Effects of continuous cycling training on motor unit firing rates, input excitation, and myosin heavy chain of the vastus lateralis in sedentary females

Sex‐related differences in motor unit behavior are influenced by myosin heavy chain during high‐ but not moderate‐intensity contractions

“…Adding a small amount of Gaussian noise to the Precision Decomposition III (PD III) derived firing times deteriorated the action potential waveforms, such as, there was a significant decrease in the peak-to-peak amplitudes (p < 0.001; 0.093 ± 0.167 to 0.002 ± 0.001 mV) (Figure 1) and the z-scores calculated from the correlation coefficients (p < 0.001; 1.471 ± 0.114 to 1.048 ± 0.197) with minor shifts in the firing times. 17,34 For the MUAP AMP versus RT relationships, there were significant interactions for the slopes (p = 0.003; p 2 = 0.374) and y-intercepts (p = 0.001; p 2 = 0.406). For the 70% MVC, the slopes were greater for the males (0.00677 ± 0.00224 mV/%MVC) than the females (0.00313 ± 0.00255 mV/%MVC; p = 0.002; g = 1.52), however, there were no sex-related differences for the 40% MVC (males = 0.00482 ± 0.00193 mV/%MVC, females = 0.00302 ± 0.00266 mV/%MVC; p = 0.084; g = 0.775).…”

“…Therefore, individuals with greater amounts of type I %MHC displayed smaller increases in MUAP AMPS with increments in RT. 8 Previously, significant relationships have been reported between %MHC isoform content and firing rate characteristics during a moderate-intensity contraction 17,23 ; however, neither the slopes nor y-intercepts for the MFR versus RT relationships were correlated with type I %MHC. A possible explanation for the divergent findings may be the importance of training status.…”

“…A possible explanation for the divergent findings may be the importance of training status. 17 It has been reported that hybrid fiber expression is more prevalent for untrained individuals 48 and indirect evidence suggests that MUs recruited during a 40% MVC possess type II fibers. [49][50][51] Thus, it is possible the sedentary training status for individuals 8,52,53 resulted in some MUs that express type IIx properties and obscured the relationships for type I %MHC with firing rate characteristics during a 40% MVC.…”

“…15,16 Subsequently, the individual firing behavior of the MU is influenced by the respective physical properties. [17][18][19] In addition, it is suggested that input excitation to the MU pool will adjust in response to MU force twitches when producing a desired muscle force output. 15,16,20 Since MU twitch forces differ relative to the fiber type, 18,21,22 it is theorized that the fiber-type composition of the muscle influences firing rate behavior at a targeted force.…”

Chronic training status affects muscle excitation of the vastus lateralis during repeated contractions

“…It is well documented training elicits adaptations that are dependent on the mode of exercise. 3 , 4 Numerous studies have reported acute and chronic aerobic- (AT) and resistance-training (RT) alters MU: firing rates, 5 , 6 , 7 , 8 synchronization, 9 , 10 recruitment thresholds, 11 , 12 excitability, 13 , 14 and MU pool output 6 , 15 during submaximal voluntary contractions. Therefore, it is plausible that EMG amplitude (EMG RMS ) would be sensitive to alterations in MU control strategies as a function of chronic training status; however, previous literature has been mixed.…”