Unaccustomed eccentric exercise, in which a muscle is lengthened while generating tension, is well known to cause injury and pain. A rapid training effect has been demonstrated in a number of eccentric exercises. The mechanism for both the damage and the training has been unknown. Morgan proposed that the damage is caused by sarcomere length instabilities during operation on the descending limb of the sarcomere length-tension curve and that the training effect is an increase in the number of sarcomeres connected in series in a muscle fiber, thus avoiding the descending limb (Biophys. J. 57: 209-221, 1990). We tested this proposal by exercising rats on a treadmill set at either an incline or a decline of 16 degrees, an exercise that has previously been shown to cause damage in untrained rats and a training effect. The vastus intermedius muscles were fixed and were digested in acid, and the fiber and sarcomere lengths of representative fibers were measured. From these measurements, the mean number of sarcomeres per fiber was found for the different training regimes. A clear and repeatable difference was found, supporting Morgan's prediction of more sarcomeres after decline running, although with some differences in response that depended on the age of the rats.

Rats were trained for 5 days by running on either an inclined or declined treadmill. Three days later, the rats were anesthetized, and angle-torque curves were plotted for the vastus intermedius muscles. The maximum active torque was generated at significantly greater muscle lengths for muscles from decline-trained rats compared with incline-trained rats. Sixteen muscles were then fixed and acid digested, and fiber lengths and sarcomere lengths were measured. The estimated average number of sarcomeres in series was greater in muscle fibers from decline-trained animals. Fourteen other muscles underwent a test series of lengthening contractions, all from the same knee angle. Torque fell less and the optimum angle shifted less for muscles from decline-trained animals, showing that the decline-trained muscles were more resistant to changes in mechanical parameters that indicate damage. These results support but do not prove the proposal that the lesser damage from a series of eccentric contractions seen in muscles trained by prior eccentric contractions is due to a greater number of sarcomeres in series.

No abstract

No abstract

No abstract