Effects of long-term spaceflight on mechanical properties of muscles in humans
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* Daniel Lambertz
* Chantal Pérot
* Rustem Kaspranski
* Francis Goubel
## Abstract
The effects of long-term spaceflight (90–180 days) on the contractile and elastic characteristics of the human plantarflexor muscles were studied in 14 cosmonauts before and 2–3 days after landing. Despite countermeasures practiced aboard, spaceflight was found to induce a decrease in maximal isometric torque (17%), whereas an index of maximal shortening velocity was found to increase (31%). In addition, maximal muscle activation evaluated during isokinetic tests decreased by 39%. Changes in musculotendinous stiffness and whole joint stiffness were characterized by means of quick-release movements and sinusoidal perturbations. Musculotendinous stiffness was found to be increased by 25%. Whole joint stiffness decreased under passive conditions (21%), whereas whole joint stiffness under active conditions remained unchanged after spaceflight (−1%). This invariance suggests an adaptive mechanism to counterbalance the decrease in stiffness of passive structures by an increased active stiffness. Changes in neural drive could participate in this equilibrium.
* isometric force
* isokinetic contraction
* musculotendinous stiffness
* musculoarticular stiffness
a number of studies have documented that the microgravity environment encountered during spaceflight or simulated by using models of weightlessness induces alterations in skeletal muscle function (14). The majority of knowledge, however, comes from animal studies. It has been shown that, as a consequence of reduction in activity, muscular atrophy preferentially occurs in antigravity muscles such as the soleus muscle (38). In the absence of weight-bearing activity, strength loss is the most evident consequence of atrophy. This is also reflected by changes in fiber size and/or fiber type. For instance, many studies indicate a relative increase in fast-twitch fibers in the soleus muscle. This fiber-type transition phenomenon also affects muscle mechanical properties, leading to an increase in shortening velocity (for a review, see Refs. 14 and 38) and a decrease in stiffness (8).
In humans, loss of muscle mass and force has been reported after spaceflight or prolonged bed rest, whereas the velocity characteristics measured in muscle groups were not always significantly modified (13). Recently, it was demonstrated that the unloaded shortening velocity measured in single human soleus fibers shifted toward higher velocities after simulated or real microgravity (42, 43). Thus the first aim of the present study was to investigate changes in force and velocity characteristics in human muscles as a result of a long-term spaceflight (90–180 days). During such a long stay in a space station, cosmonauts had to perform physical exercises to prevent muscular decay. This may interfere with probable adaptations due to microgravity itself. On the other hand, it seems that no one has raised the question of investigating changes in stiffness of human musculotendinous systems after exposure to weightlessness. Muscle and joint stiffness are important parameters for movement control because their value determines the resistance to an external perturbation. Furthermore, muscle stiffness can be modulated through changes in neural activation (26). The clinical literature indicates that disuse induces an increase in muscle and joint stiffness and a decrease in the range of motion (1). This may make normal movement more difficult and may alter neuromuscular performance because stiffness governs the mechanics of the interaction between the musculoskeletal system and the external environment. If such changes occur during spaceflight, daily work in a space station could become critical. Therefore, the second aim of the present work was to determine whether stiffness properties of human muscle groups and joints were modified after a long-term spaceflight. The experiments were performed on an ankle ergometer before and after spaceflight, offering the possibility to test contractile and elastic properties of the human ankle plantar flexors. A preliminary report of this work has already been published (18).
## MATERIALS AND METHODS
### Testing Machine
The ankle ergometer used for this study has been described in detail by Tognella et al. (39) but will be reviewed briefly here. The ergometer consisted of two main units: *1*) a power unit that contained the actuator, its power supply unit, position and torque transducers, and its associated electronics; and*2*) a driving unit controlled by a 486 personal computer equipped with a 12-bit analog-to-digital board. Angular displacement was measured with an optical digital sensor, and angular velocity was captured from a resolver bound to the rotor, except for velocities >15.70 rad/s that required a tachometer. Angular torque was obtained by use of a strain-gauge torque transducer. A specific menu-driven software controlled all procedures and recorded mechanical variables and electromyograms (EMG; 1-kHz sampling frequency) for later analysis. A dual-beam oscilloscope gave the subject visual feedback on the procedure in progress.
### Subjects
The experiments were performed on four cosmonauts who participated in EuroMir '94, '95 and '98-E space missions. Thanks to a special agreement between the French and the Russian space agencies, further experiments were done on 11 cosmonauts (Mir missions EO 19–24). From these 15 cosmonauts, one cosmonaut from Mir mission EO 23 was rejected because he had a heart attack during his space travel and consequently no postflight experiments were possible. The remaining 14 cosmonauts (C1 to C14) performed two sets of experiments 28–42 days before flight [baseline data collections (BDC); BDC 1, BDC 2] and immediately after spaceflight [return (R); R+2/R+3] of 180 days, except for C2 and C3, for whom flight duration was 90 days. The cosmonauts were familiarized with the experiment during a preliminary session before starting the preflight tests. The experimental protocol was approved by the committee of hygiene, safety, and ethics at the University of Compiègne and by the medical boards of the Mir missions.
### Experimental Protocol
The subject laid comfortably on an adjustable table with his left foot attached rigidly to the actuator of the ankle ergometer and his shoulders maintained by special shoulder holders, so that the subject could not move. The horizontal bimalleolar axis coincided with the axis of rotation of the actuator. The muscle group under study consisted of the triceps surae, which is composed of the soleus muscle and the gastrocnemius muscles. The knee was extended to 120° to minimize the contribution of the gastrocnemii. The ankle was placed at 90°, i.e., neutral position. Surface EMGs were recorded from the soleus muscle, the gastrocnemius muscles, and the tibialis anterior by use of standard Ag/AgCl surface electrodes. The electrodes were placed over the belly of each gastrocnemius muscle, 2 cm below the insertion of the gastrocnemii on the Achilles tendon for the soleus, and over the belly of the tibialis anterior. The EMG signals were recorded differentially, amplified, and band-pass filtered (1 Hz to 1 kHz).
The maximal motor direct response (Mmax) was used to normalize the EMG signals and thus to account for different conditions of skin and surface electrodes impedance in signals recorded on different days. The Mmax responses were elicited by applying a supramaximal electrical stimulus to the sciatic nerve with the cathode located in the poplitea fossa and the anode placed on the thigh, proximal to the patella. The stimulus intensity was adjusted so as to obtain the Mmax response on each part of the triceps surae.
In a first test, the maximal voluntary contraction (MVC) was determined in plantar flexion under isometric conditions. The subject was asked to develop a MVC against the actuator. Three trials were carried out, and the best performance was considered as the true MVC of the day.
Second, quick-release tests were performed. As in isolated muscle, the aim was to determine the characteristics of the so-called series elastic component (SEC). A major proportion of the series elasticity resides in the tendon (passive component of the SEC), whereas cross bridges constitute the active component of the SEC (37). In the present experiment, the quick-release method was adapted to measure musculotendinous stiffness of the plantar flexors. Quick-release movements from the neutral position were achieved by a sudden releasing of the moving parts of the device while the cosmonaut maintained a submaximal plantar flexion torque. Three trials were performed at 25%, 50%, and 75% of MVC. Because of a technical problem that resulted in considerable changes in the inertia parameter of the actuator between pre- and postflight tests, *cosmonaut C14* was excluded from this part of the experiment.
Then, trials of sinusoidal oscillations were imposed on the ankle joint to characterize joint dynamics in plantar flexion. The mechanisms underlying joint dynamics arise from the interaction of a number of subsystems, including muscle, tendon, and joint, i.e., musculoarticular structures as a whole. The dynamics of human joints deal then with the relation between angular position of the joint and the torque acting about it. Thus, to determine joint dynamics, sinusoidal oscillations of the type used widely in engineering frequency-domain analyses were applied. The duration of the sinusoidal oscillations was 4 s, and the displacement amplitude was 3° peak to peak around the neutral position. Frequencies of 4–16 Hz were successively imposed, using steps of 1 Hz except from frequencies higher than 12 Hz for which steps were 2 Hz. During such experiments, the subject had to maintain a constant level of torque equal to 50% of his MVC in plantar flexion. To mask the oscillations in the torque trace, the torque signal used as visual feedback for the subject consisted of a low-pass filtering of the signal. Thus all subjects were able to keep their level of force production at the target value without any difficulties. Sinusoidal perturbations with no participation of the subject (i.e., 0% of MVC) were also performed on *cosmonauts C4*–*C14* once in BDC 1 or BDC 2.
Finally, isokinetic trials at different constant velocities ranging from 0.52 to 3.65 rad/s, in steps of 0.52 rad/s, were performed. A number of cycles were collected that were each carried out by developing maximal contractions in alternated plantar flexion and dorsiflexion. The range of motion around the neutral position (90°) was fixed just below the physiological range of each subject. This was determined at the beginning of the experimental session and was presented to the subject on the control oscilloscope.
To prevent muscle fatigue, resting periods between the different trials were observed. These resting periods were standardized in terms of intratest (1-min) and intertest (3- to 5-min) periods. Thus total duration of the experiment was ∼1 h 30 min.
Because of the different directional changes in the two stiffness measures observed in the first eight subjects, the experimental protocol was slightly changed for *cosmonauts C9*–*C14* to get more detailed information about these opposite changes in stiffness. For this reason, a sinusoidal perturbation test at 25% of MVC was added. Because of the limited time for crew testing, these cosmonauts did not perform isokinetic tests.
Postflight tests were done in the same way as preflight tests, using the maximal preflight MVC as reference value.
### Data Processing
#### Motor direct response.
The Mmax responses were digitized at a sampling frequency of 10 kHz and averaged. Then, Mmax was quantified in area and duration. The area divided by the Mmax duration gave the power value of the Mmax response. Mmaxpower was calculated for the triceps surae by summing soleus and gastrocnemius Mmax.
#### Quick-release tests.
In human muscles, musculotendinous stiffness is measured by adapting classical methods to characterize SEC in isolated muscle (20,35). SEC characteristics were measured at the beginning of the movement, i.e., when elastic elements are supposed to recoil. Thus their stiffness (*S*) was calculated as the ratio between variation in angular acceleration Θ̈ (as a derivative of angular velocity Θ˙) and angular displacement, Θ, multiplied by the corresponding inertia value (I) as expressed by the formulaS=ΔΘ¨/ΔΘ·I Equation 1In this equation, inertia is assumed to be constant. This can be verified easily by considering the transition between the static phase and the dynamic phase. At this moment static torque (T) equals dynamic torque and acceleration is maximal (Θ̈max). ThenI·Θ¨max=T Equation 2Stiffness calculation was carried out within the first 20 ms at the beginning of the movement. During this time lapse, no reflex changes in muscle activation (e.g., unloading reflex) (4,20) are possible, which was confirmed by inspecting the recorded EMGs visually. Another artifact could originate from a shortening of the contractile component (CC), leading to an underestimation of*S* (*Eq. 1 *). This effect can be minimized by using release velocities higher than the maximal shortening velocity of CC. In the present experiment, releases were always given as fast as possible, but the effective velocity when stiffness is measured was found to decrease with the required torque. Then, the lowest velocities (5.00 rad/s at 25% of MVC) were somewhat smaller than the maximal shortening velocity of the plantar flexors found in the literature (6.00–7.00 rad/s) (41). In any case, stiffness was then related to the corresponding isometric torque initially exerted by the subject. The slope of the linear stiffness-torque relationship so obtained was defined as a stiffness index of the musculotendinous system (SIMT).
#### Sinusoidal perturbation tests.
These tests were used to construct frequency-response functions (i.e., Bode diagrams) in an attempt to obtain a general characterization of musculoarticular stiffness. The parameters of interest were angular displacement and torque. The subsequent analysis only considered torque modulated at the driving frequency. In so doing, nonlinearities are neglected (27). Classically, averaged displacement-to-torque amplitude ratios (i.e., gain) and the phase shift between displacement and torque were plotted against the imposed frequencies. As in other studies (for review, see Ref.27), frequency-dependent changes in gain and phase shifting reflected the classical features of a mixed mechanical contribution from inertia (I), viscosity (B), and elasticity (K) of the musculoarticular system. Such a feature can be observed providing that a frequency range of 4–16 Hz is imposed. So, by using identification techniques (30), a second-order model including such parameters was adjusted to the Bode diagram as expressed by the formulaT(t)=I·d2Θ(t)dt2+B·dΘ(t)dt·(α)+K·Θ(t)·(α) Equation 3where α is the level of muscle activation and *t* is time. So, for each level of MVC, K, B, and I could be determined.
First, K was calculated in passive condition (Kp: 0% MVC) and at 50% MVC for C4 to C14. Second, the K values of C9 to C14 were related to the maintained torque by using data obtained at 0%, 25% and 50% of MVC. Best fits for these stiffness-torque relationships were obtained by a linear modelK=a·T+b Equation 4where the slope was defined as a stiffness index of the musculoarticular system (SIMA). Of further interest was the intercept point (IP) described by the parameter *b*.
#### Isokinetic tests.
Parameters collected during isokinetic trials were angular velocity, torque, and EMGs. A processing module allowed the experimenter to select periods of plantar flexion with a velocity plateau of correct duration and low variability in maximal torque. Furthermore, root mean square (RMS) values, used for expressing the power of an EMG signal, of the agonist EMGs were calculated and normalized with respect to Mmax power values. It was also verified that antagonist EMG activity remained low. Three cycles with the highest RMS agonist activity were chosen to calculate a mean value in maximal torque and angular velocity. Plotting maximal T against Θ˙ led to a torque-velocity relationship. As previously reported (18), the T-Θ˙ curve was fitted by a logarithmic model such asT=a+b·log(Θ˙) Equation 5and angular shortening velocity at low torque (T = 10% of MVC) was chosen as an index of maximal shortening velocity (VImax) in plantar flexion (see discussion).
#### Statistics.
Mean differences in preflight MVC were analyzed by using an unpaired Student's *t*-test. A multiple regression analysis with least squares was used to study the reproducibility between preflight tests. The analysis was first applied to the slopes (SI) of the stiffness-torque relationships for BDC 1 and BDC 2 tests. This regression analysis offers the possibility to state directly significant changes on the SI parameter by calculating the correlation coefficient *r* for the difference in SI between BDC 1 and BDC 2. That means that, for noncorrelated differences, an equal slope can be adjusted by the BDC 1 and BDC 2 data set (see, e.g., Fig. 4). When differences in SI were significant, the preflight test with highest MVC was retained for further analysis. Second, this analysis was applied to the logarithmic model of the torque-velocity relationship. Thus the preflight test showing highest maximal RMS was retained when the difference between BDC 1 and BDC 2 was found to be significant.
A Wilcoxon's signed-rank test for paired differences was used to test changes in all mechanical parameters (MVC, VImax, SIMT, Kp, SIMA, and IP) due to spaceflight. This statistical analysis is appropriate for considering individual changes and for estimating a trend for the population (increase or decrease) by using the smallest Wilcoxon auxiliary variable *W*. Furthermore, the pooled data set inside the population was analyzed by running a Student's *t*-test for unpaired changes to compare the numerical means of the preflight and postflight data (MVC, VImax, SIMT, Kp, SIMA, and IP). All statistical analyses were conducted at *P* < 0.05. Values were presented as means ± SD.
## RESULTS
### Force Production in Isometric and Isokinetic Conditions
Because of individual variability, mean MVC was found to be 101.13 ± 20.12 N · m in BDC 1 and 111.80 ± 23.65 N · m in BDC 2. This difference was not significant so BDC 1 and BDC 2 MVC were averaged for each subject. Postflight maximal isometric torque in plantar flexion, collected near landing (R+2/R+3), indicated a decrease in MVC. More precisely, decreases in MVC were found for 12 subjects ranging from 2 to 37%, whereas 2 subjects had slightly increased MVC (0.2% and 0.8%). For the population, mean MVC was found to be significantly higher in preflight condition than in postflight condition (108.90 ± 19.70 vs. 90.90 ± 26.48 N · m). The Wilcoxon signed-rank test, which takes each subject into account, revealed a significant decrease of 17% for the whole population. A summary of individual and mean values is given in Fig. 1.
[Fig. 1.](http://jap.physiology.org/content/90/1/179/F1)
Fig. 1.
Changes in maximal voluntary contraction (MVC). The histograms represent the individual MVC values of each cosmonaut from baseline data collections (BDC; solid bars) and 2–3 days after return (R+2/R+3; open bars) test and mean MVC. *Preflight MVC was significantly higher than postflight MVC (*P* < 0.05).
As expected, maximal torque during plantar flexion movements in isokinetic conditions was found to decrease when angular velocity increased. Figure 2 *A*illustrates typical torque-velocity relationships for BDC 1 and BDC 2 data in plantar flexion. As described above, fitting was operated by using a logarithmic model (see *Eq. 5 *). This led to significant correlation coefficient (*r*) values ranging between 0.871 < *r* < 0.997 (*n* = 7). The multiple regression analysis revealed no significant changes between the preflight tests for each cosmonaut. Preflight individual VImax were thus averaged and compared with postflight VImax. VImax was found to increase for seven of the eight tested cosmonauts (Fig. 2 *B*). This increase ranged from 4 to 75%, whereas *cosmonaut C5*showed a decrease in VImax (37%). Mean VImaxin preflight conditions was significantly lower compared with postflight mean VImax (5.06 ± 1.08 rad/s vs. 6.63 ± 1.93 rad/s). The Wilcoxon signed-rank test indicated that the increase of 31% between preflight and postflight VImaxwas significant. Individual and mean data are summarized in Fig.3. Normalized RMS values of the EMGs were found to decrease significantly for seven of the eight subjects, ranging from 22 to 68%, whereas *subject C6* increased his RMS ∼26%. Mean normalized RMS in plantar flexion decreased from 0.23 ± 0.10 to 0.14 ± 0.07 for pre- and postflight, respectively. This corresponds to a significant decrease of 39%.
[Fig. 2.](http://jap.physiology.org/content/90/1/179/F2)
Fig. 2.
Typical torque-angular velocity relationships from isokinetic movements in plantar flexion. *A*: BDC 1 (■) and BDC 2 (□) data from*cosmonaut C3. B*: BDC (■) and R+2 (○) data from *cosmonaut C4*. Maximal shortening velocity index (VImax) data calculated at 10% of the torque-to-MVC ratio are also indicated. Correlation coefficients were, in *A*, *r* = 0.963 and 0.997 for BDC 1 and 2, respectively and, in *B*, *r* = 0.979 and 0.983 for BDC and R+2, respectively.
[Fig. 3.](http://jap.physiology.org/content/90/1/179/F3)
Fig. 3.
Changes in VImax. The histograms represent the individual VImax values of each cosmonaut from BDC (solid bars) and R+2/R+3 (open bars) test and mean VImax. *Preflight VImax was significantly lower than postflight VImax (*P* < 0.05).
### Elastic Properties of the Muscle-Tendon Complex
Concerning the musculotendinous system, Fig.4 illustrates typical stiffness-torque relationships in which best fit was a linear regression whatever the time of test (preflight or postflight). For each individual curve, the correlation coefficient (*r*) was found to be significant. A multiple regression analysis was performed to study the reproducibility of the measurements during preflight tests. It revealed that SIMT parameters obtained in BDC 1 and BDC 2 could be averaged, except for C2 and C8. For these subjects, the multiple regression analysis revealed significant differences in SIMT, and the preflight test with the highest MVC was retained for further analysis. Thus, for C2 and C8, preflight SIMT was calculated from BDC 2 data. On the other hand, the effect of exposure to microgravity on musculotendinous stiffness was tested by comparing SIMT values in preflight and postflight conditions. Twelve of the thirteen cosmonauts presented an increase in SIMT ranging from 6 to 100%, whereas one cosmonaut (C11) showed a decrease of 13%. Statistical analysis, taking each subject into account, revealed that the increase in SIMT was significant for the whole population. Individual and mean data are summarized in Fig. 5.
[Fig. 4.](http://jap.physiology.org/content/90/1/179/F4)
Fig. 4.
Typical stiffness-torque linear relationships from quick-release tests. BDC 1 (■) and BDC 2 (□) data (*A*) and BDC (■) and R+2 (○) data (*B*) from *cosmonaut C3*. Correlation coefficients were, in *A*,*r* = 0.978 and 0.982 for BDC 1 and BDC 2, respectively, and, in *B*, *r* = 0.996 and 0.957 for BDC and R+2, respectively.
[Fig. 5.](http://jap.physiology.org/content/90/1/179/F5)
Fig. 5.
Changes in musculotendinous stiffness index (SIMT). Histograms represent the individual SIMT values of each cosmonaut from BDC (solid bars) and R+2/R+3 (open bars) test and mean SIMT. *SIMTfrom pre- and postflight was significantly different (*P*< 0.05).
Furthermore, changes in SIMT due to spaceflight were characterized for the population by pooling the individual data. The SIMT changed from 3.34 ± 0.24 rad−1 to 4.15 ± 0.15 rad−1. That corresponded to an increase of 24%. The same result was obtained when running the*t*-test to compare the means in SIMT from preflight and postflight data. Mean SIMT was found to be significantly different between preflight and postflight (3.38 ± 0.46 rad−1 vs. 4.21 ± 0.38 rad−1), which corresponds to an increase of 25%.
### Elastic Properties of the Musculoarticular System
Figure 6 illustrates, for a given subject, a classical gain diagram and displays a number of characteristic features. There is an increase at low frequency, a resonant volley at intermediate frequencies, and a decrease in gain with a slope of −40 dB/decade at high frequency. These features correspond to the response of a second-order system, and joint dynamics have been frequently modeled by using a parametric model of the form indicated in *Eq. 3 *. Figure 6 also shows the typical behavior for two levels of activation. One can clearly distinguish differences in gain increase and a shift of the resonant frequency to higher values by increasing the level of activation. Fitting by a second-order model was always satisfactory. For pre- and postflight data, the correlation coefficient (*r*) was between 0.725 and 0.970 (*n* = 11). So, musculoarticular stiffness was analyzed in terms of Kp, SIMA, and IP.
[Fig. 6.](http://jap.physiology.org/content/90/1/179/F6)
Fig. 6.
Typical example of a gain diagram of the musculoarticular system. Data from *cosmonaut C12* obtained in preflight experiment at 25% MVC (dashed line) and 50% MVC (solid line) are adjusted by a second-order model (*r* = 0.936 in both cases). The gain is expressed in terms of compliance.
First, passive musculoarticular stiffness (at 0% MVC) was investigated for *cosmonauts C4* to *C14*. Eight cosmonauts decreased their Kp (from 3 to 37%), whereas three cosmonauts exhibited increases (from 5 to 22%). For the population, postflight data were significantly lower than preflight data. Mean Kp changed from 39.12 ± 2.68 to 34.78 ± 2.82 N · m · rad−1, which corresponded to a significant difference of 11%. Figure7 summarizes these data.
[Fig. 7.](http://jap.physiology.org/content/90/1/179/F7)
Fig. 7.
Changes in passive musculoarticular stiffness (Kp). Histograms represent the individual Kpvalues of each cosmonaut from BDC (solid bars) and R+2/R+3 (open bars) test and mean Kp. *Kp from pre- and postflight was significantly different (*P* < 0.05).
Second, for *cosmonauts C9* to *C14*, musculoarticular stiffness, calculated at 0, 25, and 50% MVC, was related to torque. The stiffness-torque relationships were then used to calculate SIMA and IP (see *Eq. 4 *). For preflight results, BDC 1 and BDC 2 data were grouped for each subject. Correlation coefficients (*r*) were found to be significant (0.930 < *r* < 0.999, *n* = 5). For postflight data, correlation coefficients were found to be significant for *cosmonauts C10* to *C14* (0.998 < *r*< 0.999, *n* = 3), whereas *cosmonaut C9* did not show a significant correlation (Table1). The Wilcoxon signed-rank test revealed that SIMA did not change as a result of spaceflight. Furthermore, SIMA mean values were not significantly different (4.74 ± 0.61 and 4.67 ± 0.49 rad−1 for pre- and postflight, respectively). Concerning IP, these intercept values were generally close to the passive stiffness values (Kp) previously calculated. A comparison between Kp and IP is given in Table 1.
View this table:
[Table 1.](http://jap.physiology.org/content/90/1/179/T1)
Table 1.
Estimation of passive musculoarticular stiffness from sinusoidal perturbation tests
Third, SIMA after exposure to microgravity were characterized for the population (C1 to C14) by pooling all the individual stiffness data. The SIMA varied from 4.83 ± 0.16 (preflight) to 4.78 ± 0.14 rad−1 (postflight), i.e., remained unchanged (−1%) as shown in Fig. 8. The value of IP was found to change from 46.95 ± 6.66 to 37.33 ± 5.21 N · m · rad−1, which corresponded to a significant decrease of 21% (Student's *t-*test). As mentioned above, the IP values were once more close to mean passive stiffness Kp, which was 39.12 ± 3.97 and 34.78 ± 4.20 N · m · rad−1 for pre- and postflight, respectively.
[Fig. 8.](http://jap.physiology.org/content/90/1/179/F8)
Fig. 8.
Musculoarticular stiffness-torque relationship. BDC 1 and BDC 2 (■, solid line) data and R+2/R+3 (○, dashed line) data for the population. Slopes reflect SIMA, whereas intercepts represent passive stiffness. Correlation coefficient *r* was 0.978 (BDC) and 0.986 (R+2/R+3).
## DISCUSSION
Changes in muscular contractile and elastic properties as a result of microgravity are well documented in the literature, at least for animal studies. For instance, studies from our laboratory showed that speeding of the soleus muscle of the rat after hindlimb suspension was accompanied by a decrease in SEC stiffness (8). However, different patterns of muscle stiffness adaptation were found in animals and humans when the effects of training were studied. For instance, endurance training was found to increase stiffness in both human and rat muscles (19, 29). On the other hand, after a period of plyometric training, Almeida-Silveira et al. (3) reported a decrease in stiffness in the rat soleus muscle, whereas Pousson et al. (34) reported an increase in stiffness in human muscles. These studies show that alterations in mechanical properties of human muscles according to changes in functional demand cannot be systematically predicted by using animal data and fully justify the present experiment.
### Force Production in Isometric and Isokinetic Conditions
The measurement of maximal isometric force depends on many factors, such as motivation of the cosmonauts to give their maximal effort. MVC values obtained in preflight conditions correspond to the data reported in the literature (16, 25). As found by other teams who also worked on the EuroMir '94 and '95 missions (5, 45), our postflight data indicated for each subject a significant decrease in MVC. Zange et al. (45), by means of a NMR technique, reported a reduction in muscle mass of the lower limbs of ∼11% and attributed these morphological data to a decrease in muscle cross-sectional area.
The larger fall in MVC found in the present study (mean decrease 17%) as well as in maximal power demonstrated by Antonutto et al. (5) (mean decrease 45% at R+2/R+3 after 180 days spaceflight) could also be due to the effects of weightlessness on*1*) the intrinsic characteristics of the recruited motor units and *2*) changes in the motor unit recruitment pattern. Additionally, a decline in strength was also proved by Koryak (28) after a period of bed rest (dry water immersion) and was attributed to a decrease in neural input. The observed decrease in normalized RMS values after spaceflight support this proposition that microgravity interferes substantially with the normal neural drive.
In isokinetic conditions, the kinetics of the torque-velocity relationship (see Fig. 2 *A*) are in agreement with the literature (7, 41). However, adjustment of data often lead to differences in estimation of maximum shortening velocity depending on the mathematical model. Furthermore, experimental data are missing in the low-torque range. Therefore, to avoid the influence of the model when quantifying changes in maximal shortening velocity, an index of maximal shortening velocity (VImax) was used by calculating velocity at low torque (T = 10% of MVC).
It is well documented that variations in torque-velocity relationships can be interpreted in terms of differences in muscle fiber-type distribution and force-velocity characteristics of fast-twitch (FT) and slow-twitch (ST) fibers. The influence of weightlessness on the dynamics of contraction was demonstrated by using different human and animal models, and in all cases an increase in maximal shortening velocity was obtained (for a review, see Refs. 14 and 38). From this point of view, the observed increase in VImaxleads to favor the hypothesis of a fiber-type transition from ST to FT fibers. However, increases in shortening velocity as a result of microgravity can also result from geometric alterations inside muscle structures, i.e., increase in filament spacing (42, 43) or a decrease in the pennation angle of muscle fibers (10).
### Elastic Properties of the Muscle-Tendon Complex
SIMT values found in this study for the human ankle plantar flexors in BDC 1 and BDC 2 conditions are in agreement with those found when a controlled-release technique (23) or a single-stretch technique (6) is used. Furthermore, taking SIMT for characterizing possible changes in SEC stiffness has several advantages: *1*) to obtain a normalized stiffness index, independent of the demanded torque level, and *2*) to avoid the use of MVC or cross-sectional area measurements for normalizing SEC stiffness data.
SEC stiffness is classically separated into two fractions: an active fraction (muscle fibers) and a passive fraction (tendon). Some results suggest that slow and fast fibers may well have different elastic characteristics. For instance, it was demonstrated that, when a training technique increases the percentage of fast fibers in the soleus muscle of rats, its SEC stiffness decreases (3). The opposite mechanical change (i.e., an increase in SEC stiffness) was also associated with a relative increase in ST fibers (19). Such results are in accordance with those obtained at other levels of observation, i.e., motor units (33) and skinned fibers (31, 40, 44). Therefore, initially, our working hypothesis based on animal data (8) considered a possible muscle fiber-type transition from ST to FT fibers, which would lead to a decrease in SEC stiffness. On the other hand, changes in SEC stiffness are not only related to the active part, but also the passive part of the SEC must be considered. A decrease in tendon stiffness of the soleus after hindlimb suspension was demonstrated by Almeida-Silveira et al. (2). Unfortunately, these animal data are in total discrepancy with the quasi-systematic increase in SIMT reported in the present experiment. Moreover, in humans, fiber-type transitions due to microgravity seem to be less marked than in animal experiments, at least for flights of short duration (22, 42). Associating individual results from isokinetic and quick-release tests did not reveal a clear relationship between changes in SIMT and VImax (Fig.9), leading to the idea that other factors must be responsible for the increase in SEC stiffness.
[Fig. 9.](http://jap.physiology.org/content/90/1/179/F9)
Fig. 9.
Paired changes in gain values between SIMTand VImax. Data set from *cosmonauts C1* to*C8*, who performed isokinetic as well as quick-release tests. Correlation coefficient *r* was not significant (*r* = 0.26; *P* > 0.05).
One element of explanation could be a change in posture, leading the ankle joint of the cosmonauts to adopt another position during spaceflight. Thus testing the plantar flexor muscle group in preflight neutral position after landing should induce a change in muscle length, which should lead to an increase in stiffness provided that the muscle group is relatively stretched in neutral position. In fact, according to Clément et al. (9), it seems that cosmonauts adopt a dorsiflexion position during spaceflight. Then, in postflight, a shortening of the plantar flexors will occur when the preflight neutral position is reached.
On the other hand, the increase in SIMT could also result from changes in muscle activation and/or motor unit recruitment pattern after spaceflight. Such alterations have already been hypothesized by some authors. Koryak (28) suggested that simulated microgravity can induce a reduction in motor drive, impeding maximal activation of the muscle. The above-reported decrease in postflight maximal RMS values recorded during isokinetic trials support this hypothesis. In addition, spaceflight seems also to induce changes in motor unit recruitment pattern. The decrease in maximal explosive power after long-term spaceflight was considered by Antonutto et al. (5) to reflect a slower motor unit recruitment. On the other hand, disuse is known to induce a decrease in the maximal firing rate of motor units, such a decrease being greater for low-threshold motor units (12). Thus, if also present in plantar flexor muscles after spaceflight, this decrease should modify motor unit recruitment when a submaximal torque is maintained. In fact, this decrease in maximal firing rate should imply the recruitment of a larger number of motor units to develop the same target torque in preflight and postflight conditions. Consequently, despite the reported decrease in maximal activation, the muscle should be relatively overactivated when performing a submaximal task in postflight conditions. Because SEC stiffness measurements were performed by using preflight target torques as reference values, this change in motor unit recruitment should participate to the observed increase in stiffness after spaceflight. Therefore, analyses including a neurophysiological approach must be designed to detail the mechanisms of adaptation for the different components of the neuromuscular system. In any case, these data indicate that the absence of gravity induces changes in musculotendinous stiffness. It is obvious that increased stiffness has important functional consequences in terms of movement control (36). For instance, force control will be more difficult because, for a given variation in displacement, variations in force will become more prominent.
### Elastic Properties of the Musculoarticular System
As in other studies (e.g., Ref. 27), the mechanical behavior of the ankle joint was accurately described by a second-order model including elastic, viscous, and inertial parameters.
The musculoarticular stiffness values from preflight tests found in this study in passive conditions (Kp) as well as SIMA and IP are in good agreement with values reported by other investigators (27, 37, 39).
Changes in musculoarticular stiffness after long-term spaceflight indicated a significant reduction in Kp. Kpdesignates a passive resistance of the ankle joint to the imposed movement when muscle is assumed to be in a resting state. Thus this passive elastic stiffness not only reflects the combined effects of passive elastic structures including skin, muscle, tendon, ligament, and the articular surface, but also the giant protein titin can be a source of adaptation (24). Clinical studies have shown an increase in human knee joint stiffness (21) as a result of immobilization. The effects of simulated microgravity (hindlimb suspension) in soleus muscle of rats were described as an increase in passive tension, which was attributed to musculotendinous units but also in part to the joint (17). All these studies indicate opposite results to those found in the present tests. However, it can be hypothesized that spaceflight-induced changes in the mechanical properties of passive musculoarticular structures are similar to those found in other collagenic structures (e.g., tendons) after simulated microgravity (2). This might be attractive to explain the observed decrease in passive musculoarticular stiffness. Other hypotheses concerning changes in human musculoarticular structures after spaceflight can be made, but they will be speculative because no detailed data of morphological and biochemical properties are available at the moment.
Concerning the normalized musculoarticular stiffness in active condition (SIMA), this value is assumed to reflect the combined effects of musculotendinous stiffness and passive ankle joint stiffness. As seen above, as a result of spaceflight, musculotendinous stiffness increases, whereas passive ankle joint stiffness decreases. It seems that Kp “sees” these changes and adapts to minimize changes in whole musculoarticular stiffness. Such an interaction between two types of stiffness was proposed by Farley and Morgenroth (15) to explain leg stiffness adjustment during hopping, taking into account ankle joint stiffness. Thus, by using the pooled data for SIMT and the intercept (IP), it was found that SIMT and IP altered about the same amount but in the opposite directions. Consequently, SIMA should not change, which was confirmed for the pooled data.
### Possible Effects of Countermeasures
It is evident that cosmonauts have to practice physical exercises on board to maintain muscle strength. For the present experiment, personal communications indicate that all crew members used a cycle ergometer, a treadmill, and devices for muscular reinforcement, but no detailed information about their use was obtained. Despite this lack of information, it is interesting that muscle function was modified to the magnitude reported in the present study, given the countermeasure programs and the in-flight activities. On the other hand, the use of eccentric exercises aboard the Mir station could contribute to the increase in musculotendinous stiffness because this kind of adaptation of the SEC was demonstrated in humans under terrestrial conditions (35). The observed increase in SEC stiffness should then be considered as an adaptive response to both microgravity and exercise. However, this kind of exercise would lead to an intensified neural drive (11). In fact, as mentioned above, the opposite adaptation is generally hypothesized. Finally, exercises aboard could also interact with the above-discussed decrease in passive musculoarticular stiffness because passive motion during a period of immobilization is known to limit the increase in joint stiffness (32). Nevertheless, these remarks underline the necessity to know the exact extent and types of countermeasures to make a clear distinction between the relative role of microgravity and exercise in the observed adaptations during human spaceflight.
To our knowledge, this is the first detailed study describing changes in contractile and elastic properties in human ankle plantar flexors after long-term spaceflight. We report a decrease in maximal force production with an increase in shortening velocity. These results are in agreement with observations in animal as well as in human studies. The most unexpected findings were, however, the increased musculotendinous stiffness and the reduced passive musculoarticular stiffness, which are in general contradiction to other microgravity models. We therefore think that the major factor to explain the increase in musculotendinous stiffness would be an increase in submaximal muscle activation brought about by the absence of gravity. The decrease in passive musculoarticular stiffness proves that disuse atrophy caused by spaceflight is different in nature from that caused by immobilization. Concerning active musculoarticular stiffness, no comparable data of simulated or real microgravity are available. Finally, the efficiency of countermeasures was found to be limited. Physical activity in a space station should therefore include an appropriate combination of exercise modes to counteract neuromuscular perturbations. Further studies, for instance of reflex activities, are necessary to elucidate the mechanisms of adaptation as well as to verify the above-mentioned hypotheses.
## Acknowledgments
We express gratitude to the crews, their backups, the medical staff of Y. A. Gagarin Cosmonauts Training Center at Star City, Moscow, and the staff of the European Space Agency. The technical assistance of Alain Mainar and Clotilde Vanhoutte for performing the experiments was highly appreciated.
## Footnotes
* This study was supported by the Centre National d'Etudes Spatiales (CNES).
* Address for reprint requests and other correspondence: F. Goubel, Université de Technologie de Compiègne, Département Génie Biologique CNRS UMR-6600, F-60205 Compiègne cedex (E-mail: francis.goubel{at}utc.fr).
* 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.
* Copyright © 2001 the American Physiological Society
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