When you exercise, bodily movement vigorous enough to produce muscle soreness is causing structural damage to the contracting muscle fibers and their surrounding membranes. This allows for increases in muscle enzyme leakage into the blood, which act as easy markers for detecting tissue damage directly related to trauma. Inflammation usually follows quickly and surrounds the traumatized area as the body tries to protect the injury from becoming more intense by making movement painful and difficult. It helps to know just what is occurring when you damage muscle versus tendonous tissue, in order to understand that the recovery process should never be short-changed, in particular when tendons are involved.
What Exercise Does to Damage Muscle
Some exercises and movements are more damaging to intact muscle tissue than others. Activities that cause a forceful lengthening of the fibers (eccentric movement) will usually produce more concerted damage than those only requiring contraction since that is what muscle tissue was fabricated to do: contract forcefully.
Another way of looking at this is: Abnormally lengthened fiber undergoes increased tension because the elongated fiber must endure an increased load-to-fiber ratio; that is, there is not enough tissue acting against a resistance. Conversely, a contracted muscle has an increased fiber-to-load ratio and can more easily handle the load because much more tissue is available to move a resistance.
A popular activity utilizing mostly concentric movement is cycling. Compare this type of movement to activities that produce extraordinary lengthening of muscle tissue--running or hiking downhill, descending stairs, lowering weights back to a starting position--and it's easy to see why in eccentric movement micro-tears occur. This then begins a cascade of events starting with bleeding at the site. Bleeding adds leaked fluid volume to the already crowded area of swelled fiber tissue. Increased swelling produces increased pressure against nerves in the immediate area, signaling discomfort, pain, weakness, and reduced range of motion. An injured muscle also fatigues more readily than an intact, fully-functioning muscle.
What Exercise Does to Damage Tendonous Tissue
Tendons can become irritated and inflamed, leading to fraying of the fibrous material if they are caused to rub against hardened surfaces in a crowded joint capsule, for example, in the shoulder of a swimmer or baseball pitcher. An inflamed tendon then swells, taking up more room than nature intended in areas not designed to accommodate lots of tissue. Tendons, all of which attach muscle to bone, can also become overly-stressed and even torn from insertion into the muscle if too great a resistive load is placed on the muscle fibers. And finally, tendons can be damaged simply by applying repeated pressure against them, as with someone constantly leaning on their knees, causing the main connective tissue running over the kneecap, the patellar tendon, to become inflamed.
This condition, from the constant pressure of excessive force against the knees, can develop in high jumpers, participants in basketball and volleyball, weightlifters who excessively and incorrectly stress their knees with deep knee bends and leg lifts at the wrong angle of attack, and anyone else having to endure the force of body weight transmitted through the knees against gravity and immovable ground.
Tendonous tears, as with muscle tears, come in various degrees of severity. It is usually an easier road to healing if a muscle sustains a minor to moderate tear than a tendon. Muscle has more tissue to help it sustain a resistive load. Appropriate rest and physical rehabilitation can help the damaged tissue rejoin together, though scar tissue and calcium deposits can ensue. However, a tear in a stringy tendon, even one somewhat enhanced due to the adaptive processes in training, could much more easily produce a complete or near-complete separation requiring surgical repair.