Muscle activity in the leg is tuned in response to ground reaction forces

During walking and running, the human body reacts to its external environme nt. One such response is to the impact forces that occur at heel strike. Th is study tested previous speculation that the levels of muscle activity in the lower extremities are adjusted in response to the loading rate of the i mpact forces. A pendulum apparatus was used to deliver repetitive impacts t o the heels of 20 subjects. Impact forces were of similar magnitude to thos e experienced during running, but the loading rate was varied by 13% using different materials in the subjects' shoes. Myoelectric patterns were measu red in the tibialis anterior, medial gastrocnemius, vastus medialis, and bi ceps femoris muscles. Wavelet analysis was used to resolve intensity of the myoelectric patterns into time and frequency space. Substantial and signif icant differences in the myoelectric activity occurred between the impact c onditions for the 50 ms before and the 50 ms after impact, reaching 3 ms in timing, 16% in wavelet number, and 154% in the intensity of the muscle act ivity.