Objective: To study the electromyogram (EMG) profile --- including ON, OFF and peak timing locations --- of the lateral gastrocnemius muscle over a wide range of walking speeds (0.5–2.1 m/sec) in healthy young adults.

Design: We studied gastrocnemius muscle-firing patterns using an electromyographic surface electrode in 15 healthy subjects ambulating on a treadmill at their normal walking speed and three paced walking speeds (0.5, 1.8 and 2.1 m/sec). Initial heel contact was determined from a force-sensitive switch secured to the skin over the calcaneous.

Results: For all speeds, the gastrocnemius firing pattern was characterized by a main peak, occurring 40–45% into the gait cycle, that increased in amplitude with walking speed. Speeds >=1.3 m/sec produced a common electromyographic timing profile, when the profile is expressed relative to the stride duration. However, at 0.5 m/sec (a speed typical of individuals with upper-motor neuron lesions), the onset of gastrocnemius firing was significantly delayed by 3–6% of the gait cycle and was prolonged by 8–11% of the gait cycle.

Conclusion: Many patients with upper motor neuron lesions (e.g., stroke and traumatic brain injury) walk at speeds much slower than those commonly described in the literature for normal gait. At the slow walking speed of 0.5 m/sec, we have measured noticeable changes in the electromyographic timing profile of the gastrocnemius muscle. Given the importance of appropriate plantar flexor firing patterns to maximize walking efficiency, understanding the speed-related changes in gastrocnemius firing patterns may be essential to gait restoration.

American Journal of Physical Medicine and Rehabilitation, Vol. 83, No. 7, pp. 507–514, 2004