The gastrocnemius muscle stiffness and human balance stability/Dvilypio blauzdos raumens standumas ir zmogaus pusiausvyros stabilumas

The aim of this study was to assess the dependence of postural stability on stiffness of the leg muscles. The stiffness of medial head of the relaxed and actively contracted gastrocnemius muscle was measured. The posturogram was registered during quite standing with relaxed and actively contracted leg muscles. The contraction of the leg muscles significantly increased maximal shift of center of pressure (COP) in transverse and sagittal direction, the mean velocity of COP, and muscle stiffness. Multiresolution analysis based on discrete wavelet transform using 4th order Daubechie family wavelets was used for COP signal decomposition. The power of component of higher frequency significantly increased with active maximum contraction of the leg muscles. Therefore, COP oscillation frequency tends to be higher when subject actively contracts the leg muscles. High correlation observed between muscle stiffness and mean velocity. High coefficient of determination was established between the power of component and mean velocity in high frequency scale. Thus it can be stated, that active leg muscle contraction increases muscle stiffness and decreases postural stability during quiet stance; also it increases COP oscillations in high frequency scales.