Many training programs are flawed in their approach due to a lack of understanding of how repetitions net specific outcomes.
An understanding of the repetition continuum is important when looking to elicit specific qualities or characteristics. This is known as the SAID PRINCIPLE (Specific Adaptations To Imposed Demands).
In the simplest of terms higher repetitions will trigger greater adaptations of stamina while lower repetitions will improve neurological qualities and strength. Reps in the middle will trigger hypertrophy or increases in the size of muscle.
Strength qualities and stamina qualities ,in general, can not be developed at the same time because of the fiber types involved. One can only serve one master at a time !
Strength qualities take much longer to develop. The trainees training age is as important as chronological age when developing the training template. The athlete with a young training will realize rapid adaptation and training will be filled with increases in training load and personal records (PR's) As the training age increases improves slow.
To a degree, strength training will improve stamina performance but training with lighter loads will not improve strength UNLESS the trainee has a lower level of initial strength and a low training age. This is why young athletes do not need to go to momentary muscular failure or fatigue to get strong! They require little complexity in their program design. I have often said that if you can not get a high school athlete stronger you are not very good at your craft.
In contrast, the intermediate trainee will need smarter programming and progression. Doing high reps with 50% of the 1 rep max will make them better at that specific load but nothing for pushing their 1 rep max higher or building strength.
At the risk of making this more complex, volume is an important factorial along with recovery intervals between sets. Prilepin's work with weightlifters revealed optimal results with sets and repetition ranges. Seek out Prilepin’s chart to learn more regarding optimal sets and loading parameters.
Strength / 1-5 repetitions. 5 x 5 , 5 x 3, 5 x 1
Hypertrophy / 6-8 repetitions 3 x 10, 3 x 8, 4 x 6
Stamina /12 or more repetitions. 2 x 12 , 1 x 20 or greater