The precision of maximum strength assessments (1 repetition maximum; 1RM) is important to evaluate the functional capacity and to prescribe and monitor the training load. Several factors can affect the precision of 1RM tests, including the warm-up procedure. General and specific warm-up routines are recommended to enhance performance. The effects of a specific warm-up have already been acknowledged in improving performance. However, the effects of a general warm-up (GWU) are unclear but seem to depend on its ability to increase muscle temperature while avoiding fatigue. Furthermore, temperature elevation is dependent on both the duration and the intensity of the activity, which may eventually affect 1RM performance. The objective of this study was to investigate the effect of different intensities and durations of GWU on 1RM performance. Sixteen strength-trained men were tested for 1RM leg press after 4 GWU conditions after specific warm-up: short duration and low intensity (SDLI; i.e., 5 minutes at 40% V[Combining Dot Above]O2max), long duration and low intensity (LDLI; i.e., 15 minutes at 40% V[Combining Dot Above]O2max), short duration and moderate intensity (SDMI; i.e., 5 minutes at 70% V[Combining Dot Above]O2max), long duration and moderate intensity (LDMI; i.e., 15 minutes at 70% V[Combining Dot Above]O2max), and the control (CTRL) no-GWU condition. Leg press 1RM values were higher (on average 3%) when subjects performed LDLI (367.8 ± 70.1 kg; p = 0.01), compared with the other 4 conditions. After the LDMI condition, 1RM values were lower (on average -4%) than in the other 4 conditions (345.6 ± 70.5 kg; p = 0.01). There were no differences between SDMI, SDLI, and CTRL (359.4 ± 69.2 kg, 359.1 ± 69.3 kg, and 359.4 ± 70.4 kg, respectively; p = 0.99). According to our results, long-duration low-intensity general warm-up seems be appropriately to improve 1RM performance in strength-trained individuals.