Early neural responses to strength training

Abstract

The neural adaptations that accompany strength training have yet to be fully determined. Here we sought to address this topic by testing the idea that strength training might share similar mechanisms with some forms of motor learning. Since ballistic motor learning is accompanied by a shift in muscle twitches induced by transcranial magnetic stimulation (TMS) toward the training direction, we sought to investigate if these changes also occur after single isometric strength training sessions with various contraction duration and rate of force development characteristics (i.e., brief or sustained ballistic contractions or slow, sustained contractions). Twitch force resultant vectors and motor-evoked potentials (MEPs) induced by TMS were measured before and after single sessions of strength training involving the forearm muscles. Participants (n = 12) each performed three training protocols (each consisting of 4 sets of 10 repetitions) and served as their own control in a counterbalanced order. All three training protocols caused a significant (P < 0.05) shift in TMS-induced twitch force resultant vectors toward the training direction, followed by a gradual shift back toward the pretraining direction. The strongest effect was found when training involved both ballistic and sustained force components. There were no large or consistent changes in the direction of twitches evoked by motor nerve stimulation for any of the three training protocols. We suggest that these early neural responses to strength training, which share similar corticospinal changes to motor learning, might reflect an important process that precedes more long-term neural adaptation that ultimately enhance strength.

Motor learning and cross-limb transfer rely upon distinct neural adaptation processes

“…Substantial performance gains are exhibited within a single session, which simplifies the use of brain stimulation methods such as TMS to assess the neural underpinnings of adaptation (Carroll et al 2008). Moreover, the neural responses to ballistic motor training are similar to those observed after strength training (Selvanayagam et al 2011). Accordingly, the model provides a window into the mechanisms underlying an important physical attribute that often limits function in old age and in patients with neurological disorders.…”

Interhemispheric interactions associated with unilateral ballistic motor tasks

“…Our results are consistent with previous data from our laboratory, where twitch direction shifted significantly after training was performed at a distance of 90° away from the baseline twitch direction (Selvanayagam, et al, 2011). While some previous TMS studies, which involved repetitive ballistic thumb movements, reported that twitch directions shifted toward the training direction after 30 minutes of training at 180° away from baseline (Bütefisch et al, 2000;Bütefisch, et al, 2004;Classen, et al, 1998;Giacobbe, et al, 2011;Kaelin-Lang et al, 2005), we showed early neural adaptation occurred with a session of just 40 ballistic wrist contractions at 90° distance away from baseline (Selvanayagam, et al, 2011). The shift in twitch directions is likely to reflect the reweighting of connectivity among the forearm muscles to favour net force production in the training direction (Selvanayagam, et al, 2012a).…”

“…In Chapter 5, we oriented the wrist into training and testing wrist postures and examined the resting twitch directions in these postures before and after a session of ballistic training. We sought to discover if ballistic contractions induce an adaptation toward the training direction in extrinsic or muscle-based reference frames, using the paradigm described by Selvanayagam et al (2011). We chose two wrist postures in our study so that a shift of twitch direction in one direction would imply extrinsic representation of use-dependent learning, whereas a shift in the other direction would imply a muscle-based representation.…”

“What trains together, gains together”: strength training strengthens not only muscles but also neural networks

“…The experiment was designed based on a previous motor control study of Classen et al (6) who showed that short-term practice of directionspecific thumb movements can change the direction of stimulation-evoked thumb twitches. As in this earlier study, Selvanayagam et al (12) used suprathreshold transcranial magnetic stimulation (TMS) to induce directionally consistent movements of the forearm. After the initial test, subjects underwent three different strength training sessions involving either 1) brief contractions or 2) sustained ballistic contractions or 3) slow, sustained contractions.…”

“…This could be done by identifying the neural mechanisms underlying the changes in twitch torque direction and, in the next step, inhibit their formation. As Selvanayagam and coworkers (12) have transferred a well-explored motor learning paradigm into the field of strength training, comparisons can be easily made. For instance, LTP was shown to play a role in the skill acquisition after repetitive thumb movements on the basis of pharmacological evidence (3).…”