Skeletal muscle function

Skeletal muscle is the muscle you move when you want to: the one in your arm, in your leg, in your jaw. It is famous for contracting and producing movement, and that is true, but the description falls short. It also stops movement, and you are using that right now: standing or sitting up means resisting gravity all the time with tiny adjustments.

Inside, the machinery is always the same. The command travels down the motor nerve with a chemical messenger called acetylcholine, and inside the fiber calcium is released, which is the start signal. Then the sarcomeres, the units that repeat along the length of the fiber like the links of a chain, shorten: heads of a protein called myosin latch onto filaments of another protein called actin and pull. Many tiny pulls at the same time add up to movement.

And there is a third job almost nobody credits it with: muscle generates heat, and in doing so it helps maintain homeostasis, the body's internal balance. That is why your temperature rises when you exercise, and why, in extreme cold, shivering is the body triggering random contractions to warm itself up.