Prolonged immobilization lasting 14 days in humans causes a 2-3% reduction in muscle volume.

Study: Leucine supplementation does not attenuate the decline in daily muscle protein synthesis rates or preserve leg muscle mass during leg immobilization in young or older adults: a double-blind randomized trial

When people’s legs are stuck in a cast for just 3 days, their muscles get a little smaller—about 1.2%. This supports the idea that being immobile for 14 days would make muscles shrink even more, likely by 2–3%.

Study: Low‐volume resistance exercise attenuates the decline in strength and muscle mass associated with immobilization

When people had to keep their leg still for 14 days, their muscles got smaller — just like the claim says. The study didn't say exactly how much smaller, but it proved muscles definitely shrank.

Study: The muscle proteome reflects changes in mitochondrial function, cellular stress and proteolysis after 14 days of unilateral lower limb immobilization in active young men

When people’s legs are stuck in a brace for two weeks, their thigh muscles shrink a lot — in this study, by nearly 10%. The claim says they shrink just 2-3%, which is much less, but still true that they shrink. So the study confirms that immobilization causes muscle loss, just more than the claim suggested.

Study: Does initial skeletal muscle size or sex affect the magnitude of muscle loss in response to 14 days immobilization?

After being immobilized for 14 days, people lost more muscle than the claim says — about 4% to 10%, not just 2-3%. But it still proves that not moving makes your muscles shrink.