Strength training is thought to train the nervous system. This makes sense as the ability to recruit muscle fibers, the ability to better coordinate between agonist and antagonist muscles, speed, power, and the ability to “tone down” protective mechanisms like the golgi tendon organs all seem to be neural qualities. In addition, strength gains are made from training seemingly independently of increases in muscle mass, especially for beginners. Having said that, there are some challenges with finding these neural training effects as a result of strength training in research (see http://wp.me/pZf7K-55 , http://wp.me/pZf7K-5d , and http://wp.me/pZf7K-5j for an overview of this).
Palmer et al, in the August issue of the Journal of Applied Physiology, conducted a study to investigate the impact of strength training on the brain. The authors studied recreationally active individuals (mean age of 24) and had them perform 16 sessions of unilateral plantar flexion training over four weeks. This training consisted of six sets of six maximum voluntary contractions, each lasting four seconds (i.e. perform the contraction and hold it for four seconds). The subjects could monitor their force output throughout. Only the dominant leg was trained during this study.
At the end of this study, the strength in the dominant (i.e. trained) leg increased by almost 40% as a result of training. Strength was also increased in the untrained leg by 30%. What is interesting is that there was some imaging evidence of changes to the corticospinal tract (CST) as a result of the training suggesting that the CST fibers controlling the foot may have increased their myelination, increased axon density/reorganization which was correlated with the increases in strength. Now, these changes occurred in the hemisphere of the brain that controlled the trained leg, it should be noted that the authors did not find these changes in the hemisphere that controlled the untrained leg (which also got stronger during the study).
The changes to the structure of the CST is extremely interesting. To my knowledge this is the first time this has been documented. Now, the changes were small (less than 2%), the subject pool was very small, and the fact that there were no changes to the untrained leg’s hemisphere seems to conflict with the changes to the trained leg’s hemisphere.
As the authors note, it’s an interesting preliminary study. From my perspective, examining these kinds of changes as a result of multi-joint exercises might be really interesting. One could make the argument that a multi-joint exercise (like a squat or power clean) would have a greater degree of neural involvement than an essentially single-joint exercise.
Palmer, H.S., Haberg, A.K., Fimland, M.S., Solstad, G.M., Iversen, V.M., Hoff, J., Helgerud, J., and Eikenes, L. (2013). Structural brain changes after 4 wk of unilateral strength training of the lower limb. Journal of Applied Physiology, 115: 167-175.