The myofibrillar complex and fatigue: a review.

The basis for all biological movement is the conversion of chemical energy to mechanical energy by different classes of motor proteins. In skeletal muscle this motor protein is myosin II, a thick filament-based molecule that harnesses the free energy furnished by ATP hydrolysis to perform mechanical work against actin proteins of the thin filament. The cyclic attachment and detachment of myosin with actin that generates muscle force and shortening is Ca2+ regulated. Intense muscle activity may lead to metabolically induced inhibitions to the function of these myofibrillar proteins when Ca2+ regulation is normal, a phenomenon referred to as myofibrillar fatigue. Studies using single muscle fibers at room temperature or lower have shown that myosin motor function is inhibited by the accumulation of the ATP-hydrolysis products ADP, Pi, and H+ as well as by excess generation of reactive oxygen species (ROS). These metabolically induced impairments to myosin motor function reduce muscle work and power output by impairing maximal Ca2+ activated force, the Ca2+ sensitivity of force, and/or unloaded shortening velocity. Based on uncertainties about their inhibitory effect on muscle function at more physiological temperatures, the influence of ATP-hydrolysis product and ROS accumulation on myofibrillar protein function of human skeletal muscle remains to be clarified.