1 Repetition Maximum

Often training intensity guidelines for strength training are expressed in percentage of the 1 Repetition Maximum or 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 the 1RM is known for a certain exercise, it is possible to estimate the resistance or weight necessary for the prescribed number of repetitions.
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 the 1 repetition maximum has been determined correctly, it should not be possible to perform more repetitions with the calculated resistance and muscle failure will occur around this number of repetitions.1RM Table