Viewpoint: New Thoughts on Facilitation of Muscle Strengthening

Abstract

Increase in efficacy of muscle strengthening is an important issue in the field of sport science. Musclestrengthening is credited with potentiation of muscle strength and muscle endurance, adding to motorperformance, fatigue resistance, as well as reduction in sport injuries. Muscle strength increases after trainingresult from structural changes in muscle fibers and adaptation of neural system. Traditional muscle strengtheningguided by the principles of specificity, overload, and arrangement often highlight structural changes in musclefibers and function benefits associated with training. However, little is concerned how to level up muscularcapacity through facilitation of neuronal excitability, such as alteration in efficacy of muscle strengthening withcentral modulation of cortical neuron. Research of direct control over cortical excitability are reminiscent ofanimal studies in the laboratory using the rodents and primates. It seems not easy to tune cortical excitability ofthe human, on account of ethical issues and technique barriers in the laboratory. For advancements of transcranialmagnetic stimulation and transcranial direct current stimulation (tDCS) in the recent years, changes in corticalexcitability of human in a non-harmful and painless manner become feasible. In particular, tDCS is spotlightedfor convenient application and lower price. tDCS drives low-intensity current (< 2 mA) to the cortex throughthe skull, producing acute effect and after effect on the cortex respectively (Lang et al., 2005; Nitsche et al.,2005). Acute effect is an immediate electrophysiological response of the cortical neurons. The anodal stimulationincreases the neuronal excitability of the area being stimulated, as the positive current causes a depolarization ofthe resting membrane potential of cortical neurons. On the contrary, cathodal stimulation decreases the neuronalexcitability of the area being stimulated. Interestingly, excitation of inhibition of cortical excitability withtDCS persists for an hour once the stimulation has ended. The phenomenon is called after effect, in relation toconcentration changes in neurotransmitters of the brain that effect on the NMDA and GABA receptors. Scholarsin different fields start to use the non-invasive brain stimulation techniques in many fields with several worthnotingimplications, including depression prevention (Brunoni et al., 2016; Nitsche, Boggio, Fregni, & PascualLeone,2009), pain relief (Brasil-Neto, 2016; O’Connell, Wand, Marston, Spencer, & Desouza, 2011), and motorrestoration of neurological victims (D’Agata et al., 2016; Stagg et al., 2012). Of particular note is the use of tDCSfor improving efficacy of muscle strengthening (Hendy, Teo, & Kidgell, 2015; Washabaugh, Santos, Claflin, &Krishnan, 2016). Despite some inconsistencies in parametric setting of the electrical stimulation, positive currentis always applied over the contralateral motor cortex in the training session (Langhorne, Bernhardt, & Kwakkel,2011). The increase in cortical excitability with the positive current is thought to associate with a superiorefficacy of muscle strengthening. Aforementioned evidence collectively gives us some inputs for future study,such as how to optimize efficacy of muscle strengthening of different training modes with tDCS? How to resistSports ＆ Exercise ResearchVol. 18, No. 3, i-v (September, 2016)DOI:10.5297/ser.1803.editorialivIng-Shiou Hwang vexercise-induced fatigue with tDCS? How to train the muscles that are difficult to be exercised with traditionalapproaches? Can tDCS help with skill advancement and injury prevention of the athletes? The message ofthe work is hereby to provide potential readers of the Journal a preliminary thought to promote athletes’competitiveness with practical values.