Effects of 20 days bed rest on muscle morphology

Using ultrasound, muscle thickness and fascicle angles from aponeurosis were evaluated before, during and after 20 days bed rest (BR). Subjects were healthy adults (4 women and 4 men). Measurements were carried out before and after BR and after 10 weeks of recovery, respectively. Muscle measurements were taken at nine sites in trunk and upper and lower extremities, respectively. For the m. triceps brachii, m. vastus lateralis, and m. gastrocnemius medialis, fascicle angles from the aponeurosis as well as muscle thickness were measured. There was a high statistical significant correlation between muscle thickness and cross-sectional area for quadriceps muscles, suggesting applicability of muscle thickness for evaluation of muscle size. Muscle thickness decreased in muscles of the lower extremity by 2.1-4.4 % after bed rest. In triceps brachii and vastus lateralis muscles, there were no prominent changes in muscle thickness and fascicle angles. It was concluded that muscle morphology deteriorates with changes in muscle architecture by bed rest but the response is small and muscle-specific. It was also suggested that bed rest affects not only muscle mass but muscle tone as well.     

Effects of 20 days of bed rest on physiological cross-sectional area of human thigh and leg muscles evaluated by magnetic resonance imaging

The present study was to investigate the effects of 20 days of bed rest on morphological characteristics of lower limb skeletal muscles. Ten sedentary volunteers (5 males and 5 females) were participating in this study. Magnetic resonance imaging techniques were used to measure the physiological cross-sectional areas (PCSAs) of the major muscles and muscle groups of the lower limb. Consecutive images were taken from the right thigh and leg of subjects, and muscle volumes (MV), muscle length, and fiber length were calculated. PCSA of each muscle was determined as MV times the cosine of the angle of fiber pennation divided by fiber length. PCSA of knee extensor and flexor muscles were significant reduced during and after bed rest. MV and PCSA of individual muscles in the knee extensors decreased by -5.1 % to -8.0%. In knee flexors, MV and PCSA in biceps femoris (long head), semitendinosus, semimembranosus, and sartorius decreased during and after bed rest. MV and PCSA in medial and lateralis [correction of andateralis] gastrocnemius, and soleus were remarkably reduced by -9.4 to -10.3% after bed rest. The results suggest that there is a great variability of muscle atrophy in the lower limb muscle groups or individual muscle after bed rest and that the plantar flexors primarily affected.     

Effects of five days of bed rest with intermittent centrifugation on neurovestibular function

Objectives:

We tested whether intermittent short-radius centrifugation was effective for mitigating alteration in balance and gait following bed rest.

Methods:

Ten male subjects were exposed to 5 days of 6° head-down tilt bed rest with: (a) no countermeasure; (b) daily 1-g centrifugation for a continuous 30-min period; and (c) daily 1-g centrifugation for six periods of 5 min. During and after the bed rest, subjects were asked to scale the severity of neurovestibular symptoms that followed centrifugation or 80º head-up tilt. Following the bed rest, equilibrium scores were derived from anterior-posterior sway while standing on a foam pad with the eyes open or closed while making pitch head movements, and gait was evaluated by grading subjects’ performance during various locomotion tasks.

Results:

At the beginning of bed rest, one single 30-min period of centrifugation induced more severe neurovestibular symptoms than six periods of 5-min centrifugation. After bed rest, although equilibrium scores and gait performance were not significantly altered, subjects felt less neurovestibular dysfunction with orthostatic stress when centrifugation was used.

Conclusion:

Centrifugation was effective at reducing the severity of neurovestibular symptoms after bed rest, but this decrease was not different between one or multiple daily sessions.     

Effects of 17 days of head-down bed rest on hydro-electrolytic regulation in men

Prolonged periods of head-down bed rest (HDBR) are commonly used to mimic the effects of microgravity. HDBR has been shown to produce, as in space, a cephalad redistribution of circulating blood volume with an increase in central blood volume which induces the early adaptations in blood volume regulating hormones. Changes in atrial natriuretic peptide (ANP), arginine vasopressin (AVP), renin activity and aldosterone have been observed. Many reports describe these endocrine adaptations but few investigations of rhythms are in the literature. We proposed to evaluate the circadian rhythms of the hormones and electrolytes involved in the hydro-electrolytic regulation during a HDBR study which was designed to simulate a 17-day spaceflight (Life and Microgravity Spacelab experiment, LMS, NASA).     

Electrical impedance measurements in the arm and the leg during a thirty day bed rest study

The need to detect, follow and understand the effects of gravity on body fluid distribution is a constant stimulus to the quest for new techniques in this area of research. One of these techniques is electrical bioimpedance spectroscopy (BIS). Although not new, this is a technique whose applications to biomedical research are fairly recent. What is new is the development of instrumentation that has made practical the use of impedance spectroscopy in the biomedical setting, particularly in studies involving human subjects. The purpose of this paper is to report impedance spectroscopy observations made on a subject who was submitted to bed rest for a period of thirty days. These observations were made as part of a study on muscle atrophy during a thirty day head down bed rest. Since bed rest studies are very costly in human and financial terms, and technically difficult to realize, we felt that even though the present study deals only with a single case it was worthy of reporting because it illustrates kinds of questions impedance spectroscopy may help to answer in microgravity research.     

Effects of 14 days of head-down tilt bed rest on cutaneous vasoconstrictor responses in humans.

This study tested the hypothesis that head-down tilt bed rest (HDBR) reduces adrenergic and nonadrenergic cutaneous vasoconstrictor responsiveness. Additionally, an exercise countermeasure group was included to identify whether exercise during bed rest might counteract any vasoconstrictor deficits that arose during HDBR. Twenty-two subjects underwent 14 days of strict 6 degrees HDBR. Eight of these 22 subjects did not exercise during HDBR, while 14 of these subjects exercised on a supine cycle ergometer for 90 min a day at 75% of pre-bed rest heart rate maximum. To assess alpha-adrenergic vasoconstrictor responsiveness, intradermal microdialysis was used to locally administer norepinephrine (NE), while forearm skin blood flow (SkBF; laser-Doppler flowmetry) was monitored over microdialysis membranes. Nonlinear regression modeling was used to identify the effective drug concentration that caused 50% of the cutaneous vasoconstrictor response (EC(50)) and minimum values from the SkBF-NE dose-response curves. In addition, the effects of HDBR on nonadrenergic cutaneous vasoconstriction were assessed via the venoarteriolar response of the forearm and leg. HDBR did not alter EC(50) or the magnitude of cutaneous vasoconstriction to exogenous NE administration regardless of whether the subjects exercised during HDBR. Moreover, HDBR did not alter the forearm venoarteriolar response in either the control or exercise groups during HDBR. However, HDBR significantly reduced the magnitude of cutaneous vasoconstriction due to the venoarteriolar response in the leg, and this response was similarly reduced in the exercise group. These data suggest that HDBR does not alter cutaneous vasoconstrictor responses to exogenous NE administration, whereas cutaneous vasoconstriction of the leg due to the venoarteriolar response is reduced after HDBR. It remains unclear whether attenuated venoarteriolar responses in the lower limbs contribute to reduced orthostatic tolerance after bed rest and spaceflight.     

Effects of 20 days horizontal bed rest on maintaining upright standing posture in young persons

The effects of 20 days horizontal bed rest (BR) on postural reflex were studied by measuring fluctuation of center of gravity in the body during two legs or one leg upright standing in 10 young volunteers. The fluctuation was decided as total moving distance of the center recorded during 60sec standing on a force plate. The stability was measured by the moved area. After BR, the moving distance increased during two legs standing with open eyes (p<0.05), but statistically unchanged with closed eyes. The moving area decreased during right one-leg standing with closed eyes (p<0.05), but unchanged during left one-leg standing. Despite with open eyes the increased distance suggested that postural reflexes to maintain upright position were probably decreased by increased unsuitable feedback informations from the visual receptor deconditioning during BR. The decreased area during right one-leg standing with closed eyes also suggested that the declined standing posture reflex was probably related to more rapidly lowered functions for maintaining standing position in the dominating leg than in the other.     

Cardiorespiratory deconditioning with static and dynamic leg exercise during bed rest.

Bed rest deconditioning was assessed in seven healthy men (19-22 yr) following three 14-day periods of controlled activity during recumbency by measuring submaximal and maximal oxygen uptake (VO2), ventilation (VE), heart rate, and plasma volume. Exercise regimens were performed in the supine position and included a) two 30-min periods daily of intermittent static exercise at 21% of maximal leg extension force, and b) two 30-min periods of dynamic bicycle ergometer exercise daily at 68% of VO2max. No prescribed exercise was performed during the third bed rest period. Compared with their respective pre-bed rest control values, VO2max decreased (P less than 0.05) under all exercise conditions; -12.3% with no exercise, -9.2% with dynamic exercise, but only -4.8% with static exercise. Maximal heart rate was increased by 3.3% to 4.9% (P less than 0.05) under the three exercise conditions, while plasma volume decreased (P less than 0.05) -15.1% with no exercise and -10.1% with static, but only -7.8% (NS) with dynamic exercise. Since neither the static nor dynamic exercise training regimes minimized the changes in all the variables studied, some combination of these two types of exercise may be necessary for maximum protection from the effects of the bed deconditioning.     

Decrease of bone mineral density and muscle and/or strength in the leg during 20 days horizontal bed rest

Ten individuals underwent 20 days of horizontal bed rest for this study of the influence of muscle mass and strength on bone mineral density. Muscle mass volume and cross sectional area were measured using magnetic resonance imaging and bone mineral density was determined by dual energy X-ray absorptiometry before and after bed rest. Measurements were made at various parts of the leg, including the knee. Gender differences were also determined. Results are presented and discussed.     

Gender differences in muscle strength after 20 days bed rest

In the previous studies, when Bed rest (BR) was prolonged over 2 weeks, muscle mass and strength began progressively to become reduced. There are many publications investigating the changes in skeletal muscles during inactivity. However it is still unclear whether the changing degrees of muscle mass and strength not only in antigravity muscles but also in non-antigravity muscles differ between males and females. So, the purpose of this study is to investigate gender difference in the effect of 20 days of BR (BR 20) on regional muscle mass and strength of the arm and leg.     

Effects of arm and leg loading on sprint performance

The effects of loading on sprint kinematics were examined in 24 male students. The moment of inertia of either the arms or legs was increased by up to 50% of their unloaded values and the time for distances of 0.5–15 m and 15–30 m from a sprint start was measured. An increase in leg loading was associated with a gradual decrease in velocity of both sprint phases, while the change associated with arm loading was modest and significant only in the second phase. The decrease in sprint velocity was predominantly due to a reduction in stride rate, while the stride length remained almost unchanged. It was concluded that leg loading affected sprint velocity more than arm loading, and also that the velocity was reduced due to a decrease in the stride rate rather than in the stride length. Accepted: 10 November 1997     

Effects of 17 days bed rest on the maximal isometric torque of the flexors and extensors of the ankle

Researchers examined the extent to which short-term bed rest affects maximal isometric force produced by the plantar and dorsal flexors of the ankle. Results indicate an increase in average values of maximal isometric torque throughout the study, a significant increase in isometric EMG between control and bed rest subjects, and a significant isometric EMG augmentation in bed rest subjects by the end of the study. The effects of training on maximal isometric torque and muscle function impairment are discussed.     

Changes in muscle size and architecture following 20 days of bed rest

Five healthy men carried out a program of head-down bed rest (BR) for 20 days. Before and after BR, a series of cross-sectional scans of the thigh were performed using magnetic resonance imaging, from which volumes of the quadriceps muscles were determined and physiological cross-sectional areas (PCSA) were calculated. Muscle thickness and pennation angles of the triceps brachii, vastus lateralis, and triceps surae muscles were also determined by ultrasonography. During BR, subjects performed unilateral isokinetic knee extension exercises every day. The contralateral limb served as a control. Decrease in PCSA after BR was greater in the control (-10.2 +/- 6.3%) than in the trained limb (-5.2 +/- 4.2%). Among the quadriceps, vastus intermedius in the control limb was predominantly atrophied by BR with respect to the volume and PCSA, and the rectus femoris showed the greatest training effect and retained its size in the trained limb. 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. Neither trained nor control muscles showed significant changes in pennation angles in any muscles after BR, suggesting that muscle architecture does not change remarkably by muscle atrophy by up to 10%.     

Core temperature circdian rhythm during 35 days of horizontal bed rest

The present study evaluated whether the previously reported alterations in core temperature circadian rhythm associated with bed rest might be attributable to increased heat loss from the skin. Infra-red thermograms were obtained at weekly intervals during 5 weeks of bed rest and after 4 weeks of active recovery. Tympanic temperature (Tty) was measured at hourly intervals from 0800 to 2300 hrs on similar occasions during bed rest. There were no significant changes in mean tympanic temperature or amplitude of Tty circadian rhythm during the 5 week bed rest period. Skin temperature decreased progressively during bed rest (P<0.005), with distal regions being the most affected.     

Cardiorespiratory responses to physical work during and following 17 days of bed rest and spaceflight.

To determine the influence of a 17-day exposure to real and simulated spaceflight (SF) on cardiorespiratory function during exercise, four male crewmembers of the STS-78 space shuttle flight and eight male volunteers were studied before, during, and after the 17-day mission and 17 days of -6 degrees head-down-tilt bed rest (BR), respectively. Measurements of oxygen uptake, pulmonary ventilation, and heart rate were made during submaximal cycling 60, 30, and 15 days before the SF liftoff and 12 and 7 days before BR; on SF days 2, 8, and 13 and on BR days 2, 8, and 13; and on days 1, 4, 5, and 8 after return to Earth and on days 3 and 7 after BR. During 15 days before liftoff, day 4 after return, and day 8 after return and all BR testing, each subject completed a continuous exercise test to volitional exhaustion on a semirecumbent (SF) or supine (BR) cycle ergometer to determine the submaximal and maximal cardiorespiratory responses to exercise. The remaining days of the SF testing were limited to a workload corresponding to 85% of the peak pre-SF peak oxygen uptake (Vo2 peak) workload. Exposure to and recovery from SF and BR induced similar responses to submaximal exercise at 150 W. Vo2 peak decreased by 10.4% from pre-SF (15 days before liftoff) to day 4 after return and 6.6% from pre-BR to day 3 after return, which was partially (SF: -5.2%) or fully (BR) restored within 1 wk of recovery. Workload corresponding to 85% of the peak pre-SF Vo2 peak showed a rapid and continued decline throughout the flight (SF day 2, -6.2%; SF day 8, -9.0%), reaching a nadir of -11.3% during testing on SF day 13. During BR, Vo2 peak also showed a decline from pre-BR (BR day 2, -7.3%; BR day 8, -7.1%; BR day 13, -9.0%). These results suggest that the onset of and recovery from real and simulated microgravity-induced cardiorespiratory deconditioning is relatively rapid, and head-down-tilt BR appears to be an appropriate model of this effect, both during and after SF.     

Effects of 20 days horizontal bed rest on kinesthesia during knee flexion and two-point discrimination in skin of young subjects

Investigating the effect of 20 days bed rest (BR) on kinesthesia and two-point discrimination, 10 young volunteers participated in this study as subjects. Angle position sensation of right knee was measured in the prone position during flexion and extension monitored by a goniometer after two-point discrimination in skin was determined on the same lower leg. Flexed constant error was unchanged but directional constant error and absolute error were increased after BR (p<0.05). Two-point discrimination was unchanged after BR, which brought about a decline of the orientation of moving the joint indicated as over shooting of the angle during knee flexion, while it did not affect superficial sensation observed by two-point discrimination. Probably, an adjustment of deep sensation to the knee joint is lowered by the reduction in kinesthesia as well as the sensory disturbance during BR, which is independent on information from superficial sensation.     

Changes in lower extremity muscle function after 56 days of bed rest

Preservation of muscle function, known to decline in microgravity and simulation (bed rest), is important for successful spaceflight missions. Hence, there is great interest in developing interventions to prevent muscle-function loss. In this study, 20 males underwent 56 days of bed rest. Ten volunteers were randomized to do resistive vibration exercise (RVE). The other 10 served as controls. RVE consisted of muscle contractions against resistance and concurrent whole-body vibration. Main outcome parameters were maximal isometric plantar-flexion force (IPFF), electromyography (EMG)/force ratio, as well as jumping power and height. Measurements were obtained before and after bed rest, including a morning and evening assessment on the first day of recovery from bed rest. IPFF (-17.1%), jumping peak power (-24.1%), and height (-28.5%) declined (P < 0.05) in the control group. There was a trend to EMG/force ratio decrease (-20%; P = 0.051). RVE preserved IPFF and mitigated the decline of countermovement jump performance (peak power -12.2%; height -14.2%). In both groups, IPFF was reduced between the two measurements of the first day of reambulation. This study indicates that bed rest and countermeasure exercises differentially affect the various functions of skeletal muscle. Moreover, the time course during recovery needs to be considered more thoroughly in future studies, as IPFF declined not only with bed rest but also within the first day of reambulation. RVE was effective in maintaining IPFF but only mitigated the decline in jumping performance. More research is needed to develop countermeasures that maintain muscle strength as well as other muscle functions including power.     

Regulation of PDH in human arm and leg muscles at rest and during intense exercise

To test the hypothesis that pyruvate dehydrogenase (PDH) is differentially regulated in specific human muscles, regulation of PDH was examined in triceps, deltoid, and vastus lateralis at rest and during intense exercise. To elicit considerable glycogen use, subjects performed 30 min of exhaustive arm cycling on two occasions and leg cycling exercise on a third day. Muscle biopsies were obtained from deltoid or triceps on the arm exercise days and from vastus lateralis on the leg cycling day. Resting PDH protein content and phosphorylation on PDH-E1 alpha sites 1 and 2 were higher (P < or = 0.05) in vastus lateralis than in triceps and deltoid as was the activity of oxidative enzymes. Net muscle glycogen utilization was similar in vastus lateralis and triceps ( approximately 50%) but less in deltoid (likely reflecting less recruitment of deltoid), while muscle lactate accumulation was approximately 55% higher (P < or = 0.05) in triceps than vastus lateralis. Exercise induced (P < or = 0.05) dephosphorylation of both PDH-E1 alpha site 1 and site 2 in all three muscles, but it was more pronounced at PDH-E1 alpha site 1 in triceps than in vastus lateralis (P < or = 0.05). The increase in activity of the active form of PDH (PDHa) after 10 min of exercise was more marked in vastus lateralis ( approximately 246%) than in triceps ( approximately 160%), but when it was related to total PDH-E1 alpha protein content, no difference was evident. In conclusion, PDH protein content seems to be related to metabolic enzyme profile, rather than myosin heavy chain composition, and less PDH capacity in triceps is a likely contributing factor to higher lactate accumulation in triceps than in vastus lateralis.     

Exercise-induced pyruvate dehydrogenase activation is not affected by 7 days of bed rest

To test the hypothesis that physical inactivity impairs the exercise-induced modulation of pyruvate dehydrogenase (PDH), six healthy normally physically active male subjects completed 7 days of bed rest. Before and immediately after the bed rest, the subjects completed an oral glucose tolerance test (OGTT) and a one-legged knee extensor exercise bout [45 min at 60% maximal load (W(max))] with muscle biopsies obtained from vastus lateralis before, immediately after exercise, and at 3 h of recovery. Blood samples were taken from the femoral vein and artery before and after 40 min of exercise. Glucose intake elicited a larger (P ≤ 0.05) insulin response after bed rest than before, indicating glucose intolerance. There were no differences in lactate release/uptake across the exercising muscle before and after bed rest, but glucose uptake after 40 min of exercise was larger (P ≤ 0.05) before bed rest than after. Muscle glycogen content tended to be higher (0.05< P ≤ 0.10) after bed rest than before, but muscle glycogen breakdown in response to exercise was similar before and after bed rest. PDH-E1α protein content did not change in response to bed rest or in response to the exercise intervention. Exercise increased (P ≤ 0.05) the activity of PDH in the active form (PDHa) and induced (P ≤ 0.05) dephosphorylation of PDH-E1α on Ser2?3, Ser2?? and Ser3??, with no difference before and after bed rest. In conclusion, although 7 days of bed rest induced whole body glucose intolerance, exercise-induced PDH regulation in skeletal muscle was not changed. This suggests that exercise-induced PDH regulation in skeletal muscle is maintained in glucose-intolerant (e.g., insulin resistant) individuals.     

Effects of short-term and prolonged bed rest on the vestibulosympathetic reflex

The mechanism(s) for post-bed rest (BR) orthostatic intolerance is equivocal. The vestibulosympathetic reflex contributes to postural blood pressure regulation. It was hypothesized that muscle sympathetic nerve responses to otolith stimulation would be attenuated by prolonged head-down BR. Arterial blood pressure, heart rate, muscle sympathetic nerve activity (MSNA), and peripheral vascular conductance were measured during head-down rotation (HDR; otolith organ stimulation) in the prone posture before and after short-duration (24 h; n = 22) and prolonged (36 ± 1 day; n = 8) BR. Head-up tilt at 80° was performed to assess orthostatic tolerance. After short-duration BR, MSNA responses to HDR were preserved (Δ5 ± 1 bursts/min, Δ53 ± 13% burst frequency, Δ65 ± 13% total activity; P < 0.001). After prolonged BR, MSNA responses to HDR were attenuated ～50%. MSNA increased by Δ8 ± 2 vs. Δ3 ± 2 bursts/min and Δ83 ± 12 vs. Δ34 ± 22% total activity during HDR before and after prolonged BR, respectively. Moreover, these results were observed in three subjects tested again after 75 ± 1 days of BR. This reduction in MSNA responses to otolith organ stimulation at 5 wk occurred with reductions in head-up tilt duration. These results indicate that prolonged BR (～5 wk) unlike short-term BR (24 h) attenuates the vestibulosympathetic reflex and possibly contributes to orthostatic intolerance following BR in humans. These results suggest a novel mechanism in the development of orthostatic intolerance in humans.