Originally Posted by Donkeypuncher
Why do you need to vary reps?
Depending on what your aims are various rep schemes promote different muscle responses.
Strength ranges: 3 - 5 reps
Hypertrophy ranges: 8 - 12 reps
Muscle endurance ranges: 30 - 40 reps
Most trainees always seem to operate in the 8 - 12 in their days.