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Fatigue of single mouse fibers during repeated high-frequency stimulation results initially from decreased Ca 2+ sensitivity while free myoplasmic calcium concentration ([Ca 2+ ] m ) increases, followed by decreasing [Ca 2+ ] m . Recovery of active force with low-frequency stimulation is slow and persistent fatigue results from low [Ca 2+ ] m . However, the consequences of intermittent submaximal contractions are not known. The aim of the present study was to investigate the changes in [Ca 2+ ] m and active force during intermittent submaximal contractions and subsequent recovery. Single fibers of mouse flexor digitorum brevis muscles at 32 °C were stimulated with 40 or 50 Hz, for 350 ms every 2 s for 2 min and then every 1 s until &lt; 40% of initial force. Values obtained during the intermittent stimulation were compared with a control force-[Ca 2+ ] m relationship. A "P"-shaped pattern in the force-[Ca 2+ ] m relationship was observed during intermittent stimulation. Early in the intermittent stimulation, [Ca 2+ ] m increased while active force decreased. Subsequent force potentiation was accompanied by increased Ca 2+ sensitivity. Later, as active force declined, [Ca 2+ ] m decreased significantly (p &lt; 0.001). This was followed, in the final phase, by a significant decrease in Ca 2+ sensitivity determined by [Ca 2+ ] m at half-maximal force (Ca 50 ) (p = 0.001). Low-frequency fatigue persisted during recovery while Ca 50 was not significantly different from prefatigue (p &gt; 0.5). In conclusion, the main mechanism of fatigue is due to decreases in both [Ca 2+ ] m and Ca 2+ sensitivity following the initial force potentiation. The intermittent submaximal contractions resulted in persistent low-frequency fatigue seen during recovery, which was explained by depressed [Ca 2+ ] m with no change in Ca 2+ sensitivity.

Role of Ca2+ in changing active force during intermittent submaximal stimulation in intact, single mouse muscle fibers