Bed rest induces neural and contractile adaptations in triceps surae.

The electromechanical adaptations of the triceps surae muscle were examined in one subject confined to bed for 5 wk due to strong orthostatic headaches. The torque produced by the triceps surae and the electromyographic (EMG) activities of the slow soleus (Sol) and fast lateral gastrocnemius (LG) were studied in response to voluntary and electrically induced contractions. Bed rest caused a 46% decrease in the maximal voluntary contraction (MVC) torque. This loss of strength was associated with a deficit of 33% in central activation and 19% in the estimated muscle force-generating capacity. The EMG activity during MVC was found to be reduced in both muscles, but to a greater extent in the LG (-51%) than in the Sol (-32%). The decrease in amplitude of the maximal M wave (Mmax) recorded in response to supramaximal electrical stimulation was greater in Sol (-28%) than in LG (-12%). After bed rest, the twitch time to peak torque was slightly prolonged (+9%) and the maximal rate of tension development (dT/dt) was reduced by 31% in electrically induced and 24% in voluntary contractions. Although muscle fatigability during a 60-s sustained MVC was not affected by the bed rest period, the relative contributions of the two muscles differed from control. These results indicate that muscular adaptations were more prominent in Sol while neural adaptations were more significant in LG. Furthermore, the neural changes almost completely recovered within 2 wk, whereas the contractile alterations required at least 5 times longer to return to the control values.