Acute effects of active and passive warm-up, proprioceptive neuromuscular facilitation (PNF), and ratings of perceived exertion (RPE) were compared during hip-joint range of motion (ROM). Two active warm-up treatments included (a) achieving a respiratory exchange ratio (RER) of 1.00 and (b) achieving 60% of heart rate reserve (HRR). Hydrocollator pads (HP) served as the passive warm-up treatment. These treatments and a control were randomly assigned to increase hamstring muscle temperature of the dominant leg. Warm-up treatments were administered to 12 men (mean 25.3 years) with a minimum of 24 hours interspersed between each treatment. A timed PNF (slow-reversal-hold) technique was conducted after each warm-up treatment. Tukey tests (p < 0.05) showed ROM for RER (107.4°) was greater than all other treatments. ROM for HRR (102.8°) and HP (103.4°) did not differ from each other but were greater than the control (98.8°). Ratings of perceived exertion were lowest for RER (4.0) and highest for control (8.5). Ratings of perceived exertion for HRR (6.0) and HP (6.5) were similar. In conclusion, an active warm-up before PNF stretching appears to be the most effective treatment to increase hip ROM. Results of RPE reinforce that active warm-up reduces the resistance to stretch. In a field setting, it is estimated that a warm-up of 70% of HRR would duplicate the muscle readiness equivalent to an RER of 1.00 before PNF stretching.