THE EFFECTS OF TESTING TECHNIQUE ON THE PERFORMANCE OF - PDF document

THE EFFECTS OF TESTING TECHNIQUE ON THE
THE EFFECTS OF TESTING TECHNIQUE ON THE PERFORMANCE OF CHEST PROTECTORS IN TAE KWON DO F. Tsui and M.T.G. Pain Loughborough University, Loughborough, UK E-mail: F.Tsui@lboro.ac.uk, Web: www.lboro.ac.uk INTRODUCTION In WTF competition Tae Kwon Do (TKD), points are scored by delivering forceful blows to the torso and head. As such, each competitor is required to wear head and chest protection. While there have been many advances in head protection from helmet studies motivated by American football and ice hockey, limited research exists on chest protectors, or ‘hogus’ as they are called in TKD. Even more limited are the methods that have been developed to assess their effectiveness. The British Standards requires that trunk protectors be placed on a rigid anvil and tested with a rigid impactor. However, this type of test is severely limited. Pain et al. (2008) showed that rugby shoulder pads tested in a similar fashion deformed in a point-elastic manner and only tested the pad’s material properties. Even during a tackle, force was only reduced over the bony bits of the acromio-clavicular joint and showed no change over the surrounding soft tissue. Thus, the pad performance was dependent on the properties of the colliding bodies as they were in series. This test also failed to account for the actual injury mechanisms that the pads were trying to prevent. The following explores how different testing techniques can affect the performance of a TKD hogu. METHODS Individual hogu performances were assessed using the ‘Standard’ method and ‘Kicking’ method. Surface forces were measured at 500 Hz using two Tekscan F-Scan sensors (Boston, MA), while the transmitted forces were collected differently in each test. The ratio between peak surface and transmitted forces were labeled as the Protective ratio or ‘P-ratio’. In total, four hogus were tested; three were commercially available (Adidas, Adidas Electronic, Kwon), while the fourth was made from inserting medium-grade Confor foam (C-45) into the shell of the Adidas Electronic hogu. The ‘Standard’ method re-created the tests outlined by the British Standards. The hogu was placed on a force plate (Kistler 2981B12) set at 2000 Hz, while a modified 2.75-kg shot put was released from a height of 0.45 m by an electromagnet to produce an impact energy of 12 J. The impact location was controlled by the placement of the foot sensors. In the ‘Kicking’ method, two expert martial artists performed rear-leg roundhouse kicks to the instrumented hogus worn by BOBXL (Century Fitness), a martial arts training dummy (Figure 1a). Its base was set flush against a wall and a high-density foam was inserted between its back and the wall to minimize rocking of the overall unit. 11 Vicon MX cameras operating at 250 Hz were used to capture 11 opto-reflective markers to obtain joint kinematics for the hip, knee, ankle and foot. Impact force data were captured by a miniature force plate strapped to BOBXL’s chest (Figure 1b). This plate was 150 mm x 100 mm and consisted of four tri-axial force transducers (Piezotronics) recording at 2000 Hz. Force and motion data were both captured by Vicon Nexus software. (a) (b) Figure 1. (a) instrumented hogu on BOBXL; (b) mini force plate strapped to BOBXL’s chest. RESULTS AND DISCUSSION Overall, hogu performance in the ‘Standard’ test (Table 1) did not mimic the ‘Kicking’ test (Table 2). For a given pad, the former produced higher surface and transmitted forces, higher P-ratios and lower contact ti

mes than the latter. This was attribu
mes than the latter. This was attributed to the rigid nature of the anvil and impactor and not the kicks themselves as foot velocities were near the peak linear foot velocities of 13-18 m/s reported for other elite martial artists (Lai & Wei, 2002). Impact forces were also similar to the literature but could not be compared due to disparities in methodology. Higher contact times in kicks were explained by the visco-elastic nature of both the foot and BOBXL. Table 1. Hogu Performance for ‘Standard’ Method. Pad Tekscan Force Plate P-Ratio Force (kN) CT (ms) Force (kN) CT (ms) Adidas 2.40 ± 0.18 21.4 2.78 ± 0.10 20.2 1.16 Adidas Electronic 2.08 ± 0.16 24.2 1.75 ± 0.08 24.5 0.84 Kwon 2.75 ± 0.22 17.8 3.56 ± 0.35 12.8 1.30 Confor 1.66 ± 0.11 47.8 1.15 ± 0.06 46.0 0.70 For all kicks, the pads performed quite consistently, while the performance in the ‘Standard’ method was dependent on the hogu. In particular, the Kwon and Adidas increased in force as the impacts were transmitted through the pad (i.e. P-ratio �1). This was likely due to the: a) material deforming too quickly to attenuate the rigid impact, as supported by the lower contact times; and, (b) under-sampling due to multi-plexing and low sampling frequency of the sensors. While sensor performance was a concern, it was present for all trials suggesting that the relative difference between P-ratios would likely remain similar (i.e. Confor still better than Kwon). The relationship between P-ratios were consistent except for the Adidas Electronic hogu, which did well in the ‘Standard’ method but scored poorly during kicks. Two possible explanations were that the materials in this hogu: a) provided adequate local resistance, but transmission increased since the foot was more massive with a larger contact area; and/or, b) were strain-rate dependent since, at impact, the shot put was travelling at ~2.95 m/s versus 12+ m/s for the majority of all kicks. CONCLUSIONS Current performance tests of hogus are merely an examination of their material properties and do not reflect their performance during typical use. As such, it is important to develop a performance test which stresses the hogu as it would be used in competition. REFERENCES 1. Pain, MTG et al. J Sports Sci, 26 (8), 855-862, 2008. 2. British Standards, BS EN13277-3:2000. 3. Lai, WB & Wei, X. ISBS, Cáceres, Spain, 2002. Table 2: Hogu performance for the ‘Kicking’ method for both subjects. CT represents contact time. Subject 1. Age: 36, Mass: 93.6 kg, Height: 1.74 m Pad Foot Velocity (m/s) Tekscan Force Plate P-ratio Force (kN) CT (ms) Force (kN) CT (ms) Adidas 12.19 ± 0.50 1.92 ± 0.33 41.6 ± 2.8 1.02 ± 0.22 42.4 ± 5.2 0.53 ± 0.09 Adidas Electronic 12.00 ± 0.55 1.63 ± 0.34 40.8 ± 4.1 0.90 ± 0.20 45.0 ± 1.7 0.57 ± 0.17 Kwon 11.76 ± 0.58 1.52 ± 0.29 82.6 ± 26.8 0.86 ± 0.22 59.4 ± 4.7 0.57 ± 0.16 Confor 12.13 ± 0.45 1.52 ± 0.48 91.7 ± 22.1 0.69 ± 0.21 48.2 ± 5.6 0.46 ± 0.07 Subject 2. Age: 25, Mass: 82.4 kg, Height: 1.84 m Pad Foot Velocity (m/s) Tekscan Force Plate P-ratio Force (kN) CT (ms) Force (kN) CT (ms) Adidas 13.59 ± 0.47 1.95 ± 0.41 42.6 ± 2.5 1.24 ± 0.18 36.0 ± 2.1 0.65 ± 0.15 Adidas Electronic 14.76 ± 0.32 1.41 ± 0.27 49.8 ± 20.4 1.02 ± 0.12 31.6 ± 3.5 0.74 ± 0.12 Kwon 14.21 ± 0.55 1.59 ± 0.48 105.4 ± 19.8 1.13 ± 0.16 31.2 ± 4.8 0.78 ± 0.28 Confor 14.47 ± 0.53 1.74 ± 0.42 108.2 ± 3.9 0.87 ± 0.28 34.8 ± 3.6 0.53 ± 0.20