For plantar flexor muscles, we examined isometric muscle fatigue at two different ankle positions in resistance trained female Sprague Dawley rats. Resistance training consisted of repeated stretch-shorten contractions (5×10 repetitions, 5 d/wk for 6 wks) of maximally active muscles produced by manual ankle rotations between about 140 degrees and 40 degrees. In two groups of rats, muscle fatigue was produced by 40 repeated concentric contractions over two different ranges of ankle motion [40 degrees to 90 degrees (long muscle lengths, n = 5) and 70 degrees to 120 degrees (short muscle lengths, n = 5)] by rotation at 50 os-1 immediately after an isometric preload contraction (duration 1.9 s, rest period 12.5 s). After 30 min of recovery, a single contraction was performed. Isometric forces were measured either at 40 degrees (ISO40 degrees-rats) or 70 degrees (ISO70 degrees-rats). Overall, resistance training increased the isometric force per gram muscle weight by 21%. In ISO70 degrees-rats, declines in force during fatigue were similar for training (60.7 +/- 8.1%, mean +/- SD) and control (67.5 +/- 6.7%). However, in ISO40 degrees-rats, less fatigue was observed after training (59.6 +/- 10.9%) compared to control (78.4 +/- 8.8%, P = 0.02). After recovery for 30 min, the isometric forces expressed as a percentage of initial values were similar in ISO70 degrees-rats for training (79.5 +/- 6.8%) and control (80.3 +/- 4.7%) but in ISO40 degrees-rats they were higher after training (95.4 +/- 6.9%) compared to control (84.7 +/- 6.2%, P = 0.03). Fatigue and recovery from fatigue were improved by resistance training (5d/wk for 6 wks) of rat plantar flexor muscles but only at relatively long muscle lengths.