Motor Unit Force Potentiation and Calcium Handling Protein Concentration in Rat Fast Muscle After Resistance Training

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.

Keywords: calcium release; excitation contraction coupling; fatigue; muscle fibers; strength training; window of opportunity.

FIGURE 1

Examples of isometric twitch responses of fast fatigable (FF) and fast fatigue resistant (FR) motor units of medial gastrocnemius muscle obtained from untrained (thin lines) and resistance-trained animals (thick lines).

FIGURE 2

Comparison of fast (A–C), FF (D–F), and FR (G–I) motor units’ contractile parameters for pre and post tetanic conditioning stimulation measurements in control (C) and weight-lifting trained (WL) animals. Data are presented as mean values and standard deviations. FF, fast fatigable; FR, fast fatigue resistant; pRFD, peak rate of force development; CT, contraction time; NP, non-potentiated; P, potentiated. The impact of factors G, group (control vs. weight lifting); Con, contraction (twitch non-potentiated vs. potentiated state); and G × Con, interaction between the group and contraction were expressed as significant (p < 0.05) or not significant (ns.) p values, presented within panels. Post hoc analysis results: # = significant difference between pre and post measurements. Non-potentiated twitch force and contraction time values are from the previous study (Łochyński et al., 2016) and are included for comparison. Note much substantial shortening of the contraction time of the potentiated twitch (much higher partial eta squared values for the ANOVA’s effect of group, Table 2) in FR [(I), large effect size] than FF [(F), small effect size] motor units. In addition, when control and weight-lifting rats were analyzed with two-sided post hoc comparisons, effect sizes expressing the magnitude of decrease in the contraction time of potentiated twitches were also higher in FR (Cohen’s d = 0.65) than FF (Cohen’s d = 0.48) motor units. These observations coincided with increased potentiation of twitch force and pRFD in FF (D,E) but not FR (G,H) motor units.

FIGURE 3

Relationships between the non-potentiated twitch contraction times, twitch:tetanus force ratios, and force ratios of the potentiated to non-potentiated twitches for FF (A–C) and FR (D–F) motor units. C, control group; WL, weight lifting group.

FIGURE 4

Peak force potentiation of 40 Hz tetanus after resistance training (A–C), and force profile of 40 Hz tetanus during repeated fatiguing stimulation (D). (A–C), * denotes significant differences (p < 0.05, post hoc comparisons) in force augmentation between trained and untrained rats. (D), * denotes significant differences (p < 0.05, post hoc comparisons) between forces of trained and untrained rats for the whole population of fast motor units. Note that force was initially slightly potentiated in trained rats and declined steadily in untrained animals. C, control group; WL, weight lifting group; FR, fast fatigue resistant motor units; FF, fast fatigable motor units; F, whole population of fast motor units.

FIGURE 5

Distribution of triadin muscle concentrations in trained and untrained rats.