Journal of Biomedical Technology and Research

Influence of 7-Day Simulated Microgravity on the Neuromuscular Function, and Architecture of the Human Triceps Surae Muscle and the Effect of Low-Frequency Functional Electrostimulation Training

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Influence of 7-Day Simulated Microgravity on the Neuromuscular Function, and Architecture of the Human Triceps Surae Muscle and the Effect of Low-Frequency Functional Electrostimulation Training

Koryak Yuri

Department of Sensory-Motor Physiology and Countermeasures State Scientific Center — Institute of Biomedical Problems RAS, 123007 Moscow, RUSSIA

 

Corresponding author: Koryak Yuri, Department of Sensory-motor Physiology and Countermeasures, State Scientific Center - Institute of Biomedical Problems of the Russian Academy of Sciences, Khoroshevskoye Shosse 76-A, 123007 Moscow, Russia, E-mail: yurikoryak@mail.ru

Citation: Koryak Yuri (2015) Influence of 7-Day Simulated Microgravity on the Neuromuscular Function, and Architecture of the Human Triceps Surae Muscle and the Effect of Low-Frequency Functional Electrostimulation Training. J Biomed Tech Res 2(1): 103. http://dx.doi.org/10.19104/jbtr.2015.101

 

Abstract 

 

The effect of a 7-day “dry” water immersion (DI) with countermeasures (functional electrical stimulation – FES) on the function and architecture of the human triceps surae muscle was studied in six healthy young men subjects [mean age 22.8 (SEM 0.8) years; height 1.84 (SEM 0.1) m, and mass 1.84 (SEM 0.1) kg]. During DI, subjects performed FES training 4 muscle groups of both lower extremities [the quadriceps femoris muscles, the hamstrings, the tibialis anterior, the peroneal, and the triceps surae muscles] every day. Before and after DI with FES-training, a internal architecture [lengths and angles of fascicles, and muscle thickness] of human triceps surae were determined. Triceps surae muscle architecture was measured in vivo by use of B-mode ultrasonography. The ankle was positioned at 15 °dorsiflexion (-15 °) and 0, +15, and +30 ° plantarflexion, with the knee set at 90 °. At each position, longitudinal ultrasonic images of the medial (MG) and lateral (LG) gastrocnemius and soleus (SOL) muscles were obtained while the subject was relaxed (passive) and performed maximal voluntary isometric plantarflexion (active), from which fascicle lengths and angles with respect to the aponeuroses were determined. The maximal plantarflexion torque was increased by 11.3 % (p < 0.05) after the 7-day DI with FES training, corresponding by five subjects. In the passive condition, fascicle lengths changed from 36, 47, and 39 mm (ankle, -15 °) to 27, 31, and 28 mm (ankle, +30 °); pennation angle changed from 31, 20, and 23 ° to 49, 29, and 34 ° for MG, LG, and SOL, respectively. After DI in the passive condition, fascicle lengths decreased by 16, 37, and 24 %, pennation angle increased by 38, 35, and 34 % for MG, LG, and SOL, respectively. Decreases in muscle thicknesses in leg muscles were not prevented by the present exercise protocol, suggesting a need for specific exercise training for these muscles. Trained muscles showed significant changes in pennation angles and fibre length after DI with by FES training, suggesting that muscle architecture does change remarkably by muscle atrophy.

Keywords: Immersion; Triceps surae muscle; Muscle contraction; Ultrasonography; Muscle architecture 

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Introduction 

 

A number of studies have indicated that sudden exposure to microgravity environment causes a decrease in the tone of the skeletal muscles [1,2], reduction of muscle strength [3], perceptual and coordination disorders in the neuromuscular systems [4-6], shift of the spinal reflex mechanisms [7], and degradation of joint position sense [8,9]. It is accepted that the major factor responsible for all of these changes is the sudden elimination of the proprioceptive information from the muscle and tendon in response to absence of load-bearing. Accordingly, the anti-gravitational or the postural muscle should be the principal target for the action of unloading.

Following of space flight, there was an extensive loss of muscle mass in the extensors composed of predominantly slow-twitch fibers, moderate loss in the extensors composed of predominantly fast-twitch fibers, and the least loss in the flexors composed of predominantly fast-twitch fibres[10-12]. These differences are based on the distribution of muscle fiber types. Recent experiments [6,13,14] show that weightlessness simulated by water immersion changed the recruitment order of motor units during isometric contraction in the hip flexor muscles, and that there were considerable differences in the change of recruitment order of motor units among muscles, seemingly based on the different distributions of muscle fiber types.

Gravitational loading appears to be necessary for the maintenance of human lower limb skeletal muscle size and force [15-18]. Studies simulating microgravity have shown that exercise countermeasures can attenuate, but not completely prevent the loss of muscle mass and force [19-21]. The muscle groups most affected by exposure to microgravity appear to be the antigravity extensors of the knee and ankle [19,22]. Among these, the plantarflexors seem to be the most affected [19,22], likely due to their greater mechanical loading under normal gravitational conditions. Most notable after exposure to microgravity is a disproportionate loss of force as compared to that of muscle size [20,22,23], indicating that factors other than atrophy contribute to muscle weakness. There is a few of studies on the effects of disuse [24] or simulated microgravity [16] on muscle architecture.

The internal architecture of a muscle is an important determinant of its functional characteristics [25]. There is a few of studies on the effects of disuse [24] or simulated microgravity [16,17] on muscle architecture.

Muscle fibresare packed in bundles (fascicle) that extend from the proximal to distal tendons, comprising a whole muscle. In pennate muscles, fascicles are arranged obliquely with respect to the tendon, and this angulation (pennation angle) changes by contraction. The forces exerted by muscle fibresare therefore modified at the fascicle level to characterize the force-generating capabilities of a muscle.

The triceps surae muscles are the main synergists for plantarflexion [26], but they have different architectural properties, such as muscle length, fascicle length, and pennation angles [27]. In addition, the gastrocnemius muscles are two-joint muscles crossing both the knee and ankle joints, whereas the soleus is a single joint plantar flexor. Consequently, the relationships among joint angles (knee and ankle), muscle (fascicle) lengths, and pennation angles are highly specific to individual muscles. Information on muscle architecture related to joint positions is essential for the study of muscle functions.

The purpose of the present study was to investigate the internal architecture of the triceps surae [medial (MG) and lateral (LG) and soleus (SOL) muscles] in relation to the functional characteristics of the plantar flexors after 7 days of dry water immersion with exercise countermeasures (long-term low-frequency functional electrical stimulation training).

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Subjects and Methods 

 

Subjects and “Dry” Water Immersion Protocol

The subjects were six healthy men-volunteers aged 20 to 24 years (22.8 ± 0.8). Their average height and mass were 1.84 ± 0.1 m, and 79.3 ± 4.2 kg (means ± SE), respectively. They were paid volunteers. Selection of subjects was based on a screening evaluation that consisted of a detailed medical history, physical examination, complete blood count, urinalysis, resting and cycle ergometer electrocardiogram, and a panel of blood chemistry analysis, which included fasting blood glucose, blood urea nitrogen, creatinine, lactic dehydrogenase, bilirubin, uric acid, and cholesterol. All of the subjects were evaluated clinically and considered to be in good physical condition. No subject was taking medication at the time of the study, and all subjects were nonsmokers. None of them had a habit of exercise on a regular basis.

Prior to the experiment, details and possible risks of the protocols were explained to the subjects, and written informed consent was obtained from each of them. The subjects were familiarized with the protocol and procedures studies of muscle function by using a isokinetic dynamometer ‘Biodex Multi-Joint System 4’ (model USA) 8-10 days before the experiment. Each subject served as his own control.

The experimental protocol was approved by the Russian National Committee on Bioethics of the Russian Academy of Sciences and was in compliance with the principles set forth in the Declaration of Helsinki (1976).

“Dry” water immersion (DI) was used to simulate microgravity effect [28]. This model seems to be a useful method for ground based investigations. As it has been shown in prevision studies [1] a close similarity exists between the effects of short time real microgravity and immersion. However, the dimension of these changes is different. The alterations during and after immersion are more marked than those of equal duration spaceflights.

Briefly, the subject was horizontally positioned (an angle which make the body and horizontal line, e.g. 5 deg head-up position) in a special bath in a “suspended” state (the law of Archimedes), on special water-proof highly elastic fabric film, which acted only as an insulation between the skin and the water. There was no bed to support the subject’s body. The folds of the water-proof material came together with water along the mid-line of the subject’s body. Water temperature was constant (33.4° C) and maintained automatically at this level throughout the experiment (Figure 1). The exposure duration was 7-d and the subjects were kept under medical observation throughout the exposure. During the 7-d of DI, the subjects remained in the experimental (horizontal) position continuously for all activities including voiding, and eating.

Figure 1: A. The position subject of the test “dry” water immersion: free time (left panel) and during lunch (right panel). B. A scheme of dipping of the man in “dry” water immersion environment [on: Shulzenko & Vil-Villiams, 1976]

 

The functional properties of the neuromuscular system were evaluated twice: 8-10 days before the beginning DI and after it ended. Each subject served as his own control. The test protocol was identical for both pre- and post-DI tests.

A nursing staff was present for subjects' transportation, maintenance of hygiene including toilet and shower, provision of food and medical care, as well as support of subjects' needs within the constraints of the protocol. The subjects were supervised 24 h × d-1.

Force Measurements

The subjects sat on a couch with the back supported and the lower limbs fully extended. All measurements were carried out on the right leg, dominant in all the subjects, with the foot positioned at 90 ° relative to a footplate. Subjects were familiarized with the exercise device on two occasions before the start of DI.

Subjects performed a series of isometric plantarflexion contractions on an isokinetic dynamometer (Biodex) at ankle angles of zero deg (neutral ankle position: the footplate of the dynamometer perpendicular to the longitudinal axis of the tibia). All measurements were carried out with the knee joint flexed at 90 °. The position of the joint (configuration) and the range of motion of limbs were recorded for each subject and reproduce after immersion. Before testing isokinetic dynamometer (Biodex) was calibrated by external weights.

Functional Electrical Stimulation

Principle of training: Functional electrical stimulation (FES) is applied to 4 muscle groups of both lower extremities. “Dry” electrodes (Axelgaard Ltd, USA) are placed on the skin above the quadriceps femoris muscles, the hamstrings, the tibialis anterior, the peroneal, and the triceps surae muscles. The synchronous stimulation of antagonistic muscle groups prevents unwanted joint movements.

The electrical stimulus was provided by the STIMUL LF-1 stimulator (Russia). The technical equipment consists of electrode trousers carrying stimulation electrodes for the 12 channels, and 2 interconnected 6 channel stimulators carried on a belt.

During BR, subjects executed a FES-training during 3 hours per day with periods of 1 s « on » and 2 s « off » and a frequency of 25 Hz and amplitude of stimulus from 0 up to 45 V for training. Used biphasic rectangular by 1 ms pulse width. The intensity level is adjusted to 20-30 % maximal tetanic force. After initialization procedure, the system begins automatic training. The intensity level stimulation is determined by a threshold of bearableness of subjects. FES-training of muscles of the examinee was carried out directly in a bath.

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Ultrasound Scanning

Joint position settings and torque measurement: Each subject’s right foot was firmly attached to an isokinetic dynamometer (Biodex, USA), and the lower leg was fixed to a test bench (Figure 2). The ankle joint was fixed at 15 ° dorsiflexion (-15 °) and 0° (neutral position, the tibia perpendicular to the sole of the foot), +15 °, and +30 ° plantarflexion. The knee joint was positioned at 90 °. Thus the following measurements were performed in 4 conditions. In each condition, the subject was asked to relax the plantarflexor muscles (passive condition), and passive plantar flexion torque was recorded from the output of the dynamometer by a PC computer. After performance in the passive condition, the subject was encouraged to perform maximal voluntary isometric plantarflexion (active condition), and torque output was recorded (isometric maximal voluntary contraction – MVC). The subjects were verbally encouraged to perform static contractions with the ankle plantarflexor with an effort 50 % of MVC at the neutral ankle position (0 °), and they were encouraged to hold contraction for about 2-3 s. Three contractions were performed by allowing a 1 min rest interval between bouts. Subjects were given visual feedback of the target and elicited force on a computer screen.

Figure 2: Experimental set-up. Position of the subject on the dynamometer. Subject performing a muscle function test using isokinetic dynamometer and ultrasound scanning MG and LG muscles. The ankle and the knee of the tested leg are fixed at 90 ° (neutral anatomic position) and 60 °, respectively.a, dynamometer; b, dynamometer footplate; c, marker [arrow] was placed between the skin and the ultrasonic probe as the landmark to confirm that the probe did not move during measurements; d, ankle joint and dynamometer footplate axis; e, velcro straps for fixating the thigh; f, ultrasonic system.  Position of ultrasonic transducer at research of MG (left panel) and LG (right panel) muscle. 

 

Measurement of lengths, and angles of fascicles, and thickness muscle

Fascicular lengths and pennation angles of human the triceps surae muscle were measured in vivo from sonographs taken during rest (passive) and active (contracting) conditions. All measurements were carried out on the right foot. In all measurements, subjects were instructed to relax your leg muscles. A real-time B mode ultrasound apparatus (SonoSite MicroMaxx, USA) with a 60 mm, 7.5 MHz linear-array probe was used to obtain sagittal sonographs of the MG, LG and SOL in the right leg at rest and at 50 % of plantarflexor MVC at the neutral ankle position.

Longitudinal ultrasonic images of the MG, and LG, and SOL were obtained at the proximal levels 30 % (MG and LG) and 50 % (SOL) of the distance between the popliteal crease and the center of the lateral malleolus. Each level is where the anatomic cross-sectional area of the respective muscle is maximal [29]. Images were recorded at 50 % shin length (90 ° flexion at the hip and knee joint), according to procedures previously described [30,31]. After identification of these levels on the surface of the muscle, marker was placed. The marker was a 1.0-mm copper wire encased in a soft plastic material and placed on the surface along the lower leg circumference at a fixed distance (Figure 1). The position marker has been reported in each of the subject and was reproduced at the time of testing after immersion. This site was clearly marked to provide a standardized measurement site and ensure that measurements after immersion were taken from the same external site.

After identification and marking of the proximal and distal insertions of the muscle, a 7.5-MHz linear-array probe with a 38-mm scanning length was positioned perpendicular to the dermal surface along the midsagittal plane of the triceps surae muscle at the site corresponding to the thickest portion of the muscle, identified by ultrasound. This site was clearly marked to provide a standardized measurement site and ensure that measurements at all velocities were taken from the same external site. The probe was coated with a water-soluble transmission gel to provide acoustic contact without depressing the dermal surface. The axially oriented transducer was placed perpendicularly to the muscle examined to ensure close contact and was transposed from the central to the lateral position along the marker placed on the muscle surface. During scanning, the pressure of the transducer on the skin was minimized to prevent muscle compression.

The fascicle pennation angle ( Q) was measured from the angles between the echo of the deep aponeurosis of each muscle and interspaces among the fascicles of that muscle [31] (Figure 3).

Figure 3: Ultrasonic images of longitudinal sectional of medial (MG; top) and lateral (LG; bottom) gastrocnemius muscles. Ultrasonic transducer was placed on skin over the muscle at 30 % (MG and LG) distance between the popliteal crease and the center of the lateral malleolus. Fascicle length ( L ) was determined as length of a line drawn along ultrasonic echo parallel to fascicle. Fascicle angles (Θ) were determined as angle between echoes obtained from fascicles and deep aponeurosis in ultrasonic image. 

 

The length of fascicles (L) across the deep and superficial aponeurosis was measured as a straight line [32] (Figure 3). Shorter fascicle lengths and steeper fascicle angles in the active compared with the rest (passive) conditions show internal shortening of fascicles by contraction (rLmuscle) [33]. The rLmuscle was estimated by the following formula, i.e.

                                                                rLmuscle = Lr.cos Θr – Ls.cos Θs, while

Lr and Ls — are fascicle lengths in rest (passive) and active conditions (strength 50 % MVC);

Θr and Θs — are fascicle angles in rest (passive) and active conditions, respectively.

The distance between aponeuroses (muscle thickness) was estimated from the fascicle length and pennation angle using the following equation:

                                                                         Muscle thickness = L x sin α

where L, and α is the pennation angle of each muscle determined by ultrasound.

In the present study, ultrasonic measurement was repeated three times for each subject and averaged values were used. The coefficients of variation of three measurements were in the range of 0–2 %. All ultrasonic images were processed with use of the software package Dr. ReallyVision (Alliance Ltd. –Holding, Russia).

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Statistics

 

The statistical analyses were performed using Statistica software for Microsoft Windows (StatSoft, version 6.1, Tulsa, OK). All data were expressed as means ± standard error of the mean (SE) within the text and the table. Differences in pennation angles, fascicle lengths and thicknesses between rest and 50 % MVC and between different ankle angles were tested using two-way analysis of variance tests. Tukey's test was used to determine significant differences between mean values. One-way analysis of variance (ANOVA) was used for comparison of muscle thickness, pennation angles, and fascicle lengths. A level of p < 0.05 was selected to indicate statistical significance within the text and the table.

Table 1: Pennation angle [Q ], fibre length [L], and thickness [H] at rest and at the 50 % MVC before and after 7-d “dry” immersion with long-term electromyostimulation trainings

 

Results 

 

Changes in maximal muscle strength

Maximal plantarflexion torque increased on the average by 11.3 % (150 ± 17.3 vs 167 ± 6.7 H; p < 0.05) after the 7-day DI with application by FES-training, corresponding by five subjects and one has decreased for 9.6 % (155 vs 140 H; p > 0.05) (Figure 4).

Figure 4: Changes in maximal plantar flexion torque of individual subjects after DI with FES training.

 

Architectural characteristics at rest

Thicknesses of MG, LG and SOL at rest (~18, 16 and 15 mm, respectively) did not change significantly in response to changes in muscle length resulting from changes in ankle joint angle (Figure 5).

Figure 5: Medial (MG), and lateral (LG), and soleus (SOL) muscles thickness as a function of changes in ankle at rest. 

 

In all three muscles at rest, pennation angle and fascicle lengths were ankle angle dependent. In all three muscles, as ankle angle increased from -15 to +30 °, the pennation angle increased: in MG from 31 ± 2.8 to 49 ± 1.7 ° (58%, p < 0.01), in LG from 20 ± 2.1 to 28.5 ± 1.6 ° (43 %, p < 0.05) and in SOL from 22.8 ± 1.4 to 34 ± 2.2 ° (49 %, p < 0.01), respectively (Figure 6).

In all three muscles, as ankle angle increased from -15 to +30 °, fascicle lengths decreased: in MG from 36 ± 1.2 to 27 ± 2.1 mm (25 %, p < 0.01), in LG from 46.8 ± 0.6 to 31.2 ± 1.9 mm (33 %, p < 0.01) and in SOL from 39.2 ± 1.2 to 28.2 ± 2.0 mm (28 %, p < 0.01), respectively (Figure 6).

Figure 6: Changes in the triceps surae complex architecture. Medial (MG), and lateral (LG), and soleus (SOL) muscles fascicle length ( L ) and pennation ankle (Θ) as a function of changes joint ankle at rest. 

 

Architectural characteristics after immersion

In all three muscles after immersion, pennation angle and fascicle lengths were ankle angle dependent. In all three muscles, as ankle angle increased from -15 to +30 °, the pennation angle increased: in MG from 26 ± 2.8 to 36 ± 2.8 ° (38%, p < 0.05), in LG from 15 ± 1.4 to 20.2 ± 1.0 ° (35 %, p < 0.05) and in SOL from 18.5 ± 1.6 to 24.8 ± 1.7 ° (34 %, p < 0.01), respectively (Figure 6).

In all three muscles, as ankle angle increased from -15 to +30 °, fascicle lengths decreased: in MG from 30.2 ± 1.7 to 25.5 ± 1.9 mm (16 %; p > 0.05), in LG from 40.2 ± 0.6 to 25.2 ± 2.9 mm (37 %, p < 0.01) and in SOL from 29.2 ± 2.9 to 22.2 ± 2.1 mm (24 %, p < 0.05), respectively (Figure 6).

Architectural characteristics during graded isometric force

Pennation angle and fascicle lengths decreased as a function of contraction intensity in all three muscles (Figure 7). Pennation angle in MG, LG and SOL gradually decreased from 49 ± 1.4 to 40.5 ± 2.4 ° (17 %, p < 0.01), from 26.2 ± 1.8 to 20.5 ± 1.9 ° (22 %, p < 0.05) and from 36 ± 5.3 to 29.8 ± 3.9 ° (17 %), respectively. The fascicle lengths in MG, LG and SOL decreased gradually from 26.3 ± 2.6 to 21.5 ± 1.9 mm (18 %), from 36 ± 2.9 to 28.2 ± 2.2 mm (22 %, p < 0.05) and from 32.2 ± 2.2 to 25.5 ± 2.1 mm (21 %, p < 0.05), respectively (Figure 7).

Figure 7: Architectural characteristics during graded isometric force. Changes at rest and during 50 % MVC at the neutral position in MG, LG, and SOL. Values are means ± SE.

 

A comparison of the mean values of values of Θ mean, L, and thickness, at rest and at 50 % MVC are presented in table 1. As shown in this Table, in the transition from rest to 50 % MVC at the neutral ankle position (0 °), the thickness of MG decreased (no significant difference) at about 5 mm but the thicknesses of LG and SOL decreased gradually from 13 to 9 mm (31 %, p < 0.01) and from 17 to 13 mm (no significant difference), respectively .

Comparison between actual and architectural changes after immersion

Pennation angles during 50 % of maximal isometric contraction intensity: Estimated by after immersion, pennation angles during 50 % of isometric plantarflexion moment differed by 8.9 ° (17.3 %, p < 0.05), 5.7 (21.8 %, p < 0.05), and 6.2 ° (17.2 %, p < 0.05) from the corresponding actual pennation angles in MG, LG and SOL, respectively (Fig. 7). In MG, pennation angle values were higher than the corresponding actual values at contraction intensities by 50 % of MVC but in LG and SOL values were systematically lower than the corresponding actual values.

Fibre lengths during 50 % of maximal isometric contraction intensity: Estimated by after immersion, fascicle lengths during 50 % of isometric plantarflexion moment differed by 4.8 mm (18.2 %, p < 0.01), 7.8 mm (21.7 %, p < 0.01) and 6.7 mm (20.8 %, p < 0.01) from the corresponding actual fascicle lengths in MG, LG and SOL, respectively (Figure 7).

Internal shortening of fascicles

Shorter fascicle lengths and steeper fascicle angles in the active compared with the passive condition show internal shortening of fascicles by contraction. Before DI ΔLmuscle the MG has found 7.9 mm after has decreased and has made 7.8 mm, and in SOL 5.9 vs 5.6 mm. Significant increased in ΔLmuscle from 0.9 to 3.3 mm (p < 0.05) were found by LG.

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Discussion 

 

This study describes, the architecture of the human TS [medial (MG) and lateral (LG) gastrocnemius and soleus (SOL) muscles] in vivo, both at rest and during graded [50 % MVC] isometric plantar flexions.

The results obtained in vivo indicate that human MG, LG, and SOL architecture drastically changes both as a function of ankle joint angle at rest and as a function of the force developed during isometric contractions at a fixed joint angle. At rest, when changing the ankle joint angle from -15 to +30 °, MG pennation angle increased from 31 to 49 °, LG – from 20 to 28,5 °, and SOL – from 22.8 to 34 °; the fascicle length decreased from 35.5 to 26.8 mm, LG – from 46.8 to 31.2 mm, and SOL – from 39.2 to 28.2 mm. These results indicate that the fascicle length and pennation angle of the human TS cannot be assumed to remain constant with changing muscle length [34-36]. The decrease fascicle length and increase pennation angle with increasing muscle length may be ascribed the taking up of the slack characterizing these structures [35]. In the present study, the decrease the fascicle length [cf. above] occurring from -15 to +30 ° of passive plantarflexion also suggests that muscle fibres became progressively slack with increasing ankle joint angles. Using ultrasound, Ichinose et al. [37] have shown on the human vastus lateralis that the slack of muscle fibres, at rest, is a function of knee joint angle. These authors observed that when the knee is fully extended, muscle fibresare remarkably slack, for they decrease by about 35 % in length when contracting only by 10 % of the MVC. In the present flexion also suggests that muscle fibresbecame progressively slack with increasing ankle joint angles.

The present study showed that from rest to 50 % MVC, GM pennation angle increased from 37.2 to 49.0 °, whereas fiber length decreased from 31.5 to 26.9 mm; LG – 21.8 to 26.2 °, whereas fiber length slightly increased from 35.8 to 36.0 mm; SOL – 28.2 to 36.0 whereas fiber length decreased from 36.2 to 32.2 mm with no significant change in the distance between the aponeuroses. This finding agrees with the predictions of Gans & Bock [25] according to whom, the thickening of pinnately arranged fibres is compensated for by the change in fiber angle during contraction; thus surfaces of origin and insertion remain parallel and equidistant.

The present study aimed to elucidate the effects of chronic unloading on the mechanical properties of human muscle and to examine the potential preventive effects of FES-training performed during the period of unloading on mechanical properties. Our findings show that 6 days of unloading resulted in a reduced structural and increased contractile properties of human muscle, and, although the exercise regimen (FES-training) performed did attenuate these detrimental effects, it did not completely prevent them. The present study may be considered unique in terms of the duration of unloading and use FES-training; many studies have investigated physiological adaptations to longer periods of unloading [38,39].

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 [18,19,40-42]. In the absence of weight-bearing activity, strength loss is the most evident consequence of atrophy. These alterations are also accompanied by changes in the mechanical properties of muscle in humans by unweighting such as immersion [43-46], and bed rest [16,17,20,39,44], and spaceflight [47-50].

According to Kirenskaya et al. [6], and Sugajima et al. [51] weightlessness induced by unloading gave rise to characteristic changes in the recruitment order of motor units during voluntary isometric contraction. The order is not completely fixed, being variable under different conditions. Of the factors controlling the order, the proprioceptive input to the motoneurons is known to be most important, especially to the voluntary muscle contraction. Weightlessness releases the musculature from its weight-bearing task and should reduce the proprioceptive inputs from tendon and muscle spindles.

The major findings of this study were that, after 7 day DI with of FES-training, maximal voluntary isometric torque (MVC) by the plantar flexor muscles increased. Previous studies have documented decrease of the contractile properties of skeletal muscles during DI [5,44-46]. The present exercise training resulted small increased (~11 %) in maximal voluntary plantarflexion torque whereas absence of preventive actions results in decrease in MVC more than on 40 % [50] and in Po more than on 30 % [41,45,46,50].

However efficacy of FES-training for increased the contractile properties of skeletal muscles during unweighting has been suggested in previous studies [52-54]. The insignificant increase in force of contraction in the present study can be assumed it is defined by slack intensity impulses.

It is well known that the smaller motoneurons innervating muscles are more readily activated than the larger cells innervating units, as the strength of the contraction increases progressively. The smaller units consist of slow twitch muscle fibres (type I) and the larger units consist of fast twitch fibres (type II). In submaximal voluntary contractions, type I fibres the motor units are activated by the synaptic current impinging on the motor neuron. The situation is completely different in contractions triggered by FES, because the muscle fibres of the motor units are activated by an electric current which is applied extracellularly to the nerve endings, and larger cells with lower axonal input resistance are more excitable [55, 56]. In fact, when the stimulus is applied from outside the cell, the electric current must first enter through the membrane before it depolarises the cell, but the extracellular medium shunts the current, and the smaller motor units will not be activated during submaximal FES because of their higher axonal input resistance. Therefore, the smaller motor units do not adapt to training with submaximal FES. However when use electrical stimulation high training intensity, larger force FES-training to be more efficient exercise [57]. In present study average intensity impulses during training was essentially insufficient for activation of small motor units (e.g. subject 4 vs 1, Figure 8).

Figure 8: Dynamics of change of amplitude stimulus pulses during training.

 

The increase in the maximal torque was accompanied by changes of internal architecture the MG, LG and SOL which have been in part prevented by preventive exercises (FES training). Both fascicle length and pennation angle were reduced after DI with FES training, this strongly suggests a loss of both in-series and in-parallel sarcomeres, respectively. The functional consequence of the decreased fascicle length was a reduced shortening during contraction. The loss of in-series sarcomeres would mean that this is likely to have implications both on the force length and force velocity relationships of the muscle. The observation of a smaller pennation angle during contraction after DI with FES training will partially compensate for the loss of force, because of a more efficient force transmission to the tendon. The reduced initial resting Θ probably, grows out reduction decreased tendon stiffness or of the muscle-tendon complex that finds confirmation in substantial growth ΔLmuscle of LG (with 0.9 up to 3.3 mm after DI) during contraction. This observation is consistent with the findings of Kubo et al. [17].

Moreover, reduction of number consistently connected sarcomeres allows to assume, that the size of developed reduction of a fibre will be reduced. This supervision will be coordinated to the results received earlier in conditions immobilization of finiteness [58-60].

Smaller pennation angle an inclination of a fibre during reduction of a muscle after DI with use of FES training, apparently, in part compensates loss of force which is constant "satellite" of gravitational unloading muscular, the device [40,45-48,51,61] because of more effective transfer of the force developed by fibres to a sinew. Reduced Θ an inclination of a fibre, probably, grows out reduction of rigidity of a tendon or muscle-tendon complex that finds confirmation in substantial growth ΔLmuscle of LG (with 0.9 up to 3.3 mm after DI) during reduction of a muscle and proves to be true earlier received data [17].

Muscle thickness of LG, and MG significantly decreased after DI with FES-training in the trained limb. This result undermines the contention that the degree of muscle atrophy is related to the relative amount of slow twitch fibres within a muscle, since LG and MG has relatively higher percentage of fast twitch fibres [62,63]. A decrease in muscle thickness of LG, MG, in the trained limb appears contradictory considering that the trained limb did exercises during DI. The reason for this is not clear, but it clearly points to the fact that specific training is required for the maintenance of the contractile properties and architecture of skeletal muscles during DI.

The increase in the maximal voluntary torque after DI with FES training allows to assume, that FES training, apparently, promotes increase stream muscular afferentation [64] in conditions of his deficiency at gravitational unloading the muscular device caused long immersion that can promote also to the certain role in maintenance and normalization of activity of control systems by any movements (by a principle of a feedback). Tetanic electrical stimulation applied over human muscle generates contractions by depolarizing motor axons beneath the stimulating electrodes. However, the simultaneous depolarization of sensory axons can also contribute to the contractions by the synaptic recruitment of spinal motoneurons. Upon entering the spinal cord, the sensory volley recruits spinal motoneurons, leading to the development of central torque. This recruitment is consistent with the development of persistent inward currents in spinal motoneurons or interneurons [65-67]. Persistent inward currents lead to sustained depolarizations [plateau potentials], and it is becoming increasingly clear that they play an important role in regulating cell firing in normal [67-69]. Maximizing this central contribution may be beneficial for increased muscle force.

 Conclusions, from the present results, follows, first, that the architecture different lead the triceps surae muscle considerably differs, reflecting, probably, their functional roles, second, various changes fibre length and pennation angle between different muscles, probably, are connected to distinctions in ability to develop force and elastic characteristics of tendon or muscle-tendon complex and, at last, in the third, FES training has preventive an effect on stimulated muscles: in part reduces loss of force of reduction of the muscles, the caused long unloading. The received data, allow concluding, that use of FES trained renders the expressed preventive action, essentially reduces depth and rate of atrophic processes in muscles.

Present results suggest that the structural adaptations to immersion (unloading) likely to contribute to a reduced force loss. On this background used FES training of muscles in conditions of unloading allows to increase contractile function.

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