Interval Training Decoded
Rest duration between sets; the special case of the incomplete recovery effect.
1/Intermittent training with complete recovery (eg: fractional training or repetition training). Since recovery is complete between reps, the impact of each single rep is as if that rep was done on its own. If we have 3 sets (s1, s2, s3), then the cumulative effect of these sets as a single workout will be linear due to the complete recovery. The total impact of the workout will be (s1+s2+s3). Of course this theory is not as simple as it sounds. Despite taking every effort to make each recovery complete, there has to be an impact on the next repetition due to the effect of the previous rep. Otherwise the length of the recovery interval would have to be exceptional and the length of the whole session could take an infinite amount of time. Too, if recovery was genuinely complete, then the runner could run an infinite amount of reps without tiring, and we all know that is not what happens. The special case of repetition training done flat-out for a number of units, requires a long recovery, but we know it is not possible to have a genuinely complete recovery.
2/Intermittent training with incomplete recovery (eg: interval training). Due to the impact of the preceding rep(s) and the incomplete recovery, the impact of each succeeding rep is higher than in a straight linear formation. The effort of each set will be compound due to the aggregated effect of the previous ones. If we have 3 sets (s1, s2, s3), then the total effect will be higher than a simple linear addition of s1+s2+s3. Due to the incomplete recovery, the impact is not linear but logarithmic.
The impact of each successive set will include the additional burden of that set having been completed subsequent to an earlier set (or sets). So the impact of s2 will include a percentage carry-over burden from s1. The rep s2 is no longer simply a repeat of s1. In other words, s1+s2 is really s1+(s2+percent of s1). Since s3 is also not done in isolation, but is performed after s1+s2, then the effect on the runner of s3 is not as it would be if s3 were performed as a standalone, but in the form of the cumulative effect when s3 is performed after the preceding sets. In such a situation, s3 really means (s3+percent of s1+percent of s2).
The main goal of interval training theory is the use of an incomplete recovery so that each set places a higher and higher cumulative onus/stimulus on the runner. This cannot be achieved in a system that uses complete recovery.
In intermittent training with complete recovery (as in fractional or repetition workouts), the only requirement is that the recovery is long enough to allow the next rep to be run at the required intensity (ie: race pace, or vVO2 pace, or whatever).
In direct contrast, in intermittent training with incomplete recovery (eg: interval training), there is the compounding effect of multiple sets that will place greater onus/stimulus on the runner than a linear aggregation.
Each set will increase the effect/stress of the subsequent one precisely due to incomplete recovery.
The decision to determine what precisely is an incomplete recovery duration is taken from the workout design and must;
a/ be short enough to promote an incomplete recovery from the first set to the last
b/ be long enough that the runner doesnt get into a state of deep exhaustion yet still is able to complete the session without reaching a state where he is not able to hold the target average pace, or needs to reduce the number of sets or even cut the session short entirely.
This is not to imply that the average workout pace cannot be accurate or pre-determined, it is not the prime interest. The pace is of interest only in that it contributes to the level of stimulus intensity.
Irrespective of the enormous variations in design that an intermittent session can have (number of reps, rep distance, pace intensity, etc), the most vital element in interval training is that the recovery interval must be incomplete; whether short, passive/standstill or active/run or jog.
In contrast, in intermittent training of the fractional/repetition type, including that of the modern vVO2max workout, the interval recovery may be active or passive, but needs to be long enough to permit the main goal of this workout; the pursuit of the defined target pace. This is usually related to the target pace for the target distance, or in more recent times, aimed at improving the physiological parameter, vVO2max.
In interval training (with incomplete recovery), a crucial element is that as each succeeding rep is run, it includes some percentage (or fraction) of the effort stimulus of the previous one(s) so that the effort, the strain on the runner, becomes accumulative.
Due to the accumulative effect, brought about purely by incomplete recovery, at some point during the session the athlete reaches an effort state that is more than the sum of the reps he/she has completed. The accumulated burden of the previous reps means the effort of each successive rep is more than that due to the distance run, or the pace achieved; the effort is greater than that normally required for the pace used in the workout.
For example; when a runner runs 10x500m interval training (with incomplete recovery), by the end of the workout the runner should have covered 5km in an average pace faster than current 5k PB. Despite incomplete recovery, the runner should manage a faster-than-5k PB pace average for the session. Along with the stresses of the higher intensity (greater than 5k race pace), a session done in this way may benefit other physiological areas like aerobic power in a way that could not be achieved if the session had been run as a continuous 5k, or run as an intermittent session with near-complete recovery breaks.