Last time you did an interval workout, did you have a specific number of reps decided before the workout? Nearly all coaches prescribe and nearly all runners perform a specific number of reps for interval workouts, with that number decided before the workout begins. However, the number of reps is not all that important, and is usually an arbitrary decision. There’s no magic in doing five or six or ten reps.
Especially in the context of group or team training, it is not optimal for everyone to run the same number of reps. What is important is causing fatigue during the workout, because fatigue is what your body responds and adapts to. Some runners may fatigue after four reps, some after eight, and some after eleven.
Do as many reps as it takes to cause a sufficient amount of fatigue on that day, until you feel that you couldn’t do another rep without catastrophic physiological events occurring (in less scientific terms, stop the workout before you throw up or collapse from heat exhaustion). Workouts should not be done to failure, until you’re throwing up on the side of the track or the road, or until you collapse. But they should cause enough fatigue, enough stress, to which to adapt. You should always walk away from a workout feeling like you’re in control of the workout rather than the workout controlling you. It’s the coach’s (or self-coached runner’s) job to assess when enough reps have been completed to reap the reward of the workout.
Another reason to train with unlimited reps is to avoid limiting yourself. If you focus on one rep at a time without any preconceived idea as to how many reps you’ll do, you’re forced to stay in the moment and focus only on the rep you’re running. When you do that, you may end up completing the workout running more reps than what you thought you could. If you decide before the workout that you’ll do eight reps, guess what happens when you get to rep seven or eight—you feel tired, because your brain thinks you’re close to the end of the workout.
Anticipation of an end point affects your feelings of fatigue, your associative thought processes, and even your neuromuscular efficiency.[i] Deciding beforehand (or being told by a coach) how long the workout is going to be also affects your pacing, because optimal pacing strategy is strongly influenced by the duration of exercise.[ii] If you leave the workout open-ended (or if a coach does not tell you how many reps you’re going to do), you’ll be surprised at what you can accomplish. Reset your limits by not placing limits on your workouts… or on yourself.
[i] Baden, D.A., McLean, T.L., Tucker, R., Noakes, T.D., and St. Clair Gibson, A. Effect of anticipation during unknown or unexpected exercise duration on rating of perceived exertion, affect, and physiological function. British Journal of Sports Medicine, 39:742-746, 2005.
[ii] Tucker, R. and Noakes, T.D. The physiological regulation of pacing strategy during exercise: a critical review. British Journal of Sports Medicine, 43(6):e1, 2009.