Short-term strength training does not change cortical voluntary activation.
ABSTRACT 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.
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ABSTRACT: The purpose of this study was to assess cortical activation associated with the cross-education effect to an immobilized limb, using functional magnetic resonance imaging. Fourteen right-handed participants were assigned to two groups. One group (n = 7) wore a cast and strength trained the free arm (CAST-TRAIN). The second group (n = 7) wore a cast and did not strength train (CAST). Casts were applied to the nondominant (left) wrist and hand. Strength training was maximal isometric handgrip contractions (right hand) 5 d·wk(-1). Peak force (handgrip dynamometer), muscle thickness (ultrasound), EMG, and cortical activation (functional magnetic resonance imaging) were assessed before and after the intervention. CAST-TRAIN improved right handgrip strength by 10.7% (P < 0.01) with no change in muscle thickness. There was a significant group × time interaction for strength of the immobilized arm (P < 0.05). Handgrip strength of the immobilized arm of CAST-TRAIN was maintained, whereas the immobilized arm of CAST significantly decreased by 11% (P < 0.05). Muscle thickness of the immobilized arm decreased by an average of 3.3% (P < 0.05) for all participants and was not different between groups after adjusting for baseline differences. There was a significant group × time interaction for EMG activation (P < 0.05), where CAST-TRAIN showed an increasing trend and CAST showed a decreasing trend, pooled across arms. For the immobilized arm of CAST-TRAIN, there was a significant increase in contralateral motor cortex activation after training (P < 0.05). For the immobilized arm of CAST, there was no change in motor cortex activation. Handgrip strength training of the free limb attenuated strength loss during unilateral immobilization. The maintenance of strength in the immobilized limb via the cross-education effect may be associated with increased motor cortex activation.Medicine and science in sports and exercise 08/2011; 43(8):1394-405. DOI:10.1249/MSS.0b013e318210783c · 4.46 Impact Factor
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ABSTRACT: Aging-related weakness is due in part to degeneration within the central nervous system. However, it is unknown how changes to the representation of corticospinal output in the primary motor cortex (M1) relate to such weakness. Transcranial magnetic stimulation (TMS) is a noninvasive method of cortical stimulation that can map representation of corticospinal output devoted to a muscle. Using TMS, we examined age-related alterations in maps devoted to biceps brachii muscle to determine whether they predicted its age-induced weakness. Forty-seven right-handed subjects participated: 20 young (22.6±0.90 years) and 27 old (74.96±1.35 years). We measured strength as force of elbow flexion and electromyographic activation of biceps brachii during maximum voluntary contraction. Mapping variables included: 1) center of gravity or weighted mean location of corticospinal output, 2) size of map, 3) volume or excitation of corticospinal output, and 4) response density or corticospinal excitation per unit area. Center of gravity was more anterior in old than in young (p<0.001), though there was no significant difference in strength between the age groups. Map size, volume, and response density showed no significant difference between groups. Regardless of age, center of gravity significantly predicted strength (β = -0.34, p = 0.005), while volume adjacent to the core of map predicted voluntary activation of biceps (β = 0.32, p = 0.008). Overall, the anterior shift of the map in older adults may reflect an adaptive change that allowed for the maintenance of strength. Laterally located center of gravity and higher excitation in the region adjacent to the core in weaker individuals could reflect compensatory recruitment of synergistic muscles. Thus, our study substantiates the role of M1 in adapting to aging-related weakness and subtending strength and muscle activation across age groups. Mapping from M1 may offer foundation for an examination of mechanisms that preserve strength in elderly.PLoS ONE 02/2014; 9(2):e89371. DOI:10.1371/journal.pone.0089371 · 3.53 Impact Factor
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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