Comparing perturbation rejection of karate experts and novices in shiko-dachi stance

A person's movement stability and robustness to perturbation has a direct correlation to their quality of life [1], with exercise training helpful in reducing fall risk [2]. Perturbation rejection from experts [3] may shed insight on how to maintain balance after perturbation. Karate consists of tactics to maintain balance for the self while removing balance for their opponent, making this an interesting aspect to study. Do karate experts’ biomechanical reaction differ from novices when perturbed during shiko-dachi? Three karate practitioners and three novices were instructed to take the shiko-dachi (defensive stance) where centre-of-mass (COM) is lowered and the legs out to the side in a widened stance. They are pushed with an instrumented stick, then allowed to return to balance, three times in a row. The pushes were delivered at the back (T10 vertebrae) and at the right shoulder (humerus head). The centre of pressure (COP) is estimated from force plate. The resting support polygon (SP) was calculated (Fig, top). The COP trajectory area (COPA), calculated as the area under an ellipsoid fit over the COP trajectory [4], is estimated for both resting (Fig, resting), as well as over the three pushes (Fig, during perturbation). To normalize for the stature of participants, the ratio of the COPA to the SP is also plotted (Fig, bottom). Fig: Resting SP and COP trajectory range of different novices (left) and expert (right) response to being pushed from behind (top) or from the right (middle), with peak applied force denoted. Resting SP area and the SP:COPA ratio before and during perturbation is also plotted.Download : Download high-res image (251KB)Download : Download full-size image When standing in the shiko-dachi stance, karate practitioners tend to keep a wider stance (shoulder to feet width difference of 194%), whereas novices tend to take a much narrower stance (94%), as well as significantly lower COM (resting to stance COM height difference of 15%) compared to novices (5%). These differences can be observed in the SP area box plots as a comparison between the two groups (Fig, bottom left). The expert tends to sway a bit more during resting, with a higher COPA:SP ratio compared to novices, while during perturbation, the inverse is true (Fig, bottom). Individuals with prior experience in karate shift their posture significantly more when entering a defensive pose, and is also less rigid in their stance. The posture and resting sway, combined with higher general levels of fitness [2] allow them to counter the applied pushes with smaller COP trajectory range, and thus they are at a lesser danger of falling over and not require alternative recovery strategies. Future work will analyze addition stances: sanchin-dachi (three battles), zenkutsu-daci (forward), and nekoshi-dachi (cat), over 9 push locations.