Increased amplitude of cutaneous reflexes during human running as compared to standing

Abstract

The amplitude of H-reflexes is decreased during walking as compared to standing and a further reduction is seen during running as compared to walking. Does a similar reduction occur for reflexes elicited by cutaneous stimulation? To answer this question, the electromyographic (EMG) responses in biceps femoris (BF) and tibialis anterior (TA) to a 20 ms train of 5 electrical pulses, were recorded. This stimulus was applied to the sural nerve at the ankle, either at 16 different phases of the step cycle in human volunteers running on a treadmill at 8 km/h or at different isometric contraction levels of TA and BF in the same subjects during standing, imitating the postures of different phases of the step cycle. The mean latency of the main responses in BF of all subjects was 76 ms. Similar responses (P2), with a latency of 79 ms were seen in TA in 6 of the 10 subjects. For a constant strength of stimulation (2 times perception threshold) during isometric contractions at different levels in early stance imitation, the mean reflex ratio's (reflex/background) of BF and TA responses were 1.07 and 0.53, respectively, while the ratio's for these 2 muscles during running were 1.78 and 1.1. The higher reflex ratio's in running were primarily due to the large facilitatory responses, which were present during most of the step cycle but rarely during voluntary contractions in the subjects during standing. At the end of the swing phase, however, the responses in BF and TA were predominantly suppressive, as were most of the responses to stimulation applied to the standing subjects. The difference in reflex amplitudes between running and standing depended on stimulus intensity. At 1.5×T (where T = perception threshold) the responses were similar for both the standing and run conditions while they were significantly different for pulses at 2.5×T. The latter difference was due to an increment in the depth of suppression at high stimulus intensities during standing. It is concluded that running is a task during which there is, in comparison with standing and with the exception of a brief period at end swing, a general enhancement of sural nerve facilitatory reflexes. It is suggested that these reflexes contribute to the normal activation of muscles such as BF and TA during the step cycle.