When doing any form of resistance training there are a huge number of factors that can affect the results you will get.
These factors include the choice of exercise, frequency of training, the intensity of training and training volume. In the short-term, how many repetitions we complete of each given exercise is thought to affect what changes will occur post-exercise.
Traditionally, people have viewed lower repetitions (3-6) as increasing strength, intermediate repetitions (8-12) as increasing size and high repetitions (15+) as helping to ‘tone’. Firstly the ‘tone’ viewpoint is flawed as the only way to improve muscle tone is to increase muscle mass and lose body fat. Performing exercises within the repetition range of 15+ will improve muscular endurance, allowing you to perform movements with that body part for longer.
Many studies, particularly in untrained men, have looked at the effects these repetition ranges have, and found low repetitions to be the best. The intermediate repetition range group increased muscle size as predicted, however, the low repetition group have similar hypertrophic effects but also showed significantly greater strength increases than both other groups. This seems to show in the majority of studies, suggesting that if we are looking to improve either strength, size or both, then lower repetitions (3-6) would appear to be best. However, if you are looking to improve muscular endurance then performing exercises with over 15/20 repetitions is undeniably best.
Two studies have shown an interesting finding that may be useful for those who are unable to train with heavy weight through injury or other reasons. They found that training with only 30% of your 1RM to fatigue was as effective at increasing muscle size as training at 90% or 80% to fatigue. One study looked into why this may be and found that when training at 30% of your 1RM the same level of protein synthesis is produced as when training heavier, BUT only if the athlete goes to complete fatigue. This lower weight group also increased their strength from this training, but not by as much as the groups who used 80% and either performed 1 repetition or 3 repetitions. A study into occlusion training has found similar results. Occlusion training involves limiting the blood flow to the working muscles by tightening a strap etc over the origin of the muscle. A study found that occlusion training with weight as low as 20% of your 1RM can produce the same hypertrophic improvements as all other forms of training. This method should again be considered for injured athletes during rehabilitation, as it is much safer.
In conclusion, it appears there is only 1 way to improve muscular endurance and that is by performing high repetitions during a set. If an athlete is healthy and competent then it would appear that training with low repetitions is optimal as it improves strength significantly more than any other group and increases muscle volume as much as the intermediate group. However, if an athlete is unable or unwilling to use heavy weights, then some strength improvements can be seen in the intermediate repetition range groups whilst producing high levels of muscular hypertrophy. As I have mentioned there are other ways to produce muscular hypertrophy however these should be used sparing, limited to rehabilitation or athletes with extenuating circumstances as they don’t produce the same increases in strength necessary for a healthy well-rounded person.