Muscle fibers need energy to contract. Unfortunately, when energy is supplied from amino sources it is usually due to the unavailability of other nutrients and results in muscle breakdown and size loss. This group of aminos is called Branch Chain Aminos and include the main energy providing amino, Leucine. You will learn more about protein and aminos later in the sections that deal with Nutrition. Related to the muscles are tendons and ligaments.
Tendons are groupings of a type of connective tissue called collagen.
The purpose of a tendon is to connect the muscle to the bone. Some tendons such as the deep flexor tendon of the leg also contain small percentages of muscle cells. In most well trained athletes, tendons can stretch as much as 8-10 percent beyond their normal size without injury. Once the tendon stretches too far, that is beyond 8-10 percent, some of the cross link fibers begin to break and the tendon can fail. Tendons, like muscles, become stronger with increased flexion or exercise.