Does the number of repetitions matter when lifting weights?

Historically it was postulated that the number of repetitions performed at a given magnitude of load will result in specific adaptations. The theory states that heavy load training (1-5 repetitions) optimises increases in maximal strength, moderate load training (8-12 repetitions) optimises increases in muscle hypertrophy, and low-load training (15+ repetitions) optimises increases in local muscular endurance. Evidence indicates that alterations in training load can influence the acute metabolic, hormonal, neural, and cardiovascular responses to training. Current research fails to support some of its underlying presumptions about physiological outcome, whereby muscular adaptations can be obtained, and in some cases optimised, across a wide spectrum of loading zones.

Strength is broadly defined as the ability to produce maximum force against an external resistance. Current evidence supports the existence of a "strength zone" for increasing 1 repetition maximum, and the dose-response relationship between load and strength gains appear valid.

Muscle hypertrophy refers to the growth of muscle tissue, which can manifest in a variety of ultrastructural adaptations. This is where previous thinking becomes clouded where it has always been advised to perform 3 sets of 9-11 repetition maximum. Current evidence suggests there should be a minimum threshold of 30% of 1 repetition maximum, and this should be performed until failure. Training with low-loads tends to produce discomfort and a higher rating of perceived exhaustion, therefore using moderate loads is likely to be more enjoyable. Training with high loads requires more sets to achieve comparable hypertrophy to moderate loads.

Local muscular endurance is defined at the ability to resist muscular fatigue when using a submaximal resistance, and always been thought to improve when training upward of 15 repetitions. Current evidence remains uncertain as there is a plethora of research with conflicting outcomes. Further research is therefore necessary to shine light on optimal local muscle endurance training.

Schoenfeld, B.J. et al. (2021). Loading recommendations for muscle strength, hypertrophy, and local endurance: A re-examination of the repetition continuum. Sports (Basel). 9 (2): 32.