Hierarchy of different muscles in postural control

The effects of muscle spindle activation on different postural muscles were examined in eight volunteers, using a force platform technique. Small electrical vibrators were placed symmetrically over the muscles concerned and the effects of vibration frequencies of 20, 40, 60, 80 and 100 Hz were studied at a constant amplitude of 0.4 mm (peak-to-peak). Significant responses were observed in most muscles. The responses were observed (in maximum response order) from: the neck, triceps surae, gluteus, abdominal, hamstring, quadriceps, lumbar and tibialis anterior muscles. The response direction did not follow anatomical gonistic vis-à-vis antagonistic distribution of the muscles studied, indicating that it is the functional properties of the muscles that determine posture stabilization. The large inter-individual variability in responses, but the consistency in intra-individual muscle responses indicates that the subjects used afferent muscles to a varying extent in postural control. The vibration-induced activation of the muscles was presumably derived by activation of stretch-sensitive secondary endings of muscle spindles that control postural stability.