Muscle volume, strength, endurance, and exercise loads during 6-month missions in space.

INTRODUCTION: Decrements in muscular strength during long-duration missions in space could be mission-critical during construction and exploration activities. The purpose of this study was to quantify changes in muscle volume, strength, and endurance of crewmembers on the International Space Station (ISS) in the context of new measurements of loading during exercise countermeasures. METHODS: Strength and muscle volumes were measured from four male ISS crewmembers (49.5 +/- 4.7 yr, 179.3 +/- 7.1 cm, 85.2 +/- 10.4 kg) before and after long-duration spaceflight (181 +/- 15 d). Preflight and in-flight measurements of forces between foot and shoe allowed comparisons of loading from 1-g exercise and exercise countermeasures on ISS. RESULTS: Muscle volume change was greater in the calf (-10 to 16%) than the thigh (-4% to -7%), but there was no change in the upper arm (+0.4 to -0.8%). Isometric and isokinetic strength changes at the knee (range -10.4 to -24.1%), ankle (range -4 to -22.3%), and elbow (range -7.5 to -16.7%) were observed. Although there was an overall postflight decline in total work (-14%) during the endurance test, an increase in postflight resistance to fatigue was observed. The peak in-shoe forces during running and cycling on ISS were approximately 46% and 50% lower compared to 1-g values. DISCUSSION: Muscle volume and strength were decreased in the lower extremities of crewmembers during long-duration spaceflight on ISS despite the use of exercise countermeasures. in-flight countermeasures were insufficient to replicate the daily mechanical loading experienced by the crewmembers before flight. Future exercise protocols need careful assessment both in terms of intensity and duration to maximize the "dose" of exercise and to increase loads compared to the measured levels.

Exercise in space: human skeletal muscle after 6 months aboard the International Space Station.

The aim of this investigation was to document the exercise program used by crewmembers (n = 9; 45 +/- 2 yr) while aboard the International Space Station (ISS) for 6 mo and examine its effectiveness for preserving calf muscle characteristics. Before and after spaceflight, we assessed calf muscle volume (MRI), static and dynamic calf muscle performance, and muscle fiber types (gastrocnemius and soleus). While on the ISS, crewmembers had access to a running treadmill, cycle ergometer, and resistance exercise device. The exercise regimen varied among the crewmembers with aerobic exercise performed approximately 5 h/wk at a moderate intensity and resistance exercise performed 3-6 days/wk incorporating multiple lower leg exercises. Calf muscle volume decreased (P < 0.05) 13 +/- 2% with greater (P < 0.05) atrophy of the soleus (-15 +/- 2%) compared with the gastrocnemius (-10 +/- 2%). Peak power was 32% lower (P < 0.05) after spaceflight. Force-velocity characteristics were reduced (P < 0.05) -20 to -29% across the velocity spectrum. There was a 12-17% shift in myosin heavy chain (MHC) phenotype of the gastrocnemius and soleus with a decrease (P < 0.05) in MHC I fibers and a redistribution among the faster phenotypes. These data show a reduction in calf muscle mass and performance along with a slow-to-fast fiber type transition in the gastrocnemius and soleus muscles, which are all qualities associated with unloading in humans. Future long-duration space missions should modify the current ISS exercise prescription and/or hardware to better preserve human skeletal muscle mass and function, thereby reducing the risk imposed to crewmembers.

Adaptation of the proximal femur to skeletal reloading after long-duration spaceflight.

UNLABELLED: We studied the effect of re-exposure to Earth's gravity on the proximal femoral BMD and structure of astronauts 1 year after missions lasting 4-6 months. We observed that the readaptation of the proximal femur to Earth's gravity entailed an increase in bone size and an incomplete recovery of volumetric BMD. INTRODUCTION: Bone loss is a well-known result of skeletal unloading in long-duration spaceflight, with the most severe losses occurring in the proximal femur. However, there is little information about the recovery of bone loss after mission completion and no information about effect of reloading on the structure of load-bearing bone. To address these questions, we carried out a study of the effect of re-exposure to Earth's gravity on the BMD and structure of the proximal femur 1 year after missions lasting 4-6 months. MATERIALS AND METHODS: In 16 crew members of the International Space Station (ISS) making flights of 4.5-6 months, we used QCT imaging to measure the total, trabecular, and cortical volumetric BMD (vBMD) of the proximal femur. In addition to vBMD, we also quantified BMC, bone volume, femoral neck cross-sectional area (CSA), and femoral neck indices of compressive and bending strength at three time-points: preflight, postflight, and 1 year after mission. RESULTS: Proximal femoral bone mass was substantially recovered in the year after spaceflight, but measures of vBMD and estimated bone strength showed only partial recovery. The recovery of BMC, in the absence of a comparable increase in vBMD, was explained by increases in bone volume and CSA during the year after spaceflight. CONCLUSIONS: Adaptation of the proximal femur to reloading entailed an increase in bone size and an incomplete recovery of vBMD. The data indicate that recovery of skeletal density after long-duration space missions may exceed 1 year and supports the evidence in the aging literature for periosteal apposition as a compensatory response for bone loss. The extent to which this compensatory effect protects against fracture remains to be seen.

Cortical and trabecular bone mineral loss from the spine and hip in long-duration spaceflight.

UNLABELLED: We measured cortical and trabecular bone loss using QCT of the spine and hip in 14 crewmembers making 4- to 6-month flights on the International Space Station. There was no compartment-specific loss of bone in the spine. Cortical bone mineral loss in the hip occurred primarily by endocortical thinning. INTRODUCTION: In an earlier study, areal BMD (aBMD) measurements by DXA showed that cosmonauts making flights of 4- to 12-month duration on the Soviet/Russian MIR spacecraft lost bone at an average rate of 1%/month from the spine and 1.5%/month from the hip. However, because DXA measurements represent the sum of the cortical and trabecular compartments, there is no direct information on how these bone envelopes are affected by spaceflight. MATERIALS AND METHODS: To address this, we performed a study of crewmembers (13 males and 1 female; age range, 40-55 years) on long-duration missions (4-6 months) on the International Space Station (ISS). We used DXA to obtain aBMD of the hip and spine and volumetric QCT (vQCT) to assess integral, cortical, and trabecular volumetric BMD (vBMD) in the hip and spine. In the heel, DXA was used to measure aBMD, and quantitative ultrasound (QUS) was used to measure speed of sound (SOS) and broadband ultrasound attenuation (BUA). RESULTS AND CONCLUSIONS: aBMD was lost at rates of 0.9%/month at the spine (p < 0.001) and 1.4-1.5%/month at the hip (p < 0.001). Spinal integral vBMD was lost at a rate of 0.9%/month (p < 0.001), and trabecular vBMD was lost at 0.7%/month (p < 0.05). In contrast to earlier reports, these changes were generalized across the vertebrae and not focused in the posterior elements. In the hip, integral, cortical, and trabecular vBMD was lost at rates of 1.2-1.5%/month (p < 0.0001), 0.4-0.5%/month (p < 0.01), and 2.2-2.7%/month (p < 0.001), respectively. The cortical bone loss in the hip occurred primarily by cortical thinning. Calcaneal aBMD measurements by DXA showed smaller mean losses (0.4%/month) than hip or spine measurements, with SOS and BUA showing no change. In summary, our results show that ISS crewmembers, on average, experience substantial loss of both trabecular and cortical bone in the hip and somewhat smaller losses in the spine. These results do not support the use of calcaneal aBMD or QUS measurements as surrogate measures to estimate changes in the central skeleton.

Effects of 17-day spaceflight on knee extensor muscle function and size.

It is generally held that space travelers experience muscle dysfunction and atrophy during exposure to microgravity. However, observations are scarce and reports somewhat inconsistent with regard to the time course, specificity and magnitude of such changes. Hence, we examined four male astronauts (group mean approximately 43 years, 86 kg and 183 cm) before and after a 17-day spaceflight (Space Transport System-78). Knee extensor muscle function was measured during maximal bilateral voluntary isometric and iso-inertial concentric, and eccentric actions. Cross-sectional area (CSA) of the knee extensor and flexor, and gluteal muscle groups was assessed by means of magnetic resonance imaging. The decrease in strength (P<0.05) across different muscle actions after spaceflight amounted to 10%. Eight ambulatory men, examined on two occasions 20 days apart, showed unchanged (P>0.05) muscle strength. CSA of the knee extensor and gluteal muscles, each decreased (P<0.05) by 8%. Knee flexor muscle CSA showed no significant (P>0.05) change. The magnitude of these changes concord with earlier results from ground-based studies of similar duration. The results of this study, however, do contrast with the findings of no decrease in maximal voluntary ankle plantar flexor force previously reported in the same crew.