Decelerator problems of the knee are emphasized since the lower leg musculature is known to atrophy in response to weightlessness. However, other important decelerator functions are served by the shoulder muscles, in particular the rotator cuff muscles. Problems in these muscles often result in tears and dislocations as seen in baseball pitchers. It is noteworthy that at least one device currently exists that can measure concentric and eccentric muscle loading including a submaximal simulated free weight exercise (i.e., force-controlled) and simultaneously record integrated EMG analysis appropriate for assessment of all muscle functional activities. Studies should be undertaken to provide information as to the performance of maximal and submaximal exercise in space travelers to define potential problems and provide rationale for prevention

Stauber, William T.

English

NASA Technical Reports Server

N91-10582
A Unique Problem of Muscle Adaptation From Weightlessness -
The Deceleration Deficiency
William T. Stauber, Ph.D.
Department of Physiology and Neurology
West Virginia University Medical Center
Morgantown, West Virginia
Skeletal muscles perform multiple functions in
man including mechanical, thermogenic, protein
storage, and even cosmetic. It is generally the
mechanical function that has priority in the discussion
of muscle physiology, since all interactions of man
with his environment require coordinated muscle
actions and without which we could not even breathe.
From the viewpoint of the space traveler, it is most
likely that the mechanical function of muscles needs
to be given the most consideration especially
considering the known effects of weightlessness on
muscle atrophy. The mechanical actions of muscle can
be separated into three categories: motor, spring,
and shock absorption. It is primarily the latter two
that are discussed in this presentation since they both
concern the action of muscles in opposing external
loads or eccentric loading. The use of eccentric
loading (muscle lengthening while maintaining
tension) as a diagnostic aid and a therapeutic
intervention in musculoskeletal disorders has been of
recent interest and is discussed as it relates to space
travel.
A few years ago, while sitting on a panel
evaluating some research sponsored by NASA (ref. 1), I
became aware that the only established benefit of an
exercise program for astronauts was treadmill running
with a bungee cord simulating a one-g environment.
Even 2 hours of bicycle ergometry did not have the
positive effect of a treadmill run. Was there a
difference in the type of exercise that could account
for this differential response? It was obvious that
certain muscle groups, particularly the extensors of
the lower limb, are normally involved in deceleration
of the body. Dr. Cavanagh has reported that a bas-
ketball player lands with approximately 9 times his
body weight with no difficulty. Yet, this force must be
absorbed primarily by the muscles of the lower limbs.
Even during normal locomotion, a sizable force must
be decelerated with each heel strike; descending
stairs requires even more force absorption. Thus,
deceleration and muscle-lengthening activities are
important physiological events when gravitational
effects are operational - more so if emergency egress
requires the individual to drop from any significant
height out of the space vehicle in a gravity-influenced
environment.
From a physiological standpoint, two unique
events occur when a muscle is stretched while
maintaining tension: (1) the tension per myofiber is
greater than that produced even during maximal
isometric conditions (refs. 2 and 3) and (2) sensory
receptors responding to both length (stretch) and
tension are stimulated simultaneously. Since tension
is generally accepted as the stimulus for muscle
growth (ref. 4), muscles undergoing eccentric
contractions and, thus, experiencing higher forces
during normal locomotion should be stronger or more
susceptible to atrophy when the stimulus is removed.
Documentation for both conditions exists (refs. 5 to 7).
Likewise, additional sensory input is derived from
muscle-lengthening activities (e.g., walking) as
compared to muscle-shortening activities (e.g., bicycle
exercises). Could it be possible that the decrease in
ambulation performance seen in astronauts and
cosmonauts upon return to Earth (ref. 6) was due to a
combination of muscle atrophy and sensory
derivation?
With the advent of machines that can test
muscle activities in all modes of dynamic and static
muscle function in combination with
electromyographic (EMG) data, it should soon be
possible to document deficits and plan rehabilitative
or preventative exercise programs suitable for
in-flight use. One such device, the Kin/Corn, has been
used by us for approximately 4 years. The machine (a
robotic dynamometer) employs standard hydraulic
and computer systems, both of which might be
expected to be present on an aircraft. The first series
of studies on the Kin/Com involved testing subjects
without any deficits or symptoms. These led to some
surprising observations. Force oscillations were noted
to occur in the knee extensor musculature only during
_'-_,_ NOT FILMED
57
https://ntrs.nasa.gov/search.jsp?R=19910001269 2020-03-19T20:28:42+00:00Z
eccentric contractions of maximal effort. Further
investigations supported the hypothesis that these
force oscillations were induced by actions of sensory
receptors because they were of a frequency
excessively high for voluntary control and were
diminished with cooling (ref. 8) - a known inhibitor of
stretch reflexes.
The presence of force oscillations during
eccentric exercise resulted in force-velocity
relationships (Stauber, unpublished observations)that
were not expected from the studies on isolated
muscles or calculations from the Hill equation. The
extremely high forces predicted from in vitro
experiments did not occur - perhaps because of reflex-
mediated inhibitions. Deviations from the predicted
force-velocity relationships have also been observed
to occur during concentric exercises of the quadriceps
muscles when performed at very slow speeds (ref. 9).
Thus, complex neurologic control mechanisms exist
for intact muscles which need to be studied before the
function of sensory input and muscle output can be
fully understood.
In another series of experiments, a patient
population was identified which had a deficit in the
eccentric loading capability of their quadriceps
muscles that occurred along with their knee pain.
Exercises designed to alleviate this deficit also relieved
their knee pain (ref. 10). These observations
supported the theory that a decelerator deficit can
occur and lead to musculoskeletal problems.
What was the mechanism for such a deficit?
The accepted rationale for the occurrence of anterior
knee pain syndrome was a biomechanical disorder
resulting from large patellofemoral reaction forces
causing an irritated and inflamed surface to scrape
past another. However, in our study, these forces
would have been created during the concentric
loading where there was not a deficit. Alternatively,
this problem might have to do with the control of the
muscles during muscle-lengthening activities causing
patellar malalignment.
Evidence for a motor control problem was
confirmed when an individual without symptoms but
having a history of multiple subluxations of one
patella was tested with EMG recordings along with
the force records. As with the patients in the study
(ref. 10), he demonstrated a marked force drop only
during eccentric exercises and in the range of motion
between 30 ° and 60 ° of knee flexion. The EMG
recording illustrated that this force deficit was
preceded by a silent period in muscle action potentials
even though the subject was attempting a maximal
effort (ref. 11). Since there was no such absence of
EMG's during the concentric exercise, primary
i
neuromuscular disease can be ruled out. Instead,
some type of motor control problem below the level
of conscious activation of muscles seemed operative.
There have been reports that this
predisposition to subluxing patellae has a genetic
component and is often present in members of the
same family (ref. 12). Next, we tested three
generations of one family because the youngest
member had a marked force deficit and had been the
subject for the EMG study mentioned previously. The
other members of the family were asymptomatic at
the time of the test, but one member of each
generation nevertheless demonstrated some
measurable deficit, although the magnitude of the
deficit was quite variable. Could it be possible that
the problem arises if the individual with such a deficit
either becomes fatigued or loses muscle strength
because of atrophy? Unfortunately, insufficient
evidence is available at this time to answer this
question. However, the incidence of patellar
subluxations reported by the subjects used for EMG
testing always occurred when the subject was most
fatigued (i.e., at the end of a day of water skiing, etc.).
In addition, Dr. Walsh of the Houghston Clinic
reported (personal communication) that anterior
knee pain did result in patients who were immobilized
for orthopedic problems not related to the knee, as if
a loss of a certain amount of protective force
capability might place these asymptomatic but
predisposed individuals into the symptomatic group.
Could this also be a potential problem for space
travelers - especially related to their ability to perform
an emergency egress where a subluxed patella might
prevent escape from the area around a disabled
spacecraft? This possibility certainly needs further
investigation.
In summary, the focus of this report has
centered around decelerator problems of the knee,
since the lower leg musculature is known to atrophy
in response to weightlessness. However, other
important decelerator functions are served by the
shoulder muscles, in particular the rotator cuff
muscles. Problems in these muscles often result in
tears and dislocations as seen in baseball pitchers.
During this workshop, we have seen photographs of
astronauts holding satellites. Would fatigue in their
shoulder muscles have the potential for shoulder
subluxations and how could these be prevented?
Obviously, many questions have been raised
that need further documentation. Since this
workshop has been designed around providing
information as to the existence of problems that
might need exercise prescriptions as well as indicating
which devices might be used to measure and exercise
58
spacetravelersin an attemptto mitigate potential
problems,it is noteworthythat at least one device
currently existsthat can measureconcentric and
eccentricmuscleloading including a submaximal
simulatedfreeweight exercise(i.e.,force-controlled)
and simultaneouslyrecordintegrated EMGanalysis
appropriatefor assessmentof all musclefunctional
activities. Studiesshouldbe undertakento provide
informationasto the performanceof maximaland
submaximalexercisein spacetravelers to define
potential problems and provide rationale for
prevention.
References
. Herbison, G. J.; and Talbot, J. M.: Research
Opportunities in Muscle Atrophy. Life
Sciences Research Office, Federation of
American Societies for Experimental Biology
(Bethesda, Md.), 1984, pp. 1-83.
. Katz, B'.: The Relation Between Force and
Speed in Muscular Contraction. J. Physiol.
(London), vol. 96, 1939, pp. 46-64.
. Curtin, N.; and Davies, R. E.: Chemical and
Mechanical Changes During Stretching of
Activated Frog Skeletal Muscle. Cold Spring
Harbour Symposia on Quantitative Biology,
vol. 37, 1973, pp. 619-626.
4. McDonagh, M. J. N.; and Davies, C. T. M.:
Adaptive Response of Mammalian Skeletal
Muscle to Exercise With High Loads. European
J. Appl. Physiol., vol. 52, no. 2, 1984, pp. 139-
155.
. Sale, D. G.; and MacDougall, J. D.: Isokinetic
Strength in Weight-Trainers. European J.
Appl. Physiol., vol. 53, no. 2, 1984, pp. 128-132.
.
7,
.
.
10.
11.
Sandier, H.: Effects of Inactivity on Muscle.
Inactivity: Physiological Effects, H. Sandier
and J. Vernikos, eds., Academic Press, Inc.
(Orlando, Fla.), 1986, pp. 77-98.
12.
Convertino, V.A.: Exercise Responses After
Inactivity. Inactivity: Physiological Effects, H.
Sandier and J. Vernikos, eds., Academic Press,
Inc. (Orlando, Fla.), 1986, pp. 149-192.
Hart, D. L.; Miller, L. C.; and Stauber, W. T.:
Effect of Cooling on Force Oscillations During
Maximal Eccentric Exercise. Exp. Neurol., voi.
90, 1985, pp. 73-80.
Perrine, J. J.; and Edgerton, V. R.: Muscle
Force-Velocity and Power-Velocity
Relationships Under Isokinetic Loading. Med.
Sci. Sports Exer., vol. 10, 1978, pp. 159-166.
Bennett, J. G.; and Stauber, W.T.: Evaluation
and Treatment of Anterior Knee Pain Using
Eccentric Exercise. Med. Sci. Sports Exer., vol.
18, 1986, pp. 526-530.
Stauber, W. T.: Measurement of Muscle
Function in Man. Sports Physiotherapy,
International Perspectives in Physical Therapy,
V. Grisogono, ed., Churchill Livingstone
(London) (in press).
Fox, T. A.: Dysplasia of the Quadriceps
Mechanism. Surg. Clin. N. America, vol. 55,
1975, pp. 199-226.
59


A unique problem of muscle adaptation from weightlessness: The deceleration deficiency

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server