Neuromuscular adaptations during long-term bed rest
The weightless environment encountered during human spaceflight virtually eliminates the mechanical loading of the human body. The accompanying physical inactivity sets in motion a cascade of changes that affects practically every physiological system in the human body. Of particular medical and operational concern are the decrements in skeletal muscle strength (force, power and endurance capacity) of the legs and the demineralization of weight- bearing bones. By now, it is acknowledged that these functional impairments may be prevented by adequate muscle exercise. This thesis addresses the study into the neuromuscular adaptations in the quadriceps femoris muscle as a consequence of bed rest-induced physical inactivity. Bed rest is hereby used as a simulation model of human spaceflight. The nature and progression of the adaptations, but also the preventative effect of physical were studied. In the Berlin bed rest study, the used training paradigm comprised combined resistance and vibration exercise, i.e. strength training exercises were performed against a mechanically vibrated platform. We were interested in the changes that occur during – particularly in the early stage of – bed rest. Hence, besides conducting experiments pre- and post bed rest, we also performed seven experiments – with an increasing time interval between experiments - during the eight weeks of bed rest. To disentangle the neural activation of the muscle from more intrinsic muscle characteristics both voluntary and electrically stimulated contractions were assessed. Voluntary motor control was assessed using two procedures: by means of superimposed electrical stimulation and by means of recording the electrophysiological properties of the quadriceps muscle by means of a sophisticated high-density surface electromyography methodology (HD-sEMG). The results of the studies described in this thesis have contributed to a better understanding of the underlying mechanisms and the time course in which they contribute to the various manifestations of muscle weakness as a result of physical inactivity imposed by strict bed rest. Most notably were the findings of a linear reduction in voluntary isometric knee extension strength, and an increase in relative muscle fatigability. These adaptations were predominantly the result of a linear decay in the cross- sectional area of the quadriceps femoris muscle and a reduced blood flow as a consequence of bed rest. Changes in the intrinsic contractile characteristics of the quadriceps femoris towards a faster muscle also progressed linearly in time. An unexpected finding across experiments was that the adopted longitudinal study fully prevented neural deconditioning. Vigorous resistive vibration exercise training during bed rest appeared a suitable gravity-independent countermeasure that offset or substantially mitigated most of the adaptive changes in quadriceps femoris muscle that evolved during bed rest in the absence of this countermeasure.