The leading cause of muscle damage related to exercise stems from mechanical factors. Muscle tension and active strain on lengthened fibers during eccentric contractions produce direct trauma to the muscle tissue. This differs greatly from the proposed oxidative stress placed on muscle tissue with pro-longed, intense endurance exercise. As it is known, dynamic muscle function in acyclical, impact sport occurs as a sequence of active eccentric muscle actions followed by active concentric muscle actions known as the stretch-shortening cycle (SSC). Since eccentric contractions contribute to the SSC, it is not a surprising phenomenon that muscle damage occurs during prolonged or intense exercise such as distance running, plyometrics and resistance training. These activities are commonplace in training programs for most athletes.

The hypothesis that attempts to explain the physiology behind eccentric-based mechanical muscle damage is that fewer motor units are recruited to handle the same load and force per cross-sectional area of muscle. This demonstrates that eccentric work requires fewer muscle fibers to do the same amount of concentric work and that less energy is required to perform the eccentric work. Tension per unit of active muscle mass is also greater, which results in more damage to the muscle. At the cellular level, Z-line streaming, which is explained as ?disorganization of the area that joins the repeating contractile elements of the myofibrils together? and myofibrillar disruption are direct manifestations that muscle damage has occurred. Furthermore, calcium homeostasis and excitation-contraction coupling are impaired, and examination of eccentrically damaged muscle shows damage to the sarcolemma, T-tubules, myofibrils and the cyoskeleton. All of these structural changes to the muscle fiber are actually present as soon as five to 15 minutes post exercise.

The Impact of Muscle Damage on Performance

It is common and normal to experience pain and muscle stiffness with a new training program. This phenomenon, as we all know, is known as delayed onset of muscle soreness (DOMS) and is associated with muscle fiber injury. DOMS is most prevalent at the beginning of the sporting season or a new training program when athletes are returning to training following a period of reduced or varied activity.