Abstract :Muscular endurance training increases muscular endurance; but its ability to improve metabolic and cardio respiratory performance remains unclear. Purpose: The aim of the current Investigation was to evaluate the effect of supplementing an aerobic exercise training program with a muscular endurance training program on various cardio respiratory and metabolic measurements. Methods: Fourteen aerobically active men performed an incremental exercise test to determine the onset of blood lactate accumulation (OBLA), gas exchange threshold (GET), and maximal oxygen uptake (VO2max). Maximal strength was measured using 1-repetition max (1-RM) for leg press (LP), leg curl (LC), and leg extension (LE). Eight participants supplemented their aerobic activity (EX group) with 8 weeks of muscular endurance training, while six continued their regular aerobic activity (CON group). Results: No significant group differences were observed for all pre-training variables. Following eight weeks of training no significant differences in body mass, GET, and VO2max were observed for either group. However, the EX group showed a significant improvement for VO2 at OBLA and the percent of VO2max at which OBLA occurs. LP, LC, and LE 1-RM assessments for the EX group showed a significant improvement. Conclusion: Muscular endurance training did not improve GET and VO2max, but significantly increased VO2 at OBLA and percent of VO2max at which OBLA occurs, which suggests that supplementing aerobic activity with muscular endurance training can improve sub maximal endurance performance via improvements in OBLA.