A comprehensive conditioning program incorporates interval work along with endurance (“steady-state”) training.  While the optimal mix for a given athlete depends on numerous factors, interval training is imperative because it enables athletes to work at greater intensity.   To understand the importance of intensity, consider this very brief summary of muscle fiber characteristics:

An individual’s muscle fibers display characteristics along a spectrum from pure slow to pure fast.  A given fiber may be categorized “slow” or “fast” based on its myosin components, but within that slow or fast type, there is quite a bit of variation.  Not all fibers of the same type exhibit identical characteristics.  Spectrum is the key idea.

Slower muscle fibers are preferentially recruited for lower-intensity, steady-state type work.   Faster fibers are preferentially recruited for high intensity, short-duration efforts. 

Training works because it produces physiological adaptations.  An effective conditioning program increases the volume, density and enzyme content of muscle cell mitochondria.  It also increases capillary density.   Collectively, conditioning increases oxygen supply to cells, oxygen extraction by cells, and waste product removal from cells, benefiting multiple energy systems.   However, these adaptations are not universally distributed to every muscle every time you exercise.  The frequency and intensity of muscle cell contractions are correlated with these adaptations.   Interval training recruits a greater portion of muscle fibers, so that conditioning adaptations are more broadly distributed.  


Using Interval Training

Perhaps the most important parameter for designing an interval training program is the work-to-rest ratio, because it directly influences intensity.  For example, the power output during work intervals in :45 work / 1:15 rest will, in general, be higher than power output during :45 work / :15 rest.    During a WOD, an athlete’s “race pace” is their maximum sustainable aerobic power output plus maximum sustainable anaerobic power output.    Therefore, it makes sense to develop both energy systems.   The work-to-rest ratio is the key parameter for balancing energy system contribution to power output. 

Again, the idea of a spectrum is useful.  Classic anaerobic training utilizes a 1:5 work to rest ratio.   An example of this would be :30 work / 2:30 rest.  The long rest periods allow for more complete recovery from strenuous work, clearance of lactic acid and return to equilibrium, permitting another very high intensity work interval.  The point of developing anaerobic power is to increase top-end power output, so full recovery between work intervals is imperative.   Shorter rests preclude recovery, and necessitate a shift in energy production to aerobic power, reducing intensity.  Therefore, for pure anaerobic sessions, we suggest a 1:5 work to rest ratio, and further, keep all working sessions under 45-60 seconds, so that the high power-generating capacity of the glycolytic system does not “time out”. 

For aerobic power, we typically recommend a 1:1 or lower work-to-rest ratio to target aerobic power.   For example, something like :30 work / :30 rest  is a classic aerobic-dominant interval.   The short rest periods do not allow much recovery, so that if an athlete is relying on anaerobic power, they will soon be forced to shift gears to aerobic energy.   We have also seen athletes derive substantial benefit from intervals where the work period is longer than the rest period.  Examples of this include 1:40 work / :20 rest or 3:00 work / 1:00 rest, and as many other combinations as your creativity will allow.    

By keeping the work interval longer than the rest interval, the aerobic system takes priority, mirroring the demands of WODs.  In general, though, intervals should remain short enough to permit athletes to work above their race pace for most of the session.   There is a lot of room for coaches to design training sessions with intervals between the 1:1 classic aerobic and 1:5 classic anaerobic workout.  The best mix of work-rest ratios depends on the athlete and their goals.  The following chart is a reference point for manipulating the work-rest intervals to elicit the desired stimulus.  

Intervals chart.jpg


It’s easy to measure steady state conditioning: just run the clock and count the distance, watts, calories or whichever numerator you’re using.  Measuring intervals is a bit trickier.   Consider two athletes, Dan and Dave, who each complete the following aerobic interval session:

  • 18 rounds, for meters
  • Row :30 / Rest :30

Dan comes out blazing and completes 175m for his first interval.  But he’s exhausted and continues downward to 127m for his last interval.   His average per interval is 153m.   Dave was more consistent; he rowed 150-155m for every interval, but also finished with an average distance of 153.   The chart summarizes the relevant information.  Who did it better, Dan or Dave?   

Dan and Dave.jpg

The two measurements that define an athlete’s performance in an interval training session are (1) average work done per interval and (2) range (difference between highest and lowest interval).  The average work completion should be as high as possible, and the range should be as low as possible.  The idea is not to go out blazing and be exhausted by minute 3 (the Dan approach) but to work at the fastest pace you can sustain (like Dave).  Using the average and the range enable a coach to track athlete progression.   Whether the session is targeting aerobic power, anaerobic power, or a combination of both, the rule is the same: maximize the average, minimize the range.    Therefore, Dan’s coach needs to emphasize consistency and pacing, and probably bias Dan's training toward aerobic capacity, while Dave’s coach should be targeting power output.  Interval work often reveals opportunities for improvements that steady state training and WODs overlook.



We have explained the importance of intensity to a training program, and the role of intervals in facilitating increased intensity and broader adaptations to training.    While a conditioning program should include a mix of intervals and endurance work, the exact mix depends on the individual athlete.  Common variables include goals, age, fitness level and experience.    Use these guidelines, along with some trial and error, to find the right balance for yourself and you.   Got questions? We can help.