Phase-dependent modulation of proximal and distal postural responses to slips in young and older adults

Background. Phase-dependent modulation of postural responses plays an impor tant functional role in integrating reflexes into ongoing locomotion behavi ors. This study tested the hypotheses that proximal and distal postural res ponses are modulated differently according to the phases of the gait cycle in young adults and that there is a decline in this modulatory ability with normal aging. Methods. Thirty-three healthy young adults (age = 25 +/- 4 years) and 32 he althy older adults (mean age 74 +/- 14 years) participated. Subjects walked across a movable force plate with its movement timed to heel strike or mid stance to simulate a forward slip occurring at different times during the g ait cycle. Surface electromyography was recorded from bilateral leg, thigh, hip, and trunk muscles. Kinematic data were collected from the right (pert urbed) side of the body. Results. Postural responses to the heel strike slips occurred more frequent ly, and were of shorter latency, longer burst duration, and greater burst m agnitude, than those in response to the midstance slips. Whereas the early and predominant postural responses came from bilateral tibialis anterior, r ectus femoris, and biceps femoris muscles in heel strike slips, early postu ral responses were observed in bilateral erector spinae muscles in midstanc e slips. The late postural responses in midstance slips (from bilateral bic eps femoris muscles and medial gastrocnemius and tibialis anterior of the n onperturbed leg) assisted in foot liftoff of the perturbed leg and earlier and safe landing with the nonperturbed leg. In response to the heel strike versus midstance slips, older adults preserved the phase-dependent modulato ry abilities of the occurrence, onset latency, and burst duration of their postural responses, but not the ability to modulate burst magnitude, as com pared to young adults. Conclusion. Postural responses from the proximal and distal muscles in reac tion to different temporal phasing of slips during the step cycle showed di fferential modulation to meet the different task requirements. Older adults preserve this modulatory ability but with limited capacity. Physiological or psychological factors may influence older adults' phase-dependent modula tory capacity.