Submaximal Eccentric Training During Immobilization Does Not Prevent Serial Sarcomere Loss or Impairments in Mechanical Function in Old or Young Rats

Abstract

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. Serial sarcomere number 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. Ten 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.

Keywords: Aging; Casting; Force–length relationship; Passive force; Power; Sarcomerogenesis.

Figure 1.

Experimental setup and representative traces for mechanical testing data from an old rat. A. Experimental setup for mechanical testing and eccentric training. The rat’s left foot was secured to a foot pedal and electrically stimulated to evoke plantar flexion contractions, with the knee fixed at 90°. The ankle had a 70° to 110° range of motion. B. Unilateral cast placing the ankle in full plantar flexion. C. Raw torque traces of a maximum isometric contraction and the corresponding 60% maximum isometric contraction. The stimulation current from the 60% maximum isometric contraction was used for the remainder of that training session during the eccentric contractions. D. Raw torque trace of a maximum isometric contraction at an ankle angle of 90°. E. Raw torque trace of a maximum isometric contraction at an ankle angle of 70°. F. Raw torque and ankle angle traces for construction of the passive torque–angle relationship. Passive torque was recorded immediately prior to each length change. G–H. Raw torque, ankle angle, angular velocity, and power traces for isotonic contractions from 70° to 110° with load clamps set to 30% and 40% of the maximum isometric torque. Peak power was recorded as torque multiplied by the peak angular velocity.

Figure 2.

Differences in muscle wet weight and physiological cross-sectional area (PCSA) of the soleus (A–B) and medial gastrocnemius (MG; C–D) between the control leg versus the leg that was casted with intermittent submaximal eccentric training, and between old versus young rats. A, B, and D: ^#Effect of cast with data from young and old rats combined. ^†Effect of age with data from control and casted combined (p < .05). C. ^*Difference between control and casted (p < .05). ^†Difference between young and old rats (p < .05).

Figure 3.

Differences in muscle fascicle length (FL) and serial sarcomere number (SSN) of the soleus (A, C) and medial gastrocnemius (MG; B, D) between the control leg versus the leg that was casted with intermittent submaximal eccentric training, and between old versus young rats. A–B: ^#Effect of cast with data from young and old rats combined. ^†Effect of age with data from control and casted combined (p < .05). C–D: ^*Difference between control and casted (p < .05). ^†Difference between young and old rats (p < .05).

Figure 4.

A. Differences in maximum isometric torque as a function of joint angle from pre- to postcast and between young and old rats. *Difference from pre- to postcast (p < .05). ^†Difference between young and old rats (p < .05). ^#Difference between ankle angles of 90° and 70° (p < .05). Note that for young rats, 70° (stretched muscle length) was optimal for torque production pretraining but 90° (shorter muscle length) was optimal post-training, and for old rats 90° was optimal both pre- and post-training. B. Differences in maximum isometric torque at 90° normalized to the combined physiological cross-sectional area of the soleus and medial gastrocnemius from pre- to postcast and between young and old rats. ^#Effect of cast with data from young and old rats combined (p < .05). ^†Effect of age with data from control and casted combined (p < .05). C. Differences in peak isotonic power from pre- to postcast and between young and old rats. ^*Difference from pre- to postcast (p < .05). ^†Difference between young and old rats (p < .05). D. Differences in peak power normalized to the combined physiological cross-sectional area of the soleus and medial gastrocnemius from pre- to postcast and between young and old rats. ^*Difference from pre- to postcast (p < .05). †Difference between young and old rats (p < .05). E. Differences in passive torque as a function of joint angle from pre- to postcast and between young and old rats. ^*Difference between pre- and postcast (p < .05) with data from young and old rats pooled, as there was no interaction with age.

Figure 5.

Changes in maximum isometric torque at 90° throughout the 2-week casting with intermittent training period. Training began 2 days after application of casts and occurred 3 days per week (Monday, Wednesday, Friday), amounting to 6 training sessions in total. Casts were removed and postcast testing was completed 2 days following the final training session. Different letters denote a significant difference between time points (p < .05).