Integration of learned and naturally occurring flexor EMG in the human step cycle.

Human locomotion, as a behavioral system, is well suited for investigating the interplay of learning with innate biological mechanisms. The present work assembled, from previous findings, a test battery of operant conditioning to modify strong ongoing electromyographic (EMG) activity of the left tibialis anterior (TA), an ankle flexor muscle. An EMG segment 100-400 msec long had to occur within 700 msec, and (in different training regimens) with filtered amplitude both above and below different thresholds--all in response to a colored light that flashed at a designated time in the step cycle. Reinforcement was a high or low tone that indicated success or failure, respectively. All four subjects met training criteria (90% success rate) at a moderate amplitude, just as had been true previously for a relatively inactive knee extensor. Subject-specific limitations were found in operant control at higher and lower amplitudes. New learning was not disruptive, although ipsilateral swing duration increased, nor was it phase-dependent on any portion of the step cycle.