The effect of long-term confinement and the efficacy of exercise countermeasures on muscle strength during a simulated mission to Mars: data from the Mars500 study

Abstract

Background: Isolation and long duration spaceflight are associated with musculoskeletal deconditioning. Mars500 was a unique, high-fidelity analogue of the psychological challenges of a 520-day manned mission to Mars. We aimed to explore the effect of musculoskeletal deconditioning on three outcome measures: (1) if lower limb muscle strength was reduced during the 520-day isolation; (2) if type I or II muscle fibres were differentially affected; and (3) whether any 70-day exercise interventions prevented any isolation-induced loss of strength.

Methods: Six healthy male subjects (mean ± SEM) (34 ± 3 years; 1.76 ± 0.02 metres; 83.7 ± 4.8 kg) provided written, informed consent to participate. The subjects' maximal voluntary contraction (MVC) was assessed isometrically in the calf (predominantly type I fibres), and maximal voluntary isokinetic force (MVIF) was assessed in the quadriceps/hamstrings (predominantly type II fibres) at 0.2 and 0.4 ms^-1 using the Multifunctional Dynamometer for Space (MDS) at 35-day intervals throughout Mars500. Exercise interventions were completed 3-7 days/week throughout the 520-day isolation in a counterbalanced design excluding 142-177 days (rest period) and 251-284 days (simulated Mars landing). Exercise interventions included motorized treadmill running, non-motorized treadmill running, cycle ergometry, elastomer-based resistance exercise, whole-body vibration (WBV), and resistance exercise using MDS.

Results: Calf MVC did not reduce across the 520-day isolation and MDS increased strength by 18% compared to before that of 70-day exercise intervention. In contrast, there was a significant bilateral loss of MVIF across the 520 days at both 0.2 ms^-1 (R ² = 0.53; P = 0.001) and 0.4 ms^-1 (0.4 ms^-1; R ² = 0.42; P = 0.007). WBV (+ 3.7 and 8.8%) and MDS (+ 4.9 and 5.2%) afforded the best protection against isolation-induced loss of MVIF, although MDS was the only intervention to prevent bilateral loss of calf MVC and leg MVIF at 0.2 and 0.4 ms^-1.

Conclusions: Mars500 induced significant loss of quadriceps/hamstrings MVIF but not calf MVC. Collectively, these data suggest that muscles with predominantly type I fibres were affected less by isolation compared to type II dominant muscles. MDS and WBV afforded the best protection against isolation-induced loss of strength and thus may have virtue in exploration class missions.

Keywords: Confinement; Ground-based analogue; Intervention; Mars500; Muscle strength; Spaceflight; Spaceflight analogue.

Fig. 1

Mean (±SEM) bilateral leg press maximal voluntary isokinetic force (MVIF) at 0.2 and 0.4 ms⁻¹ reduced with time during Mars500. Bilateral MVIF was measured every 35 ± 3.6 days in each subject during the 520-day isolation period at 0.2 and 0.4 ms⁻¹ contraction rates. Individual (a, b) and mean (±SEM) (c, d) MVIF forces at 0.2 and 0.4 ms⁻¹ are shown respectively

Fig. 2

a Change in calf MVC across Mars500 was significantly associated with starting calf MVC. Calf MVC was measured every 35 ± 3.6 days in each subject during the 520-day isolation period. b, c Individual changes in MVC where the intervention on average protected against loss of strength and d where no exercise intervention was completed. e Mean (±SEM) calf MVC was increased relative to baseline (day 0) following candidate 70-day exercise interventions

Fig. 3

a–c Individual data from exercise interventions that on average protected against loss of strength at 0.2 ms⁻¹ and d where no exercise intervention was completed. e Mean (±SEM) bilateral leg MVIF at 0.2 ms⁻¹ change following candidate 70-day exercise interventions

Fig. 4

a–d Individual data from exercise interventions that on average protected against loss of strength at 0.4 ms⁻¹ and e where no exercise intervention was completed. f Mean (±SEM) bilateral leg MVIF at 0.4 ms⁻¹ change following candidate 70-day exercise interventions