Explored here is the question of whether astronauts can perform extravehicular activities effectively, efficiently, and productively. The loss of muscle mass, movement control, central nervous system function, muscle atrophy and fatigue, all consequent to weightlessness exposure, are discussed. The author recommends more research in these areas

Edgerton, Reggie

English

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Exercise Issues Related to the Neuromuscular Function and
Adaptation to Microgravity
Reggie Edgerton, Ph.D.
Department of Kinesiology
University of California, Los Angeles
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Introduction
One of the basic problems in life sciences facing
NASA is performance. There is the issue of
performance in space as well as when astronauts
return to Earth. Can they function safely in their two
environments, particularly during the adaptive
phases? My general impression is that the operational
question has been, "Can one perform the tasks
required and survive?" I would like to suggest that we
assess the efficiency and effectiveness of performance
in space more quantitatively while maintaining an
acceptable margin of safety.
There are remarkable expectations for
productivity from humans in space even though there
are few data to suggest that humans can meet those
expectations. When this issue has been raised
previously, the usual response is essentially, "We
know that it can be done. We have done it."
Generally, I would agree that a number of specific
tasks can be done. But given the magnitude of the
work to be done, particularly during extravehicular
activities (EVA's), and given the high cost of labor in
space (thousands of dollars per hour), we should be
concerned with more than simply surviving and
whether or not you can perform some function. We
should be concerned about productivity. How
efficiently and effectively can it be done? How can
the productivity of man in space be improved?
Neuromuscular Function
As one goes from one g to virtually zero g, the
central nervous system must make rapid and accurate
adjustments. Based on my discussions with astronauts
who have had some experience in space, I believe that
it is remarkable how well and rapidly the central
nervous system can adapt to weightlessness. Within
hours, they can perform exceptionally well under
unique circumstances. However, it is important to
recognize that we're expecting virtually flawless
performance continuously over a prolonged period of
time, particularly during the construction phase of
the Space Station when EVA is required. These EVA
tasks require detailed manipulation of instruments in
a pressurized suit and glove in which the fingers are
difficult to control. For these reasons, we need to
understand how the central nervous system manages
these skill requirements. After 4 or 5 hours of EVA,
can one perform with the proper safety margin given
the consequences of making a mistake? On Earth, a
variety of mistakes can be made in attempts to
complete a task and rarely will the results be fatal.
However, during EVA particularly, mistakes must be
avoided.
Concerns related to movement skills are
justified also by the likelihood of a dramatic change in
mass of some muscles. Some experiments on rats
suggest that the amount of atrophy that occurs within
1 week of hindlimb suspension is almost as much as
that seen in 4 weeks normally. Most of the space-
related data on protein metabolism are consistent
with the view that there are marked changes in
muscle within the first few days of flight. Experiments
on rats demonstrate that about 35 percent of the mass
of a slow muscle is lost within a week of exposure to a
microgravity environment.
The loss in muscle mass is related to movement
control, in that 35 percent less tension will be
produced when these muscles are activated. Thus, the
nervous system must adjust its neural commands in
movement and in postural control. It appears that the
central nervous system is quite capable of making the
necessary adjustments. More muscle fibers can be
recruited for a given task to compensate for the loss in
force potential. So, in summary, it appears that even
though remarkable changes occur in the muscle tissue
in flight, the nervous system is able to adjust
remarkably well to weightlessness and upon return to
one g. Maintenance of posture is a potential point of
concern, largely because of the loss of muscle mass
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noted previously.Manyof the muscles involved in
maintaining the position of the head, the shoulders,
the trunk, and the hip consist of a large proportion of
slow-twitch fibers. These muscles are the most
susceptible to a loss in mass in a variety of models of
atrophy• This atrophy is unlikely to be a problem in
space, but it probably will be a problem upon return
to a one-g environment. This requires a readaptation
of the musculature to avoid problems in the realign-
ment of the vertebrae, which could eventually be
manifested as low back pain.
It is commonly assumed that prolonged periods
in space will result in "disuse" and probably increased
fatigability. However, there are a number of experi-
ments which suggest that the fatigability of atrophied
fibers does not increase necessarily. For example, the
soleus muscle of a rat that atrophies 35 percent in 1
week is no more fatigable than a normal soleus
muscle. However, it is likely that upon return to one
g, the astronaut will be more fatigable than when
doing the same amount of exercise prior to flight.
This may occur because in order to compensate for the
small muscle mass, one has to recruit more muscle
fibers, and those additional muscle fibers that are
recruited are the more fatigable ones. Generally,
there are sound bases neurologically on which to
develop hypotheses to explain some of the
observations related to the neuromuscular system.
Further, there seem to be reasonable ways to address
and solve these particular problems.
Injury
There is some evidence that there are
adaptations in tendons and bones as welt as muscle as
a result of space flight. Injuries may not be a problem
during space flight because generally the muscle
forces produced will be less than at one g. To produce
more force, one recruits more muscle fibers. In low-
level activity, theoretically, one is using the lowest
threshold motor units, which consist of the muscle
fibers that are the most susceptible to atrophy during
flight. Interestingly, the largest fibers normally are
the strongest ones and are used the least often and
atrophy the least in space. Obviously, the total
amount of activity or the total amount of force can
affect how much a muscle fiber atrophies in space, but
there are other factors to consider as well. Some
• muscle fibers are more sensitive to the changes
imposed by space flight than others. For example, it
appears that some muscle fibers can be activated for a
few seconds a day and still be maintained, whereas
other muscle fibers must receive activation for longer
periods of time. It is not clear why these differences in
sensitivity exist among fibers and muscles.
The NASA needs to know exactly what is
needed to maintain the normal size of muscle fibers.
One of the exciting aspects of this problem is that it is
technically feasible to solve; NASA has an opportunity
to attack this problem given the vast amount of useful
and basic information available. Although muscle
atrophy is a recognized problem in space endeavors, it
can be managed effectively if NASA supports an ag-
gressive and coordinated effort among its investi-
gators.
Many of the issues noted in this paper can be
addressed using animal models of space flight.
However, eventually, there must be full participation
of the astronaut corps. A simple and direct way to
address the problem in astronauts is to study muscle
tissue taken from needle biopsies. Despite the fact
that it has not been a preferred approach by NASA, it
is a direct one. It is economically very feasible, it is
safe, and most importantly, it represents the best way
to solve the problem.
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Exercise issues related to the neuromuscular function and adaptation to microgravity

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