Training intensity guidelines for strength training are mostly expressed in a percentage of the 1 Repetition Maximum. Often abbreviated to 1RM, the 1 repetition maximum is the highest amount of weight or resistance with which someone can make one repetition with proper form in a particular exercise. When you know 1RM for a certain exercise, it is possible to estimate the necessary amount of weight to reach a prescribed number of repetitions or intensity.

Example

For example, when someone can perform one repetition with 100kg on the leg press and this person wants to train at 80% of the 1 repetition maximum at the right number of repetitions, this person, according to the table below, should be able to perform 7-9 repetitions at 100 * 80% = 80kg.

When you have determined the 1 repetition maximum, you should not be able to perform more repetitions with the calculated weight. Muscle failure will occur around this number of repetitions.

Do note however, that determining the 1RM, this is test of maximum muscle strength, not muscular endurance. The 1 RM is not as suitable to deteremine the resistance with which to train at 20-30 or more repetitions. Being able to bench press 100kg one time does not mean that you can press 50kg 30 times or more. Conversely, being able to bench press 50kg for 30 repetitions does not mean you are able to press 100kg once.