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Anaerobic Training and Electromyography Studies

Electromyography (EMG) is a common research tool used to examine the magnitude of neural activation within skeletal muscle. Two kinds of EMG are commonly used in research and applied settings: surface EMG and intramuscular (needle or fine wire) EMG. Surface EMG requires placement of adhesive electrodes on the surface of the skin where they are able to monitor a large area of underlying muscle (152). Surface EMG is often more effective for monitoring superficial muscle, as it is unable to bypass the action potentials of superficial muscles and detect deeper muscle activity. Also, the more body fat an individual has, the weaker the EMG signal is likely to be with use of this methodology. In comparison, with intramuscular EMG, the skin surface is numbed, and a needle electrode, or a needle containing two fine-wire electrodes, is inserted through the skin and positioned into the belly of the muscle itself. Fine-wire electrodes emphasize a specificity of assessment in that they are located in a muscle of interest and accurately record localized motor unit action potentials (85). Because of its invasiveness, intramuscular EMG is primarily adopted in research settings or under clinical conditions. While it is often difficult to determine the specific underpinning mechanism(s) (i.e., increased recruitment, discharge rate, or synchronization; Golgi tendon organ inhibition) affecting EMG output, an increase in EMG signal indicates greater neuromuscular activity. 

An important consideration when examining the neuromuscular system is the training status of an individual. Neural adaptations (improved motor learning and coordination) predominate in the early phase of training without any concomitant increases in muscle hypertrophy (73, 75-77). In addition, the onset of hypertrophy is associated with a decline in EMG activity (145). It appears that as an individual’s training status advances, there exists an interplay between neural and hypertrophic mechanisms that contribute to further gains in strength and power.

Sale (166, 167) reported that dramatic increases in neural adaptation take place in the early part of a training program (6 to 10 weeks). As the duration of training increases (>10 weeks), muscle hypertrophy then occurs, and it is these structural changes that contribute to strength and power gains more than neural adaptations. Eventually muscle hypertrophy plateaus as accommodation to the training load occurs. However, at that time, if an athlete incorporates new variation or progressive overload into the training plan, neural adaptations will once again contribute to the performance improvements by acting to tolerate the “new” physical insult from training. This pattern is replicated with every stepwise change in the training demand, and as athletes progress in training, the type of program used may be one of the most important factors to consider (77, 80, 161). Neural factors are especially important for strength gains in programs that use very high training intensities (>85% of 1-repetition maximum [1RM]) (145). Training programs designed to elicit muscular power also provide a potent stimulus to the nervous system and result in higher post training EMG activity (149).

Electromyography studies have also yielded some interesting findings regarding neural adaptations to anaerobic training: progressive overload into the training plan, neural adaptations will once again contribute to the performance improvements by acting to tolerate the “new” physical insult from training. This pattern is replicated with every stepwise change in the training demand, and as athletes progress in training, the type of program used may be one of the most important factors to consider (77, 80, 161). Neural factors are especially important for strength gains in programs that use very high training intensities (>85% of 1-repetition maximum [1RM]) (145). Training programs designed to elicit muscular power also provide a potent stimulus to the nervous system and result in higher post training EMG activity (149).

Electromyography studies have also yielded some interesting findings regarding neural adaptations to anaerobic training:

• Exercising muscle undergoing unilateral resistance training produces increased strength and neural activity in the contralateral resting muscle, a phenomenon known as cross-education (89). A review of the literature has shown that strength in the untrained limb may increase up to 22%, with an average strength increase of approximately 8% (147). The increase in strength of the untrained limb is accompanied by greater EMG activity in that limb (176), thereby suggesting that a central neural adaptation accounts for the majority of strength gains.
• In untrained individuals, a bilateral deficit is evident. The force produced when both limbs contract together is lower than the sum of the forces they produce when contracting unilaterally. Research has shown that the corresponding EMG activity is lower during bilateral contractions (63), suggesting that neural mechanisms are, at least in part, a contributing factor. With longitudinal bilateral training, the magnitude of the bilateral deficit is reduced. In fact, trained or stronger individuals often show a bilateral facilitation effect in which an increase in voluntary activation of the agonist muscle groups occurs (15, 171).
• The EMG activity of antagonist muscle groups has been shown to change in response to anaerobic training during agonist movements. In most instances, cocontraction of antagonist muscles serves as a protective mechanism to increase joint stability and reduce the risk of injury (96). However, when too much antagonist activity opposes agonist movement, it creates a resistance to maximal force production. A number of studies have shown reduced antagonist cocontraction following resistance training, resulting in an increase in net force without an increase in agonist motor unit recruitment (26, 76, 151). Elsewhere, sprint and plyometric training have also been shown to alter the timing of cocontractor activation (96). The specific role of altering antagonist cocontraction patterns remains unclear. Greater antagonist activity may be observed during ballistic movements that require high levels of joint stability, or when people are unfamiliar with a task and require more inherent stability (48).

Developed by the National Strength and Conditioning Association (NSCA), Essentials of Strength Training and Conditioning, Fourth Edition, is the fundamental preparation text for the Certified Strength and Conditioning Specialist^® (CSCS^®) exam as well as a definitive reference that strength and conditioning professionals will consult in everyday practice. The book is available in bookstores everywhere, as well as online at the NSCA Store.