Replacement of daily load attenuates but does not prevent changes to the musculoskeletal system during bed rest

Abstract

The dose-response effects of exercise in reduced gravity on musculoskeletal health have not been well documented. It is not known whether or not individualized exercise prescriptions can be effective in preventing the substantial loss in bone mineral density and muscle function that have been observed in space flight and in bed rest. In this study, typical daily loads to the lower extremities were quantified in free-living subjects who were then randomly assigned to control or exercise groups. Subjects were confined to 6-degree head-down bed rest for 84 days. The exercise group performed individually prescribed 1 g loaded locomotor exercise to replace their free-living daily load. Eleven subjects (5 exercise, 6 control) completed the protocol. Volumetric bone mineral density results from quantitative computed tomography demonstrated that control subjects lost significant amounts of bone in the intertrochanteric and total hip regions (p < 0.0125), whereas the exercise group showed no significant change from baseline in any region (p > 0.0125). Pre-and post-bed rest muscle volumes were calculated from analysis of magnetic resonance imaging data. The exercise group retained a larger percentage of their total quadriceps and gastrocnemius muscle volume (- 7.2% ± 5.9, - 13.8% ± 6.1, respectively) than their control counterparts (- 23.3% ± 5.9, - 33.0 ± 8.2, respectively; p < 0.01). Both groups significantly lost strength in several measured activities (p < 0.05). The declines in peak torque during repeated exertions of knee flexion and knee extension were significantly less in the exercise group than in the control group (p < 0.05) but work done was not significantly different between groups (p > 0.05). The decline in VO_2max was 17% ± 18 in exercising subjects (p < 0.05) and 31% ± 13 in control subjects (p = 0.003; difference between groups was not significant p = 0.26). Changes in blood and urine measures showed trends but no significant differences between groups (p > 0.05). In summary, the decline in a number of important measures of musculoskeletal and cardiovascular health was attenuated but not eliminated by a subject-specific program of locomotor exercise designed to replace daily load accumulated during free living. We conclude that single daily bouts of exposure to locomotor exercise can play a role in a countermeasures program during bed rest, and perhaps space flight, but are not sufficient in their own right to ensure musculoskeletal or cardiovascular health.

Keywords: Bed rest; Biomechanics; Exercise; Gravity; Simulation; Space flight.

Fig. 1

The zero-gravity locomotion simulator (ZLS) in which subjects assigned to the exercise group walked and ran during the study. Subjects assigned to the control group were suspended in the ZLS for a similar period of time but did not exercise.

Fig. 2

Average and standard deviation of the time spent by each exercise group subject running and walking during the exercise sessions (5 per week).

Fig. 3

Exercise group average and standard deviation of the time spent walking and running each week during bed rest.

Fig. 4

Percentage change in integral bone loss (vBMD [post-pre bed rest]) in regions of the hip and lumbar spine in control and exercise subjects. Significant difference between groups *p < 0.0125. Significant difference from baseline ^#p < 0.0125. F. Neck, femoral neck; Troch, trochanter; Inter, intertrochanteric region.

Fig. 5

Change in maximal knee extensor torque (nM) during repeated contractions for assessing endurance. A). Average of control subjects before (solid, straight line) and after bed rest (dashed, straight line). Regression coefficients: − 1.574 and − 0.919 Nm/repetition, respectively. B). Exercise subjects before (solid, straight line) and after bed rest (dashed, straight line). Regression coefficients: − 1.243 and − 0.662 Nm/repetition, respectively. The pre-bed rest slopes were not different between groups, but the control subjects showed significantly greater decline than the exercise group after bed rest. The solid, curved pairs of lines indicate 95% confidence intervals.

Fig. 6

Changes in calcium excretion (mg/day, solid lines) over the course of an 84-day bed rest campaign. The slopes (dashed lines) of the control and exercise subjects' individual data sets were used as derived variables and showed that control subjects (gray) had an average increase in Ca excretion (slope = 2.89 ± 10.01) while exercise subjects (black) had a decrease in Ca excretion (slope = − 1.44 ± 3.59) over the course of the study. The slopes are not significantly different from each other (p > 0.05).