Rep ranges are an important variable in the effect of each set. When combined with tempo, they determine the time under tension and the load you will be able to use. Reps, tempo, and load all combine to determine the quality that each rep/set is training, and the stimulus to that muscle tissue.
Reps and tempo when combined properly, provide a harmonious stimulus of contraction and tension in the muscle by being able to maximize the qualities of a given exercise. A good program will have pretty firm rep ranges for each exercise. This means a prescribed rep range of no more than 3 reps, for instance 8-10, which includes 8, 9, and 10. The lower the reps, the smaller the range should be.
An occasional range of say 5 on the last exercise of a workout or tri-set when the reps are 12 or greater is an acceptable practice. However, if your workout is full of 10-15’s or 5-10’s etc then it’s time to get yourself on a better program. The stimulus of a set of 5 vs 10 is so drastically different that this practice is absolutely unacceptable in a program.
A program should be designed for a goal. It is not a choose your own destiny session. This requires precision in all training variables, reps, tempo, rest, sets etc. This is why we stress that our clients adhere to their given rep ranges regardless of how challenging the set is.
Remember that not all sets are meant to take you to failure. Rest intervals and volume over a workout are important things to consider. Often times ambitious trainees will sacrifice the goal if the workout by prematurely exhausting themselves in the workout by extending sets with load or TUT that was not prescribed. This can completely diminish the benefits of the rest of the workout, and alter the training stimulus outside the goal of the program. If you do happen to horribly underestimate the weight to be used, scrap the set and do another with a more appropriate weight. Don’t perform a set that is outside the parameters of the workout stimulus, as this may interfere with the training effect.
Reps are also one of the best progression tools throughout a program. If you do not program them precisely it makes it very difficult to track progress and make the appropriate progressions from week to week in the program.