How much muscle do you lose when an injury stops you from training?

Short answer

With a leg immobilized, muscle dropped about 5% in a week and about 8% in 2 weeks, and strength fell 2 to 3 times as much. But muscle and strength lost over 3 weeks came back with 3 weeks of strength training.

How much muscle do you lose when an injury stops you from training?
'A few weeks in a cast and your muscle is gone' — is that true?
A common worry: once a leg or arm is immobilized after an injury, all the muscle you built disappears. We used studies that deliberately immobilized healthy people's legs and arms to check how much is lost, how fast, and whether it comes back.
Loss starts within 2 days
One week immobilized: 5.5% of thigh muscle
13 people had one leg held in a knee brace for 7 days and were scanned with MRI. Thigh muscle volume in the braced leg dropped 1.7% after 2 days and 5.5% after 7 days. That is a steady loss of about 0.8% a day.
At 2 weeks
Muscle down 8.4%, strength down 22.9%
24 people had one leg in a cast for 5 or 14 days. Front-thigh muscle size dropped 3.5% after 5 days and 8.4% after 14 days. Leg strength dropped 9.0% and 22.9% over the same periods, far more than muscle.
Arms lose strength first too
3 weeks immobilized: muscle 3.44%, strength 18.06%
An analysis of 45 studies that immobilized an arm found that after about 3 weeks, muscle dropped 3.44% but strength dropped 18.06%. Per day, that's 0.16% for muscle and 1.55% for strength. Arm muscle was lost more slowly than leg muscle (about 0.8% a day).
The first days are the fastest
The loss slowed down over time
An analysis pooling 318 people who stayed in bed for 5 to 120 days found that front-thigh muscle and strength fell fastest in the first few days, and the rate of loss slowed after that. Within the same leg, one week of immobilization shrank the front of the thigh by 6.7% and the back by 3.5%.
So what happens when you train again?
Muscle lost in 3 weeks came back in 3 weeks
When 8 people kept one leg off the ground for 3 weeks, strength fell 26% and front-thigh muscle volume fell 10%. After 3 weeks of strength training, both strength and muscle volume were back to where they started. 20 people trained for 4 weeks after 2 weeks of immobilization and all regained their strength, but the 9 aged 61 to 74 regained less muscle volume.
What about training the other side?
The injured side lost less strength
A 2026 analysis of 8 studies with 189 people who had one arm immobilized found that strength training the free arm meant the immobilized arm lost less strength. The effect on keeping muscle size was small and only appeared when a joint closer to the trunk was immobilized. In 8 trials, eating extra protein did not reduce muscle loss.
Bottom line: it goes fast, but it comes back
5% in a week, 8% in 2 weeks, strength 2 to 3 times more
With a leg immobilized, muscle dropped about 5% in a week and about 8% in 2 weeks, while strength dropped 2 to 3 times as much. The loss was fastest at the start. But muscle and strength lost over 3 weeks of immobilization came back with 3 weeks of strength training.