A cast holds a fracture still so bone can bridge the gap. Everything else inside the cast is also held still, and those tissues respond to stillness in ways that have to be reversed afterwards.
Muscle loses size and strength quickly
Muscle adjusts its protein content to the demands placed on it. Without contraction against resistance, breakdown outpaces synthesis and measurable size loss begins within the first week.
Strength falls faster than size, because part of the loss is neural. The pathways that recruit the muscle become less effective when they are not being used.
Losses are largest in the muscles that cross the immobilised joint and in those that normally work against gravity, such as the calf and the quadriceps.
Joint tissue adapts to the fixed position
Cartilage relies on joint movement to circulate fluid through it, since it has no blood supply of its own. Prolonged stillness reduces that exchange and the cartilage thins.
The capsule and surrounding connective tissue shorten toward whatever position the joint was held in. Collagen fibres lay down in a disorganised pattern that resists movement.
This is why stiffness after immobilisation is not simply reluctance. The tissue itself has physically reorganised around the held position.
Bone thins even as the fracture heals
Bone density is maintained by the loads muscles and body weight place on it. A limb that carries no load receives no signal to maintain mineral content, and density falls locally.
The fracture site itself is healing under a different, injury-driven process, so a bone can be knitting while the surrounding shaft loses density.
Recovering that density takes far longer than losing it, and it depends on load returning to the limb. Weight-bearing and resistance work are the signals that drive mineral back into bone.
Position sense degrades
Sensors in muscle, tendon and joint capsule constantly report limb position. When the joint stops moving, those signals become sparse and the brain's map of the limb degrades.
Restored strength with degraded position sense produces clumsy, poorly timed movement, which is part of why the period just after immobilisation carries elevated re-injury risk.
Balance tasks and controlled movement rebuild the map, but they have to be practised deliberately. Strength returns from lifting; coordination returns only from moving in varied and progressively less predictable ways.
Why practice has shifted toward earlier movement
Because these losses scale with duration, management now favours the shortest safe period of immobilisation, and often protected movement instead of complete stillness where the fracture allows it.
Rehabilitation after removal addresses range, then strength, then coordination, in that order. What is safe for a given fracture is a decision for the treating clinician.
Swelling complicates the first weeks, because a limb that has not moved drains poorly once its support is taken away. Compression and early movement address that as load resumes.
