Why does a cast cause a loss of muscle strength?

Casting is a common treatment for fractures but comes at the cost of immobilisation. Bed rest and space flight have similar effects, and those effects are usually greatest in the muscles we rely on to stay upright and walk around – the extensor muscles of the lower limbs. Reduced function may persist for months after people resume activities, and every one day of bed rest is thought to require about 1 week of rehabilitation to restore function. Immobilisation affects the electrical properties of the nerve, the electrical and mechanical properties of the muscle, and the reflexes affecting its function. It appears that the main effect of disuse is a decrease in muscle protein synthesis, rather than an increase in muscle protein breakdown.