The best way to release that tension is to tense them even more.
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Lay down anywhere and do some deep breathing.
Start at the feet and point them to the sky, tensing the ankle and curl the toes, release after holding for ten seconds.
Tense the knees by straightening the legs as much as you can and release.
Tense the thighs and release.
Breath in your abdominals as far as you can, hold it and release.
Clench your fists as hard as you can. release and straighten your arms as much as you can, tensing them release after that.
Arch your back and tense your shoulders as if you are pushing your chest out, do this as hard as you can, hold it and release.
Doing that should make your muscles less tense and relaxed, it will also help you identify when you are tensing or not.