Muscle tension during sleep – Are all muscles flaccid?

Question

A sleep doctor told me that all muscles are completely flaccid during sleep and that it is therefore nonsense to exercise the muscles. Is this true ?

Answer

If the sleep physician were right, all people would die as soon as they fall asleep. Swallowing, breathing and eye muscles are active all the time, whether we are asleep or awake. Breathing is natural and swallowing must occur to remove saliva and keep the airways clear. Swallowing alone activates 52 muscles, muscles of mastication, muscles in the mouth and throat. This is most noticeable in the tongue. With correct posture, which is adjusted and constantly maintained by the tongue muscles alone, the tongue maintains a basic shape. The posture is comparable to the hand. When the back of the hand is placed on the table, parts of the palm and, of course, the fingers remain facing upward (even while sleeping). The muscles in the hand provide this shaping.
Likewise, the heart and other organs are constantly dependent on muscle activity. However, in the skeletal muscles of the body, strong muscle relaxation occurs during periods of REM sleep, dream sleep. During eight hours of sleep, this would be the case for about one and a half to two hours. During the other time, there is at least a basic tension in all muscles.

Maintenance during sleep is necessary for quick reactions to potential threats or disturbances. This mechanism probably has evolutionary roots – our ancestors had to be ready to react quickly to threats even during sleep.
So, it would be more correct to say that there are phases in sleep (during REM sleep) when some muscles relax completely and the other remains active.

Additional info: there is scientific evidence that even complete relaxation of skeletal muscles occurs only when the brain transmitters GABA and glycine dock to their respective recognition sites. If one of the two transmitters is missing or one of the receptors is defective, the muscles do not rest even during sleep. This could be an explanation for the occurrence of “restless leg syndrome” (restless leg movements during sleep).

Source: Patricia Brooks and John Peever (University of Toronto): Journal of Neuroscience, doi: 10.1523/JNEUROSCI.0482-12.2012 © wissenschaft.de – Gesa Seidel.