Submaximal eccentric training during immobilization does not prevent serial sarcomere loss or impairments in mechanical function in old or young rats

The age-related loss of muscle mass is partly driven by a reduction in serial sarcomere number (SSN), and further SSN loss occurs during immobilization. SSN is associated with optimal force and power production and muscle passive tension, thus immobilization-induced SSN loss is especially a concern for older individuals who are often subjected to forced muscle disuse with illness and injury. We previously showed that submaximal eccentric resistance training increased SSN and improved muscle function in old rats. The present study investigated whether this training could prevent the losses of SSN and function when performed intermittently during immobilization. 10 old (32 months) and 10 young (8 months) rats underwent unilateral casting of the plantar flexors in a shortened position for 2 weeks. Thrice weekly, casts were removed for isokinetic eccentric resistance training. Pre- and post-training we assessed in-vivo maximum isometric torque at ankle angles corresponding to stretched and neutral muscle lengths, the passive torque-angle relationship, and isotonic power. The soleus and medial gastrocnemius were harvested for SSN measurements, with the untrained leg as a control. In old and young rats, muscles of the casted leg had smaller muscle wet weights (20-40%), physiological cross-sectional area (16-20%), and SSN (7-29%) than the control leg. Furthermore, maximum isometric torque (37-46%) and isotonic power (≈70%) decreased, and passive torque increased (+≈400%) from pre- to post-training for both age groups. Thus, irrespective of age, submaximal eccentric resistance training 3 days/week was ineffective for preventing the losses of muscle contractile tissue and mechanical function during casting.