Submaximal eccentric resistance training increases serial sarcomere number and improves dynamic muscle performance in old rats

The age-related loss of muscle mass is partly accounted for by the loss of sarcomeres in series. Serial sarcomere number (SSN) influences muscle mechanical function including the force-length and force-velocity-power relationships, and the age-related loss of SSN contributes to declining performance. Resistance training biased to active lengthening (eccentric) contractions increases SSN in young muscle, however, we showed maximal eccentric training in old rats did not alter SSN and further worsened performance. A submaximal eccentric training stimulus may be more conducive to adaptation for aged muscle. Therefore, the purpose of this study was to assess whether submaximal eccentric training can increase SSN and improve mechanical function in old rats. Twelve 32-month-old male F344/BN rats completed 4 weeks of submaximal (60% maximum) isokinetic eccentric plantar-flexion training 3 days/week. Pre- and post-training, we assessed in-vivo maximum isometric torque at a stretched and neutral ankle angle, the passive torque-angle relationship, and the isotonic torque-angular velocity-power relationship. The soleus and MG were harvested for SSN measurements via laser diffraction, with the untrained leg as a control. SSN increased 11% and 8%, and muscle wet weight increased 14% and 13% in the soleus and MG, respectively. Maximum isometric torque and shortening velocity were unaltered, but there was a shift towards longer muscle lengths for the optimal angle of torque production, a 42% reduction of passive torque, and 23% increase in peak isotonic power. Eccentric training at 60% maximum was beneficial for aged muscle, increasing SSN and muscle mass, reducing muscle passive tension, and improving dynamic performance.