Impact of endurance concentric contraction training on acute force deficit following in vitro lengthening contractions

The purposes of this study were to (1) determine whether endurance training employing solely concentric contractions would reduce force deficit following lengthening contractions, and (2) to determine if aged skeletal muscle would respond similarly from training, compared with young animals. Young (3-month) and old (23-month) male Fischer 344 rats were randomly assigned to either a control or an exercise training group. Exercise training consisted of 10 weeks of treadmill running (15% grade, 45 min/day, and 5 days/week) such that by the end of training the young and old rats were exercising at 27 and 15 m/min, respectively (∼70% V̇O_2max). After training, the percent decrease in maximal isometric force (P_o) of the diaphragm muscle was measured in vitro at 26°C immediately after a series of 20 lengthening contractions (20% strain from optimal muscle length, one contraction every 5 s), and at 5 and 10 min post-injury. At 10 min post-injury, P_o averaged ∼66% of initial P_o in the exercise group (young and old combined) and ∼61% of initial P_o in the control groups (young and old combined) (P < 0.05). This difference represents an attenuation of force deficit by ∼13% in the trained animals, compared with controls. These findings indicate that 10 weeks of endurance training employing solely concentric contractions is beneficial in minimizing acute force deficit after a bout of lengthening contractions in both young and old rats.