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Adjustments in motor unit properties during fatiguing contractions after training

Publication . Vila-Chã, Carolina; Falla, Deborah; Velhote, Miguel C.; Farina, Dario

The objective of the study was to investigate the effect of strength and endurance training on muscle fiber membrane properties and discharge rates of low-threshold motor units of the vasti muscles during fatiguing contractions. Methods: Twenty-five sedentary healthy men (age (mean T SD) = 26.3 T 3.9 yr) were randomly assigned to one of three groups: strength training, endurance training, or a control group. Conventional endurance and strength training was performed 3 dIwkj1, during a period of 6 wk. Motor unit conduction velocity and EMG amplitude of the vastus medialis obliquus and lateralis muscles and biceps femoris were measured during sustained isometric knee extensions at 10% and 30% of the maximum voluntary contraction before and immediately after training. Results: After 6 wk of training, the reduction in motor unit conduction velocity during the sustained contractions at 30% of the maximum voluntary force occurred at slower rates compared with baseline (P G 0.05). However, the rate of decrease was lower after endurance training compared with strength training (P G 0.01). For all groups, motor unit discharge rates declined during the sustained contraction (P G 0.001), and their trend was not altered by training. In addition, the biceps femoris–vasti coactivation ratio declined after the endurance training. Conclusions: Short-term strength and endurance training induces alterations of the electrophysiological membrane properties of the muscle fiber. In particular, endurance training lowers the rate of decline of motor unit conduction velocity during sustained contractions more than strength training

2012Journal article

Open access

Changes in H reflex and V wave following short-term endurance and strength training

Publication . Vila-Chã, Carolina; Falla, Deborah; Velhote, Miguel C.; Farina, Dario

This study examined the effects of 3 wk of either endurance or strength training on plasticity of the neural mechanisms involved in the soleus H reflex and V wave. Twenty-five sedentary healthy subjects were randomized into an endurance group (n 13) or strength group (n 12). Evoked V-wave, H-reflex, and M-wave recruitment curves, maximal voluntary contraction (MVC), and time-to-task-failure (isometric contraction at 40% MVC) of the plantar flexors were recorded before and after training. Following strength training, MVC of the plantar flexors increased by 14.4 5.2% in the strength group (P 0.001), whereas time-to-task-failure was prolonged in the endurance group (22.7 17.1%; P 0.05). The V wave-to-maximal M wave (V/Mmax) ratio increased significantly (55.1 28.3%; P 0.001) following strength training, but the maximal H wave-to-maximal M wave (Hmax/Mmax) ratio remained unchanged. Conversely, in the endurance group the V/Mmax ratio was not altered, whereas the Hmax/Mmax ratio increased by 30.8 21.7% (P 0.05). The endurance training group also displayed a reduction in the H-reflex excitability threshold while the H-reflex amplitude on the ascending limb of the recruitment curve increased. Strength training only elicited a significant decrease in H-reflex excitability threshold, while H-reflex amplitudes over the ascending limb remained unchanged. These observations indicate that the H-reflex pathway is strongly involved in the enhanced endurance resistance that occurs following endurance training. On the contrary, the improvements in MVC following strength training are likely attributed to increased descending drive and/or modulation in afferents other than Ia afferents.

2012Journal article

Open access