Changes in the cutaneous reflex during co-contraction of the antagonist muscles in the lower limb

Task-dependent changes in the cutaneous reflex in the upper and lower leg muscles were examined in normal human subjects (n=11). After instruction, the subjects were asked to selectively contract agonist muscles (SC task) and to co-contract antagonistic muscles (CC task) for the ankle or knee joint while standing. The cutaneous reflex was elicited by applying non-noxious electrical stimulation to the superficial peroneal nerve at the ankle joint (200 Hz, 5 pulses). The EMG signal was rectified, averaged (n=10), cumulatively summated up to 150 ms after the end of the stimulation artifact, and then divided by the time interval for the summation (ACRE(150)). A strong inhibitory effect was determined at a latency of 50 ms and was followed by a facilitatory effect after the electrical stimulation during the SC task in all muscles. In contrast, it was found that the early inhibition and the later facilitation tended to be decreased and increased during the CC task, respectively. A linear regression analysis between the ACRE(150) and the background EMG revealed that the regression slopes were significantly decreased during CC task except for the tibialis anterior (TA) and biceps femoris. The reflex ratio (ACRE(150)/background EMG) was also negative for the SC task in all muscles tested, but was significantly reduced or showed a positive value for the CC task. These results suggest that the brain may control the cutaneous reflex pathways to enhance the facilitatory effects of the thigh and ankle extensor muscles during the CC task. This reflex action during the CC task may serve to prevent an undesirable fall in the center of gravity in response to a sudden tactile sensation to the dorsal surface of the foot.