Neural plasticity is a two-way street: Counteracting age-related neural deterioration with strength training

Changes in the nervous system with age are largely responsible for decreased force generating capacity (FGC), i.e., maximal strength and rate of force development. Emphasizing studies applying near-maximal/maximal contraction and stimulation intensity during testing, and training intensity above 60 % maximal strength, this review uncovered that efferent neural drive, comprising motor unit recruitment and firing frequency, consistently appeared to be a main contributing factor for the decline and subsequent improvement in FGC with age. However, identifying single steps along the efferent neural drive pathway is challenging. Structural changes in the brain and corticospinal excitability alterations may be partly responsible for the reduced FGC, although these are seldom investigated in relation to FGC. Further, methodological constraints associated with measurements of corticospinal excitability challenge firm conclusions. Conduction velocity in the corticospinal tract is slower with age and intracortical inhibition increases, the latter may be improved following strength training. Peripheral factors - loss of spinal α-motoneurons, reduced spinal α-motoneuron excitability, and reduced conduction velocity - also contribute, albeit without considerable improvements following strength training. Despite methodological constraints associated with corticospinal measurements, we conclude that the efferent neural drive enhancement observed following strength training is likely a result of central nervous system adaptations, as peripheral adaptations appear to be negligible. It therefore seems essential to apply training that specifically targets efferent neural drive enhancement with older age. Heavy loads are imperative for efferent neural drive improvements and should be recommended to maintain or improve efferent neural drive and maximal FGC in older adults.