The subjects in this study were seven male cosmonauts who participated in Space Station «MIR» missions («MIR» missions EP 1822 ^{24 26}27). Prior to the flight, their average age, height, body mass were 45.1 ± 2.0 years (range 43-54), 176.0 ± 2.3 cm (range 167-182), 79.9 ± 2.0 kg (range 70-86), respectively (Table 1). The duration of the space flight was by mean 213.0 ± 30.5 days (range 115-380). Each subject performed two sets of experiments 30 days before space flight (baseline data collections) and immediately after space flight (return) of 3 days.

The cosmonauts reported to the laboratory twice. During the first visit, the cosmonauts were familiarized with the experimental set-up and procedures. The cosmonauts gave their written informed consent to participate in this study. On a subsequent day, the cosmonauts were involved in testing contraction properties of the ankle extensor muscles. Each subject served as his own control.

The mechanical responses of the TS muscle were recorded by method tendometry (59) (Figure 1, top panel, left) which made it possible to measure the force of a single muscle contraction by the degree of tension change in muscle distal tendon (44, 46-48). Measurement of muscle tension using a strain-gauge transducer is based on the physical law of the resolution of forces according to the parallelogram principle (Figure 1, top panel, insert). If a strain-gauge transducer is pressed to the tendon, the transducer causes it to bend at an angle. The force (F₁) that is directed along the muscle axis to the proximal point of attaching and originates during the muscle contraction is oppositely directed and equal to the force (F₂) that is directed to the distal point of the tendon attachment. F₁ which is directed across the tendon, operates at the point of the transducer and tendon contraction. If the angle at which the tendon bends is constant, the force (F) recorded by the strain-gauge dynamometer is proportional to F₁ (or F₂). A rigid dynamometer is needed for recording the muscle force using a strain-gauge transducer, because any deformation under tendon pressure will change the transducer position and alter the tendon angle. A steel dynamometer ring was used in our transducer.

The subject was seated comfortably on a special chair in a standard position (knee joint angle between tibia and sole of foot of ~ 90 deg). The position of the seat was adjusted to the individual and then firmly secured. A rigid leg fixation provided the isometric regimen of the muscle contraction. The dynamometer that is a steel ring with a saddle-shaped special block was tightly attached to support the Achilles tendon of the muscle. The degree of pressure between the tendometrical sensor and the tendon was constant for all the subjects and amounted to 5 kg.

After careful preparation of the skin including shaving excess hair, abrading the skin with fine sandpaper, and cleaning the skin with an isopropyl alcohol swab to reduce impedance below 5 kΩ, pairs of surface Ag-AgCl electrodes (∅ 8 mm, center to center distance 25 mm and a recording area of 50 mm2), were placed over the muscle bellies of medial (MG) and lateral (LG) gastrocnemius, and soleus (Sol) muscles. The electrodes were placed longitudinally with respect to the underlying muscle fiber arrangement and located according to the recommendations by SENIAM (surface EMG for noninvasive assessment of muscles). For the soleus muscle, electrodes were attached to the skin over the middle part of the soleus muscle, belly and placed ~ 6 cm distally of the stimulation electrodes The large grounding electrode (7.5 cm x 6.5 cm) was located on the proximal portion of the leg between the upper recording electrode and the stimulating electrodes. Recordings of the electrical responses (EMG) of the skeletal muscle during slow MVC were made during for a short period (~ 0.2 s). EMG signals were amplified (x 1,000) and digitized (bandwidth of 0–2,000 Hz) at a sampling rate of 5 kΩ. During contraction the EMG signal was recorded on FM tape and simultaneously displayed on an oscilloscope to assess its quality. EMG signals were bandpass filtered between 20 and 500 Hz, and smoothed using a moving root-mean- square filter with a time constant of 50 ms.

A single, square-wave, supramaximal transcutaneous electrical stimulus of 1 ms duration was delivered to the n. tibial. Transcutaneous electrical stimuli were delivered to the tibial nerve using a high-voltage (maximal voltage = 400 V), constant current stimulator (model «ESU-1», USSR). The cathode (active electrode) was a metal probe (∅ 1 cm) with the tip covered in a saline-soaked sponge, which was pressed over the tibial nerve in the popliteal fossa which is the place of the lowest resistance. The anode (passive electrode) was a 6 x 4 cm rectangular self-adhesive electrode that was positioned between the tibial tuberosity and the patella. The large grounding electrode (7.5 cm x 6.5 cm) was located in the proximal portion of the leg between the peak-up and stimulating electrodes. Single stimuli were administered to the tibial nerve at a low current (amperage = 20 mA) to determine the optimal probe location based on the visual inspection of the compound muscle action potential (M-wave) and H-reflex of the soleus muscle that were monitored on an cathode-ray storage tube oscilloscope. A single stimulus was given every 30 s. Once the location was determined and marked, the maximal M-wave was achieved with incremental (5 mA) amperage increases until a plateau in the peak-to-peak M-wave was observed after three successive amperage increases. To assure a supramaximal stimulus, 120 % of the stimulus that elicited the maximal M-wave was used during the evoked twitch procedures.

The isometric tetanic contractions of the TS muscle were induced by electrical stimulation of the n. tibial using supramaximal rectangular pulses with a frequency of 150 impulses⋅s⁻¹ (52). All the recordings were made in a room at constant temperature (~ 22 ± 1 C).

The contractile properties of the TS muscle were tested twice: pre- (~ 30-days) and post-space flight (~ 3-4-days) and the test protocol were identical for both pre mission and post mission tests.

The whole protocol was executed by one investigator. For all cosmonauts, the right leg was studied. The contractile properties of the TS muscle were estimated according to the mechanical parameters of a voluntary and electrically evoked contraction (maximal isometric twitch and tetanic contractions, double stimulation).

The maximal isometric peak twitch force (P_t) was measured from the tendogram of the ts muscle isometric twitch response to a single electrical stimulus applied to the tibial nerve. The time from the moment of stimulation to peak twitch (TPT), the time from contraction peak to half-relaxation (1/2 RT) and total contraction time (TCT) - the time from moment of stimulation to the total muscle relaxation - were calculated from the tendogram of the isometric twitch (Figure 1, top panel, right).

The rate of development of increased muscle tension was calculated from the tendogram by the isometric voluntary contraction after the instruction «to exert the fastest and greatest tension» using a relative scale, i.e. the time of reaching 25, 50, 75, 90 % of maximum tension (36, 37, 43, 44). Similarly, the rate of rise of the evoked contraction, in response to electrical stimulation of the n. tibial with a frequency of 150 impulses⋅s⁻¹ (43, 44) (Figure 1, top panel, right). The maximum rates of voluntary contraction (dP_vc/dt), and evoked contraction (dP_ec/dt) development were obtained by differentiation of the analog signal.

Electromechanical delay (EMD) comprises an important portion of the rate of tension development phase of the contraction (28), where changes in motor unit activity patterns have been demonstrated to play a critical role in the maximal rate of force development (84).

The experiments were divided into two protocols.

To preserve the muscle contractile properties in long-term space missions, cosmonauts used physical training (PT). Details of the training program and performance tests have been provided elsewhere 2476. PT which were mostly of a locomotor type and included a warm-up (walking on a treadmill for 5 min), and low (2 min), moderate (2 min) and maximum (1 min) intensity running. PT were performed for 1.5–2 hours. Four-day microcycles, each including 3 days with PT and 1 day of rest, were followed in the PT program 2476.

A treadmill and a cycle ergometer were the main PT tools, training the cardiovascular and respiratory systems 33. The treadmill device could be used in a passive (subject driven) or active (motorized) mode of operation, which was selected by the crewmember during each exercise session. Crewmembers used a subject-loading device to fix themselves to the treadmill, which provided varying levels of loading relative to body weight (typical load was ~ 70 % of body weight) during use. In this way, the crewmembers could complete running or walking exercise while partially loaded. The cycle ergometer provided a load of 50–225 W at a pedaling rate of 40–80 rpm. The crewmembers also had access to bungee cords, which they could use to provide resistance-type exercise for various muscle groups.

Mean data of the changes in the TS muscle tension properties are shown in Figure 2 (top panel). Isometric P_t increased by mean 14.8 % (pre 106.0 ± 7.8 N compared to post 121.6 ± 17.6 N) after space flight (p > 0.05). MVC decreased by a mean of 41.7 % (pre 520.0 ± 66.7 N compared to post 303.1 ± 50.0 N; p < 0.05), and isometric muscle tetanic (P_o) decreased by a mean of 25.6 % (pre 738.6 ± 65.7 N compared to post-space flights 549.4 ± 37.3 N; p < 0.05), after space flight, respectively. The P_d (Figure 2, top panel) increased significantly by a mean of 49.7 % (pre 32.2 ± 4.6 % compared to post-space flight 46.7 ± 5.4 %; p < 0.01).

A summary of individual values is given in Figure 2 (middle panel). Post-flight maximal isometric MVC in plantar flexion, collected near landing (immediately after space flight (return - R); R+3 days), indicated a decrease in MVC. More precisely, decreases in MVC were found for each cosmonaut from baseline data collections ranging from 22 % to 65 %, respectively). For the population MVC was found to be significantly higher in pre-flight condition than in post-flight condition (p < 0.05).

Post-flight maximal isometric muscle tetanic (P_o) in plantar flexion indicated a decrease in P_o (Figure 2, lower panel). More precisely, decreases in P_o were found for each cosmonaut from baseline data collections (baseline data collections) ranging from 8 to 41 %, whereas 2 subjects (1, and 7, respectively) had had concerning smaller decrease P_o (8, and 11 %, respectively). For the population P_o was found to be significantly higher in pre-flight condition than in post-flight condition.

The analysis of the data shows, that there is no connection between basic value of force of reduction of a muscle and degree of drop force properties muscle after space flight. For example, cosmonauts 3 and 4, the highest parameters, having up to space flight, MVC, after space flight differ on depth of drop of force of reduction. There are no such connections and for a parameter P_o (Figure 2).

Figure 3 shows a 55 % increase (p < 0.05) of iEMG and a 71 % increase (p < 0.05) of the iEMG/MVC ratio for a slow MVC after the space flight. The increase in both IEMG and iEMG/MVC are indicators of decreases in efficiency of the degree of electrical activation in the muscle after flight. The degree of activation required to generate a force is estimated by the EMG/force relation. The slope of the EMG/force relation was decrease significantly after space flight (p < 0.05).

The mean changes in force of the ts muscle contraction during doublet stimulation, in which a second pulse was applied at various intervals, are presented graphically in Figure 3. The greatest force of contraction under these conditions was at an interval of between 4 and 10 ms and decrease or increase in the interval were accompanied by a considerable decline. The relative increase in force as a result of doublet stimulation was significantly lower after long-term space flight in comparison with the control (baseline data collections) value (p < 0.01-0.001).

The data of the change of mean time of isometric twitch contraction as the opposite value to contraction velocity for the TS muscle after long-term space flight, is given in Figure 5. As is seen from the data analysis, exposure to microgravity conditions was accompanied by a statistically significant decrease of the muscle contraction and increased relaxation (1/2 RT) velocity. Thus, TPT increase by a mean of 7.7 % (pre 130 ± 2 ms compared to post 140 ± 6 ms; p < 0.05) after space flight. 1/2 RT was shorter after space flight than in the control value. Thus, 1/2 RT reduced by a mean of 20.6 % (pre 97 ± 4 ms compared to post 77 ± 4 ms; p < 0.05), but TCT increase by a mean of 7.5 % (pre 456 ± 25 ms compared to post 490 ± 31 ms; p < 0.01).

The same subjects showing increase in TPT also showed significant increase in the twitch-to-tetanic ratio by a mean of 46.7 % (pre 0.15 ± 0.01 compared to post 0.22 ± 0.02; p < 0.05) (Table 2).

Individual data of each cosmonaut from baseline data collections and collected near landing (R +3) are summarized in Figure 5 (middle and low panel). Post-flight TPT, collected near landing, indicated a increased in TPT. More precisely, increases in TPT were found for 5 subjects ranging from 8 to 24 %, whereas 2 subjects (3, and 4, respectively) had slightly decreased TPT (6, and 12 %, respectively).

On the other hand, the effect of exposure to microgravity on relaxation (1/2 RT) velocity was tested by comparing 1/2 RT values in pre-flight and post-flight conditions. Seven cosmonauts presented an increase in 1/2 RT ranging made an interval 2 % to 37 %, whereas the one subjects (cosmonaut 5) had slightly decreased 1/2 RT (2 %). Statistical analysis, taking each subject into account, revealed that the decrease in 1/2 RT was significant for the whole population. Individual data are summarized in Figure 5.

Mean changes in the rate of development of isometric tension of the TS muscle are given in Figure 6. The decrease in the MVC (42 %) was associated with a significant slowing of the rate of tension development during a voluntary isometric contraction (Figure 6, left curve; p < 0.01-0.001); maximal dP_vc/dt significant decrease by a mean of 36.2 % when normalized as the percentage MVC (Figure 6, insert; p < 0.001).

Analysis of the force-time curve of the electrically evoked contractions did not reveal significant differences (Figure 6, right curve; p > 0.05) while the maximal dP_ec/dt was significant increased by a mean of 43.7 % after space flights (Figure 6, insert; p < 0.001). These mechanical dissociations suggested that some state beyond the membrane processes was changed.

The mean changes in EMD under space flight are shown in Table 2. The EMD increased significantly, by 34.1 ± 1.3 % after space flight (pre 31.4 ± 2.8 ms compared to post 42.1 ± 3.4 ms; р < 0.05).

The PMT decreased significantly by a mean of 18.8 % (pre 256.4 ± 24.1 ms compared to post 207.6 ± 19.6 ms; p < 0.01), and TRT decreased by a mean of 14.1 % (pre 289.1 ± 25.0 ms compared to post 248.4 ± 20.5 ms; p < 0.01) after space flight.

The coefficients of correlation between the EMD and MVC before space flight were −0.67 (p < 0.05) and strongest correlations were found between EMD and ballistic voluntary force (r = 0.90; p < 0.01). The coefficients of correlation decreased between the EMD and MVC after space flight (r = −0.57; p = 0.05), and between the EMD and ballistic voluntary force (r = −0.70; p = 0.07).

EMD during evoked contraction did not differ significantly before and after space flight (Table 2).

Comparison of EMD during evoked and voluntary contractions revealed a longer EMD during voluntary both pre- and post-space flight. For instance, pre-space flight value EMD during voluntary contractions were by a mean of 31.4 ± 3.1 ms in contrast to 10.3 ± 0.4 ms during evoked contractions (p < 0.05); post-flight values were by a mean of 42.1 ± 3.1 ms and 12.43 ± 0.3 ms, respectively (Table 2).

In previous work, we examined the responses of the human TS muscle to 120 days of complete bed-rest 44. Figure 7 compares the results from this previous work to the present study. Inter-subject variability appears to be similar for each model. These data indicated no differences in the relative changes between the two models for MVC, and P_t, and P_o, and force deficit, TPT and 1/2 RT. On the other hand, our data demonstrate that the countermeasures employed during space flights have not been completely successful in preventing reductions in alterations in neuromuscular performance. A better understanding of how humans respond to space flight is a requirement for the development of more effective countermeasures.

We anticipated that the changes in isometric P_t would correspond to those in MVC, namely a decrease with microgravity. In contrast, we observed a nonsignificant increase in P_t after long-term space flights. These unexpected findings are not unique. Edgerton et al. 20 found a nonsignificant increase in P_t after 6 mo hindlimb immobilized skeletal muscles of a nonhuman primate (Galago senegalensis), and Koryak 4546 after 7 days ‘dry’ water immersion in the human TS muscle. These changes are quite similar to these observed in our study. These changes in twitch tension may reflect alterations in stiffness (extensibility) in human muscles. A decrease in muscle extensibility (stiffness), which has been observed after disuse/space flight 3062 would be the changes in isometric P_t. In contrast, P_o decreased after space flight. Thus there is a tendency for the twitch-to-tetanus ratio to increase after space flight. These changes in the twitch-to-tetanus ratio may reflect alterations in muscle extensibility. We speculate that in our experiment, space flight is the cause of a decrease in muscle extensibility.

After prolonged space flights the electrically evoked contractions (P_o) decreased significantly at a rate of 26 % of normal. The P_o is a direct measure of the force-generating capacity of a muscle and has been considered to reflect the number of active interactions between actin and myosin 17. Disuse produced a decline in P_o214546. This decline could reflect a decrease in the number of active cross-bridges and be expected to decrease the work capacity. Two hypothesis may be suggested to account for the observation. First, the total number of cross-bridges could have been smaller after the period of space flight. Second, the force output per cross-bridge could have been decreased. However, according to Steven et al. 77 have shown that when it was expressed per cross-sectional area, the force was unchanged after disuse. This would indicate that the first hypothesis of a decrease in the maximal number of cross-bridges was more appropriate to our results, rather than a change in density.

As a whole the mechanisms responsible for the force loss with microgravity are not well understood. However, changes in the force might have been caused by changes in different architectural properties, such as muscle length, and fascicle length, and pennation angles 2785. Consequently, the relationships among joint angles, muscle lengths, and pennation angles are highly specific to muscle. Muscle architecture, together with intrinsic properties such as also fiber composition, also affects functional characteristics of muscle, e.g., maximal shortening velocity and P_o70. In addition, considering that of the TS muscle our experimental environment was not extended and tensed, which makes possible the associated of such a condition with physiological shortening, it may be assumed that the total number of successively connected sarcomeres was considerably decreased 25. These may have contributed to the reduced in muscle layer thickness and fascicle angles. In fact, according to Clément et al. 16 it seems that cosmonauts adopt a dorsiflexion position during space flights. Then, in postflight, a shortening of the plantar flexors will occur when the preflight neutral position is reached.

The measurement of MVC depends on many factors, such as motivation of the cosmonauts to give their maximal effort. MVC values obtained in pre-flight conditions correspond to the data report in the literature 45789. As found by other teams who also worked on the Space Station «MIR» mission 589, our post-flight data indicated for each subject a significant decrease in MVC. For instance, Zange et al. 89, by means of a NMR technique, reported a reduction in muscle mass of the lower limbs of ~ 11 % sand attributed these morphological data to a decrease in muscle cross-sectional area. Our post-flight data indicated for each subject a significant decrease in MVC. The larger fall in MVC in the present study could also be due to the effects of weightlessness on i) the intrinsic characteristics of the recruited motor units and ii) changes in the motor unit recruitment pattern. Such alterations have already been hypothesized by some authors. Additionally, a decline in strength was also proved by Lambertz et al. 62 after a space flight on root mean square values, used for expressing the power of an EMG signal. These studies indicate decrease in normalized mean square values after space flights support this proposition that microgravity interferes substantially with the normal neural drive.

The much larger reduction in MVC (mean decrease 42 %) when compared with to changes in P_o after a prolonged space flights of 3 mo on Space Station «MIR» (mean decrease 24 %) may indicate an inability of the central nervous system to activate the TS muscle normally. Whether this was due to a lack of motivation on the part of the subjects, or to an involuntary reduction in neural drive, is difficult to distinguish. Additionally, a decline in maximal power was also proved by Antonutto et al. 5 after 180 days space flight and was attributed to a decrease in neural input. The subjects certainly appeared well motivated and had no discomfort or knee stiffness before performing the test which could have account for the low MVC. The increase in force deficit would suggests a decline in central drive in the control of voluntary muscle by the motor nervous system. In fact, during MVC, the EMG activity has been found to be significantly changed by the inactivity itself 214873. Moreover, observation of amplitude changes after inactivity has suggested that fewer motor unit were activated in disused muscle 73, and maximal firing frequency of motor unit has been found to be decreased 21. In fact, this decrease in maximal firing rate should imply the recruitment of a larger number of motor units to develop the same target strength in pre-flight and post-flight conditions.

On the other hand, a decrease in maximal firing rate could be explained by changes in proprioceptive afferents on the motoneurons 64. This suggests that in future studies in human cognizance must be taken of the initial physiological states of the muscle that are to be disused to access the extent to which neural and muscle function is affected by loss of voluntary movement. Additionally, the data of Antonutto et al. 45 suggest that microgravity causes a fundamental alteration in motor control. They observed two-legged muscle power to decline considerably more than could be explained by the loss in muscle mass. Moreover, the loss of explosive leg power was associated with a substantial reduction in the EMG activity of the muscle. The data of Recktenwald et al. 71 also suggest that microgravity induced a reorganization of motor recruitment motor units. These changes of motor control, may be one contributing factor of microgravity to the reduced extensor muscle torque and maximal power previously experienced by cosmonauts/astronauts after space flight 534 and be change in architecture as muscle fibers, and in a muscle-tendon complex, as previously is shown on astronauts after space flight 53 and after stay in head-down tilt bed-rest 4160.

During bed-rest, subjects underwent the exact same physiological testing performed by the cosmonauts during this flight. Mechanical responses the TS muscle were obtained and analysed using identical procedures. By directly comparing the results from these two studies we have attempted to assess the validity of the ground-based, head-down tilt bed rest procedure as a model of human space flight. The reductions in both MVC and P_o did not vary by more than a few per cent between space flight and bed rest model. Thus, the results of this comparison indicate that in general, mechanisms of force production in TS muscle respond in a similar manner to either bed rest or space flight. We should point out that one difference between the bed rest and space flight studies was that the cosmonauts were required by «MIR» flight surgeons to perform aerobic countermeasure exercise during the flight. It is impossible to evaluate how this may have affected the comparison between bed rest and space flight. We would concur with that conclusion based on the similarities between the bed rest where subjects did not perform exercise countermeasures and space flight results. We studied contractile properties the TS muscle obtained from adult subjects before and after 120 days head down tilt bed rest 444749. During bed rest subjects did perform exercise countermeasures. Training had caused a decrease of 3 % in MVC, and P_t, and in P_o of 14 %, and of 9 %, respectively. The force deficit had decreased significantly by 10 %. These experimental findings indicated that neural as well as muscle adaptation occurred in response to head-down tilt with countermeasures.

The results showed that a slow contracting muscle group 4382 was affected by microgravity. These data extend previous finding concerning the effects of short-term voluntary ‘dry’ water immersion 45, and long-term bed-rest 4447 on the mechanical characteristics of the TS muscle. The functional components of the twitch duration are the TPT and the 1/2 RT. Significant increases in maximal isometric twitch TPT and decreased 1/2 RT were maintained during a 120-day bed-rest. In the present study, the difference between the post-flight and pre-flight TS muscles was ~ 8 %. The underlying mechanistic basis of TPT and 1/2 RT involves the competitive interaction of factors involved with activation (Ca²⁺ release kinetics), cross-bridge cycling, Ca²⁺ uptake by the sarcoplasmic reticulum (SR), and changes the active fraction (muscle fibers) or the passive fraction (tendons) of the series-elastic component.

A likely explanation of the change in TPT in the real microgravity limbs is a relatively greater atrophy of type I fibres, which has been found to make up the majority of the TS muscle 40. However, since microgravity produces muscle atrophy both in fast and slow skeletal muscles and, in addition, have been shown to cause fibre type-specific changes in the contractile properties 29, other factor(s) may be affected that alter fibre type composition. Additionally, it is tempting to suggest that the decreased 1/2 RT of the space flight TS muscles is probably due to an increased number of fast SR Ca²⁺-ATP-ase pumps, as observed in previous hindlimb-suspension studies 77.

One additional effect to consider is the fibre type distribution. Is shown, that after space flight percentage of fibers no significantly changed. Observed a small increase in the percentage of fast type IIa fibres post-space flight 86. In this connection it would be possible to expect augmentation of increase the rate of rise of evoked twitch contraction a muscle. However, as show our data time-to-peak of a muscle after flight is increased. With these data are compounded and earlier received data in simulated conditions 4648495051.

The rapid nature of the isometric changes, i.e., twitch duration, may be related to alterations in SR function 1126. The primary factor (mechanism) of the explanation for these the changes may be a reduction in the rate at which Ca²⁺ is dissociated from the myofibrillar proteins 9. Dissociation would occur more slowly if the rate of Ca²⁺ re-uptake by the SR was decreased. Such a decrease has been found following disuse 34. A reduced rate of Ca²⁺ dissociation from myofibrillar proteins might be expected due not only to increase the time course of the twitch response but also allow more force to be generated, since cross-bridges will continue to be formed while Ca²⁺ is available in the sarcoplasm. These effects on the SR would be difficult to observe as the effects on P_t would be masked by atrophy but are of interest on the assumption that the twitch changes are due to SR alterations. The changes in the kinetics the mechanical responses at paired stimulation (see Figure 2, C) might also be explained by altered development of Ca²⁺ kinetics in the muscle used in the experimental. At any given interpulse interval the relative increase in force of contraction after long-term space flight effect was significantly less compared to the pre-flight value.

The rate of rise of evoked contraction (the shape of the force-time curve) in response to electrical stimulation of the n. tibialis with a frequency of 150 impulses⋅s⁻¹ calculated according to the relative scale showed only minor changes due to microgravity, which agrees with the observations 87 of relatively constant mechanics of tetanic contraction theory of muscle contraction 74. Similar to the flight condition (a 120 days), we have previously shown that head-down tilt no significant changes in the force-velocity characteristics of human the TS muscle 4447 and this observation agrees with the data obtained earlier by Witzmann et al. 87, who have shown that there were no significant changes in the force-velocity characteristics of rat soleus, extensor digitorum longus or superior, and vastus lateralis muscles after 21 days of immobilization. If the shape of the force-time curve is determined by the net rate of formation and disruption of the cross-bridges 74 which are proportional to the activity of myo-ATP-ase 75, then it may be assumed that the cycling of cross-bridges and activity of myo-ATP-ase varied slightly (or not at all) during the effects of microgravity. Therefore, one can assume that inactivity of myo-ATP-ase is reason for changing the kinetics of contraction after weightlessness. This supports the hypothesis that the changes of isometric contractile properties of the microgravity skeletal muscles being in microgravity result not from a change of contractile proteins but from any other factor/s, possible, from change in the function on SR. The increase of the normalized rate of tension development (dp/dt), that was recorded during prolonged space travel was consistent with the finding that myo-ATP-ase activity and maximal velocity of shortening have been enhanced during space flight which agrees with the observations to disuse 80. It would therefore seems reasonable to conclude that disuse, for example weightlessness or head-down tilt, in crew prolonged space travel has little effect on either cross-bridge cycling or myosin activity 17.

In addition to, it is known, the time course of force development during electrically evoked tetani did change whereas the maximal rates tetanic force development (dP_ec/dt) was increase (see Figure 6). The cause of the space flights-induced increase in the maximal unloaded shortening velocity is unknown. 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 66 or a decrease in the pennation angle of muscle fibers 2485.

On the other hand, the microgravity-induced increase in the maximal unloaded shortening velocity of the TS muscle could be due to at least three possible mechanisms: i) an increase in the fast type myosin heavy chains (MHC) protein isoform content; ii) the de novo expression of a faster MHC isoform, and/or iii) an increase in the fast myosin light chains (MLC). Previous studies at both the whole muscle 14 and single-fiber 10 level have report good correlations between the MHC isoform composition and maximal unloaded shortening velocity. Hence, the 44 % increase in the maximal rates tetanic force development (dP_ec/dt) in the flight triceps surae muscles could be due to the corresponding increase in the relative fast type MHC protein isoform content. Also possible that some of the increase in maximal velocity may have been due to alterations in the essential MLC isoform composition of the flight muscles 86. Increased maximal shortening velocity might have occurred due to the selective loss of thin filaments. Such alterations have already been hypothesized by some authors. Riley et al. 72 shown that a 17 day space flight (STS-78) induced disproportionate loss of actin filaments in human soleus muscle. This structural change was responsible for post-flight increase in the maximal shortening velocity and for a reduced peak stiffness in the muscle 86. These alterations would be expected to have greater effect on shortening velocity than on force 86.

During many everyday activities, the ability to produce high muscle force per time unit is more important than the ability to generate high force. The rate of force development depends on many factors, in particular, on the duration of the excitation-contraction process, on the force-velocity properties of muscle fibers (even during isometric contraction due to tendon structure deformity) and SEC stiffness. Therefore, decreased MTC stiffness may be the reason for the decreased rate of development of the TS muscle contraction after unloading, since SEC stiffness in known to be an important factor determining the rate of muscle force development 9. In other words, as indicated in the Introduction, the time of SEC stretching by contractile elements determines the EMD value 38. Therefore, changes in MTC (SEC) stiffness after hyperactivity/hypoactivity mainly explain changes in EMD. In addition, all the structures of the SEC, classically composed of an active part (located in myofibrils) and a passive part (mainly aponeurosis and tendon) 88, could contribute differently to EMD. Nordez et al. 69 used the noninvasive methodology (a real-time ultrasonography) determine the relative contribution of the passive part of the SEC (47.5 ± 6.0 % of EMD) and each of the two main structures of this component (aponeurosis and tendon, representing 20.3 ± 10.7 % and 27.6 ± 11.4 % of EMD, respectively).

This study demonstrated that the rate of isometric voluntary contraction development performed after the instruction to exert the fastest contraction had decreased after unloading, which confirms the theoretical relationship between the decreased muscle MTC stiffness and decreased rate of transmission of the contractile force and therefore the rate of contraction development. This data was obtained for the first time and is indicative of the increase of MTC stiffness after PT under conditions of muscular system unloading. Our results in the present study indicate that PT during space flight results in an increase in MTS (MTC), whereas a lack of PT is associated with a decrease of this parameter 53. Therefore, PT decreases the EMD under muscular system unloading conditions.

The changes in elastic properties due to PT have been well documented. Endurance training resulted in an increase in the SEC stiffness in the soleus muscle of rats, associated with an increase in type I fibres 31. Both jump and endurance training also appear to increase both collagen concentration 2358 and muscle passive stiffness. The soleus rat muscles submitted to plyometric training had more fast twitch fibres and a lower SEC stiffness than controls 383. Malisoux et al. 63 reported that human subjects given 8 weeks of maximal effort stretch-shortening cycle exercise training tended to have an increase in the proportion of type II fibres in their vastus lateralis muscles. Kubo et al. 60 reported that the MTS is greater in long distance runners than in untrained individuals.

The results obtained correlate well with data on the decreased EMD after isometric training under bed-rest conditions 6061. The EMD changes during training are mainly attributed to changes in the tendon structure, but it should be noted that tendon stiffness is always increased during any physical activity, both during endurance 12 and isometric training 6061. Furthermore, EMD changes correlate very closely with muscle MTC and especially with the active SEC fraction during training 2263.

This study demonstrated that the EMD is sensitive enough and may serve as an indirect marker for measurements of muscle MTC stiffness in order to determine the chronic adaptation of the musculo-tendinous structure of the muscle to the mechanical unloading during PT under real or simulated microgravity conditions. The data obtained demonstrates that a 180-day of actual microgravity led to decrease the TS mechanical properties and although the adverse effects were reduced thanks to PT, they were not completely prevented by the training program. This permits us to suppose that the volume of exercises and especially the intensity of exercises performed did not exceed the threshold level required for the complete prevention of changes in mechanical properties.

The mechanical muscle responses obtained and the list of physical exercises received from the study subjects, as well as from members of long-term (6-month) space missions confirm the importance of PT in preserving muscle function and the capacity to work during long-term stays under microgravity conditions. Within the framework of this study, the PT program contained mainly low intensity exercises. The inclusion of exercises with higher load and higher intensity into the training process would contribute to a more effective exercise program for the training of skeletal muscles and it would reduce the total training time under zero gravity conditions. In general, PT under microgravity conditions allows subjects to create an increased functional reserve and reduces the effect of unloading observed under real microgravity conditions.

In conclusion, the results presented here show that mechanical properties in human the TS muscle were altered after prolonged space flight. The comparison of the mechanical alterations recorded during voluntary contractions and in contractions evoked by electrical stimulation of the motor nerve (for all seven cosmonauts studied MVC produced more decreased then P_o after the space flight), suggests that weightlessness not only modifies the peripheral processes associated with contractions, but also changes central and/or neural command of the contraction.