Neuromuscular adaptation and training effects of whole body vibration training

Kurzfassung

Whole body vibration training (WBVT) is currently been tested as a potential exercise countermeasure against the losses of leg muscle mass and strength during long term space flight. Vibration causes a direct mechanical stimulus to leg muscles. An increase in muscle activity is also expected via the so called tonic vibration reflex. WBVT causes a moderated increase in respiratory oxygen consumption (Rittweger et al. 2002, Int J Sports Med). A combination of resistive strength training plus whole body vibration was successfully applied to counteract leg muscle atrophy (Blottner et al. 2006, Eur J Appl Physiol, Mulder et al. 2006, Eur J Appl Physiol). However, resistive training alone also efficiently counteracts muscle loss during bed rest (Alkner & Tesch 2004, Eur J Appl Physiol). In three recent studies we examined a) whether the energy consumption of an isometric contracting calf muscle increased by a superimposed vibration of the pedal, b) whether WBVT increased the EMG activity of the m. rectus femoris during slow knee bends and whether this increase persisted over 5 consecutive days of trainings, and c) whether WBVT in upright standing position with moderately bended knees and an extra load of 15% body weight was capable to prevent the muscle volume loss caused by 14 days of 6° head down tilt (HDT) bedrest. We examined local energy metabolism in the right calf muscle of 20 subjects by 31P magnetic resonance spectroscopy. Twenty subjects performed 3 min static plantar flexion exercise at 40% MVC with and without superimposed 20 Hz vibration at ±2 mm amplitude. Both exercise types were performed with and without applied arterial occlusion (AO). Vibration plus contraction caused a significantly higher consumption in phosphocreatine than static contraction alone (p<0.05). This difference, however, was moderate and only visible under AO. Natural muscle perfusion may be increased during vibration which would allow an aerobic ATP formation for the extra energy costs during vibration. During 5 subsequent days, we tested the combination of WBVT plus knee bends (n=20) on the EMG activity of m. rectus femoris of both legs in comparison with resistive training alone (n=19). On days 1, 3, and 5, the integrated EMG amplitude was determined after filtering of motion artefacts by vibration for each knee bend cycle and was averaged for every training set. On the first training day, significantly higher EMG amplitudes (p<0.01) were found during vibration than during knee bends only. Furthermore, a steeper increase in EMG amplitude during the 10 sets of exercise was found (p<0.01) but no significant decrease in median frequency. However, during subsequent days of training the effects of vibration on EMG became smaller. At day 5, the EMG during knee bends plus vibration was not anymore significantly different compared to EMG at normal knee bends. After 14 days of HDT bed rest in 8 subjects the volumes of knee extensors and flexors and of plantar flexors were decreased by about 5%. WBVT in upright standing position was not capable to reduce these losses in muscle volume. An optimum WBVT to counteract losses in muscle volume by immobilisation probably needs individually adapted vibration frequencies and amplitudes to reach maximum activation of each target muscle group. Furthermore, an optimum training protocol probably needs a variation in the vibration stimulus in order to minimise the reduction in muscle activation due to neuromuscular adaptation.