Tendons are bands of fibrous connective tissues made of parallel collagen fibers. Collagen allows the tendons to be strong and flexible. This is necessary because tendons are stretched tightly when activated.
Tendons connect muscles to bones and can withstand tremendous amounts of tension that the body encounters. Tendons allow movement of bones when the muscle is contracted, which causes the connecting tendon to pull on the bone. The connection between the bone and tendon is tough to break due to the tendon actually growing into the bone.
Over-strained tendons are a common injury seen in sports. If a tendon is overused repeatedly, a condition known as tendinitis occurs.