Isometric training at longer muscle-tendon complex lengths: A potential countermeasure to impaired neuro-muscle-tendon function during space travel.

Manned space exploration to distant destinations, including Mars, continues to be an aspiration of humankind. Space travel does, however, present many challenges to the body, amongst which adaptation to microgravity is perhaps the largest. For instance, both short and long manned spaceflight missions have shown substantial deleterious effects on muscle size and neuromuscular function. Although the neuro-muscle-tendon system is responding primarily to the load to which it is subjected, resistive exercise countermeasures with dynamic contractions during space travel do not entirely mitigate the space travel-induced deteriorations in neuro-muscle-tendon function, probably owing to a lack of overall accumulation of sufficient mechanical stress. The aim of this review is to evaluate the evidence for isometric resistance training at longer muscle-tendon complex lengths to mitigate microgravity-induced deterioration in neuro-muscle-tendon function better than conventional resistance-training programmes. It has been shown that specific joint positions, associated with a longer muscle-tendon complex, require larger internal muscle forces for the same external torque, thus requiring more muscle activation and imposing more tendon strain than during conventional dynamic resistance training. Isometric resistance training also confers the advantage of requirement of less voluminous equipment, in comparison to that required for dynamic resistive exercise. This factor is particularly important for space travel owing to the physical space and mass constraints. In addition, isometric contractions allow for easier monitoring and progression in exercise prescription compared with dynamic contractions.