Skeletal muscles, exposed to a prolonged period of specific functional demands, respond adaptively. Electrical stimulation, when employed as a technique for subjecting selected muscles to altered use, enables precise entrainment of the pattern of functional activity. In this investigation, the vastus lateralis muscle in a group of volunteers was stimulated. The stimulation program typical of a phasic type of activity (high frequency, high current amplitude, short pulse duration) intermittently subjected the stimulated muscles to brief periods of intense activity, followed by relatively long pauses. The activation-relaxation time ratio chosen was 1 to 13. It was determined to prevent the muscles from fatiguing. The effects of the chronic stimulation program were established by measurements of the time course of contraction and relaxation and fatigue of the vastus lateralis muscle. Chronic phasic electrical stimulation increased the speed of muscle contraction by 15% while the fatigue characteristics remained unchanged.