It is well recognized that skeletal muscle is incapable of maintaining the continuous power output needed for significant circulatory assistance unless it is rendered fatigue resistant by electrical stimulation. Most studies using conditioning patterns related to the pumping rate of the natural heart have suggested that such patterns can produce complete transformation to a slow phenotype. Such a transformation has two undesirable accompaniments: a reduction in the contractile speed of the muscle and an extreme reduction in its power-generating capacity. We have investigated the chronic effect on rabbit muscle of intermittent high frequency contractions either alone or overlaid on a continuous background of 10 Hz. If the amount of stimulation were the overriding stimulus for transformation, then the rate and/or extent of transformation should, if anything, be greater with the additional activation. If, on the other hand, components of the transformation could be modulated independently by the pattern of impulses, then the reduction in power output might be avoided to some extent by a pattern somewhat analogous to a strength-training exercise regime. Our results suggest that high force contractions may indeed preserve power output without compromising the acquisition of fatigue resistance.