Keynote Lecture, 18th ISB, Zürich, Switzerland, 2000Muscles in microgravity: from fibres to human motion
Introduction
A primary objective of the international space programmes is to install permanently manned bases on the Moon and to undertake manned explorations of Mars within the next 15–20 years. For these projects to become a reality, the negative effects of microgravity on the human muscle system must be overcome. Indeed, since the beginning of the space-flight era, weightlessness was shown to lead to substantial changes of muscle function. These changes, globally defined as deconditioning, consist mainly of loss of muscle mass, force and power, increased muscle fatigability, and abnormal reflex patterns (for reviews see Desplanches, 1997; Edgerton and Roy, 1983). They are due to a combination of factors among which an increased degradation of muscle proteins, and substantial changes of the neuromuscular control of movement, both brought about by the absence of the constant pull of gravity, play a major role. So, during long-term space missions, muscle deconditioning could limit the crews’ ability to work in space, and/or on the surface of the Moon or Mars, and/or to rapidly egress the spacecraft in an emergency landing. Furthermore, muscle atrophy and weakness are of particular concern when the transition from zero g to one g occurs, as the musculoskeletal system after several days to months in microgravity suddenly has to bear the force of gravity.
The aim of the paragraphs that follow is to review briefly the changes of muscle structure and function which occur in microgravity, focusing mainly, but not exclusively, on studies in humans. The structural modifications that follow simulated or actual microgravity will be reported in the first section; the second will be devoted to the functional changes and the final one to some possible countermeasures. The concluding section will address some topics of interest for future research.
Before addressing these specific questions, a few lines must be devoted to the means by which microgravity can be simulated on ground. These belong essentially to three main categories. (i) Bed rest studies in humans, in which healthy subjects are confined rigorously in bed, usually with a 6° head down tilt. (ii) Lower limb suspension in humans or animals, wherein one leg is maintained in a flexed position by means of appropriate straps (in humans) and the subject is free to walk on crutches, or the weight of the rear part of the body is sustained by a harness (in animals), so that the hind limbs do not support any load and the animal can move on his front limbs. (iii) “Dry water immersion” in humans, wherein healthy subjects are immersed in water from which they are separated by means of a layer of impermeable tissue. Their body weight is therefore supported very nearly completely by the Archimedean lift, but the subjects’ skin is dry, a fact which permits rather long periods of “immersion”.
Section snippets
Structural alterations
The results of studies performed in simulated or actual microgravity consistently show that human and animal muscles undergo substantial atrophy, due to a decrease in fibre size, with no change in fibre number (Desplanches et al., 1987; Ferrando et al., 1995; Roy et al., 1991; Templeton et al., 1984; Thomason and Booth, 1990). These studies also show that, in humans, atrophy is considerably greater for postural muscles, i.e. for those muscles that on ground support the weight of the body, as
Functional alterations
It should not come as a great surprise that the structural changes summarised in the preceding section lead to substantial modifications of the muscle functional characteristics. Indeed, the results of simulation studies on ground or during space-flight on animal muscles have shown that, whereas the mechanical properties of fast muscles are generally unaffected, those of slow muscles, such as the soleus, change towards the fast muscle type. For example, in the soleus: (i) the time to peak
Countermeasures
As shown in the preceding section, the maximal explosive power of the lower limbs during maximal all-out” efforts of very short duration was reduced to about 67% after 31 days and to about 45% (of pre-flight values) after 180 days. At variance with these findings, the maximal aerobic power in these same subjects was reduced only to about 80% of pre-flight values, or less, independently of the flight duration (Antonutto and di Prampero, unpublished observations).
During flight, the
Conclusions and recommendations for future research
The data reported and discussed in the preceding sections, show four major discontinuities. (i) The vast majority of the results, for both animal and human muscles under simulated or actual microgravity, have so far been obtained on weight bearing, predominantly slow muscles, such as the soleus. This approach may yield a restricted view of the changes affecting skeletal muscles in microgravity. Future studies should therefore be directed to investigate also the responses to microgravity of
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