Muscle fibers are long thin cells, made mostly of protein that lie next to each other in a muscle.

When they receive a signal from the brain. Muscle fibers can contract^* completely or not at all. The strength of a muscle contraction depends on how many fibers contract at the same time.

To contract many times, muscle fibers need oxygen. Blood vessels are always found near muscle fibers. The blood flowing^* through these vessels keeps bringing oxygen and removing waste and heat from muscle fibers as they contract.

As soon as muscle fibers contract, they must relax. For a muscle to stay contracted, different sets of fibers must contract one after the other. That is why arms start to shake and get tired after holding something for a long time. The fibers are using oxygen faster than they get it. As they run out of oxygen, they cannot continue to contract. Finally, the whole muscle must relax.

Exercise makes muscles stronger by making muscle fibers larger and by increasing the number of muscle fibers.