Background Sensori-motor insoles are supposed to influence muscle activity of the ankle stabilizing muscles during stance phase of human gait by imposing pressure upon tendon structures. For this purpose, specially shaped elements are integrated in the insoles. In particular, activation of m. peroneus longus as a main pronator of the foot is supposed to prevent ankle sprains. Nevertheless, no evidence for a potential effect of sensori-motor insoles could be shown until now.
The objective of this study was to examine changes in muscle activity of the peroneus longus (PL) and tibialis anterior (TA) muscle caused by sensori-motor insoles with a lateral wedge.
Methods For 32 healthy subjects the activity of the peroneus longus and tibialis anterior muscle was recorded by means of surface EMG (Noraxon Telemyo). A randomized, double blinded cross-over-design was used. Subjects walked in several sub-tests both in neutral shoes with sensori-motor insoles and with dummy insoles. Sensori-motor insoles (Shore 35, Springer AG, Berlin) had a convex-formed spot that was placed above the skin of the peroneus longus tendon, 8 mm distal to the inferior retinaculum. Pressure sensors under the heel and forefoot were used to identify gait phases.
In a former study changes in muscular activity peaks of the peroneus longus muscle could be found mainly in the period between 14% and 34% stance phase.1 Therefore, integrated EMG was calculated for both PL and TA for the periods of 0–15% (loading response), 15–50% (mid stance) and 50–100% (terminal stance). Sub-tests with sensori-motor insoles and dummy insoles were compared for each subject by statistical means (paired t-test). Effect size and post hoc power were calculated using G*Power 3.1.
Results PL activity in mid stance was statistically significant higher (p=.0083, post hoc power =.98) with sensori-motor insoles (18.4±12.17 uV*s) compared to dummy-insoles (10.0±7.6 uV*s). Effect size (Cohen's d) was .80, indicating a large effect. For other gait phases and for TA activity no significant effects (p<.05) could be found.
Discussion As changes in peroneus activity could be found only in mid stance when both the body weight is transferred to the foot and the sensori-motor spots impose pressure on the skin we conclude that increased muscular activation is due to increased afferent input. In both initial contact and terminal stance the sensori-motor spots do not influence afferent input and motor activation pattern of peroneus longus. As the ankle joint is sensitive to disturbances (supination) during mid stance an increased activity of pronator muscles might help to stabilize the foot and possibly help to prevent ankle sprains.
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