Postural modulation of the segmental reflex: Effect of body tilt and postural sway

This purpose of this study was to clarify the relationship between segmental reflex excitability and posture and to investigate potential mechanisms responsible for modulation of the H-reflex (HR) in unsupported standing. Soleus (S) and lateral gastrocnemius (LG) HRs were recorded from subjects (N = 12S, N = 11 LG) while their static posture was altered from supine to vertical (5 positions). This was compared to an unsupported standing position in which the subjects naturally underwent a small degree of postural sway, a dynamic posture condition. Although individual profiles suggested varied relationships between the S and LGHR and the angle of body tilt, the group data did not reveal significant differences. There was, however, a significant (p < .01) decrease in the S (43%-49%) and LG (34%-46%) HR when subjects stood without support compared to all static postures. This decrease occurred even though the tonic or background activity of the S and LG was present only when subjects were free standing. To determine whether weight-bearing was responsible for the HR depression, 3 additional conditions were compared (N = 3), supported standing without weight-bearing (90 degrees NWB), supported standing with weight-bearing (90 degrees WE), and free standing. Again, S and LGHRs were depressed only when subjects were free standing. Presynaptic inhibition presumably accounts for the depression of the HR in unsupported standing. Data from 8 of the subjects were collected under the same 6 conditions using a shorter interstimulus interval (1 Hz stimulus instead of 0.1 Hz) which produced low frequency depression (LFD) of the S and LGHR. LFD reduced the amplitude of the S HR an average of 43% (p < .05) when subjects were in a supported static position but only 21% when subjects were free standing. Although tonic activity of the S was present only when subjects were free standing, in 2 (of 8) individuals the EMG in free standing was not measurably different from static conditions. In these individuals, free standing still depressed the SHR by 35%; however, the shorter stimulus interval now produced the same degree of LFD when subjects were free standing (35%) as when they were standing with support (37%). The data suggest that 2 presynaptic mechanisms, although independent, interact to control spindle afferent feedback when subjects are free standing. Postural sway appears to be necessary to reduce the gain of the HR when subjects are standing, whereas, LFD is influenced by the degree of muscle activation.