Concomitant effects of fatigue and potentiation in rat medial gastrocnemius fast motor units

During contractile activity, force can be reduced (i.e., fatigue/performance fatigability) or increased (i.e., potentiation) in a stimulation frequency and task-dependent manner, through the parallel action of various potentiating and fatiguing mechanisms. Potentiation and fatigue can be observed concomitantly, with the most characteristic example being increased submaximal force in parallel with maximal tetanic force reduction. The majority of these observations come from isolated whole muscle models, with less data available at the motor unit level. Here, we investigated the coexistence of fatigue and potentiation in rat medial gastrocnemius fast fatigable (i.e., FF) and fast fatigue resistant (i.e., FR) motor units in situ, using a 180-s repetitive isometric contractile protocol including twitches, unfused tetani of different fusion degrees (30 Hz, 40 Hz) and maximal tetanic contractions (150 Hz). Potentiation of twitch and unfused tetanic force was apparent in both motor unit types in parallel with reduced maximal tetanic force. Considerable overlap in potentiation magnitude between FF and FR units was observed, across stimulation frequencies. FF units exhibited potentiation more rapidly compared to FR units.