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Which muscles remain active during REM sleep?

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During REM sleep, the body experiences a state of atonia, which is a temporary paralysis of most skeletal muscles. However, certain muscles remain active to support vital functions and the characteristic features of this sleep stage:12

Diaphragm: This essential muscle remains active to maintain breathing throughout the sleep cycle.1
Eye muscles: These muscles are responsible for the rapid, irregular eye movements that define this phase.12
Muscles of the face and extremities: While gross body motility is low, these areas may exhibit periodic twitching during REM sleep.1

Atonia primarily affects gravity-bearing muscles that are not required for respiration or eye movement, a mechanism researchers believe helps prevent individuals from physically acting out their dreams.2