Coexistence of potentiation and fatigue in skeletal muscle

Twitch potentiation and fatigue in skeletal muscle are two conditions in wh ich force production is affected by the stimulation history. Twitch potenti ation is the increase in the twitch active force observed after a tetanic c ontraction or during and following low-frequency stimulation. There is evid ence that the mechanism responsible for potentiation is phosphorylation of the regulatory light chains of myosin, a Ca2+-dependent process. Fatigue is the force decrease observed after a period of repeated muscle stimulation. Fatigue has also been associated with a Ca2+-related mechanism: decreased peak Ca2+ concentration in the myoplasm is observed during fatigue. This de crease is probably due to an inhibition of Ca2+ release from the sarcoplasm ic reticulum. Although potentiation and fatigue have opposing effects on fo rce production in skeletal muscle, these two presumed mechanisms can coexis t. When peak myoplasmic Ca2+ concentration is depressed, but myosin light c hains are relatively phosphorylated, the force response can be attenuated, not different, or enhanced, relative to previous values. In circumstances w here there is interaction between potentiation and fatigue, care must be ta ken in interpreting the contractile responses.