Strength is increased by work against high resistance — ‘strength training’ and ‘resistance training’ are synonymous. After warming up against lesser loads, strength athletes during a high-intensity phase of training will perform, in a weight-room session, no more than perhaps ten efforts against loads almost as great as they can possibly manage. Strength trainers characterize loads in terms of the individual's ‘repetition maximum’ (RM). A 1 RM load can be managed just once in a session, a 10 RM load can be tackled 10 times within a period measured in minutes. Working against loads less than 60% of 1 RM is not considered capable of increasing strength at all.
(By contrast, in endurance training, any one stride or stroke or pedal-turn probably involves only 10-15% of the maximum force the limb can produce; however, not 10 but at least 10 000 such actions are performed in the course of an endurance training session.)