Stretching human muscles makes them stronger.

WHEN SKELETAL MUSCLE IS STRETCHED while activated, force increases significantly. After the end of the stretch, force decays rapidly and reaches steady state. This steady-state force is higher than the force produced at the corresponding length during purely isometric contractions. This increase in force observed after the stretch, referred to as steady-state force or residual force enhancement, has been observed since early 1950s (1), but its mechanisms are still a matter of debate. Residual force enhancement has been typically associated with sarcomere nonuniformities and the so-called " popping sarco-meres hypothesis " (5). However, recent studies have shown that the residual force enhancement cannot be explained uniquely by popping sarcomeres (11), and evidence suggests that it may be linked to cross-bridge kinetics and passive elements inside muscle fibers (3). Residual force enhancement has been observed in a variety of experimental preparations, including myofibrils (11), fibers (2, 9), and isolated muscles (1). Recent studies performed with humans experimented " in vivo " suggest that this phenomenon has important functional implications. These studies show that the residual force enhancement is present in small muscle groups following maximal and submaximal electrical stimulation (4, 10), as well as voluntary contractions (4, 6). The residual force enhancement following electrical stimulation presents characteristics that are similar to those observed during experiments with isolated preparations. However, the residual force enhancement obtained after voluntary activation, and its physiological importance for human muscles, still needs investigation. One of the reasons for such limited knowledge is that the residual force enhancement following submaximal, voluntary contractions depends on the level of muscle activation , and it also varies considerably among individuals (6). The study performed by Pinniger and Cresswell, published in this issue of the Journal of Applied Physiology (7), adds important information regarding the connection between the residual force enhancement and contractions performed in physiological conditions. The authors evaluated the residual force enhancement in large leg muscles of healthy individuals; in this case, the authors measured the joint torques produced around the ankle as an indicative of muscular forces. During the experiments, submaximal plantar flexion and dorsiflexion activation was maintained at ϳ25% of maximal effort, by providing the subjects with direct visual feedback of the electromyography signals from the soleus or tibialis anterior muscles. During activation, muscles were stretched by changing the ankle angle within their working range (corresponding to the ascending limb of their force-length relationship). Force enhancements of 7 …