The neural mechanisms responsible for strength improvement in the early phase of strength training are unknown. One hypothesis is that strength increases because of increased neural drive to the trained muscles. Here, we used twitch interpolation to assess voluntary activation before and after a 4-wk strength training program.
Twelve volunteers performed unilateral strength training for the right wrist abductors (three times per week). Control subjects (n = 11) practiced the same movement without resistance. We assessed voluntary activation of the trained muscles during wrist abduction and extension contractions using twitch interpolation with motor nerve and motor cortical stimulation.
Strength training increased wrist abduction maximal voluntary contraction (MVC) force for the trained hand by 11.0% (+/-8.7, P < 0.01). MVC of the untrained wrist was unchanged. There were no significant changes in wrist extension MVC force in either group. During submaximal wrist abduction, but not extension contractions, the average size of the superimposed twitches produced by cortical stimulation was significantly larger after strength training (P < 0.01). Furthermore, the direction of the twitches produced by cortical stimulation during wrist abductions and maximal wrist extension shifted toward abduction (P = 0.04). There were neither significant changes in voluntary activation measured during MVC with motor nerve or motor cortical stimulation nor changes in the amplitude of evoked EMG responses to motor cortical or motor nerve stimulation.
Four weeks of strength training produced a small increase in MVC that was specific to the training direction. Although maximal voluntary activation did not change with short-term strength training, the changes in direction and amplitude of cortically evoked twitches suggest that motor cortical stimulation (and presumably volition) can generate motor output more effectively to the trained muscles.
[Show abstract][Hide abstract] ABSTRACT: The neural adaptations that mediate the increase in strength in the early phase of a strength training program are not well understood; however, changes in neural drive and corticospinal excitability have been hypothesized. To determine the neural adaptations to strength training, we used transcranial magnetic stimulation (TMS) to compare the effect of strength training of the right elbow flexor muscles on the functional properties of the corticospinal pathway. Motor-evoked potentials (MEPs) were recorded from the right biceps brachii (BB) muscle from 23 individuals (training group; n = 13 and control group; n = 10) before and after 4 weeks of progressive overload strength training at 80% of 1-repetition maximum (1RM). The TMS was delivered at 10% of the root mean square electromyographic signal (rmsEMG) obtained from a maximal voluntary contraction (MVC) at intensities of 5% of stimulator output below active motor threshold (AMT) until saturation of the MEP (MEPmax). Strength training resulted in a 28% (p = 0.0001) increase in 1RM strength, and this was accompanied by a 53% increase (p = 0.05) in the amplitude of the MEP at AMT, 33% (p = 0.05) increase in MEP at 20% above AMT, and a 38% increase at MEPmax (p = 0.04). There were no significant differences in the estimated slope (p = 0.47) or peak slope of the stimulus-response curve for the left primary motor cortex (M1) after strength training (p = 0.61). These results demonstrate that heavy-load isotonic strength training alters neural transmission via the corticospinal pathway projecting to the motoneurons controlling BB and in part underpin the strength changes observed in this study.
The Journal of Strength and Conditioning Research 09/2010; 24(11):3123-32. DOI:10.1519/JSC.0b013e3181f56794 · 2.08 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This review summarizes evidence from randomized controlled trials to examine whether strength training influences anxiety, chronic pain, cognition, depression, fatigue symptoms, self-esteem, and sleep. The weight of the available evidence supported the conclusion that strength training is associated with reductions in anxiety symptoms among healthy adults (5 trials); reductions in pain intensity among patients with low back pain (5 trials), osteoarthritis (8 trials), and fibromyalgia (4 trials); improvements in cognition among older adults (7 trials); improvements in sleep quality among depressed older adults (2 trials); reductions in symptoms of depression among patients with diagnosed depression (4 trials) and fibromyalgia (2 trials); reductions in fatigue symptoms (10 trials); and improvements in self-esteem (6 trials). The evidence indicates that larger trials with a greater range of patient samples are needed to better estimate the magnitude and the consistency of the relationship between strength training and these mental health outcomes. Plausible social, psychological, and neural mechanisms by which strength training could influence these outcomes rarely have been explored. This review revealed the high-priority research need for animal and human research aimed at better understanding the brain mechanisms underlying mental health changes with strength training.
American Journal of Lifestyle Medicine 09/2010; 4(5):377-396. DOI:10.1177/1559827610368771
[Show abstract][Hide abstract] ABSTRACT: Practicing skilled tasks that involve the use of the hand and fingers has been shown to lead to adaptations within the central nervous system (CNS) underpinning improvements in the performance of the acquired task. However, neural adaptations following a period of strength training in the hand is not well understood. In order to determine the neural adaptations to strength training, we compared the effect of isometric strength training of the right first dorsal interosseous (FDI) muscle on the electromyographic (EMG) responses to transcranial magnetic stimulation (TMS) over left M1. The specific aim of the study was to investigate the corticospinal responses, including latency, motor-evoked potential amplitude (MEP), and silent period duration (SP) following 4 week of strength training of the FDI muscle. Sixteen healthy adults (13 male, three female; 24.12±5.21 years), were randomly assigned into a strength training (n=8) or control group (n=8). Corticospinal measures of active motor threshold (AMT), MEP amplitude, and SP duration were obtained using TMS during 5% and 20% of maximal voluntary contraction force (MVC) pre and post 4 week strength training. Following training, MVC force increased by 33.8% (p=.01) in the training group compared to a 13% increase (p=.2) in the untrained group. There were no significant differences in AMT, latency, or MEP amplitude between groups following training. However, in the trained group, there was a 16 ms reduction in SP duration at 5% of MVC (p=.01) and 25 ms reduction in SP duration at 20% of MVC (p=.03). These results demonstrate a task dependent adaptation in corticospinal inhibition via a reduction in cortical SP duration that may in part underpin the strength increases observed following strength training.
Human movement science 10/2010; 29(5):631-41. DOI:10.1016/j.humov.2010.01.004 · 1.60 Impact Factor
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