File Name: SLS1_14.DOC

EXPERIMENT SLS1 BSP14

CONTRACTILITY PROPERTIES OF SKELETAL MUSCLES

[PIs:]{.underline}

[Russia:]{.underline}

V.S. Oganov, Institute of Biomedical Problems, Moscow

[Co-Investigators:]{.underline}

L.M. Murashko, Institute of Biomedical Problems, Moscow

O.E. Kabitskaya, Institute of Biomedical Problems, Moscow

[US:]{.underline}

D. Riley, Wisconsin Medical College, Milwaukee, Wisconsin

R. Edgerton, University of California, Los-Angeles, California

[Introduction]{.underline}

It has been found that in weightlessness skeletal muscles of man develop
certain changes, in spite of countermeasures used. During space flight
these changes include a decline in the velocity and precision of motor
acts, decrease of muscle mass and tone, strength and endurance of
muscles of, primarily, legs and trunk \[5, 9\].

Cosmos rat experiments have demonstrated that weightlessness-induced
changes in skeletal muscles are adaptive in character and that,
depending on their involvement in antigravitational function, fiber
composition and biomechanics, their adaptation varies in pattern and
degree \[2, 5\]. It has been also shown that adaptive changes in
physiological properties of muscles in space are accompanied by adequate
changes in their structure \[1, 7\], energy metabolism \[1\] and, in
certain muscle fibers, by reprogrammed synthesis of contractile and
regulatory proteins \[3, 13\].

In SLS-1 experiments, we, in collaboration with U.S. investigators,
continued studies of the mechanisms and time course variations of
adaptation of mammalian skeletal muscles to the weightless state of
varying duration and functional recovery during readaptation.

[Materials and Methods]{.underline}

The Soviet investigators were supplied with biosamples from the rats
flown in space for 9 days. The biosamples were dissected from 25 animals
divided into 5 groups: basal controls (5 rats, BC), flight animals
sacrificed at R+0 (5 rats, R+0), vivarium controls (5 rats, C+0), flight
animals sacrificed at R+9 (5 rats, R+9) and vivarium controls (5 rats,
C+9).

The following muscles were taken under study: forelimb muscles \-- m.
brachialis (BRACH), medial head of m. triceps brachii (TRIC) and
hindlimb muscles \-- extensor digitorum longus (EDL), m. soleus (SOL),
m. gastrocnemius caput laterale(GL) and m. gastrocnemius caput mediale
(GM).

Contractility of glycerinated muscle fibers was examined according to
the method of Saint-Gyorgy in our modification \[4\]. From each muscle
25 - 30 bundles containing

5 - 10 myofibers were isolated. Then muscle bundles were in sequence
placed into trays containing a contracting (ATP + Ca++), washing and a
relaxing solution. The contractility level was measured by means of a
device in which a muscle bundle was fixed by two forceps and contracted
isometrically. One of the forceps was connected

to a tension gauge (mechanotron of the 6 M x 1 B type) and the other -
to a rod.

Prior to placing muscle bundles into the above solutions, the number of
fibers in a bundle was calculated, and the diameter of the bundle (D)
and the diameter of every fiber in the bundle were measured to determine
the fiber cross-section area. Using the amplitude of isometric tension
(A) and cross-section area of the fibers, their isometric tension (in
g/mm2) Pm or contraction force was calculated. Analysis of muscle bundle
contraction also allowed evaluation of time parameters of tension
development (Tc) and relaxation (Tp). In addition, the maximum velocity
of contraction (Vc) and velocity of half-relaxation (Vp) to reach 1/2 Am
were calculated. Also, the area under the curve of isometric tension
development, or force impulse (I) was determined which was taken for
performance measure.

[Notes:]{.underline} 1. In the SLS-1 experiment, the whole triceps
muscle was dissected and the medial head was not isolated. This makes it
difficult to compare the SLS-1 data with previous Cosmos results. As
known, the medial head of m. triceps is different from other parts of
the muscle in terms of the composition of muscle fibers: fibers of
oxidative-glycolytic type are predominant in the medial head. 2. The
degree of glycerination of m. brachialis and m. triceps was inadequate,
probably, because muscle samples were too

large. 3. Some fibers were attached only to one end of the rod and we
failed to obtain a muscle bundle preparation because the muscle was
already contracted. 4. The soleus muscle preparation was not dissected
precisely along myofibers. Due to this, muscle bundle preparations were
shortened which made difficult myofiber fixation.

[Results]{.underline}

The results of this study are summarized in Tables 1 to 5. The tabulated
data show that the greatest changes developed in the soleus muscle.
Measurement of *diameters* of individual fibers demonstrated that they
diminished only in the slow-twitch soleus muscle (-10.9%, P = 0.01)
(Table 1, Figure 1), whereas they tended to increase in GM and BRACH;
the parameter remained unaltered in EDL, TRIC or GL (Table 1).

The tabulated data indicate a significant decline in the *contraction
force* (Pm) of postural SOL (-34.4%, P \< 0.05) (Table 2, Figure 2). At
R+9 the contraction force of the SOL muscle fibers returned to the
baseline. The trend of changes in EDL and both heads of the
gastrocnemius muscle (R+0) differed from that in SOL: the contraction
force increased insignificantly (Table 2, Figure 2). The contraction
force of BRACH and TRIC was unchanged. At R+9 the force parameters
recovered not only in SOL but also in GL. During recovery the myofiber
force decreased in GM, TRIC and EDL, the decrease being significant in
TRIC (-19.9%, P \< 0.05) (Table 2).

After flight the maximal *velocity of contraction* (Vc) of myofibers was
significantly reduced in SOL (-27.5%, P \< 0.01) (Figure 3). In BRACH
and GL this parameter only tended to diminish, in EDL and GM it slightly
increased and in TRIC remained unchanged (Table 3). Only BRACH showed a
recovery of the velocity parameter, while EDL, GM and SOL developed
opposite changes. After flight the contraction velocity in SOL
diminished and 9 days thereafter it increased by 20%. During the
recovery period the velocity also tended to grow in BRACH (by 7%). In
EDL and GM the contraction velocity at first increased slightly and then
decreased by 10% and 20%, respectively. In GL the parameter decreased
slightly immediately after flight and continued to go down 9 days
thereafter (by 11%).

Variations in the half-relaxation velocity (Vp) included a trend toward
a decrease: it was more marked in SOL (-18.8%) and insignificant in GM
(-3.9%) and GL (-1.4%) (Figure 4). Vp increased in the fast-twitch EDL
(32.5%), BRACH (22.5%) and TRIC (only 10%) (Table 4).

In consistency with changes in the velocity and force of muscles, their
performance also varied: force impulse of myofiber preparations
decreased, primarily, in postural muscles (-53.7%, P = 0.01) and TRIC
(-21.3%) (Figure 5). At the same time force impulse tended to increase
in EDL (69.5%, P = 0.05) and GL (32.7%) and GM (46.1%), with no
noticeable changes in BRACH. At R + 9 the performance parameter returned
to the baseline in all muscles examined, except for TRIC (Table 5).

[Discussion]{.underline}

Our observations give evidence that responses of contractility
parameters of various muscles to space flight are largely dependent on
their functional specialization and contraction biomechanics \[5\]. The
SLS-1 experiment lent support to the previously advanced hypotheses that
contractility changes in muscles are highly selective and associated
with their involvement in antigravitational function at 1 g. The
greatest changes were seen in SOL which is a very active
antigravitational muscle mainly consisting of intermediate and slow
fibers.

The lowered contraction strength of the postural antigravitational
muscles SOL and TRIC in weightlessness, i.e., in the absence of weight
(static) loading of the musculo-skeletal system, caused muscle
hypofunction. Measurement of myofiber diameters

showed that they were reduced only in the slow SOL. The reduction was
very small in the fast locomotor muscles. Morphological examinations of
muscles of Cosmos rats give evidence that actual atrophy occurred
primarily in antigravitational muscles \[1, 2\]. In the SLS-1 case only
SOL became atrophied.

SLS-1 observations are in good agreement with previous Cosmos studies.
It was shown \[2\] that in terms of a decrease in single contraction
velocity the muscles examined can be arranged as BRACH-EDL-TRIC-SOL,
which confirms a distinct antigravitational function of SOL and TRIC. In
terms of mass decline after space flight (of 18 to 21 days in duration)
the above muscles can be arranged in the same sequence. The Cosmos
experiments also revealed a significant decrease of strength properties
of postural muscles (SOL, TRIC) and no change of strength (or,
occasionally, a slight increase) in fast muscles (BRACH, EDL) \[5\].
They demonstrated a consistent increase of the contraction velocity of
the postural slow SOL, a decrease in BRACH and changes of different sign
in TRIC and EDL \[3\] (Figure 8). It is very likely that the primary
cause responsible for the reduced capability of muscle fibers to
generate strength is atrophy.

Comparison of SLS-1 findings with previous Cosmos and ground-based
findings allows the conclusion that the recent flight study has enriched
our knowledge about time course variations in physiological properties
of rat skeletal muscles. Previous experiments demonstrated that
short-term flights (5 days) led to a decline of contraction force of not
only postural muscles SOL and TRIC but also fast flexor muscles BRACH
and EDL \[6\]. However, after a 7-day flight the strength properties
varied in a more diverse manner. In the fast EDL and BRACH these changes
were insignificant. After a SLS-1 9-day flight the differentiation was
more distinct than after shorter-term flights

(5, 7 days) (Figure 6, 7). Besides, SLS-1 data are consistent with
previous findings which showed that the strength level tended to decline
as the flight time increased. In this context, it can be asserted that
as the flight time increases from 1 to 3 weeks muscle strength
properties tend to recover. The fast locomotor EDL tended to enhance its
contraction force. Similar tendencies of changes in the contractility
parameters of EDL and BRACH indicating their hyperfunction were detected
in earlier, although longer-duration space flights (3 weeks).

Analysis of the results accumulated suggests that at early stages of
weightlessness (5 days) muscle contraction becomes slower because Ca++
motility in sarcoplasmic reticulum membranes declines \[10, 12\]. The
total decrease of muscle strength observed during this time period may
be induced not only by a reduction of the mass of myofibrillar proteins
but also by a decrease of their affinity for Ca \[10, 11\].

A longer-duration space flight (18.5-21.5 days) causes more substantial
and different changes of muscle functions. Together with atrophic
processes (decrease of myofiber diameter, muscle mass and strength),
that were observed for the most part in the postural slow SOL and, to a
lesser degree, in TRIC, other adaptive changes may also develop which
include reprogrammed synthesis of myofibrillar proteins. In SOL these
changes include synthesis of \"fast\" myosins (light chains in
structure), which normally do not occur in this muscle and,
consequently, acceleration of the contraction process \[3, 13\]. As
shown by previous studies, myosin restructuring can already be discerned
at an early stage of exposure to weightlessness \[6\]. It follows from
\[8\] that similar trends developed in the SLS-1 9-day flight: decrease
in the amount of \"slow\" myosins (heavy chains in structure) and
appearance of fast myosins. However, in the SLS-1 flight, as was the
case in previous short-term flights, such changes are slight and masked
by a slower contraction process induced by inadequate coupling of
excitation and contraction processes (Figure 9).

Functional muscle changes detected in Cosmos experiments proved
reversible and, in all likelihood, biologically necessary to sustain
life in a support-free space; due

to this, they can be referred to the category of adaptive reactions.

As follows from the above SLS-1 results, muscle changes during the
recovery period also differ in different muscles (Tables 1 -5). For
instance, in SOL the force of contraction decreases immediately after
flight and recovers substantially in the course of adaptation (Figure
10). On the 7th day after the 21-day Cosmos-1129 flight the force of
contraction of SOL myofibers also increased substantially and exceeded
the vivarium control level (rebound-effect). In our opinion, this can be
attributed, among other reasons, to an enhanced sensitivity of
myofibrillar proteins to Ca++ which is one of the components of the
hypergravity stress reaction.

In summary, responses of skeletal muscles to space flight are consistent
and their magnitude and sign in different muscles depend on their
involvement in antigravitational functions and muscle biomechanics.

Comparative biomechanical analysis of flight and ground-based control
findings gives evidence that the factor responsible for functional
restructuring of postural muscles in weightlessness is the unloading of
antigravitational muscles and deficiency of the tonic component of
movements \[5\]. This is also confirmed by tail suspension results \[14,
15\].

Thus, SLS-1 results indicate that in space virtually all skeletal
muscles develop both passive adaptive changes of the atrophic type
(primarily, postural muscles) and active adaptive changes of the
regulatory type.

Muscle adaptation in space flight leads to an adequate rearrangement of
functional and metabolic profile of myofibers at the expense of
reprogrammed synthesis of myofibrillar and sarcoplasmic proteins.
However, these changes diminish significantly muscle functional
capabilities at 1 g.

[References]{.underline}

1\. Ilyina-Kakueva, E.I., Petrova, N.V., Portugalov, V.V. The Effect of
Space Flight on Skeletal Musculature and Muscle Nervous Apparatus. In:
The Effect of Space Flight Dynamic Factors on the Animal Body. Moscow,
Nauka, 1979, pp. 95-104 (in Russian).

2\. Oganov, V.S. The Effect of Weightlessness on Contractility
Properties of Functionally Different Skeletal Muscles. In: Nervous
Control of Structural and Functional Organization of Skeletal Muscles.
Leningrad, Nauka, 1980, pp. 142-162 (in Russian).

3\. Oganov, V.S., Skuratova, S.A., Murashko, L.M. et al. Changes in the
Composition and Properties of Contractile Properties After Space Flight.
Biofizika, 1982, vol. 27, No. 1, pp. 26-30 (in Russian).

4\. Oganov, V.S., Skuratova, S.A., Shirvinskaya, M.A. The Effect of
Cosmos-936 Flight on Contractile Properties of Rat Muscle Fibers. Kosm.
Biol. Aviakosm. Med., 1981, No. 4, pp. 58-61 (in Russian).

5\. Oganov, V.S. The Role of Nervous Trophics in the Adaptation of
Skeletal Muscles and Motor Function to Weightlessness. In: Mechanisms of
Neuronal Regulation of Muscle Function. Leningrad, Nauka, 1988, pp.
107-136 (in Russian).

6\. Oganov, V.S., Skuratova, S.A., Murashko, L.M. Contractile Properties
of Rat Skeletal Muscles After Cosmos-1887 Flight. Kosm. Biol. Aviakosm.
Med., 1991, No. 2, pp. 44-47 (in Russian).

7\. Portugalov, V.V. On the Mechanisms of Development of Morphological
Changes in Mammals Flown on Biological Satellites. Izv. AN SSSR, Ser.
Biol., 1978, No. 4, pp. 501-506 (in Russian).

8\. Baldwin, K.M. Effects of Zero Gravity on Biochemical and Metabolic
Properties of Skeletal Muscles of Different Fiber Types. Spacelab Life
Sciences-1, 180-Day Preliminary Results. 1992, Ch. 3, pp. 1-11.

9\. Gazenko, O.G., Grigoriev, A.I., Kozlovskaya, I.B. Mechanisms of
Acute and Chronic Effects of Microgravity. The Physiologist, 1987, vol.
30, No. 1, Suppl., pp. 1-5.

10\. Holy, X., Mounier, Y., Goblet, C. Microgravity Effects on the
Contractile Proteins of Rat Muscles. In: Physiologie Spatiale/Space
Physiology. 2nd Int. Conf., Toulouse, France, 20-22 Nov., 1985. ESA
Publ. Div., ESTEC, Noordwijk, Netherlands, 1986, pp. 61-65.

11\. Holy, X., Oganov, V.S., Mounier, Y., Skuratova, S. Compartement des
Proteines Contractiles Derats Soumis a la Microgravite. C.R. Acad. Sc.
Paris, 1986,

T. 303, Serie 111, No. 6, pp. 229-234.

12.Mounier, Y. La Systeme Musculaire en Apesanteur. In: In: Physiologie
Spatiale/Space Physiology. 2nd Int. Conf., Toulouse, France, 20-22 Nov.,
1985. ESA Publ. Div., ESTEC, Noordwijk, Netherlands, 1986, pp. 53-59.

13\. Takacs, O., Oganov, V., Hideg, J. et al. The Effect of
Weightlessness on the Composition of of Myofibrillar Proteins. J. Muscle
Res. Cell Motility, 1980, No. 1, pp. 242-248.

14\. Templeton, G.H., Padalino, M., Manton, J. et al. Influence of
Suspension Hypokinesis on Rat Soleus Muscle. J. Appl. Physiol.:
Respirat. Environ. Exercise Physiol., 1984, vol. 56, No. 2, pp. 278-286.

15\. Thompson, D.B., Herrick, R.E., Baldwin, K.M. Time Course of Soleus
Muscle Myosin Expression During Hind Limb Suspension and Recovery. J.
Appl. Physiol., 1987, vol. 63, No. 1, pp. 130-137.

[Summary]{.underline}

The effects of zero-gravity on contractile properties of skeletal
muscles of rats were studied after a 9-day space flight and 9-day
postflight readaptation period. The results gave evidence that the
greatest changes occurred in the postural soleus muscle:

the diameter of muscle fibers diminished, and isometric tension and
contraction velocity decreased. The fast locomotor muscles, i.e., EDL
and both heads of the gastrocnemius muscle, showed a trend toward an
increase in the contractile force. EDL also showed a decrease of the
contraction and half-relaxation velocity. During the recovery period
these parameters returned to the baseline.

The above observations indicate that changes in contractile properties
of muscles during flight largely depend on their functional profile.

[Table 1]{.underline}

Diameter of Muscle Fibers of Rats (μm, M [+]{.underline} m) Immediately
(R+0) and

9 Days (R+9) After Flight

  ----------- ----------- ----------- ----------- ----------- -----------
  Muscle      BC          C+0         R+0         C+9         R+9

  ----------- ----------- ----------- ----------- ----------- -----------

  ----------- ------------ -------------- ------------ ------------ ------------
  SOL         55.98±2.01   58.67±1.61\*   52.27±1.07   65.03±4.11   60.59±1.41

  ----------- ------------ -------------- ------------ ------------ ------------

  ----------- ------------ ------------ ------------ ------------ ------------
  EDL         46.48±3.08   52.35±1.82   52.51±0.85   55.21±1.12   51.40±1.39

  ----------- ------------ ------------ ------------ ------------ ------------

  ----------- ------------ ------------ ------------ ------------ ------------
  GL          61.52±1.80   63.66±0.98   65.04±1.92   65.38±1.84   60.51±2.36

  ----------- ------------ ------------ ------------ ------------ ------------

  ----------- ------------ ------------ ------------ -------------- --------------
  GM          64.00±1.66   58.17±1.74   62.19±1.68   65.49±1.37\*   63.43±1.22\*

  ----------- ------------ ------------ ------------ -------------- --------------

  ----------- ------------ ------------ ------------ -------------- ------------
  BRACH       54.13±1.73   53.09±1.91   56.19±0.86   56.28±1.14\*   58.64±0.47

  ----------- ------------ ------------ ------------ -------------- ------------

  ----------- ------------ ------------ ------------ ------------ ------------
  TRIC        54.10±1.05   55.99±1.25   55.78±1.08   55.12±1.50   56.90±1.22

  ----------- ------------ ------------ ------------ ------------ ------------

\* - Significant difference when compared to C+0 at p \< 0.01

[Table 2]{.underline}

Maximal Isometric Tension of Muscle Fibers of Rats (Pm, n mm-2 10-1, M
[+]{.underline} m) Immediately (R+0) and 9 Days (R+9) After Flight

  ----------- ----------- ----------- ----------- ----------- -----------
  Muscle      BC          C+0         R+0         C+9         R+9

  ----------- ----------- ----------- ----------- ----------- -----------

  ----------- ----------- ----------- ------------- ----------- -----------
  SOL         5.46±0.30   4.50±0.36   2.95±0.47\*   4.67±0.79   5.04±0.81

  ----------- ----------- ----------- ------------- ----------- -----------

  ----------- ----------- ----------- ----------- ----------- -----------
  EDL         6.11±0.38   3.94±0.45   5.13±0.59   4.45±0.36   3.87±0.48

  ----------- ----------- ----------- ----------- ----------- -----------

  ----------- ----------- ----------- ----------- ----------- -----------
  GL          4.66±0.39   3.55±0.39   4.50±1.16   4.69±0.30   4.78±0.54

  ----------- ----------- ----------- ----------- ----------- -----------

  ----------- ----------- ----------- ----------- ----------- -----------
  GM          4.49±0.38   3.96±0.35   4.81±0.46   5.39±0.68   4.82±0.71

  ----------- ----------- ----------- ----------- ----------- -----------

  ----------- ----------- ----------- ----------- ----------- -----------
  BRACH       3.50±0.50   2.62±0.22   2.62±0.10   2.74±0.23   2.62±0.35

  ----------- ----------- ----------- ----------- ----------- -----------

  ----------- ----------- ----------- ----------- ----------- ------------
  TRIC        2.58±0.27   2.33±0.32   2.42±0.10   2.20±0.12   1.76±0.14#

  ----------- ----------- ----------- ----------- ----------- ------------

\* - Significant difference when compared to C+0 at p \< 0.05

\# - Significant difference when compared to C+9 at p \< 0.05

[Table 3]{.underline}

Velocity of Isometric Tension Development of Muscle Fibers of Rats (Vc,
g s-1 10-3,

M [+]{.underline} m) Immediately (R+0) and 9 Days (R+9) After Flight

  ----------- ----------- ----------- ----------- ----------- -----------
  Muscle      BC          C+0         R+0         C+9         R+9

  ----------- ----------- ----------- ----------- ----------- -----------

  ----------- ------------ ------------ -------------- ------------ ------------
  SOL         31.30±2.49   33.57±2.08   24.34±1.41\*   30.28±4.43   36.47±3.70

  ----------- ------------ ------------ -------------- ------------ ------------

  ----------- ------------ ------------ ------------ ------------ ------------
  EDL         46.39±6.67   42.53±4.07   45.65±3.82   45.07±4.17   40.94±3.31

  ----------- ------------ ------------ ------------ ------------ ------------

  ----------- ------------ ------------ ------------ ------------ ------------
  GL          66.70±5.33   73.98±3.20   69.40±5.83   77.49±8.86   68.77±5.83

  ----------- ------------ ------------ ------------ ------------ ------------

  ----------- ------------ ------------ ------------ ------------ ------------
  GM          74.30±2.26   66.26±5.54   71.11±5.50   66.94±5.63   53.04±6.03

  ----------- ------------ ------------ ------------ ------------ ------------

  ----------- ------------ ------------ ------------ ------------ ------------
  BRACH       41.90±2.32   38.38±5.87   32.25±2.35   31.06±3.38   33.20±2.36

  ----------- ------------ ------------ ------------ ------------ ------------

  ----------- ------------ ------------ ------------ ------------ ------------
  TRIC        39.29±4.72   31.71±2.45   31.67±3.93   27.12±2.82   28.21±4.15

  ----------- ------------ ------------ ------------ ------------ ------------

\* - Significant difference when compared to C+0 at p \< 0.01

[Table 4]{.underline}

Velocity of Half-Relaxation of Muscle Fibers of Rats (Vc, g s-1 10-3,

M [+]{.underline} m) Immediately (R+0) and 9 Days (R+9) After Flight

  ----------- ----------- ----------- ----------- ----------- -----------
  Muscle      BC          C+0         R+0         C+9         R+9

  ----------- ----------- ----------- ----------- ----------- -----------

  ----------- ------------ ------------ ------------ ------------ ------------
  SOL         24.00±1.07   21.43±2.45   17.40±1.87   24.98±4.47   30.80±3.67

  ----------- ------------ ------------ ------------ ------------ ------------

  ----------- ------------ ------------ ------------ ------------ ------------
  EDL         41.48±6.21   31.48±5.54   41.71±7.54   40.59±5.16   33.76±7.14

  ----------- ------------ ------------ ------------ ------------ ------------

  ----------- ------------ ------------ ------------ ------------ ------------
  GL          38.27±3.61   33.31±3.86   32.85±6.39   31.03±3.82   33.51±2.71

  ----------- ------------ ------------ ------------ ------------ ------------

  ----------- ------------ ------------ ------------ ------------ ------------
  GM          32.62±4.36   32.38±3.60   31.12±2.07   36.33±2.67   33.75±4.33

  ----------- ------------ ------------ ------------ ------------ ------------

  ----------- ------------ ------------ ------------ ------------ ------------
  BRACH       25.98±3.89   18.62±2.11   22.80±2.15   20.73±1.95   22.20±2.82

  ----------- ------------ ------------ ------------ ------------ ------------

  ----------- ------------ ------------ ------------ ------------ -------------
  TRIC        19.69±2.02   17.11±2.56   18.83±1.51   20.01±1.30   14.62±1.30#

  ----------- ------------ ------------ ------------ ------------ -------------

\# - Significant difference when compared to C+9 at p \< 0.02

[Table 5]{.underline}

Force Impulse - Performance of Muscle Fibers of Rats (I, n s 10-3,

M [+]{.underline} m) Immediately (R+0) and 9 Days (R+9) After Flight

  ----------- ----------- ----------- ----------- ----------- -----------
  Muscle      BC          C+0         R+0         C+9         R+9

  ----------- ----------- ----------- ----------- ----------- -----------

  ----------- ------------ ------------ -------------- ------------ ------------
  SOL         0.744±0.10   0.622±0.08   0.307±0.07\*   0.742±0.11   0.830±0.14

  ----------- ------------ ------------ -------------- ------------ ------------

  ----------- ------------ ------------ -------------- ------------ ------------
  EDL         0.597±0.08   0.317±0.05   0.537±0.09\*   0.409±0.04   0.330±0.08

  ----------- ------------ ------------ -------------- ------------ ------------

  ----------- ------------ ------------ ------------ ------------ ------------
  GL          0.684±0.07   0.404±0.08   0.536±0.11   0.681±0.08   0.644±0.08

  ----------- ------------ ------------ ------------ ------------ ------------

  ----------- ------------ ------------ ------------ ------------ ------------
  GM          0.641±0.03   0.396±0.06   0.578±0.08   0.782±0.14   0.817±0.16

  ----------- ------------ ------------ ------------ ------------ ------------

  ----------- ------------ ------------ ------------ ------------ ------------
  BRACH       0.413±0.03   0.360±0.03   0.373±0.03   0.356±0.03   0.349±0.05

  ----------- ------------ ------------ ------------ ------------ ------------

  ----------- ------------ ------------ ------------ ------------ ------------
  TRIC        0.371±0.05   0.284±0.05   0.223±0.01   0.224±0.03   0.182±0.02

  ----------- ------------ ------------ ------------ ------------ ------------

\* - Significant difference when compared to C+0 at p \< 0.01

![](media/image1.pct){width="5.555555555555555in"
height="6.472222222222222in"}

Figure 1. Diameter of myofibers of m. soleus (SOL) and extensor
digitorum longus (EDL) of rats immediately (R+0) and 9 days (R+9) after
flight as compared to C+0 and C+9 controls. (BC = basal control)

![](media/image2.pct){width="5.555555555555555in"
height="6.930555555555555in"}

Figure 2. Isometric tension of m. soleus (SOL) and extensor digitorum
longus (EDL) myofibers of rats immediately (R+0) and 9 days (R+9) after
flight as compared to C+0 and C+9 controls. (BC = basal control)

![](media/image3.pct){width="5.875in" height="6.680555555555555in"}

Figure 3. Velocity of isometric tension development of m. soleus (SOL)
and extensor digitorum longus (EDL) myofibers of rats immediately (R+0)
and 9 days (R+9) after flight as compared to C+0 and C+9 controls. (BC =
basal control)

![](media/image4.pct){width="5.513888888888889in"
height="6.694444444444445in"}

Figure 4. Velocity of half-relaxation of m. soleus (SOL) and extensor
digitorum longus (EDL) myofibers of rats immediately (R+0) and 9 days
(R+9) after flight as compared to C+0 and C+9 controls. (BC = basal
control)

![](media/image5.pct){width="5.583333333333333in"
height="6.847222222222222in"}

Figure 5. Force impulse of m. soleus (SOL) and extensor digitorum longus
(EDL) myofibers of rats immediately (R+0) and 9 days (R+9) after flight
as compared to C+0 and C+9 controls. (BC = basal control)

![](media/image6.pct){width="7.833333333333333in"
height="5.277777777777778in"}

Figure 6. Variations in isometric tension of myofibers from muscles of
different profile

![](media/image7.pct){width="7.652777777777778in"
height="4.458333333333333in"}

Figure 7. Variations in maximal contraction velocity of myofibers from
muscles of different profile

![](media/image8.pct){width="5.486111111111111in"
height="6.930555555555555in"}

Figure 8. Variations in isometric tension and contraction velocity of
myofibers from different muscles related to their involvement in
antigravitational function

![](media/image9.pct){width="5.472222222222222in"
height="8.11111111111111in"}

Figure 9. Variations in isometric tension of myofibers from muscles of
different profile in flights of varying duration

![](media/image10.pct){width="5.527777777777778in"
height="8.416666666666666in"}

Figure 10. Variations in SOL and EDL during postflight recovery of
different duration