Reflex modulation in human movement and posture.
Human soleus H-reflex gain was measured in supine lying and in standing vertically while stabilised
by a backboard. H-reflex amplitude was less in stabilised standing than in supine lying. The reduction
was partly due to the effect of gravitational load. When the same load was applied (by compression of
the body between shoulders and feet) while lying supine the corresponding reduction was 70%. The
results are discussed in relation to possible gravitational load receptors.
In a second series of experiments a collapsible landing platform was used to differentiate between
reflex and programmed contributions to EMG activity in landing from a jump. Post-landing activity
of the calf muscles was a short latency spinal reflex triggered by ankle rotation. In the rectus femoris
muscle, activity was programmed for short falls and had a reflex component in longer falls. When the
collapsible platform caused a landing to occur at a time later than anticipated, reflex gain was
increased in the gastrocnemius, biceps femoris and rectus femoris muscles. Experimental results were
consistent with the time that would be required for descending pathways to modulate the reflex gain
and an appropriate model is proposed.