Muscle maintenance by volitional contraction against applied electrical stimulation

Muscle training exercises are needed for muscular endurance during spaceflight. This study was designed to investigate effects of volitional contraction against applied electrical stimulation on the muscular endurance of the proximal upper extremity. Thirteen healthy sedentary men were allocated into two groups. One group participated in a hybrid (HYB) exercise regimen in which the biceps brachii was stimulated as he volitionally extended his elbow, and the triceps brachii was stimulated as the volitionally flexed his elbow. The second group underwent a similar regimen in which the electrical stimulation (ELS) was alternatively delivered to the biceps brachii and then to the triceps brachii with the limb fixed. Forty-second surface electromyography (EMG) recordings at 50% maximum voluntary contraction (MVC) were made as baseline data at just before starting the training regimen, and again conclusion. The median frequency (MF) and mean power frequency (MPF) slopes with time were determined using power spectrum analysis. There were statistical significance only for the triceps in which the MF and MPF slopes in the HYB Group became less negative over the period of study (from -45.7±14.7 and -47.0±8.6%/min at baseline to -36.9±10.7 and -36.8±7.0%/min at the end of training, respectively). The corresponding values for these slopes in the ELS Group showed opposite tends with less marked changes of borderline significance for MF and of statistical significance for MPF. These results suggested that the HYB exercise regimen was capable of producing an improvement in triceps but not biceps brachii.