How Muscle Works in Space

Astronauts on the International Space Station train about two hours a day, six days a week, and none of it is about looking good. It is so they can stand up on their own when they land. Microgravity is the fastest known model of muscle loss in a healthy human, and what happens up there in a month takes years of sitting down here.

What gravity does while you are doing nothing

When you just stand still, your calves, quads, glutes and spinal muscles hold you up against gravity. No conscious effort, but constant work. These are the antigravity muscles, and they are built mostly from slow type I fibres designed to work for hours under a light load.

In orbit the station and everything in it is falling freely around the Earth, so the load on those muscles is close to zero. The signal that says you are needed disappears, and the body stops paying the electricity bill for tissue it is not using.

Your muscle has no idea whether you are in orbit or on the couch. It only knows how much force you asked of it in the last few days.

The numbers: how fast it goes

• On early missions, without serious exercise hardware, crews lost up to 20% of muscle mass in 5 to 11 days of flight.
• With no countermeasures, calf and thigh muscle volume drops roughly 0.5–1% per day for the first two weeks, then the rate slows.
• Strength falls faster than size: on a six-month mission without adequate training, leg strength drops 20–30%.
• Bone loses 1–1.5% of density per month at the hip and spine, about ten times faster than postmenopausal osteoporosis.
• Around two litres of fluid shifts toward the head, which is where the puffy face and thin legs come from. The spine decompresses and astronauts temporarily gain 3 to 5 cm of height.

The fibre profile changes too. Slow type I fibres atrophy the most and partly take on the properties of faster ones, so the muscle fatigues sooner. It is the same pattern seen in people after a long hospital stay.

The protocol that actually holds muscle in orbit

There are three machines on the station: a resistance device using vacuum cylinders that delivers up to about 270 kg of resistance, a treadmill with a harness pulling you down at 60–70% of body weight, and a stationary bike. The schedule looks like this:

1. About 60 minutes of resistance work a day: squats, deadlifts, presses, calf raises. The same basic lifts as in any gym.
2. About 30 minutes of cardio on the treadmill or bike, with intervals a few times a week.
3. A total of 2 to 2.5 hours blocked out in the daily schedule, 6 days a week.
4. Nutrition: 1.2–1.7 g of protein per kilogram of body weight per day, plus 800 IU of vitamin D, since there is no sunlight.

With all that in place, muscle loss over a six-month mission comes down to a few percent. The most common reason someone still loses more is calories: crews often eat 10–15% less than they burn without meaning to, and in a deficit no amount of training protects muscle.

Why this matters on the ground

You do not need a rocket to reproduce the effect. A cast, surgery, a week in hospital, or simply moving a lot less will do it.

• 5 days of knee immobilisation: quadriceps cross-section down about 3.5%, strength down 9%.
• 14 days of immobilisation: leg muscle mass down 5–9%, strength down 20–25%.
• When healthy people cut from 10,000 to 1,500 steps a day for two weeks, leg muscle mass falls about 3–4% and insulin sensitivity measurably worsens.
• After 65, the same period of inactivity costs roughly twice as much, and rebuilding takes several times longer than the loss did.

The minimum that keeps muscle

1. Two strength sessions a week, 4–6 working sets per major muscle group. That is roughly a third of the volume needed to grow, but enough to maintain.
2. Load: 6–12 reps, with the last set taken to 1–2 reps short of failure. With no weights, 20–30 reps works just as well, as long as it is close to failure.
3. In a cast or after surgery: isometrics, 5 x 30 seconds, twice a day, on anything you are cleared to contract. Training the healthy leg cuts the loss in the injured one by 5–10%.
4. Steps: aim for 7,000–8,000 a day. If you sit for work, stand up every 30–45 minutes and do 2 minutes of walking or 15 bodyweight squats.
5. Protein 1.6 g/kg a day, split across 3–4 meals of 25–40 g. For an 80 kg person that is about 130 g. And do not cut calories hard while you are inactive: a deficit over 500 kcal without strength training comes straight out of muscle.

When to see a doctor

• Swelling, pain and warmth in one calf after a long period of rest or a long flight. Urgent, it can be a clot.
• Unintentional loss of more than 5% of body weight over six months.
• Weakness or numbness on one side of the body only.
• Tea-coloured urine and severe muscle pain after your first session back. Urgent.
• If you are over 65 and it takes more than 15 seconds to stand up from a chair five times, ask for a strength and sarcopenia assessment.

The short version

• Muscle is maintained by load alone: without it, losses run about 1% per day for the first two weeks, and strength falls faster than size.
• Astronauts keep it in check with 60 minutes of lifting and 30 minutes of cardio, 6 days a week, holding losses to a few percent over six months.
• On Earth, 2 strength sessions a week and 4–6 sets per major group are enough to keep what you have.
• Protein at 1.6 g/kg and enough total calories are the other half of the job, especially while you are laid up.
• Bone rebuilds far more slowly than muscle, which is why long breaks from moving cost more than they look like they do.