3. Bad sitting or kneeling positions
Many people think that we are only using our muscles when we are performing a task of strength or physical effort, but as a matter of fact, we are using them all the time, even when we are still. Our muscles need to hold a state of contraction or relaxation in order to keep our joints straight or bent in a particular position.
Also, there is something called the muscular tone, which is a level of contraction that all muscles have so we can cancel the effect of gravity on our body; without muscular tone, we would all be lying down on the ground like puppets without a puppeteer. For that reason, holding a position takes effort from our muscles; just as any other effort, what it takes to hold a position can be excessive for our muscular fibers, especially if the position is held for a long time.