In weight training, the number of repetitions, sets, and intensity determines your training outcome. Depending on your fitness goals, high reps and lows reps play major roles on both your performance and appearance.
Repetitions vs. Intensity
The number of repetitions performed is the reciprocate of the amount of intensity. The higher the intensity, the fewer the repetitions, and vice versa. In training, trying to perform high repetitions at high intensities can be very dangerous.
High repetitions (15 to 20 reps) should be used for endurance training such as marathons and cross-country skiing. This is done at lower intensity to develop muscular endurance and enhance work capacity. High reps should also be used for beginning exercisers to get them familiar with the movement patterns.
Explosive movements, such as throwing, jumping and power-lifting, require optimal range of motion and stability of the joints and muscles to prevent injuries. Such high-energy movements require less repetition in conditioning, which has the range of one to four reps.
Most people who train for power and strength do too many repetitions (eight to 15 reps) which are not required for their goals. This concept comes from many gym cookie-cutter workouts which adopted the number of reps as their gold standard, misguiding many people, including athletes who want to develop power and strength.
If you are training for endurance, your rest time should be between 15 to 30 seconds. If you are training for power and strength, your rest time should be between 30 seconds to three minutes, depending on the intensity of your workout.