Harlan Evans

Cortical and Trabecular Bone Mineral Loss From the Spine and Hip in Long‐Duration Spaceflight

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.

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†

We studied the effect of re‐exposure to Earths 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 Earths gravity entailed an increase in bone size and an incomplete recovery of volumetric BMD.

Calf muscle area and strength changes after five weeks of horizontal bed rest

Nine male volunteers participated in a 10 week meta bolic study in which subjects underwent 5 weeks of ambulatory control and 5 weeks of complete horizontal bed rest. Bed rest is a model commonly used to simu late space flight. The changes in muscle area and strength of the calf dorsiflexors and plantar flexors were measured before and after bed rest using magnetic resonance imaging (MRI) and a Cybex II dynamometer. The muscle area of the plantar flexors (gastrocnemius and soleus) decreased 12%, whereas the muscle area of the dorsiflexors was not significantly decreased. The maximal muscle strength of the plantar flexors de creased 26%; the muscle strength of the dorsiflexors was not significantly decreased. These results, which demonstrate differential muscle atrophy and a larger loss in strength relative to muscle area, have important implications in the development of exercise counter measures to be impiemented during space fiight. The results also have implications for patients who have severe orthopaedic disorders and must be bed rested for long periods of time, and for persons who are voluntarily inactive (a large number of the elderly).

Calcium absorption, endogenous excretion, and endocrine changes during and after long-term bed rest.

Negative calcium balance is a known consequence of bed rest, and is manifested in elevated urine and fecal calcium (Ca). Elevated fecal Ca can result from either decreased absorption, increased endogenous fecal excretion, or both. We measured the Ca absorption and endogenous fecal excretion in eight healthy male volunteers before and during 4 months of bed rest. Dual isotope (n = 6) or single isotope (n = 2) methods in conjunction with Ca balance were used to calculate true and net Ca absorption and endogenous fecal excretion. Stool Ca increased from 797 mg/day (mean intake 991 mg/day) to 911 mg/day during bed rest, whereas urine Ca excretion increased from 174 to 241 mg/day. True Ca absorption decreased from 31 +/- 7% of Ca intake pre-bed rest to 24 +/- 2% during bed rest, (p < 0.05) and returned toward pre-bed rest values within 5-6 weeks following reambulation. Endogenous fecal excretion did not change significantly, and therefore, most of the increased fecal Ca resulted from changes in absorption. However, in one individual, endogenous fecal Ca excretion was the major contributor to Ca loss. Ionized Ca and pyridinium crosslinks increased and 1,25(OH)2 vitamin D decreased during bed rest, similar to the decrease in Ca absorption; parathyroid hormone (PTH), calcitonin, serum albumin, phosphorus, and total serum Ca were unchanged. Although alkaline phosphatase, osteocalcin, and PTH were unchanged during bed rest, they were elevated during reambulation. These changes accompanied by increased Ca absorption and balance and decreased ionized and total serum Ca suggest a rebound in bone formation following immobilization.

Changes in intervertebral disc cross-sectional area with bed rest and space flight

Study Design. We measured the cross-sectional area of the intervertebral discs of normal volunteers after an overmight rest; before, during, and after 5 or 17 weeks of bed rest; and before and after 8 days of weightiessness. Objectives. This study sought to determine the degree of expansion of the lumbar discs resulting from bed rest and space flight. Summary of Background Data. Weightlessness and bed rest, an analog for weightlessness, reduce the mechanical loading on the musculoskeletal system. When unioded, intervertebral discs will expand, increasing the nutrtional diffusion distance and altering the mechancial properties of the spine. Methods. Magnetic resonance imaging was used to measure the cross-sectional area and transverse relaxation time (T2) of the intervertebral discs. Results. Overnight or longer bed rest causes expansion of the disc area, which reaches an equilibrium value of about 22% (range 10–40%) above baseline within 4 days. Increases in disc area were associated with modest increases in disc T2. During bed rest, disc height increased approximately 1 mm, about one-half of previous estimates based on body height measurements. After 5 weeks of bed rest, disc area returned to baseline within a few days of ambulation, whereas after 17 weeks, disc area remained above baseline 6 weeks after reambulation. After 8 days of weightlessness, T2, disc area, and lumbar length were not significantly different from baseline values 24 hours after landing. Conclusions. Significant adaptive changes in the intervertebal discs can be expected during weightlessness. These changes, which are rapidly reversible after short duration flights, may be an important factor during and after long-duration missions.

Spinal bone mineral after 5 weeks of bed rest

SummaryPatients put at bedrest for medical reasons lose 1–2% of spinal bone mineral per week. Losses of this magnitude during even shortterm space flights of a few months would pose a serious limitation and require countermeasures. The spinal bone mineral (L2–L4) was determined in 6 healthy males (precision=2%) before and after 5 weeks of complete bed rest. Only one individual had a significant loss (3%) and the −0.9% mean change for the 6, was not significant (P=0.06). The average negative clacium balance during the 5 weeks was 4 g or 0.36% of total body calcium, similar to that reported in other bed-rest studies. Spinal bone loss, however, in healthy bed-rested males is significantly less than reported for bed-rested patients, suggesting that a large loss of spinal bone mineral does not occur during space flight missions lasting 5 weeks or less.

Evaluation of muscle injury using magnetic resonance imaging.

The objective of this study was to investigate spin echo T2 relaxation time changes in thigh muscles after intense eccentric exercise in healthy men. Spin echo and calculated T2 relaxation time images of the thighs were obtained on several occasions after exercise of one limb; the contralateral limb served as control. Muscle damage was verified by elevated levels of serum creatine kinase (CK). Thirty percent of the time no exercise effect was discernible on the magnetic resonance (MR) images. In all positive MR images (70%) the semitendinosus muscle was positive, while the biceps femoris, short head, and gracilis muscles were also positive in 50% and 25% of the total cases, respectively. The peak T2 relaxation time and serum CK were correlated (r = 0.94, p<0.01); temporal changes in muscle T2 relaxation time and serum CK were similar, although T2 relaxation time remained positive after serum CK returned to background levels. We conclude that magnetic resonance imaging can serve as a useful tool in the evaluation of eccentric exercise muscle damage by providing a quantitative indicator of damage and its resolution as well as the specific areas and muscles.

EFFECTS OF GROWTH HORMONE ON BONE DENSITY IN CHILDREN WITH TURNER SYNDROME

Children with Turner syndrome (TS) frequently have asymtomatic osteoporosis by radiologic diagnosis. Growth hormone (GH) has been considered as a therapeutic or preventive measure for osteoporosis. GH also has been demonstrated to be effective in improving growth velocity in children with TS. This study was designed to determine the changes In bone density associated with GH therapy.Eight children with TS (45, X in 3; 45, X/46X, Xq in 1; 45, X/46, X, idic(X) In 1; 45, X/46, X, idic(Y) In 1; 45, X/46X, rX(p11q22); and 45, X/46X, iso(Xq) in 1) were studied between chronological ages of 7 to 13 with bone ages of 3 to 11 years. The bone density changes were assessed by dual beam photon densitometry with a program modified for children. The area studied was L2-L4. Bone density was measured every 6 months. Each patient served as her own control for 6 months before GH therapy was started. GH was administered intramuscularly three times weekly at a dosage of 0.125 mg/Kg/dose. Changes in bone density at 6 months of control period was 0.020±1.975 gm/cm2/yr (mean±SEM). During six months of GH therapy changes In bone density was 0.007±0.021 gm/cm2/yr (P > 0.05). This indicates a lack of effectiveness of GH therapy on bone density in children with TS. Further study is required to determine if this lack of effectiveness is because the children were growing or because children with TS may respond differently from the general population.

Total body sodium, calcium, and chloride measured chemically and by neutron activation in guinea pigs

Body sodium (Na), calcium (Ca), and chloride (Cl) of guinea pigs weighing between 227 and 600 g were measured by total body neutron activation analysis (TBNAA) followed by chemical analysis (CA) on 12-17 animals. Paired t test was used to compare any differences in the results obtained by the two methods. There was no significant difference in the results for the three elements. The means and standard deviation for Ca (g/100 g body wt) 1.070 +/- 0.132 (TBNAA) and 1.107 +/- 0.125 (CA); for Na 0.149 +/- 0.019 (TBNAA) and 0.143 +/- 0.021 (CA); and for Cl 0.126 +/- 0.009 (TBNAA) and 0.132 +/- 0.024 (CA). Neutron activation analysis alone, in a series of 27 animals, gave means (g/100 g body wt) and standard deviation of 1.110 +/- 0.084 for Ca, 0.120 +/- 0.009 for Cl, and 0.153 +/- 0.011 for Na. TBNAA has potential usefulness, particularly in longitudinal studies in the same animal, because of its accuracy and the rapidity and ease with which the measurements can be made.