What are the musculoskeletal consequences of complete bed rest?

• A. They lose bone mass and muscle strength.
• B. They experience no change in bone or muscle.
• C. They lose bone mass but not muscle strength.
• D. They lose 1% of total bone mass weekly (up to 30-40% overall) and 10-15% of muscle strength weekly.

Answer: (D)

Complete bed rest causes unloading of the musculoskeletal system, so bones and muscles respond to the lack of mechanical strain with rapid deterioration. When the weight-bearing stimulus is removed, bone remodeling shifts toward resorption because osteoclast activity continues while osteoblast activity diminishes, leading to a quick loss of bone mineral density. This loss accumulates over time, and a typical pattern noted in many studies is about 1% of total bone mass each week, with a possible overall decline of 30–40% if bed rest is extended. At the same time, muscles depend on regular contraction and load to maintain mass and strength. Disuse lowers protein synthesis and increases protein breakdown, producing muscle atrophy and a substantial drop in strength—often in the range of about 10–15% per week in the early period of inactivity. Because both bone and muscle are affected by complete inactivity, the scenario that best matches the observed physiology is concurrent bone loss and muscle strength decline at these rapid weekly rates. This underscores the importance of early mobilization and countermeasures to limit unloading effects during and after immobilization.