Why does a body become stiff (rigor mortis) some hours after death?
Detaching a myosin head from actin to relax a muscle requires ATP.
Detaching a myosin head from actin to relax a muscle requires ATP. After death, respiration stops and ATP is no longer made, so once the remaining ATP runs out the myosin heads stay locked onto the actin. With the cross-bridges unable to release, the muscles remain contracted and rigid — rigor mortis. It gradually passes as the muscle proteins themselves start to break down.
Key point
Relaxation needs ATP; after death ATP runs out, so myosin stays bound to actin and muscles lock (rigor mortis).
Common mistake
thinking muscles need energy only to contract, not to relax.
Memory tip
no ATP → cross-bridges can't release → stiff muscles.
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