The effects of activation history on neuromuscular responses

Alexander Zero, The University of Western Ontario

Abstract

Muscle contractility is influenced by its activation history. For example, prolonged muscle activation (e.g., minutes) reduces force; termed muscle fatigue. Conversely, brief (e.g., seconds) activation enhances submaximal force; termed post-activation potentiation (PAP). Prior muscle activation is required for both effects and thus there is a coexistence between these processes.

Here, fatigue and PAP were investigated independently, and concurrently in humans by using voluntary and electrically stimulated isometric (study two) and dynamic contractions (studies one & four), and with single motor unit recordings using intramuscular electromyography (studies two & four). For study three, isolated intact single myofibers from mice were used to make assessments of force changes and intracellular calcium transients.

Study one demonstrated that during voluntary fatiguing contractions, PAP mechanisms delayed power loss in response to stimulated low frequencies of muscle excitation (submaximal) despite a decline in stimulated high frequencies of excitation (maximal). During recovery, power in response to stimulated low frequencies was preferentially impaired. However, inducing PAP ameliorated this power loss. Therefore, the concurrent effects of fatigue and PAP are frequency-dependent, and PAP mitigates power loss of low frequencies during recovery.

Study two investigated the effect of PAP during recovery from fatigue on motor unit (MU) firing rates. During recovery, MU firing rates increased as submaximal torque generation was impaired compared to baseline. However, inducing PAP during the recovery state mitigated the requirement of higher MU firing rates. Therefore, firing rates are responsive to opposing influences on the contractile state and can make compensatory rate adjustments dependent on the active state of the muscle.

Study three, in single myofibers, showed the preferential force loss of low frequencies of excitation (submaximal) during recovery was due to reduced cytosolic Ca²⁺. However, inducing post-tetanic potentiation in this state recovered submaximal force by increasing cytosolic Ca²⁺. Therefore, adjustments in force due to activation history are principally accomplished by opposing adjustments in cytosolic Ca²⁺.

Study four demonstrated that compared to baseline inducing PAP increased MU recruitment thresholds, but decreased firing rates during contractions at 50 and 75% peak power. Therefore, MUs make compensatory adjustments in relation to the active state of the muscle during tasks requiring a high-power output.