The brain can make you stronger

Are strength gains associated with training partially attributable to changes in the way the nervous system drives muscle? There are a number of indirect lines of evidence that suggest that this is indeed the case (see review by Enoka, 1988). For example, marked increases in muscle force have been observed following strength‐training regimes that are too brief (a few weeks) to cause detectable changes in muscle itself. Likewise, when human subjects undergo strength training of a single limb, the strength of the contralateral, untrained limb often increases significantly. Similarly, Yue and Cole (1992) clearly showed that muscle strength increased substantially in subjects who performed only imaginary muscle contractions during training. Collectively, these findings support the idea that strength training, at least in the initial stages, can increase the force exerted by a limb because of neural factors.

But what neural adaptations could occur in response to training that would serve to increase strength? One possibility is that training may change the way the nervous system coordinates the activities of the multiple muscles crossing a joint such that their summed actions more effectively contribute to the development of torque in a desired direction. The most obvious manifestation of such a training‐related change in muscle coordination would be a reduction in the contributions of antagonistic muscles. Another possibility is that strength training might improve the ability of the nervous system to recruit motor units that otherwise would be quiescent even during maximal effort contractions. Similarly, strength training might increase the maximum rates of firing in motor units that normally discharge at rates below that needed to achieve maximal force. A few studies have provided some evidence that such increases in firing rate might occur with training. For example, motor unit firing rates increased significantly in a hand muscle of young healthy human subjects after 6 weeks of strength training with most of the increase occurring in the early stages of training (Patten et al. 2001).

One limitation of those studies was that motor unit firing rates were usually recorded only from brief trains of action potentials once maximum force was achieved. As a consequence, recruitment thresholds (an important clue as to the type of motor unit) were not known. Furthermore, it has not been possible to follow how the discharge behaviours of individual motor units change over the course of a period of training. This information is important as it could be that only certain motor unit types exhibit significant changes in activity with training.

In an impressive study reported in this issue of The Journal of Physiology, Del Vecchio and colleagues (2019) used multi‐electrode surface arrays to track the discharge of a large population of identified motor units possessing a broad range of recruitment thresholds before and after a 4‐week strength‐training programme involving human tibialis anterior. By placing the arrays in the same locations during different recording sessions and by taking advantage of the multiple ‘viewpoints’ furnished by the arrays that revealed distinct spatial and temporal electrical signatures of individual motor units, the authors were able to reliably identify the same motor units before and after training.

After the training, maximal voluntary force increased on average by 14%, which was accompanied by a similar increase in the average discharge rates of motor units by about 17%. Interestingly, the degree of increase in discharge rate with training was not related to recruitment threshold, suggesting that all motor‐unit types are capable of extending firing rates with training. Furthermore, the threshold forces (both absolute and normalized to maximum force) at which motor units initiated their activities diminished with training.

These results provide a clear‐cut demonstration that early increases in strength with training are caused by increases in motor unit activity. Of course, the question remains as to what mechanisms underlie these training‐related increases. Certainly, as the authors suggest, a prime suspect would be plastic changes in motor cortex spurred by its repeated vigorous activity accompanying strength training and which enhances the intensity of it excitatory drive delivered to target motor neurons. A similar possibility would be that the CNS ‘learns’ to quench some of the inhibitory synaptic input to motor neurons that often co‐occurs with excitatory inputs.

A third possibility is that the motor neurons themselves become more responsive to excitatory synaptic input with training. Some evidence of such intrinsic mechanisms was present in the data of Del Vecchio et al. (2019). Before training, motor units appeared to exhibit some degree of firing rate saturation, namely, firing rates levelled‐off even though force (and presumably excitatory synaptic drive) continued to increase. We have previously shown that when motor units saturate, artificially increasing excitatory drive does not increase firing rate, thereby implicating intrinsic mechanisms as the key culprit underlying firing rate saturation (Fuglevand et al. 2015). Indeed, a host of intrinsic ion‐channel processes have been shown to adapt during various forms of learning in a variety of neuron types (Zhang and Linden, 2003). Consequently, it seems likely that increases in motor unit firing rate with strength training were partly mediated by adaptations in motor neurons themselves. Regardless, the findings of Del Vecchio et al. (2019) are important not only for the clear demonstration that strength depends both on muscle and on brain but also because it highlights new opportunities for enhancing strength in a host of disorders for which weakness is severely debilitating.

Additional information

Competing interests

None declared.

Author contributions

Sole author.

Funding

NIH NS102259.