Abstract
Post-tetanic potentiation (PTP) of force depends on intramuscular Ca2+ levels and sensitivity and may be affected by fatigue. The aim of this study was to determine the ability of isolated fast fatigue-resistant (FR) and fast-fatigable (FF) motor units (MUs) to potentiate force evoked with single and 40-Hz electrical stimulation after 5 weeks of voluntary weight-lifting training. Tetanic contractions evoked by gradually increasing (10–150 Hz) stimulation frequency served as conditioning stimulation. Additionally, the concentration of myosin light chain kinase and proteins engaged in calcium handling was measured in rat fast medial gastrocnemius muscle. After the training, the potentiation of twitch force and peak rate of force development was increased in FF but not FR MUs. Force potentiation of 40-Hz tetanic contractions was increased in both fast MU types. After the training, the twitch duration of FR MUs was decreased, and FF MUs were less prone to high-frequency fatigue during conditioning stimulation. Muscle concentration of triadin was increased, whereas concentrations of ryanodine receptor 1, junctin, FKBP12, sarcoplasmic reticulum calcium ATPase 1, parvalbumin, myosin light chain kinase, and actomyosin adenosine triphosphatase content were not modified. After short-term resistance training, the twitch contraction time and twitch:tetanus force ratio of FR MUs are decreased, and PTP ability is not changed. However, PTP capacity is increased in response to submaximal activation. In FF MUs increase in PTP ability coexists with lesser fatigability. Further work is required to find out if the increase in triadin concentration has any impact on the observed contractile response.
Highlights
Post-tetanic potentiation (PTP) is expressed as an enhancement of twitch contraction force evoked after preceding high-intensity muscular contractile activity
Ca2+ release may be affected by modifications in triadin and junctin, the major transmembrane proteins, which form a complex with the ryanodine receptor (Ca2+ release channel) in skeletal muscle sarcoplasmic reticulum (Beard et al, 2009)
The present study showed that 5 weeks of resistance training resulted in the enhancement of twitch potentiation and peak rate of force development of FF motor units (MUs) but did not affect twitch potentiation in FR MUs in rat medial gastrocnemius muscle
Summary
Post-tetanic potentiation (PTP) is expressed as an enhancement of twitch contraction force evoked after preceding high-intensity muscular contractile activity. In only one report, increased postactivation potentiation with concomitant reduction of twitch force and twitch:maximum voluntary force ratio was found in humans after several weeks of low-intensity resistance training with vascular occlusion (Moore et al, 2004). It is largely unknown whether resistance training modifies the concentration of proteins in the skeletal muscle, which regulates force potentiation ability. We noted that short-term resistance training affects twitch contractile parameters in fast fatigue-resistant (FR) but not fast-fatigable (FF) motor units (MUs) In the former, decrease in duration of twitch contraction and relaxation times and reduction of twitch:maximum tetanus force ratio was present. Triadin negatively regulates RyR function by suppression of the depolarization-induced SR Ca2+
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