Active Voice: Resistance Training and the Nervous System
By James L. Nuzzo, Ph.D., CSCS®, and Janet L. Taylor, M.D.
James L. Nuzzo, Ph.D., CSCS®, is a postdoctoral fellow at Neuroscience Research Australia (Sydney, Australia). His research focuses on how resistance training alters the neural connections between the brain and muscles.
Janet L. Taylor, M.D., is a researcher at Edith Cowan University in Perth, Australia and at Neuroscience Research Australia. Her work focuses on the neural control of movement, especially in the areas of neural plasticity and fatiguing exercise.
This commentary presents Dr. Nuzzo’s and Dr. Taylor’s views on the topic of a research article that they and their colleagues authored. The article appeared in the November 2017 issue of Medicine & Science in Sports & Exercise® (MSSE).
Resistance training is a form of exercise that involves repeated muscle contractions against external resistance. One benefit of resistance training is that it improves muscle strength. Much of this muscle strength improvement occurs within the first month of training. However, over this time, changes in muscle size and architecture are small or non-existent. This observation has led researchers to propose that initial improvements in muscle strength from resistance training are due primarily to changes in the central nervous system.
One hypothesis has been that resistance training enhances the nervous system’s ability to “drive” muscles. This ability is called voluntary activation, and it can be tested by stimulating an individual’s brain while they perform a maximal contraction with their muscles. If the stimulation produces extra force from the muscles, it means the individual’s nervous system was not driving the muscles maximally, i.e., activation was less than 100 percent. Currently, there is no consensus if voluntary activation improves with resistance training. Moreover, the site of adaptation — brain or spinal cord — remains unclear.
In our study, published in the November 2017 issue of MSSE, we examined whether four weeks of resistance training improves voluntary activation. Also, using magnetic stimulation of corticospinal axons, we examined if resistance training alters the strength of the synaptic connections in the spinal cord. Healthy individuals were randomized into a resistance training group or control group. Individuals in the resistance training group performed 12 sessions of isometric contractions of the elbow flexors at an intensity of 70 percent to 85 percent of maximum. Individuals in the control group did not complete the training. Before and after the four-week study period, muscle strength, voluntary activation and other measures of nervous system and muscle function were assessed.
The group that performed the resistance training increased muscle strength by 13 percent. They also improved voluntary activation from 88.7 percent to 93.4 percent. However, the strength of the synaptic connections in the spinal cord was unchanged by training. The control group did not exhibit changes in strength, activation, or any other measures.
The results from this study show that four weeks of resistance training of the elbow flexor muscles improves the brain’s ability to drive the muscles to produce their maximal force. This finding helps to explain how muscles can become stronger without a change in muscle size or architecture. However, changes in strength and activation were not accompanied by increased strength of the synaptic connections in the spinal cord. Thus, the site of adaptation, and the mechanism that led to the improvements in strength and activation, remains unknown. Nevertheless, the findings suggest that health practitioners can use resistance training to improve neural drive in patients with motor impairments.