Motor unit behavior during submaximal contractions following six weeks of either endurance or strength training

Abstract

The study investigated changes in motor output and motor unit behavior following 6 wk of either strength or endurance training programs commonly used in conditioning and rehabilitation. Twenty-seven sedentary healthy men (age, 26.1 ± 3.9 yr; mean ± SD) were randomly assigned to strength training (ST; n = 9), endurance training (ET; n = 10), or a control group (CT; n = 8). Maximum voluntary contraction (MVC), time to task failure (isometric contraction at 30% MVC), and rate of force development (RFD) of the quadriceps were measured before (week 0), during (week 3), and after a training program of 6 wk. In each experimental session, surface and intramuscular EMG signals were recorded from the vastus medialis obliquus and vastus lateralis muscles during isometric knee extension at 10 and 30% MVC. After 6 wk of training, MVC and RFD increased in the ST group (17.5 ± 7.5 and 33.3 ± 15.9%, respectively; P < 0.05), whereas time to task failure was prolonged in the ET group (29.7 ± 13.4%; P < 0.05). The surface EMG amplitude at 30% MVC force increased with training in both groups, but the training-induced changes in motor unit discharge rates differed between groups. After endurance training, the motor unit discharge rate at 30% MVC decreased from 11.3 ± 1.3 to 10.1 ± 1.1 pulses per second (pps; P < 0.05) in the vasti muscles, whereas after strength training it increased from 11.4 ± 1.2 to 12.7 ± 1.3 pps (P < 0.05). Finally, motor unit conduction velocity during the contractions at 30% MVC increased for both the ST and ET groups, but only after 6 wk of training (P < 0.05). In conclusion, these strength and endurance training programs elicit opposite adjustments in motor unit discharge rates but similar changes in muscle fiber conduction velocity.

Effects of In-Season Strength Maintenance Training Frequency in Professional Soccer Players

“…Large volumes of endurance training may inhibit adaptations to strength training (15) and thus potentially quest for a larger frequency of strength maintenance training. Indeed, endurance training has been shown to lower the maximum shortening velocity of type II fibers, reduce motor unit discharge rates, and to slightly reduce peak tension development in all fiber types (9,10,30,(33)(34). In accordance with the latter findings, endurance training has been associated with reduced vertical jumping ability (5), strength (5,19), and unchanged or slightly reduced CSA of muscle fibers (9,15,(33)(34).…”

Muscular endurance training and motor unit firing patterns during fatigue

“…Maximal H to M (H/M) wave ratios of the triceps surae muscles are higher in endurance-trained compared to power-trained and sedentary individuals (Rochcongar et al 1979;Maffiuletti et al 2001), suggesting that endurance training may increase excitability of the spinal cord. Further, motor unit firing rates during a brief non-fatiguing contraction at 30 % maximal voluntary contraction (MVC) were lower after 6 weeks of aerobic cycle ergometer training (Vila-Cha et al 2010). In contrast, the same study also found that motor unit firing rates during brief non-fatiguing contractions were higher after resistance training (Vila-Cha et al 2010).…”

Time to task failure of the contralateral untrained limb after high load-low repetition eccentric and low load-high repetition resistance training

“…Previous studies have also shown that time to task failure of sustained contraction was prolonged following resistance training, most likely due to neural adaptations at level of central nerve system 8 . Cross-training is the process whereby training of one limb gives rise to enhancements in the performance of the opposite, untrained limb 9 .…”