Effects of microgravity on maximal power of lower limbs
during very short efforts in humans
G. ANTONUTTO, C. CAPELLI, M. GIRARDIS, P. ZAMPARO, AND P. E. DI PRAMPERO
Dipartimento di Scienze e Tecnologie Biomediche dell’Università di Udine,
I-33100 Udine, Italy
ments of myosin heavy chains occurring already within
the first week of spaceflight (5, 6, 12, 24, 26).
In humans, muscle force and velocity were studied by
using a Cybex dynamometer before and after the
Skylab missions (2, 3, and 4 of 28, 59, and 84 days,
respectively) during which the crews performed minimal to vigorous physical exercise to prevent muscular
decay. Nevertheless, the leg extensor force was reduced
by 25% after the 28-day Skylab-2 mission (35) and by
6.5% after the 84-day Skylab-4 mission (19). Isokinetic
(eccentric and concentric) force was also measured
before and after 5–11 days of spaceflight in 19 subjects.
At reentry, concentric muscle force of the quadriceps,
trunk flexors, and trunk extensors had decreased significantly (13, 10, and 23%, respectively) compared with
preflight values (19). However, isovelocity muscle contractions never, or only seldom, occur in everyday life.
We therefore set out to compare the effects of microgravity on maximal muscular power of the lower limbs of
astronauts and cosmonauts during short efforts, in
which either the velocity was imposed by the experimenters or both variables (force and velocity) were
dependent on the subjects’ muscle action. The experiments were performed before and after the two missions, Euromir ’94 and ’95, jointly organized by the
European and Russian space agencies.
muscle power; spaceflight; Euromir missions
MATERIALS AND METHODS
SINCE THE BEGINNING of the spaceflight era, weightlessness was shown to lead to substantial changes in
muscle function (for a review, see Ref. 11). These
changes, globally defined as deconditioning, consist
mainly of loss of muscle mass and force, increased
muscle fatigability, and abnormal reflex patterns (17,
19, 21, 27). They are likely due to an imbalance between
muscle protein synthesis and catabolism, brought about
by the absence of the constant pull of gravity, particularly in weight-bearing muscles, and similar to that
observed during immobilization of fractured limbs or
during prolonged bed rest without exercise (13, 23, 28,
31, 32). Animal studies have also shown significant
changes in rat muscle, as a consequence of 12.5–22
days of spaceflight. These changes consisted of reduction of mass and diameter of slow-twitch fibers and
decrease in muscle force, as well as molecular rearrange-
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The procedures reported in the literature to determine the
maximal power during very short all-out efforts in humans
can be subdivided into two broad groups: 1) ‘‘instantaneous’’
methods, in which the power is assessed during a single, very
short (,1-s) contraction of the extensor muscles of the lower
limbs, such as a standing high jump off both feet; and 2)
‘‘average’’ methods, in which the power is determined on a
longer time basis (5–7 s) over an even number of contractions
of the muscles of one limb at a time, such as pedaling
maximally or running upstairs (for reviews, see Refs. 7 and
13a). In this study we determined the maximal power according to both procedures by means of a newly developed
instrument, the multipurpose ergometer dynamometer [MED
(3, 37)], the technical characteristics of which are summarized below (see MED).
Subjects. Four male subjects (S1–S4), whose characteristics are reported in Table 1, were studied before and after
spaceflights of 31- to 180-day duration. An additional subject
(S5) has been studied only after reentry from the longest
mission in manned spaceflight history so far (438 days).
The MED. The MED (Fig. 1) consists of two rectangular
(3 3 0.9-m) metal frames hinged at one end. Two precision
rails are fixed to the upper frame, which can be inclined by
means of a hydraulic jack up to an angle of 30° with respect to
the lower one. A seat is fixed on a carriage that is free to move
by means of four ball bearings on the rails. An isokinetic
cycloergometer, powered by a 4-kW electric motor, is fixed to
8750-7587/99 $5.00 Copyright r 1999 the American Physiological Society
85
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Antonutto, G., C. Capelli, M. Girardis, P. Zamparo,
and P. E. di Prampero. Effects of microgravity on maximal
power of the lower limbs during very short efforts in humans.
J. Appl. Physiol. 86(1): 85–92, 1999.—The maximal power of
the lower limbs was determined in four astronauts (age
37–53 yr) 1) during maximal pushes of ,250 ms on force
platforms [‘‘maximal explosive power’’ (MEP)] or 2) during
all-out bouts of 6–7 s on an isokinetic cycloergometer [pedal
frequency 1 Hz: maximal cycling power (MCP)]. The measurements were done before and immediately after spaceflights of
31–180 days. Before flight, peak and mean values were
3.18 6 0.38 and 1.5 6 0.13 (SD) kW for MEP and 1.17 6 0.12
and 0.68 6 0.08 kW for MCP, respectively. After reentry, MEP
was reduced to 67% after 31 days and to 45% after 180 days.
MCP decreased less, attaining ,75% of preflight level, regardless of the flight duration. The recovery of MCP was essentially complete 2 wk after reentry, whereas that of MEP was
slower, a complete recovery occurring after an estimated time
close to that spent in flight. In the same subjects, the muscle
mass of the lower limbs, as assessed by NMR, decreased by
9–13%, irrespective of flight duration (J. Zange, K. Müller, M.
Schuber, H. Wackerhage, U. Hoffmann, R. W. Günther, G.
Adam, J. M. Neuerburg, V. E. Sinitsyn, A. O. Bacharev, and
O. I. Belichenko. Int. J. Sports Med. 18, Suppl. 4: S308–S309,
1997). The larger fall in maximal power, compared with that
in muscle mass, suggests that a fraction of the former
(especially relevant for MEP) is due to the effects of weightlessness on the motor unit recruitment pattern.
86
MAXIMAL MUSCULAR POWER AFTER SPACEFLIGHT
Table 1. Anthropometric characteristics of subjects,
preflight V̇O2 max , and flight duration
Subject
Age,
yr
Height,
cm
Preflight
Body Mass,
kg
Preflight
V̇O2 max ,
l/min
Mission
Duration,
days
S1
S2
S3
S4
S5
53
47
37
39
52
181
172
182
182
175
77.7
73.0
74.2
67.0
82.5
3.0
2.9
3.5
3.5
31
169
180
180
438
V̇O2 max , maximal O2 uptake. In the case of subject 5 (S5), preflight
V̇O2 max was not determined.
Fig. 1. Schematic view of multipurpose
ergometer dynamometer (MED) configured to assess maximal explosive power
(MEP). Subject sitting on carriage seat
(C-S) pushes with both feet on force
platforms (FP). Velocity (V) of consequent backward movement of C-S is
determined by means of a wire tachometer (WT). Force (F) exerted by subject
is measured by 2 load cells indwelling
into FP. Instantaneous power (Ẇ) is
calculated as Ẇ 5 F · V. Hinges (Hi)
allowing tilting up of MED’s mobile
frame, by action of hydraulic jack (HJ),
and isokinetic cycloergometer (Cy), are
also indicated. When MED is used as
an isokinetic cycloergometer, mobile
frame is positioned horizontally, and
FP are tilted upward. See text for details.
Ẇ(t) 5 F(t) · V(t)
(1)
Before use, the MED was inspected and granted approval
for use in human experiments by the appropriate European
Space Agency committee (further details on the construction
and operation of the MED can be found in Ref. 37).
Experimental protocol. The maximal power of the lower
limbs was assessed after both an instantaneous and an
average method as defined by Capelli and di Prampero (7).
The instantaneous procedure consisted of a series of six
maximal pushes with both feet on the force platforms, with a
resting interval of 2 min between pushes. To optimize the
force developed by the quadriceps femoris, the maximal
pushes were performed from a knee angle of 110° (22, 25). The
requested knee angle was obtained by adjusting the position
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the hinged end of the MED, between the rails. The electric
motor is controlled by an inverter so that the pedaling
frequency can be varied from 0.5 to 4 Hz. The power of the
motor is large enough to avoid being overridden by the
subject, even during an instantaneous all-out effort. A straingauge system (SRM Powermeter and Powercontrol II),
mounted on the chain ring, yields continuous measurements
of the force exerted by the subject on the pedals. The
mechanical power, as given by the product of the torque times
the angular velocity, is calculated on a 10-ms basis and
displayed to the subject on the front panel for visual feedback
control. When the MED is used as a cycloergometer, the
carriage seat is fixed in a given position by means of adjustable electromagnetic blocks.
Two force platforms, which, when the MED is used as a
cycloergometer, are tilted upward so as not to interfere with
the movements of the pedals and of the lower limbs, are also
fixed to the hinged end of the upper frame.
When the MED is used for instantaneous power assessment, the electromagnetic blocks are removed and the force
platforms are tilted down so that the subject pushing on them
can accelerate himself and the carriage seat (41.4 kg) backward. A couple of adjustable mechanical blocks can be appropriately positioned on the rails, thus setting the minimum
distance between the carriage seat and the platforms. The
backward movement of the carriage is stopped by two shock
absorbers mounted on the back of the main frame. The force
exerted on the force platforms is measured by two load cells
(PA40 300, Laumas), indwelling in the platforms in such a
way as to be unaffected by the point of application of the push.
The backward velocity is recorded by means of a wire
tachometer (PT8201, Celesco) connected to the carriage seat
and fixed to the frame of the MED. The analog outputs of the
force and velocity transducers are digitalized and recorded by
means of a data-acquisition system (MP 100, Biopac) for
subsequent analysis. The mechanical power (Ẇ) developed by
the subject is obtained from the instantaneous product of the
total force (F; sum of the two legs) times the backward speed
(V )
MAXIMAL MUSCULAR POWER AFTER SPACEFLIGHT
87
of the mechanical blocks, which also prevented the motion of
the carriage seat toward the platforms, thus impeding any
countermovement and, consequently, recovery of elastic energy (37). The subjects sat on the carriage seat of the MED
with arms on the handlebar and soles of the feet leaning
against the platforms. The platforms were positioned perpendicularly to the rails, and the main frame was inclined 20°
with respect to the horizontal position (see Fig. 1). The time
course of force, velocity, and power (as calculated from Eq. 1)
during a maximal instantaneous push is reported in Fig. 2.
Analysis of the time course of Ẇ allowed us to assess its peak
(Ẇpeak, kW) and mean (Ẇmean, kW) values. In turn, Ẇmean was
determined as
Ẇmean 5 (eẆ · dt)/tẆ
(2)
Fig. 3. F (N) exerted on pedals of isokinetic cycloergometer during
all-out effort of 5 s plotted as a function of time. Pedal frequency was
1 Hz, so 2 consecutive peaks result from action of right and left limb,
or vice versa. Maximal cycling power (MCP) was calculated from
average of 3 indicated cycles (1, 2, and 3).
The data were collected before flight [launch (L)] at days
L 2 230, L 2 68, and L 2 48 for S1 and S2; at days L 2 75 and
L 2 15 for S3 and S4 and after flight [return (R); at days R12,
R16, and R111 for S1; R13, R18, and R112 for S2; and R12,
R16, R112, R114, and R126, for S3 and S4]. As mentioned
above (see Subjects), S5 was tested after flight only, at days
R12, R16, and R111.
The experimental protocol was approved by the ethical
committees and by the medical boards of the two Euromir
missions, and all subjects signed an informed consent agreement.
Statistical procedures. The differences between the parameters observed in the different postflight baseline data collections were investigated by using an ANOVA. A post hoc
Bonferroni test (Systat 5.2.1 for Macintosh) was then applied
to determine the significance of the differences between the
average values obtained pre- vs. postflight from repeated
measures of any given parameter. The differences were
considered significant for P , 0.05.
RESULTS
Fig. 2. Time course of F (kN), V (m/s), and Ẇ (kW) during actual
determination of MEP. Instantaneous power was obtained multiplying force signal by speed (Ẇ 5 F · V). Arrows, duration of work phase
(tẆ ).
The average values (61 SD) of Ẇpeak and Ẇmean for
both MEP and MCP are reported in Table 2 in percentage of preflight values, together with the subjects’ body
masses. In Table 2, the absolute preflight values of
Ẇpeak and Ẇmean are reported in brackets for preflight
conditions only.
In S1, the preflight baselines of MEP and MCP were
calculated by averaging the values obtained in all
experimental sessions. For S2, however, the data obtained at L 2 48 were not considered because of his
suboptimal physical status. In the case of S3 and S4, we
observed a learning or training effect between L 2 75
and L 2 15, on the order of 10% for both MEP and MCP
for S3 and of 5 and 30% for MEP and MCP for S4,
respectively. Therefore, in these subjects, the preflight
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where tẆ is the duration of the work phase. Throughout this
report, the power assessed by means of this procedure will be
defined ‘‘maximal explosive power’’ (MEP).
The average procedure was similar to that proposed by
Ikuta and Ikai (18); it consisted of a bout of five to seven
‘‘all-out’’ pedal revolutions at the frequency of 1 Hz. These
bouts were performed from a brief period of free wheel
pedaling, either at rest or 5–7 min after mild aerobic exercise.
The power developed over the three most forceful pedal
revolutions during a single bout of 5- to 6-s duration was
calculated as described above and averaged to yield what will
be defined here as ‘‘maximal cycling power’’ (MCP). A typical
tracing obtained during a maximal cycling bout is reported in
Fig. 3, which shows that the force exerted on the pedal shaft
varies sinusoidally from zero, when the pedal is vertical, to a
maximum, when the pedal is horizontal. The maximal force
value is defined Fpeak, whereas the average force throughout
one revolution is defined Fmean. Because the pedal frequency
was constant (1 Hz), these two force values yielded two
power values: Ẇpeak and Ẇmean. It should be remembered that,
in cycling, only one limb is active at a time instead of both
simultaneously, as is the case for MEP, a fact that should be
kept in mind when the absolute force or power values
obtained by means of the two methods are compared.
88
MAXIMAL MUSCULAR POWER AFTER SPACEFLIGHT
Table 2. Individual data for maximal explosive power and maximal cycling power
MEP
Session
Body Mass, kg
Ẇmean % , kW
n
Preflight
77.7
9
R12
R16
R 1 11
74.0
75.5
74.0
100
(1.41 6 0.09)
66.0 6 7.1
77.3 6 2.1
91.5 6 0.7 (NS)
MCP
Ẇpeak % , kW
Ẇmean % , kW
Ẇpeak % , kW
100
(2.88 6 0.21)
69.4 6 6.6
80.2 6 0.7
88.5 6 2.8 (NS)
100
(0.68 6 0.02)
82.4 6 1.5
97.1 6 1.5 (NS)
102.9 6 7.4 (NS)
100
(1.16 6 0.01)
81.0 6 1.7
96.6 6 1.7 (NS)
100.9 6 4.3 (NS)
100
(3.09 6 0.12)
54.0 6 2.9
75.7 6 2.6
74.1 6 2.6
100
(0.59 6 0.02)
67.8 6 3.4
86.4 6 1.7
93.2 6 1.7 (NS)
100
(1.05 6 0.04)
75.2 6 4.8
90.5 6 2.9 (NS)
92.4 6 1.9 (NS)
100
(3.74 6 0.08)
40.9 6 7.8
60.7 6 3.5
59.4 6 2.9
74.6 6 3.5
81.6 6 2.1
100
(0.79 6 0.03)
74.7 6 1.3
93.7 6 1.3 (NS)
91.1 6 1.3
103.8 6 2.5 (NS)
94.9 6 3.8 (NS)
100
(1.33 6 0.04)
80.5 6 3.0
94.7 6 0.8 (NS)
94.7 6 1.5 (NS)
106.0 6 0.8 (NS)
100.8 6 3.8 (NS)
100
(3.03 6 0.08)
53.5 6 4.0
64.7 6 4.0
54.8 6 6.9
58.7 6 3.0
64.7 6 5.6
100
(0.65 6 0.01)
66.2 6 1.5
83.1 6 1.5
87.7 6 1.5
76.9 6 1.5
89.2 6 1.5
100
(1.16 6 0.05)
63.8 6 2.6
83.6 6 0.9
87.1 6 0.9
74.1 6 0.9
90.5 6 2.6
S1
3
3
3
S2
Preflight
73.0
R13
R18
R 1 12
71.6
70.8
74.2
Preflight
74.2
R12
R16
R 1 12
R 1 14
R 1 26
69.8
71.5
71.2
73.7
73.3
100
(1.44 6 0.13)
49.3 6 2.8
72.2 6 1.4
68.8 6 6.9
6
100
(1.70 6 0.03)
42.4 6 7.6
62.9 6 3.5
59.4 6 3.5
74.1 6 8.2
80.6 6 2.4
5
5
5
6
S3
S4
Preflight
67.0
R12
R16
R 1 12
R 1 14
R 1 26
65.1
63.9
65.1
64.1
67.4
100
(1.45 6 0.06)
48.3 6 5.5
57.2 6 4.1
47.6 6 6.9
52.4 6 4.8
56.6 6 3.4
6
6
6
6
6
6
S5
Preflight
85
R12
R16
R 1 11
82.5
79.3
82.7
100
(1.75)
44.0
53.1
49.7
5
6
5
100
(3.71)
52.5
62.5
63.5
Values are means 6 SD, with absolute values in parentheses. n, No. of observations; MEP, maximal explosive power; MCP, maximal cycling
power (n 5 3); Ẇ, mechanical power; Ẇmean and Ẇpeak , mean and peak value of MEP and MCP (in percentage of preflight), respectively; R,
recovery; NS, postflight values nonsignificantly different from preflight. In the case of S5, MEP was determined only postflight (see text for
further details).
baseline values retained for comparison were those
recorded at L 2 15.
S5 was not investigated before flight; in this case,
therefore, preflight MEP, when expressed per kilogram
of body mass, was assumed to be equal to the average
observed preflight in the other subjects. MCP was not
assessed in this subject.
Before flight Ẇpeak ranged from 3.74 to 2.88 kW for
MEP and from 1.33 to 1.05 kW for MCP, whereas Ẇmean
ranged from 1.70 to 1.41 kW for MEP and from 0.79 to
0.59 kW for MCP. Table 2 shows that Ẇpeak and Ẇmean
for both MEP and MCP were substantially reduced,
approximately by the same amount, after reentry.
Indeed, at R12 (R13) expressed as a percentage of
preflight values, MEP amounted to ,68% in S1 (31
days in microgravity) and to ,50% in the three subjects
who spent 169–180 days in microgravity (S2, S3, and
S4). Furthermore in subject S5, who remained in space
for 438 days, MEP did not seem to fall below ,50% of
the preflight value. It should be remembered, however,
that preflight MEP was not determined in S5: it was
assumed to be equal (per kg body mass) to that
observed on the other subjects.
After flight, MCP was reduced to a lesser extent than
was MEP (see Table 2). In addition, the effects of the
flight duration seemed to be less pronounced for MCP
than for MEP. Indeed, MCP at R12 (R13) (average of
Ẇpeak and Ẇmean ) amounted to ,81% of preflight after
31 days in microgravity (S1) and to ,70% after 169–
180 days (S2, S3, and S4).
The recovery of MCP after flight seemed to be rather
fast. Indeed, with the possible exception of S4, it was
essentially complete within 2 wk (Table 2). At variance
with MCP, the recovery of MEP seemed to follow a
slower time course and, as a first approximation, could
be described by a monoexponential function (Fig. 4)
y(d ) 5 AR 1 2 1 (A 2 AR 1 2) · (1 2 e2d/B )
(3)
where y(d) is Ẇmean (W/kg) at day d, A and AR12 are the
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6
6
6
6
6
MAXIMAL MUSCULAR POWER AFTER SPACEFLIGHT
corresponding individual values assessed preflight or
after reentry at R12 (or R13), and B is a time constant
in days.
By using an iterative statistical package (Systat
5.2.1), Eq. 3 was solved for each subject. A and B,
together with the determination coefficient (r2 ) and the
number of points (n) of each individual regression, are
reported in Table 3. Because the asymptote of an
exponential is reached after about five time constants,
the time (days) for complete recovery (R) was calculated
as R 5 5 · B and tends to be longer, the longer the
duration of the preceding flight.
The instantaneous procedure yielding MEP allowed
us to determine several other variables of physiological
interest, such as peak force (Fpeak; kN), peak velocity
(Vpeak; m/s); maximal acceleration during the backward
movement [(dV/dt)max; m * s22], and duration of the
positive work phase (tẇ; s). These values are reported
below in some detail. For simplicity, only nonsignificant
differences are indicated by NS.
Table 3. Coefficients of the equation describing time
course of postflight recovery of MEP (Eq. 3)
Subject
Days in
Microgravity
A,
W/kg
B,
days
r2
n
R,
days
S1
S2
S3
S4
S5*
31
169
180
180
438
18.16
19.71
22.96
21.66
20.62
9.65
21.14
22.57
166.02
66.09
0.994
0.987
0.988
0.985
0.986
9
16
30
30
16
48.2
105.6
112.8
830.1
330.0
A, individual preflight value of Ẇmean ; B, time constant (days); r 2,
coefficient of determination; n, no. of observations; R 5 5 · B. * Preflight Ẇmean/kg was not known and therefore was assumed to be equal
to corresponding average value obtained in other subjects.
Force. The preflight Fpeak ranged from 1.74 to 2.06
kN. It decreased by 11.7% after 31 days (S1 at R12),
26.2% after 169 days (S2 at R13), and 31.5 and 27.0%
after 180 days of spaceflight (S3 and S4 at R12,
respectively). The recovery of Fpeak after reentry was
faster for S1, who at R16 and at R111 had attained
values of only ,5% (NS) to 10% lower than preflight.
On the contrary, in the three subjects who spent
169–180 days in microgravity, Fpeak was still 15–24%
lower than preflight at R111 and R112, and, in subjects S3 and S4, 12–22% lower at R126.
The duration of the push phase immediately after
spaceflight remained essentially unchanged in S1 (31
days in microgravity), whereas it increased on average
by ,12% after 169–180 days in space (S2, S3, and S4).
Speed. Vpeak attained during the push phase ranged
from 2.76 to 2.29 m/s. It was substantially reduced in
all subjects after reentry: 224.2% after 31 days (S1 at
R12) and 227.8 to 235.8% after 169–180 days (S2, S3,
and S4 at R12 or R13). As was the case for Fpeak, the
recovery of Vpeak was faster in S1, who had attained the
preflight value (actually 98.6%, NS) at R111, than in
the three other subjects. Indeed, S2 recovered only to
213.4% at R112, whereas S3 and S4 were still below
the preflight values (25.7 and 224.0%, respectively) at
R126.
The preflight (dV/dt)max values were close to 1 G (9.81
m * s22 ) for all subjects (average value: 10.0 m * s22 ). In
S1, who spent 31 days in microgravity, (dV/dt)max
showed only a nonsignificant decrease at R12 (90.26%,
NS) and a complete recovery at R111 (103.4%, NS). In
the three subjects who spent 169–180 days in microgravity, (dV/dt)max dropped to 62.3, 68.4, and to 59.3% of
preflight values at R12 (R13) and recovered only to
69.8, 85.6, and to 62.2% at R112 (S2) and R126 (S3
and S4), respectively.
DISCUSSION
Maximal muscle power was determined by two different means: 1) an instantaneous procedure of ,0.3-s
duration, yielding MEP and 2) an average procedure,
consisting of a 5- to 6-s all-out pedaling bout on an
isokinetic cycloergometer, yielding MCP.
Both procedures, despite their widely different duration, depend energetically on anaerobic alactic sources
(mainly phosphocreatine splitting). In addition, in both
cases the active muscles are essentially the same
(primarily knee extensors), and the recovery of elastic
energy is negligible, a fact that is obvious for MCP and
that has been recently demonstrated for MEP (37).
Thus the main difference between MEP and MCP is the
velocity of movement, which is much higher for MEP.
With this in mind, we will now briefly discuss the
salient characteristics of both procedures with the
purpose of drawing a comparison between the two.
The power developed during very short all-out efforts
can be assumed to depend on 1) the active muscle mass,
2) the intrinsic characteristics of the recruited motor
units, and 3) a fast and coordinated recruitment pattern. If this is so, the decrease in MEP and MPC
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Fig. 4. Recovery of MEP mean Ẇ (Ẇmean; W/kg) after reentry in 1
subject (S3). Continuous curve is described by y(d) 5 AR12 1(A 2
AR12 ) · (1 2 e2d/B; see text for details and Table 3 for actual values of
coefficients of equation). s, Means; bars, SD; dashed line, asymptotic
value of Ẇmean (assumed to be equal to preflight value).
89
90
MAXIMAL MUSCULAR POWER AFTER SPACEFLIGHT
Fig. 5. MCP plotted as function of MEP assessed in same experimental session. Both variables are expressed in %preflight values. For all
subjects [S1 (k), S2 (q), S3 (s), and S4 (n)] and all experimental
sessions, data lie above identity line.
than, those in muscle mass, presumably because of
more effective motor unit recruitment patterns (30). In
addition, data recently reported by Koryak (20) suggest
that the fall in maximal voluntary contraction force of
the triceps surae after 7 days of simulated microgravity
(‘‘dry’’ water immersion) are mostly due to a reduction
in motor drive.
Ferretti (16) has recently reported a 24% average
reduction in the maximal power in 6 subjects during a
standing high jump off both feet after 41 days of
head-down tilt (26°) bed rest, accompanied by a 13.4%
mean decrease in the CSA of the extensor muscle
groups. Thus, after 41 days of bed rest, the explosive
power relative to the muscle mass amounted, on average, to 0.76/0.866 5 0.88, a value substantially larger
than that of 0.74 observed in S1, who remained in space
for a comparable period of time. The procedure utilized
in this study to determine MEP is not directly comparable to standing high jumps off both feet, the latter,
but not the former, permitting a substantial recovery of
elastic energy (37). When it is considered that Ferretti’s
bed rest study lasted 42 days with no exercise at all,
compared with a spaceflight of 31 days with 2 h/day of
aerobic exercise for S1, the large fall in MEP seems to
be a specific characteristic of spaceflight that cannot be
easily reproduced by bed rest.
The fall in MCP was substantially less than that of
MEP, as indicated in Fig. 5, in which the MCP data lie
above the identity line. In addition, when the MCP data
are expressed per unit muscle mass, as was done above
for MEP, the resulting values are 0.82/0.91 5 0.90 for
S1 and ,0.70/0.87 5 0.80 for S2, S3, and S4, i.e.,
substantially larger than those observed for MEP.
The different responses of MEP and MCP to microgravity presumably arise because, for MCP, the pedal
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observed after spaceflight could be due to any of these
three factors or to a combination thereof.
Morphological data obtained by Zange et al. (38), by
means of nuclear magnetic resonance of the subjects’
calf muscles immediately after reentry, demonstrate a
decrease in the muscle cross-sectional area (CSA) ranging from 9% in S1 to 13%, on average, for S2, S3, and
S4. In addition, in two subjects (S3 and S4) the CSA of
the thigh was estimated from measurements of the
circumference 20 cm above the upper margin of the
patella. The resulting changes in the CSA at R12 were
29.0% for S3 and 217.8% for S4, with no differences
between the left and right thigh. Because, at R12, the
body mass of S3 and S4 had decreased by 6 and 3% only
(see Table 2), respectively, the above changes in CSA of
the thigh are due essentially to changes in muscle
mass. These changes are on the same order as observed
in the same subjects for the CSA of the calf, which
amounted to 213 and 220% for S3 and S4, respectively
(J. Zange, personal communication). Thus the percent
changes observed by Zange et al. (38) in the calf can be
assumed to be a measure of the changes in CSA of all
active muscles. If an unchanged muscle length is
assumed, the observed changes in CSA must be equal to
the changes in muscle mass. Thus the percent MEP
values observed after flight can be expressed relative to
the muscle mass in percentage of preflight values. For
MEP, these calculations at R12 (R13) amount to
0.67/0.91 5 0.74 for S1 and to 0.47/0.87 5 0.54 for S2,
S3, and S4 (average).
It is therefore apparent that, in the case of MEP, the
decrease in muscle mass is much smaller than the
decrease in the power output throughout the investigated microgravity duration (31–180 days). It necessarily follows that other mechanisms must be responsible
for the observed decline in MEP.
Under all experimental conditions, the fall in MEP
was due to essentially equal declines in force and
velocity (see RESULTS ). For all subjects, with the exception of S1, the duration of the push increased after
flight, this being presumably an attempt to reduce the
fall in force by increasing its time of application. This
set of observations can be tentatively attributed to
either 1) changes in the motor unit types, which tend to
become slower; and/or 2) a slower motor unit recruitment pattern.
The former possibility seems rather unlikely in view
of the data obtained in animals after microgravity,
which show the opposite trend (11, 12, 33). Similar
‘‘slow-to-fast’’ changes in muscle characteristics have
been shown in humans as a consequence of spaceflights
of short duration (15, 39).
We therefore think that, besides the above-mentioned decrease in muscle mass, the major factor responsible for the decline in MEP is a change in the motor
unit recruitment patterns brought about by the absence of gravity. This hypothesis is similar to that put
forward by other authors as ‘‘hypogravitational ataxia’’
(17). In essence, this state of affairs is the opposite of
the early effects of muscle strength training, in which
case the changes in muscle force precede, and are larger
MAXIMAL MUSCULAR POWER AFTER SPACEFLIGHT
The authors express their gratitude to the crews; their back-ups;
the crew surgeons, Drs. Bernard Comet, Klaus Lohn, and Vladimir
Nalishiti; Dr. Benny Elmann-Larsen, Dr. Eva König, and Gen.
Sigmund Jähn of the European Space Agency; Paul Esser of Deutsche
Forschungsanstalt für Luft und Raumfahrt; and Claudio Annoni and
Ranieri Burelli of the Istituto Tecnico Industriale ‘‘A. Malignani’’ of
Udine for their invaluable help.
This study has been financially supported by the Italian Space
Agency (ASI) grants ASI-RS-100/140/172. P. E. di Prampero was the
main investigator.
Address for reprint requests: G. Antonutto, Dipartimento di
Scienze e Tecnologie Biomediche dell’Università di Udine, via Gervasutta 48, I-33100 Udine, Italy (E-mail: [email protected]).
Received 19 February 1998; accepted in final form 14 September
1998.
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