnu parameter not relevant for the phw. A - lhovrs no change. while Ihe m show tk~ direction of signifcant change (pCO.05). On yound Air hrnc Biceps fcmorir Soleus Tibialis anterior Tonpord chan~u From fora plate data stancc wing From ipsilateral fax switch stancx rving lmpubc changr
Page 18
subject of much research (5,171. To assess the role of the proprioccptive system. particularly Ia affe- rents, researchen have used ischemic leg block [5]. Based on the response following the block and the response latencies. it has been sugg&ted that the group

I1 and 111 afferents provide the peripheral . information to the nervous system. To address the hue of whether the response is generated by con- tinuous feedback or released by the CNS as a pre- programmed pattern, Deitz et al. [15] did a very interesting experiment. They applied different modes of perturbation either alone or in combina- tion and found that the first agonist burst induced by the perturbation was unaffected by the arrival of the second perturbation. When the holding stimu- lus to the swinging limb preceded the deceleration impulse of the treadmill belt, the first burst of the

gastrocnemius response to the holding stimulus was unchanged. Thus they concluded that the ini- tial response is fixed and is released by the CNS based on the feedback received. Thcse class of perturbations have received less at- tention, although one can argue that they are equally if not more important than the unexpected perturbations. he ability to anticipate perturba- tions that might occur as well as their effect on our body and take appropriate actions is possible. Rather than always hitting an obstacle duringswing phase, the limb trajectory to clear the obstacle is modified. Similar

alternative anticipatory actions can be described for other perturbations. Thus, it is unfortunate that not enough research is done in this important area. There are two major types of actic- ipatory perturbations: the ones applied to the up- per body and the ones induced by changes in the walking pattern. Some experiments from these two types of perturbations arc discussed next. Nashner & Forssberg (411 asked the subjects to pull or push a handle while the subjects were walk- ing on a treadmill. They used several experimental conditions. In one they asked the subjects to initi- ate the handle

pull or push at their own times. The results showed that subjects tended to initiate the disturbancc at heel contact. When they were asked to respond to a tone given at different times during the step cycle, the reaction times were similar for all phases of \he step cycle. The postural responses which were directionally specific, were observed to precede the onset of the arm aaivity. The rc- sponxs during the supporl phase were similar to the ones obtained during standing, while the ones obtained during the transition phase were differ- ent. Patla (421 used a different experimental paradi- .

gm to investigate the adaptation to expected per- turbation during walking. Subjects were asked to flex their arm as rapidly as possible in response to a . visual cue given during different times during the cycle. The results obtained were similar to those found by Nashner and Forssberg (411. The subjec:s showed similar reaction times during all phases of the step cyde. The postural response onset with respect to the arm activity onset was modulated. During the support phase, the postural response preceded the arm activity, while during the swing phase the response was observed following the

arm activity. Based on the mechanic; of gait and the temporal patterns, it was argued that this was func- tionally appropriate to provide body stability. For example during swing phase, the later activation of the biceps femotis (postural rtsponse) allowed the subject to shorten the swing phase and provide earlier double support for improved stability. In recent experiments when elderly subjects were ;xamined while performing rhe same task, some interesting results were obsc&cd. Unlike ybunger subjects. the elderly tended to modulare the reaction time to the visual cue given during various

phases in the step cycle (Table 1). During the single support phase (rns) when :he stabili~y Tablc2. 0nx1 of anterior deltoid latcnqwith rqxa to a rnual cue given duringstanding (St) and three phaxsol the stepcyric: HC- Heel Contna: MS- Mid Stance: and TO - Toe-off. Data is lor ten suhjccfs. Mean (ms) 198 183 239 226 Standard deviation (ms) 42 52 82 58
Page 19
requircmcnts are more critical, the subja de- the information one step duration earlier had no layed thc onset of disturbance. In contrast to the . problems shortening the stcp. When the informa- younger subjects, their walking

patterns as mca- tion to adapt the locomotor syncrgy and what the surcd from the stance and swing phasc duration did adaptation required is, arc intimately tied together not alter. Thus instead of initiating the arm move- [45]. The time rcquircd to plan and exccutc suc- mcnts at the same time and modulating thc walking cessful shortening of the step is quite different from pattcm as the youngcr subjects did, the elderly thc time necessary to successfully changc direction: chose to initiate the arm rnovcmcnts at times which although thcy an shonen thc step length with the providcd better

stability. This reprcsenu a func- contralateral foot after the ipsilateral foot has land- tionally appropriate adaptive strategy. The re- ed during walking, thcy arc unable to changc direc- sponses to cxpcctcd perturbation show similar tion [45]. Thus planningand adaptation of the loco- char~ctcristics as thc oncs due to uncxpcacd per- motor pattcrns to achieve the intended goal is con- turbations: phase and perturbation specificity. : strained by the more impnant goal of maintaining Body stability can be threatened whcn thc loco- stability. motor pattcrns havc to be quickly ad~ptcd baxd on

environmental cues such as modulation of stcp . length to provide proper placcrncnt of thc foot. V. ~onclusions Rcscarchcrs havc studied how thc subjects mod- 1 ulate step length in response to visual cues given at The following statements arc justified based on our diffcrcnt times during a trial (441. It is clcar that ; cuncnt knowledge on the postural control systcm. stability has to be of prime concern during these 1. The postural syncrgy cnlistcd by the nervous changes. This is evident when onc cxamincs the systcm is task and penurbaiion speefic. The success rates (Table 3). When thc

subjcct~ arc giv- evidence for task specificity can be clearly seen en information about changing the step length with in he differences in the balance response during the contralateral foot just whcn thc ipsilaleral foot standing and walking. For cxampie, during has landed, success rate of shortening the step walking the goal is not to maintain the centre of (30% of the normal) is considerably reduced for ' gravity within the fw:, while during standing it rhc running trials. Shortening the step involves by is mandatory. The site and type of perturba:ion definition reduction in thc forward

momentum. elicits different rcsponscs in character and orga- Given such a shon time to execute the changc in nization. The overall aim of the balance rc- which part of the lincar momentum may be con- sponse is functional'such thzt stability and the vcrted to angular momentum and cause the subject goal of the task at hand is maintained. to potentially fall forward, subjects chose not to 2. It follows from the above that a single asses;;- initiate the change. The same subjects when given mcnt technique cannot be used as a truc in- dicator of the overall integrity of the balance control systcm.

Simple temporal rncasurcs tak- en during maintcnancc of static posture arc commonly used in clinics. Before we acccpt those measures as valid indimton their correla- tion with other more comp;cx tests of balanc: must be shown. it may be that we rcquire a battery of tests to properly assess the balance control system. The challenge would then be to develop thesc tests that can bc easily and cffi- cicntly applied in a clinical setting. Table]. Pcrccnt succca rrlc in modulating stcp lcntrh with the cuntralatcral foot during locomotion when the informalion lo changc is given just rftcr the

ipiiatcral fwt has landed on the ground. Data is from !en subjects. Thc shon s~cp vu;W'olthe normal. whilc the long step was 130% of the normal during mnning and 50% and ISOX during walking. Walking Runnins Shon stcp Long step Shon stcp Long step .Avenge 88% 83 % 32% 56% Rantc (60-100) (80-100) (0-70) (I-)
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;he authors gratefully acknowledge thc support kern their individual operating granb (MRC 4343. NSERC #@I70 and #2917) and a joint rating grant from the National Health and Wcl- . Canada. . . 1. Badkc MB. Duncan PW. Patterns of npid motor rcqxma during ptural adjustments

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