宇宙空間で有効な筋力維持装置開発の研究

抄録

Weightlessness is associated with the rapid onset of osteoporosis and muscle atrophy. Muscle bulk, for example, decreases in weightlessness twice as rapidly (1%/day) as it does during strict bed rest on the ground. In the space shuttle, astronauts are stabilized with an elastic band and perform resistance exercise using an ergometer in an attempt to reverse these processes. Despite these efforts, it remains very difficult to prevent musculoskeletal deterioration.
We developed a “hybrid exercise ” technique that uses the force generated by an electrically stimulated antagonist muscle to provide resistance to a volitionally contracting agonist. In other words, the agonist performs a volitional concentric contraction against an electrically stimulated eccentric contraction of the antagonist.
To learn more about the basic effects of stimulation, we examined comfortable maximal electrical stimulation voltage on the quadriceps femoris and also measured its concentric and eccentric torque using KIN-COM&^R;. Seven sedentary young males (14 extremities) were involved in this study. Stimulation was performed 3 times a week for 6 weeks. The maximal comfortable stimulation voltage increased significantly at the midpoint (3 weeks) (from 31.81±12.01 V to 48.47±17.98 V). Electrically stimulated muscle torque also increased with increasing stimulation voltage (from 0.37±0.14 Nm/kg to 0.47±0.12 Nm/kg) over the same period. Electrically stimulated eccentric muscle contraction strengths were significantly greater (50%) than concentric contractions produced with the same stimulating voltages (p<0.001). The tension of the electrically stimulated eccentric contraction of the antagonist could be utilized effectively as a resistance of volitional contraction of the agonist at even low stimulation intensities.
Hybrid exercise has several advantages. These include: 1) activation of type I muscle fiber as a result of the volitional contraction of agonist muscles, 2) simultaneous exercise of both agonist and antagonist musculature, 3) longitudinal bone force loads, 4) requiring minimal external stabilization of the subjects, and 5) the proven safety of electrical stimulation as a physical therapy modality.
Hybrid exercise, which utilizes electrically stimulated antagonist force as a resistance of joint motion instead of gravity, would become a useful exercise method in a weightless environment.