Energy Cost of Skeletal Muscle Force Production is Reduced in the Force Enhanced State

When a skeletal muscle is actively stretched, its force is substantially greater than the force obtained for a muscle contracting purely isometrically [1]. This phenomenon has been observed for more than fifty years, is an accpeted property of skeletal muscle contraction, and has been termed force enhancement [2]. When performing experiments on force enhancement in human skeletal muscles, we observed that force enhancement was not associated with an increase in muscle activation. In fact, in the force enhanced state, a given submaximal force could be maintained with substantially lower activation than the same force in an isometric reference contraction [3]. This observation led to the idea that maybe force in the enhanced state was metabolically cheaper than in the isometric reference state. However, there is nothing in the literature on metabolic cost of muscle contraction in the force enhanced state. Therefore, the purpose of this study was to test the hypothesis that force in the enhanced state was metabolically cheaper compared to force in the isometric reference state. If this hypothesis was supported, it could point to new mechanisms that might explain force enhancement.