Abstract
Maximum cushioned shoes could reduce the peak vertical Ground Reaction Force (GRF) during running, but it may increase the foot instability in horizontal directions, especially at the initial foot contact. Sample entropy (SampEn) has been adopted to define the irregularity to quantify levels of complexity of movement and examine the fluctuations in GRF within a time series. The foot instability would be better represented by the index of SampEn within a stance phase instead of variation among different stance phases viewing instability as errors. PURPOSE: this study was aimed at examining the complexity of GRF while running on the treadmill with different types of footwear through SampEn. METHODS: 19 experienced runners were recruited and ran on the treadmill at 7.8 mph wearing different types of running shoes: max cushioned shoe, minimalist shoe, and regular shoe. GRF of 10 consecutive steps were collected by the instrumented Tandem treadmill. GRF data were further analyzed to calculate sample entropy for both the first 20% stance phase and entire stance phase. Two-way MANOVA was used to examine the effects of independent variables (shoe, step) on sample entropy measures at first 20% stance phase (SampEn20%-ML, SampEn20%-AP, SampEn20%-VT) and entire stance phase (SampEnML, SampEnAP, SampEnVT). Post hoc Tukey test was applied as needed. RESULTS: a significant shoe effect was observed on the association among dependent variables listed above (P < .05). Cushioned shoes displayed a higher SampEnAP (.1162 ± .01558) than regular shoes (.1094 ± .0258) and a greater SampEn20%-AP (.1953 ± .1556) than minimalist shoes (.1546 ± .0932). Minimalist shoe exhibited greater SampEnVT (.0614 ± .01310) and SampEn20%-VT (.0636 ± .0688) than cushioned shoes (.0577 ± .1002; .0448 ± .0305) respectively. No other significant difference was observed. CONCLUSIONS: in general, cushioned shoes displayed more fluctuations of GRF in the anterior-posterior (AP) direction compared to minimalist and regular shoes, which indicates cushioned shoes may increase the foot instability in AP direction during running. Greater fluctuations of vertical GRF when wearing minimalist shoes may indicate a reduced foot stability that could affect the impact absorption at foot touchdown and force generation in push-off.
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