Purpose The aim of this study was to compare the concentration of blood lactate [bLa-] and the subjective perception of exertion of trained men in a moderate repetition protocol (MRP) versus a high repetition protocol (HRP) equated for time under tension. Methods A sample of 40 healthy young men (aged, 23.2 ± 4.0 years; height, 177.3 ± 7.0 cm; BMI, 24.3 ± 2.2) performed two sessions of 8 sets of bicep curls with a one-week recovery interval between the trials. In the HRP protocol, 20 repetitions were performed with a cadence of 2 seconds of eccentric and 1 second of concentric, while in the MRP protocol 10 repetitions were performed with 4 seconds of eccentric and 2 seconds of concentric. Cadences were controlled by a metronome. At the beginning and end of each of the sessions, blood lactate was taken at 2, 15, and 30 minutes, and rating of perceived exertion (OMNI-RES) was assessed immediately after completion of each session. Results There were [bLa-] differences between protocols in the MRP 2 min, (5.2 ±1.4); 15 min, (3.2 ±1.2); 30 min, (1.9 ±0.6); p< 0.05, and the HRP 2 min, (6.1 ±1.6); 15 min, (3.7 ±1.1); 30 min, (2.2 ±0.6); p<0.01. OMNI-RES was higher in HRP, (8.8 ±0.7) than in MRP, (7.7 ±0.9). Additionally, a correlation was found between the RPE and [bLa-] values in the HRP protocol (rs = 0.35, p < 0.01). Conclusions Training protocols with high times under tension promote substantial increases in metabolic stress, however, our findings indicate that HRP generates more [bLa-] than MRP. In addition, there were higher RPE values in the HRP protocol compared to MRP in single-joint exercises.