Friday, August 26, 2011

Do Intervals Require More Effort?

At our cross country practices I’ve noticed that we are running a lot more interval workouts than we did last year. An interval session includes (1) a warm up period at easy pace, (2) a series of short, fast runs with rests, and (3) a cool down period at easy pace.

The series of short, fast runs with rests, or intervals, can vary widely in repetition and distance (typically inversely proportional), but commonly are run at fast pace. Typical examples of interval sessions (in meters) are: 20x100s, 16x200s, 10x400s, 6x800s or 3x1600s. Paces of course depend on athletic ability, but typically easy pace at warm up/cool down is approximately 10 min/mile and interval paces range roughly from 5 min/mile to 7 min/mile, the shorter the interval distance the faster the pace. The rest periods in between intervals can range from complete rest to walking to very easy jog, about 12-16 min/mile.

In contrast to interval workouts, our distance runs are at typically easy pace throughout, say 9-10 min/mile. Both types of workouts last about the same amount of time, approximately 1 hour. Our coach believes as do I that once a base foundation of fitness has been established (through consistent distance runs), then workouts should focus more on interval training to adapt runners to faster paces.

Whatever the effect of interval training has on performance, they definitely feel harder to do than a slow, steady state distance run. Let’s see if the math supports this feeling, shall we. Energy rates (cal/kg/hr) are derived by Ainsworth et al (1). Effort will be based on my body weight, 79 Kg (175 lb). For those that care, I mean kilocalorie when I say cal. If this loses you, forget about it because it’s a relative comparison anyway.

Let’s compare my effort for a 6 mile distance run @ 9 min/mile pace (11 cal/kg/hr) versus the following 6 mile interval session:

• 2 mile warm up @ 10 min/mile (10 cal/kg/hr)

• 12x Quarter Mile Loop
Sprint 5.5 min/mile (18 cal/kg/hr) for ¾ loop
Walk/Jog 16 min/mile (6 cal/kg/hr) for ¼ loop

• 1 mile cool down @ 10 min/mile (10 cal/kg/hr)

Math for Distance Run, 6 Miles Total

Time = 6 miles x 9 min/mile = 54 minutes

Energy = 11 cal/Kg/hr x 79 Kg x 54 min x 1 hr/60 min = 782 cal

Math for Interval Run, 6 Miles Total

Warm Up Time = 2 miles x 10 min/mile = 20 minutes

Warm Up Energy = 10 cal/Kg/hr x 79 Kg x 20 min x 1 hr/60 min = 263.3 cal

Sprint Time = 12 x 0.1875 miles x 5.5 min/mile = 12.375 minutes

Sprint Energy = 18 cal/Kg/hr x 79 Kg x 12.375 min x 1 hr/60 min = 293.29 cal

Walk/Jog Time = 12 x 0.0625 miles x 16 min/mile = 12 minutes

Walk/Jog Energy = 6 cal/Kg/hr x 79 Kg x 12 min x 1 hr/60 min = 94.8 cal

Cool Down Time = 1 mile x 10 min/mile = 10 minutes

Cool Down Energy = 10 cal/Kg/hr x 79 Kg x 10 min x 1 hr/60 min = 131.67 cal

Total Time = 20 + 12.375 + 12 + 10 = 54.375 min

Total Energy = 263.3 + 293.29 + 94.8 + 131.67 = 783.06 cal


Comparing these two 6 mile runs, they both require the same amount of time within 1 minute and they both require the same amount of energy within 1 cal. From a time, distance and energy perspective, the distance run is the same as the interval run. But I agree with coach, interval running makes runners faster!

(1) Ainsworth BE, Haskell WL, Leon AS, Jacobs DR Jr, Montoye HJ, Sallis JF, Paffenbarger RS Jr. Compendium of Physical Activities: Classification of Energy Costs of Human Physical Activities. Medicine and Science in Sports and Exercise. 1993: 25: 71-80.


  1. I agree with you and the coach. The boost in overall speed from intervals doesn't seem to come from energy expenditure, but rather from getting acclimated to higher effort levels.

  2. Even though the calculations seem straightforward, I'm still not sure about the energy rates. I need to check a few other sources. What bothers me is that my cited source says the energy and speed relationship is linear. However, I recall from Physics that energy is the quadratic of speed...E = 1/2 x mass x square of velocity. In other words, it seems to me that intervals (at faster speeds) should require quadratic (nonlinear) energy expenditure. It sure does feel that way.