Muscle

The objectives of this review to provide a complete summary of and synthesize the current space-related physiological evidence base; and to inform decision making processes around muscle performance requirements; regarding operational CM, for exploration human space missions. The aim is to aid space agencies in designing CM programmes; provide a complete summary of what muscle groups and outcomes have been assessed in the current evidence; and highlight areas of minimal data or research gaps to guide future relevant research in this area.

Space Agencies are planning to transition from International Space Station (ISS) missions to Lunar missions; including a crewed base from which to test and develop hardware; and procedures required for the longer term goal of human Mars missions. It is well documented that exposure to microgravity (μG) during spaceflight; causes adaptation in response to gravitational unloading, especially in the musculoskeletal, cardiovascular and neuro-vestibular systems.

Based on bed rest research and previous spaceflight experience; the ISS provides astronauts with 2.5 h per day for exercise using a treadmill, cycle ergometer and resistance exercise device designed and adapted for μG. Several years of refining ISS exercise countermeasures (CM); has led to astronauts completing 6-month missions with, on average, little to no change in bone mass or cardiovascular capacity; although the efficacy seems to vary widely between individuals.

Simulating human spaceflight

Based on bed rest research and previous spaceflight experience, the ISS provides astronauts with 2.5 h per day for exercise using a treadmill; cycle ergometer and resistance exercise device designed; and adapted for μG. Several years of refining ISS exercise countermeasures (CM) has led to astronauts completing 6-month missions with, on average; little to no change in bone mass or cardiovascular capacity, although the efficacy seems to vary widely between individuals.

The target population was astronauts, however, as astronauts have taken part in space agency recommended exercise program to date; there was no inactive data available from this population. Therefore, healthy terrestrial adults, with no gender restrictions, taking part in μG analog bed rest studies, included. Bed rest studies; the only terrestrial model included as they are considered the most valid ground based model for simulating human spaceflight for periods beyond a few minutes.

Therefore, to maintain the greatest level of transferability to astronauts and in keeping with our other systematic reviews; only bed rest studies that stated they were simulating human spaceflight were considered. The main finding of the review was that muscle cross-sectional area, volume, shape, size, activity, power, performance, torque, and force-based outcomes; at either regional or global level, all decline over time, based on the current evidence base.

Muscle performance requirements

Moderate effects became apparent in the following order: power and MVC during one repetition maximum (7 days), followed by volume, cross sectional area, torques and strengths, contractile work capacity, thickness and endurance (14 days), then muscle activity (15 days). Large effects became apparent in the following order: volume, cross sectional area (28 days) torques and strengths, thickness (35 days), and peak power (56 days). No large effects found for muscle activity. There were limited data for contractile work capacity and no large effects apparent. In general, lower limb and trunk muscles appeared to decline more rapidly than upper limb muscles.

Locomotion muscles such as Plantar Flexor and Quadriceps muscles also generally appeared to decline more rapidly; than other muscles groups and with larger effect sizes. The results of this review suggest that moderate effects on a range of muscle function parameters; may occur within 7–14 days of unloading, with large effects within 35 days. Combined with identification of muscle performance requirements for future exploration mission tasks, these data, may support the design of CM programmes to optimize their efficient use without compromising crew safety and mission success.

However, the data suggests CM are likely to still be needed for longer transit/orbital periods of 14–28+ days; such as a prolonged Lunar orbit, deep space exploration, or a Mars mission, as moderate effects occur between 7–14 days and large effects by 28 days for most muscle outcomes. However, if large effect sizes occur only after 28–35 days, to save resources, space agencies might consider short missions without exercise CM, or fixed periods of abstinence during longer μG exposures, if they could be confident that moderate changes in muscle performance could be reversed in-flight.