Keynote Lecture, 18th ISB, Zürich, Switzerland, 2000
Muscles in microgravity: from fibres to human motion

https://doi.org/10.1016/S0021-9290(02)00418-9Get rights and content

Abstract

In simulated or actual microgravity, human and animal postural muscles undergo substantial atrophy: after about 270 days, the muscle mass attains a constant value of about 70% of the initial one. Most animal studies reported preferential atrophy of slow twitch fibres whose mechanical properties change towards the fast type. However, in humans, at the end of a 42-days bed rest study, a similar atrophy of slow and fast fibres was observed. After microgravity, the maximal force of several muscle groups showed a substantial decrease (6–25% of pre-flight values). The maximal power during very short “explosive” efforts of 0.25–0.30 s showed an even greater fall, being reduced to 65% after 1 month and to 45% (of pre-flight values) after 6 months. The maximal power developed during 6–7 s “all-out” bouts on an isokinetic cycloergometer was reduced to a lesser extent, attaining about 75% of pre-flight values, regardless of the flight duration. In these same subjects, the muscle mass of the lower limbs declined by only 9–13%. Thus, a substantial fraction of the observed decreases of maximal power is probably due to a deterioration of the motor co-ordination brought about by the absence of gravity. To prevent this substantial decay of maximal absolute power, we propose that explosive exercise be added to the daily in-flight training schedule. We also describe a system aimed at reducing cardiovascular deconditioning wherein gravity is simulated by the centrifugal acceleration generated by the motion of two counter rotating bicycles ridden by the astronauts on the inner wall of a cylindrical space module. Finally, cycling on circular or elliptical tracks may be useful to reduce cardiovascular deconditioning in permanently manned lunar bases. Indeed, on the curved parts of the path, a cyclist generates an outward acceleration vector (ac). To counterbalance ac, the cyclist must lean inwards, so that the vectorial sum of ac plus the lunar gravity tends to the acceleration of gravity prevailing on Earth.

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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