The purposes of this study were 1) to determine the extent to which endurance training reduces the functional deficit induced by lengthening contractions in the soleus (Sol) muscle and 2) to determine whether young and old rats training at a comparable relative exercise intensity would demonstrate a similar protective effect from lengthening-contraction-induced injury. Young (3-mo-old) and old (23-mo-old) male Fischer 344 rats were randomly assigned to either a control or exercise training group [young control (YC), old control (OC), young trained (YT), old trained (OT)]. Exercise training consisted of 10 wk of treadmill running (15% grade, 45 min/day, and 5 days/wk) such that by the end of training the young and old rats were exercising at 27 and 15 m/min, respectively. After training, contractile properties of the Sol muscle were measured in vitro at 26°C. The percent decrease in maximal isometric specific force (P_o) was determined after a series of 20 lengthening contractions (20% strain from optimal muscle length, 1 contraction every 5 s). After the lengthening-contraction protocol, Sol muscle P_o was decreased by ~26% (19.6 vs. 14.6 N/cm²) and 28% (14.8 vs. 9.6 N/cm²) in the YC and OC rats, respectively. After exercise training, the reduction in P_o was significantly (P < 0.05) attenuated to a similar degree (~13%) in both YT rats (18.7 vs. 16.2 N/cm²) and OT rats (15.8 vs. 13.7 N/cm²). It is concluded that exercise training attenuates the force deficit after repeated lengthening contractions to a comparable extent in young and old rats training at a similar exercise intensity.