Postural modulation of the soleus H reflex in young and old subjects

The influence of different static postures on the soleus H reflex was assessed in 15 old (mean age = 76.3 years) and 10 young (mean age = 24.2 years) subjects. H reflex and M wave recruitment curves were obtained under 2 randomly administered conditions: (1) standing; and (2) prone. Once in place, the recording and stimulating electrodes were not removed until the completion of testing, to ensure that exact placement was maintained. A 1 msec current pulse was given transcutaneously to elicit the H reflex and M response. Static postural sway area (cm(2)) was assessed on a Kistler force platform using custom software (sample rate = 50 Hz/15 sec trials). Results demonstrated that the young subjects reduced the amplitude of the H reflex from the prone (H-max/M(max) = 73.6%) to the standing (H-max/M(max) = 59.9%) condition, whereas the old subjects did not (prone = 32.4%, standing = 38.2%). However, within the old group, 2 subgroups emerged - those who depressed the reflex similar to the young subjects (O-D, n = 6) and those who did not depress the reflex (O-ND, n = 9). Furthermore, there were significant differences in postural sway scores between the young and old, between the O-D and O-ND, but not between the O-D and young groups. These results suggest differences in the manner in which young and old subjects modulate the soleus H reflex when standing, and support the view that modulation of the stretch reflex may be important in the control of static posture.