Neural adaptations to strength training: Moving beyond transcranial magnetic stimulation and reflex studies

Abstract

It has long been believed that training for increased strength not only affects muscle tissue, but also results in adaptive changes in the central nervous system. However, only in the last 10 years has the use of methods to study the neurophysiological details of putative neural adaptations to training become widespread. There are now many published reports that have used single motor unit recordings, electrical stimulation of peripheral nerves, and non-invasive stimulation of the human brain [i.e. transcranial magnetic stimulation (TMS)] to study neural responses to strength training. In this review, we aim to summarize what has been learned from single motor unit, reflex and TMS studies, and identify the most promising avenues to advance our conceptual understanding with these methods. We also consider the few strength training studies that have employed alternative neurophysiological techniques such as functional magnetic resonance imaging and electroencephalography. The nature of the information that these techniques can provide, as well as their major technical and conceptual pitfalls, are briefly described. The overall conclusion of the review is that the current evidence regarding neural adaptations to strength training is inconsistent and incomplete. In order to move forward in our understanding, it will be necessary to design studies that are based on a rigorous consideration of the limitations of the available techniques, and that are specifically targeted to address important conceptual questions.

Acute Strength Training Increases Responses to Stimulation of Corticospinal Axons

“…The high force and high rate of force development contractions in the current study are comparable to those used in longer-term training studies (6). Changes in the nervous system are thought to occur with longer-term strength training, but findings in humans are inconsistent (6).…”

“…Changes in the nervous system are thought to occur with longer-term strength training, but findings in humans are inconsistent (6). One study reported increased MEP and CMEP twitch forces at low contraction intensities of the forearm muscles after 4 wk of radial deviation strength training (4).…”

Muscular endurance training and motor unit firing patterns during fatigue

“…In the earliest phase of muscular training, changes in force production may be due to neuromuscular adaptations related to learning optimal muscle activation patterns (for review, see Griffin and Cafarelli 2005;Carroll et al 2011). For example, increases in motor unit firing rate (Van Cutsem et al 1998;Kamen and Knight 2004) and earlier motor unit recruitment (Van Cutsem et al 1998;Keen et al 1994) occur during the first few weeks of resistance training.…”

Chronic Pain and Exercise: Studies on pain intensity, biochemistry, adherence and attitudes

“…The neural adaptations include for example increased single motor unit activity, improved correlated motor unit activity, increased spinal reflex activity, and improved efferent activity from motor cortex. However, the findings about neural adaptations to strength training are inconsistent [146].…”

“…In addition, strength PA&E leads to hormonal alterations and protein synthesis in the muscle which also stimulates increased strength [145]. Furthermore, neural adaptations has been proposed as one additional reason to increased strength [146]. The neural adaptations include for example increased single motor unit activity, improved correlated motor unit activity, increased spinal reflex activity, and improved efferent activity from motor cortex.…”