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. In general though tendons take twice as long as do muscles to become fit.
Tendon damage is common in sports.
Swelling is the body's first response to tendon injury. Swelling can cause serious, more permanent damage and should be immediately treated with ice or a doctor visit. Despite decreased pain as time continues, it may be as much as two weeks before the body begins to reinforce the tendon with new collagen. Again, the best prevention for tendon problems is exercise. The more work you are able to slowly introduce to the tendons, the stronger they will become.