What morphological changes occur in skeletal muscle following eccentric contractions? Dr. Jan Fridén, a Swedish hand surgeon working in Dr. Björn Ekblom’s laboratory of exercise physiology quantified the extent and type of muscle injury that occurred in humans following "model" eccentric contractions. Subjects were asked to pedal against a motor-driven ergometer that was moving in the opposite direction to their applied force (imagine pedaling a bicycle that was pedaling against you, Figure 1). Immediately and three and six days after the exercise bout, small biopsies were taken from their vastus lateralis muscles. The most consistent change observed in the experimental muscle biopsies was the disorganization of the myofibrillar material, especially at the Z-disk (Figure 2).

Figure 1 (left): Experimental method for inducing eccentric muscle contractions in human quadriceps femoris muscles. The individual pushes against the pedal which is being driven in the reverse direction by a torque motor. Figure 2 (right): (A) Electron micrograph demonstrating normal muscle vs. (B) muscle that has undergone eccentric injury. Note the loss of Z-disk alignment.

The nature of the disruption was relatively focal, often extending only a few sarcomeres. This myofibrillar disruption was accompanied by loss of proteins associated with the cytoskeleton such as vimentin, laminin, and desmin (Figure 2). Fridén et al. demonstrated that the ultrastructural disruption was only observed after eccentric contraction and that the magnitude of the disruption was greatly attenuated if the subjects had been previously trained by performing the same eccentric exercise.

Figure 3: Desmin immunohistochemical staining of a normal muscle (A) and a muscle that has undergone eccentric injury (B). While the normal muscle exhibits even staining in all cells, the eccentrically injured muscle has cells that demonstrate desmin loss.

Many questions regarding eccentric contraction-induced exercise remain. What are the cellular signals that initiate the damage process? What can be done to prevent the muscle damage? What can be done to facilitate recovery following muscle damage? How often should repeat exercise bouts be experienced in order to maximally strengthen the muscle but so as not to cause excessive damage?