https://phys.org/news/2018-07-image-tricorder.html

By default, our muscles are always slightly contracted. This is how we maintain posture and respond quickly to sudden movements. Our muscles reflexively tense to maintain balance and reduce damage when unexpectedly pulled or stretched. Muscle tension and stiffness are also good indicators of flexibility, strength, and general health.

Of course, our muscles evolved to do this on Earth. In weightlessness, muscles lose functionality and mass. This phenomenon is well studied, and astronauts exercise for approximately two hours a day to prevent muscle wasting away.