What happens to your muscles in space

What changes and why

On Earth, the muscles of the legs and back work all day just to hold us up and move us around. In orbit, crews float, so those “anti-gravity” muscles are barely used, and the body starts to break them down. Aerobic fitness, which depends on the heart, blood vessels and muscles together, falls as well.

• The calves are hit hardest. In early International Space Station missions (2000s, before the heavier Advanced Resistive Exercise Device (ARED) replaced the original interim Resistive Exercise Device (iRED)), four crewmembers lost 10–16% of calf muscle volume and 4–7% of thigh volume after about 6 months, while the upper arm did not change [1]. In those early studies, the U.S. space agency, the National Aeronautics and Space Administration (NASA), reports that the deep calf muscle (soleus) lost the most, about 15–19% [2].
• Strength falls too. In the same early studies, calf strength fell by about 24% [2], and in the four crewmembers strength also fell at the knee, ankle and elbow [1]. In 37 crew across station Expeditions 1–25, average strength fell 8–17%, and losses were up to 7% smaller after ARED arrived (not statistically significant) [3]. In more recent crew using ARED, average knee-extensor strength fell about 6% and aerobic fitness about 6% [4], but results vary widely between people: in a NASA dataset of 98 station crew, more than half lost over 10% of knee or ankle strength, and about a quarter lost over 20% [2].
• Aerobic fitness drops. Peak oxygen uptake (the most oxygen the body can use during hard exercise, a standard measure of fitness) fell by about 17% early in flight, then improved but did not return to preflight levels in space [5].
• Exercise in space is lighter than it looks. Even when crews run and cycle in space, the forces on their feet, measured in an early-station study, were roughly half of those on Earth (about 46% lower running and 50% lower cycling) [1].
• The back muscles shrink too, which matters for back pain and disc injury. See what happens to your spine in space.

Timeline: days to a Mars mission

1. First daysShort flights

   Muscle loss starts early. After a 17-day Space Shuttle mission (Life and Microgravity Spacelab, STS-78), four crewmembers had 7–12% less muscle volume in the back, thighs, calves and buttocks on landing day; all of it was back to preflight levels by 10 days after landing [2].

2. WeeksFirst month

   Aerobic fitness falls by about 17% within the first weeks [5]. In a 60-day bed rest study on Earth (a ground model of weightlessness), back muscles shrank about 11% without exercise and about 4% with exercise [6].

3. About 6 monthsTypical station stay

   In 46 astronauts on missions averaging about 6 months, around 600 minutes a week of aerobic and resistance exercise did not fully protect them. Changes from before to after flight ranged from 30% worse to 5% better [7]. NASA reports that a large share of long-duration crew come home with more than 10–20% loss of aerobic capacity and of muscle strength and endurance [2].

4. Mars-length missionsAbout 2 to 3.4 years (730–1,224 days)

   The same research team estimated that, with today’s exercise programs, up to 17% of astronauts would have fitness losses severe enough to limit their work on future missions [7]. NASA says there are limited data for missions longer than 4–6 months, for women and for partial gravity such as on the Moon or Mars, and rates this risk as one that requires mitigation for Mars [8].

5. Back on EarthLanding and after

   Fitness was lower 1 and 10 days after landing but had recovered by day 30 [5].

What crews do about it

• Daily exercise is part of the job. Station crews have usually exercised about 2 hours each workday, with personalized programs set by strength and conditioning specialists working with flight surgeons [9].
• Three machines. A treadmill (with a harness that holds the runner down) [10], an exercise bike and a heavy resistance machine, ARED, which provides up to 600 pound-force of resistance [9]. After that machine arrived in 2009, resistance training rose from about 33% to 46% of European Space Agency (ESA) astronauts’ in-flight exercise [10].
• Clear targets. NASA’s crew health standard requires that in-flight aerobic capacity and muscle strength be kept at or above 80% of preflight values [11].
• Harder exercise seems to help. Seven of 14 astronauts reached their preflight fitness at some point during flight or the day after landing; four of them had exercised at higher intensity than those who lost fitness [5]. Loss of fitness in space is common, but not inevitable [5].

After landing and recovery

• Structured reconditioning. NASA requires an individual reconditioning program for each crewmember, starting as they leave the spacecraft [11]. For long missions it runs about 45 days, 2 hours a day, 7 days a week from the day after landing, with fitness tests about one week and one month after landing; it can be extended [9].
• Timing of recovery. Most crewmembers return to normal activities within about 45 days [12]. Aerobic fitness had recovered by 30 days after landing in one study [5], but NASA’s research plan notes that many astronauts are not back to preflight fitness at the 30-day post-flight check [8]. Strength is largely recovered within about 30 days in most crew [3], and the full reconditioning program runs about 45 days and can be extended [9].
• Soreness on return. In the early-station studies, about half of lost calf muscle was still missing up to 2 weeks after landing, and calf strength remained low for at least 2 weeks; NASA researchers think muscle damage and soreness from suddenly carrying body weight again add to the weakness [2].

What it means for clinicians on Earth

These points are general education for clinicians, not advice about any individual patient. Coordinate with the person’s flight surgeon (the physician responsible for crew health) where there is one.

• Expect deconditioning, and expect variation. After a long mission, lower leg strength and aerobic fitness may be well below the person’s own baseline for weeks, but some people lose little [7][5]. Strength is largely recovered within about 30 days in most crew [3].
• The legs, especially the calves, lose the most muscle; upper-arm muscle size did not change in one study, although elbow strength fell [1]. Bear this in mind for falls, ankle and calf injuries, and back strain during the first weeks.
• Early limits apply. In the first week after landing, crews are usually advised not to drive or do physically demanding tasks [12]. Muscle soreness from reloading is expected [2].
• Work with the reconditioning plan. Returning crew follow a structured program run by specialists [9]; new exercise advice should fit with it.
• Short flights count too. Measurable muscle loss appeared after 17 days, though it recovered within 10 days [2]. Spaceflight participants returning from short orbital trips may notice brief weakness.
• An Earth parallel. Bed rest causes the same pattern of back and leg muscle loss, and exercise reduces it [6]. That is why space agencies use bed rest as a ground model.