The correct answer is immune system.
Muscles stiffen up as a result of stress on the body. The body's response to stress is to tighten its muscles, which serves as a form of protection against pain and harm.
When there is a rapid increase in stress, the muscles stiffen up all at once and then relax once the stress has subsided. The body's muscles are more or less always on alert as a result of chronic stress. Long-term muscle tension has the potential to cause various bodily reactions and perhaps to worsen stress-related diseases.
For instance, persistent muscle tension in the shoulders, neck, and head regions is linked to both tension-type headaches and migraine headaches.
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