The Determinants of Skeletal Muscle Force and Power: Their Adaptability with Changes in Activity Pattern
Journal of Biomechanics
A kinetic model of the cross-bridge cycle in skeletal muscle is presented with special reference to the rate limiting steps regulating the peak rate of force development (dPdt), peak force (Po), and the maximal shortening speed (Vmax). Force production in skeletal muscle is dependent on the number of cross-bridges in the strongly bound, high-force state (AM•-ADP), and during a peak isometric contraction this state is the dominant cross-bridge form. The peak force and power output of a muscle depends upon numerous factors to include: (1) muscle and fiber size and length: (2) architecture, such as the angle and physical properties of the fiber-tendon attachment, and the fiber to muscle length ratio: (3) fiber type: (4) number of cross-bridges in parallel: (5) force per cross-bridge: (6) peak dPdt: (7) force-velocity relationship: (8) fiber Vmax: (9) force-pCa2+ relationship: and (10) the force-frequency (action potential Hz) relationship. In this paper, we discuss these determinants of force and power output, and consider how they adapt to both muscle unloading (induced by hindlimb suspension) and programs of regular endurance exercise. Slow- and fast-twitch fibers have similar capacities to generate specific tension (kg cm−2). However, fast fibers show a considerably higher peak dPdt, Vmax, and power output. The high Vmax of the fast-twitch fiber is likely due to the high myofibrillar ATPase activity of the fast myosin isozyme. Both hindlimb suspension and regular endurance exercise have been shown to induce fiber type specific changes in single fiber function. For example, fiber size and the peak tetanic tension of the slow oxidative (SO), fast oxidative glycolytic (FOG), and fast glycolytic (FG) fiber types were generally unaltered by endurance exercise-training. In contrast, hindlimb suspension produced cell atrophy in all fiber types and a reduced specific tension in the SO but not the FOG or FG fiber types. Both exercise-training and HS shifted the force-pCa curve to the right, and increased the Vmax of the SO fiber type. From the standpoint of work capacity or the ability to move a load, the important functional property is power output. Peak power is obtained at loads considerably below 50% of Po, and it is correlated with the percentage of fast-twitch fibers. Peak power can be increased by both dynamic and isometric programs of exercise-training. This adaptation should improve performance as a high correlation exists between power and maximal sprinting speed in man. Future attempts to model the contribution of a particular muscle to a given movement or specific bone torque will need to consider the percentage and distribution of each fiber type, the functional differences between fiber types, and the level of daily activity generally experienced by the muscle or muscle group.