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.