Velocity-Dependent Changes in Electrical Efficiency of the Leg Extensors during Eccentric Isokinetic Muscle Actions

The mechanisms underlying velocity-specific decreases in torque production during eccentric muscle actions remain unclear. Therefore, the purpose of the present study was to examine electrical efficiency at 60 and 180°·s −1 during fatiguing, maximal eccentric isokinetic muscle actions of the leg extensors. Thirteen men visited the lab on two occasions and randomly performed 30 repeated maximal eccentric isokinetic muscle actions of the leg extensors at 60 and 180°·s − ¹ through a 90° range of motion. Polynomial regression analyses and separate repeated measures ANOVAs were used to examine the composite patterns of responses for electromyographic amplitude, eccentric torque, and electrical efficiency across the 60 and 180°·s − ¹ protocols. Eccentric torque remained unchanged across the 30 maximal eccentric muscle actions performed at 60 and 180°·s −1. Electromyographic amplitude and electrical efficiency, however, increased linearly across the 180°·s −1 protocol, but there were no changes for either electromyographic amplitude or electrical efficiency during the 60°·s −1 fatiguing protocol. These findings indicated that performing eccentric muscle actions at a fast velocity (i. e., 180°·s −1) was characterized by greater efficiency (i. e., decreased electrical efficiency) compared to eccentric muscle actions performed at a slow velocity (i. e., 60°·s −1). Thus, it is possible that eccentric muscle actions performed at fast velocities are more robust to fatigue due, in part, to greater efficiency compared to slow-velocity eccentric muscle actions.

Motor cortex tDCS does not improve strength performance in healthy subjects

The influence of transcranial direct current stimulation (tDCS) upon maximal strength performance in exercises recruiting large muscle mass has not been established in healthy populations. The purpose of this study was to investigate whether anodal tDCS was able to increase the performance during maximal strength exercise (MSEX) in healthy subjects. Fourteen volunteers (age: 26 ± 4 yrs) performed two MSEX after anodal or sham tDCS (2mA; 20min prior MSEX), involving knee extensors and flexors in concentric isokinetic muscle actions of the dominant limb (3 sets of 10 repetitions). The electrical muscle activity (sEMG) of four recruited muscles was recorded during MSEX. Anodal tDCS was not able to improve force production (i.e., total work and peak torque), fatigue resistance, or electromyographic activity during MSEX when compared to sham condition. In conclusion, anodal tDCS applied upon the contralateral motor cortex was not capable of increasing the strength performance of knee extensors and flexors in young healthy subjects.

Mechanomyographic Responses for the Biceps Brachii Are Unable to Track the Declines in Peak Torque During 25, 50, 75, and 100 Fatiguing Isokinetic Muscle Actions

This study examined the peak torque and mechanomyographic (MMG) amplitude and mean frequency (MNF) responses during fatiguing isokinetic muscle actions. On four separate occasions, twenty men (mean ± SD age = 23 ± 3 years) performed 25, 50, 75, and 100 repeated maximal concentric isokinetic muscle actions of the dominant forearm flexors. During each muscle action, the MMG signal was detected from the biceps brachii with an accelerometer. The data were examined with linear regression and one-way repeated measures analyses of variance. The results indicated that the mean percent decline in peak torque value for the 25 repetition trial (25.6%) was significantly less than that for the 50 repetition trial (45.2%). Furthermore, the mean linear slope coefficient for the peak torque versus repetition number relationship for the 50 repetition trial was significantly less than that for the 100 repetition trial. There were no mean differences among the trials for the linear slope coefficients and y-intercepts for the MMG amplitude and MNF versus repetition number relationships. When detected with an accelerometer, the linear slope coefficients and y-intercepts for the MMG amplitude and MNF versus repetition number relationships were not sensitive enough to track the decline in muscle function during fatigue.