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1 Département de Physiologie, Centre Médical Universitaire, 1211 Genève 4, Switzerland; 2 Environmental Physiology Laboratory, Department of Physiology and Pharmacology, Karolinska Institute, S-171 77 Stockholm, Sweden; 3 Institute of Advanced Biomedical Technologies, National Research Council, 20090 Segrate, Italy; and 4 Department of Exercise and Sport Science, Manchester Metropolitan University, Alsager, Cheshire ST7 2HL, United Kingdom
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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A reduction in lower limb cross-sectional area (CSA)
occurs after bed rest (BR). This should lead to an equivalent reduction in maximal instantaneous muscular power (
p) if the
body segments' lengths remain unchanged. p was
determined during maximal jumps off both feet on a force platform
before and on days 2, 6, 10, 32, and 48 after a 42-day duration BR. CSA of
thigh muscles was measured by magnetic resonance imaging before and on
day 5 after BR. Before BR, p was
3.63 ± 0.43 kW or 48.6 ± 3.3 W/kg. On days 2 and
6 after BR, p was reduced by 23.7 ± 6.9 and 22.7 ± 5.4% (P < 0.01), respectively.
Thigh extensors CSA (CSAEXT) was 16.7 ± 4.7%
(P < 0.01) lower than before. When normalized per
CSAEXT, p was reduced by only 4.8 ± 4.5% (P < 0.05). By day 48 of recovery, p had returned to baseline values. Therefore, if
p is appropriately normalized for CSA of the
extensor muscles, the reduction in CSAEXT explains most of
the decrease in p decrease after BR. Other factors
such as a deficit in neural activation or a decrease in fiber-specific tension may account for only 5% of the
p loss after BR.
muscle cross-sectional area; spaceflight simulation
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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THE MEASUREMENT OF THE
MAXIMAL instantaneous muscular power (p) of
the lower limbs, as determined during a high jump off both feet on a
force platform, relies on the assumption that, at the time instant of
its attainment, all motor units of the active muscles are
simultaneously activated (16). Within this assumption, the
p depends on the maximal rate of ATP splitting (16, 19) and, for a given segment (muscle) length, on the cross-sectional area (CSA) of the muscles contracting during the jump.
Indeed, after prolonged altitude exposure, the decrease in
p could be entirely explained by the concomitant
changes in the CSA of the thigh muscles (17).
The CSA of both thigh extensor and calf plantar flexor muscles is
decreased by bed rest (BR) and spaceflight (6, 9, 24, 25,
29). This being the case, prolonged BR and spaceflight should
result in a significant reductions of p. As no
changes in segment length occur during BR, such a reduction should be proportional to that of CSA.
Contradictory with this hypothesis is the observation that,
after prolonged spaceflight, the decrease in p was
remarkably greater than that in CSA (2). These authors
attributed their results to impairment of motor control. However, in
their study, total thigh muscle CSA, rather than knee extensors CSA,
was measured. As a result of this approach, these authors might have
underestimated the CSA decrease of the active muscle mass and thus
overestimated the decrease in p per unit of CSA
because 1) only the chain of extensor muscles, mainly of the
thigh, is active during the push phase of the jump and 2)
the decrease in CSA may be heterogeneously distributed among the
various muscles, being predominant in the extensors (24,
25).
To our knowledge, the literature contains no p data
taken after prolonged BR. Thus we carried out the present
study, which was aimed at determining the changes in
p after prolonged BR and correlating the changes in
p with the changes in the CSA of the lower limb
extensor muscles, to ascertain whether changes in muscle size could
entirely explain the expected drop of power.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Study design. Seven healthy young men, who had previously given their written, informed consent, participated in this study. At the beginning of the study, mean age was 28 ± 1 (SD) yr, height was 1.76 ± 0.01 m, and body mass was 74.7 ± 8.8 kg. The study was approved by the local ethics committee (Comité Consultatif de Protection des Personnes dans la Recherche Biomédicale, Toulouse I, France). All experiments were carried out at the Hôpital Rangueil, Toulouse, France.
The study consisted of three phases: 1) baseline control experiments before BR, including performance of magnetic resonance imaging (MRI) for CSA determination andp
measurements; 2) a 42-day, head-down tilt (
6°) BR period
without countermeasures (no deviations from the lying position were
permitted, and neither exercise nor muscle contraction tests were
allowed during this period); and 3) final experiments after
BR. These included p measurements on days
2, 6, 10, 32, and 48 during recovery, and CSA measurements (by MRI) of the thigh muscles
were carried out on day 5 of recovery.
Muscle CSA.
CSA of the lower limb muscles were computed from transaxial images
obtained by whole body MRI (Magnetom 63 SP 4000, Siemens, Germany).
Images were obtained at three levels [3/10 (+2), 5/10 (0), and 7/10
(2)] of the femur length, calculated from the femoral head to the
upper edge of the patella. Slice thickness was 10 mm, repetition time
was 700 ms, and echo time was 12 ms. Each film was digitally scanned
(StudioScan II, Agfa) at a resolution of 150-185 dpi. The
resulting files were then processed with NIH Image 1.52, a public
domain image-processing program, on an Apple Duo 230 computer. Contours
of each thigh muscle were individually drawn by hand. Total thigh CSA
(CSATOT) was expressed as the sum of all muscle CSAs
(extensors, flexors, and adductors). CSA of the extensors
(CSAEXT) included the CSAs of the quadriceps heads.
Maximal p.
p was determined during a maximal vertical jump off
both feet on a force platform, as proposed by Davies and Rennie
(11). We chose a squatting starting position, with an
angle between the thigh and the calf of ~90°, to control the range
of motion and to minimize the unavoidable negative work done by the
lower limb muscles at the onset of the push. The time course of the changes of the vertical forces was monitored by eight strain gauges located at the four corners of the platform and acquired by a computer
(ALR 486 DX 33) at a frequency of 100 Hz. Power () at time
(t) was calculated as the product of vertical force (F) times the corresponding vertical velocity of the center of gravity (v)
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(1) |
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(2) |
(t) was
retained as the p developed during the jump
(16).
The average power (a) during the whole push phase of
the jump was also determined as the integral mean of the time course of
power during the push.
The correctness of the starting position was checked by determining the
negative work performed before the push phase of the jump as the time
integral of the flexion phase (negative velocity) of the power vs. time
curve. Only jumps in which negative work was <10 J were retained.
Statistics.
Data are given as means ± SD. One-way ANOVA for repeated
measurements was used to test the significance of the
p changes as a function of time. Significant
interactions were then located by a post hoc (Bonferroni) test.
Student's t-test for paired observations was used to test
the significance of the CSA changes observed after BR. The level of
significance was set at P < 0.05. Linear regressions
were calculated by means of the least squares method.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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p and a data are summarized
in Table 1. On day 2 after BR,
absolute and specific p were 23.7 ± 6.9 and
22.7 ± 5.4% less than before BR (P < 0.05).
Similarly, a was 20.5 ± 11.0% lower at the
end of BR than before (P < 0.05). On day 6 of recovery, when the closest power determination to the CSA
measurement after BR was obtained, p was 20.9 ± 3.4 and 20.2 ± 1.6% less than before BR
(P < 0.05). The observed decrease in either
p or a after BR results from a
drop of both the maximal velocity attained during the jump (12.7±4.6
and 11.7±2.5% on days 2 and 6 after BR,
respectively; P < 0.05 for both cases) and the maximal
contraction force during the jump (14.7±5.5 and 11.8±5.2% on
days 2 and 6 after BR, respectively;
P < 0.05 in both cases).
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After bed rest, CSATOT was 12.2 ± 5.8% (P < 0.05) lower than before, whereas CSAEXT was 16.7 ± 4.7% lower than before, and the difference was significant (P < 0.01, paired t-test). The latter decrease was evident in all subjects, ranging between 11.4 and 25.8% and covered most of the decrease in CSATOT.
p, determined on recovery day 6, is plotted in Fig.
1 as a function of the CSAEXT
observed on recovery day 5. A significant linear
relationship was found (y = 0.05x + 0.024; r = 0.904, P < 0.0001). When
expressed per unit of CSATOT (Table
2), p
after BR was 9.7 ± 5.2% lower (P < 0.05) than
before BR because the decrease in p was greater than
the corresponding decrease in CSATOT. However, when
p was expressed per unit of CSAEXT, this decrease was much less (4.8 ± 4.5%) but still significant
(P < 0.05). By analogy, a,
expressed per unit of CSAEXT, was 10.1 ± 9.2% lower
than before BR (P < 0.05).
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The kinetics of recovery of p is shown in Fig.
2. This figure is a semilogarithmic plot
of the changes in p with respect to the values
before BR, as a function of the time of recovery. The highly
significant regression equation, y = 1.454 0.026x (n = 31; r = 0.84, P < 0.00001), is compatible with a simple exponential model of the p kinetics during recovery, with a
calculated half-time of the p recovery of 26.3 days.
At the end of the recovery period, p was 3.49 ± 0.44 W or 46.3 ± 4.4 W/kg, i.e., only 3.8 and 4.7% less than
in the control condition, respectively (nonsignificant).
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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In this study, for the first time, p was
determined after prolonged BR, and its changes were correlated with the
changes in CSA of the lower limb extensor muscles. Its main new finding is that the decrease in thigh muscle CSA (and thus muscle mass) after
BR explains most of the observed drop in p and that
its more prevalent occurrence in the extensor than in the flexor muscle groups explains most of the discrepancy between the change in p and that in CSATOT.
After BR, the size of the CSATOT reduction corresponded to the predictions that can be made from data in the literature (6, 9, 24, 25, 29). This decrease, however, is unevenly distributed, as it is larger in the extensor than in the other thigh muscle groups. This is in line with the observation that muscle hypotrophy preferentially affects the postural muscles of the lower limbs (24, 25). In addition, our subjects displayed structural and functional changes in the fibers of the vastus lateralis (1, 15, 23) but not in the deltoid muscle (12). The ensemble of these findings support the concept that gravity withdrawal specifically affects body supporting muscles.
The p observed before BR were similar to those
reported by others on men of similar ages and training conditions
(8, 10, 19, 31). Besides the active muscle mass and CSA,
p and a are known to depend on
the muscle fiber type composition, the muscle ATP concentration, and
either the rate of myosin ATP splitting (for p) or
ATP resynthesis through the Lohmann reaction (for a)
(16, 18, 19, 27, 28). The ATP splitting rate is affected,
among others, by the type of myosin heavy chain that is expressed in a
muscle fiber (7, 22) and, therefore, by the types of
muscle fibers that are present in a given muscle. Muscle fiber
composition, anaerobic enzyme activities, and myosin heavy chain
composition in our subjects were the same at the end of the BR as
before (15, 23) and, therefore, can be ruled out as
determinants of the observed p and
a drops.
On this basis, for an equal length of the body segments, it
appeared reasonable to assume that the reduction in
p and a after BR would
be proportional to that in CSA, so that the specific power would remain
unchanged. The decrease in CSA after BR was indeed associated with a
drop of p, and a linear relationship between
p and CSA was observed, but, in contrast with this
assumption, the decrease in p was larger
than that in CSATOT, thus reducing the
p per unit of CSATOT. However,
when p was related to CSAEXT instead of
CSATOT, its decrease was smaller but still significant. This indicates that most of the discrepancy between the decreases in
CSATOT and in p was due to the fact that
the CSA changes occurred in the active muscle mass.
By analogy, such a discrepancy also exists regarding the maximal
isometric force (Fmax) developed during voluntary
contractions, even though there is no direct relationship between force
and p. Although it is known to be proportional to
muscle CSA, Fmax is decreased after BR and/or lower limb
unloading of varying duration by a greater extent than CSA (3, 4,
14, 20, 26). Berg et al. (4) determined
Fmax in the same subjects that we used in the present
study. We normalized their values to the present CSATOT and
found that, after BR, normalized Fmax decreased from 4.25 to 3.80 N/cm2, being 10.8% lower than before BR.
The present results are similar to those obtained by others on
cosmonauts after prolonged spaceflight (2). Both studies discovered a decrease in CSATOT and p.
Antonutto et al. (2) attribute this discrepancy to motor
control alterations and maintain that, in BR, as opposed to
spaceflight, muscles are still subjected to gravitational force,
although they do not perform antigravitational work. Consequently, they
consider gravity-desensitization after prolonged spaceflight as a
plausible explanation for their findings. Those authors, however, could
measure only CSATOT. If, in the case of spaceflight, the
reduction in CSATOT is also due to a decrease in
CSAEXT, as in the present study after BR, most of the
p changes observed by Antonutto et al.
(2) would be a consequence of CSAEXT reduction
instead of motor control impairment.
Although its role is probably less than proposed, the hypothesis of
motor control alterations cannot be fully rejected on the basis of the
present results. In fact, several observations support this hypothesis.
The cosmonauts studied by Antonutto et al. (2) underwent a
physical countermeasure program, which might have reduced the degree of
muscle hypotrophy after the flight with respect to that found after BR
without countermeasures. Furthermore, in the cosmonaut, who endured a
spaceflight duration comparable to that of the present BR,
p decreased more than in the subject who showed the
greatest p drop after BR. Moreover, an alteration in
motor control is consistent with the greater motor unit activation at
any given force level observed during submaximal voluntary isometric
contractions after unloading (4, 5, 13, 26). Last but not
least, Koryak (21) found a greater decrease in maximal
voluntary isometric force than in maximal electrically-evoked tetanic
contraction force after 1 wk of simulated spaceflight and attributed
his findings to a deficit of neural activation. Indeed, an alteration
in motor control may justify the small, but significant, unexplained
fraction of change in p after BR.
In the interpretation of the p changes after BR, it
should be kept in mind that only the CSA of thigh extensor muscles was measured, whereas the entire chain of antigravitational muscles is
activated during the jump. Calf muscles are activated only in the final
phase of a jump and thus are likely to contribute very little to the
development of p. Yet this may not be the case for
the hip extensor muscles. This interpretation implies the assumption
that the CSA changes of these muscles and those of the thigh extensors
are the same. Supporting this assumption would require determination of
the CSA of the hip extensor muscles, which, to our knowledge, was never
carried out after BR.
Assuming a monoexponential process, the kinetics of recovery of
p after BR shows that a complete recovery can be
attained within 1.5 mo of reambulation in the absence of a specific
training program. The power gain is greater in the earlier phase of
recovery. The present subjects did not perform power training after
reambulation, although its performance would have probably increased
the speed of power recovery. It is noteworthy, however, that several of the structural and functional changes observed at the end of BR concerned indexes of aerobic performance (15, 30) and that this type of training would have interfered and eventually delayed recovery.
In conclusion, gravity withdrawal determines a specific hypotrophy of
the extensor muscles of the thigh. This is the main cause for the drop
in p after BR, as it explains up to 79% of the
observed changes. Other factors, such as alterations in motor control,
changes in muscle architecture, reduction in fiber-specific tension,
and, eventually, muscle damage, play a minor role at most, contributing
collectively 
21% of the observed drop in p after BR.
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ACKNOWLEDGEMENTS |
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We acknowledge the collaboration of Alain Maillet, Guillaume Weerts, and Joel Le Kernau. We thank Dr. Robert Aziza for performing MRI scanning and Dr. Philippe Levorch for hosting the force platform in his facilities.
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FOOTNOTES |
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This study was part of the HDT 94 BR project of the European Space Agency (ESA). Financial support to this study was provided by the ESA, the Swiss Federal School of Sport Sciences, the Swedish Board for Space Activities, and the Gösta Fraenckels Foundation.
Address for reprint requests and other correspondence: G. Ferretti, Département de Physiologie, Centre Médical Universitaire, 1 Rue Michel Servet, 1211 Genève 4, Switzerland (E-mail: guido.ferretti{at}medecine.unige.ch).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 19 July 2000; accepted in final form 21 August 2000.
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TOP
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METHODS
RESULTS
DISCUSSION
REFERENCES
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) and after (
)
bed rest.
