The term interval training is interchangeable with HIIT (high intensity interval training) and HIT (high intensity training), both of which are terms used to describe what is essentially work performed where the work:rest ratio’s are used. Interval training is used to develop maximal aerobic power (or anaerobic power) where the variables manipulated are VMAX & TMAX (maximal aerobic velocity, & time during which maximal aerobic velocity can be maintained).
Interval training v continuous training: do as much work in less time
The primary findings from the preliminary studies on interval training were described by Astrand & Rodahl as follows:
1. When a subject with a VO2max of 4.6L.min-1 exercised at 350W, they could maintain that work rate for 8min only. The VO2 requirement of 350W was supra-maximal. However, when the workload was halved i.e. exercising at 2.45L.min-1, the workload could be performed for 1 hour. Total VO2 during the hour was 145L. The key finding of this experiment is that sub-maximal VO2max intervals is sustainable for continuous periods.
2. When the same subject exercised again at 350W, but this time for a 3min:3min ratio, the same amount of work was performed as in experiment 1 for the 60min continuous. VO2max and heart rate were worked to maximal capacity, but the energetic cost of work was 10% higher in this experiment. In other words interval training at high power outputs is a far more efficient way of training than continuous sub-maximal training.
3. When a 30:30s work:rest ratio was performed, total VO2 during 1 hour of work was 154L, but no significant increased in blood lactate occurred. However, peak VO2 was lower than in experiment 2, and peak HR was <150b.min-1.
From the first set of experiments, we see that it is difficult to exercise continuously at high intensities particularly when anaerobic metabolism contributes to exercise. It is clear from experiments 2-3 that a greater amount of work can be performed for an aerobic:anaerobic high intensity “broken set” workloads, than in continuous training at the same workload, where early fatigue ensues.
1:2 work:rest ratio, different times with different results
The same (unlucky!) subject was put through another set of rigorous experiments. The results are summarised here, and (2-4) illustrated graphically below:
1. The subject cycled at 400W continuously, but was exhausted at 3min
2. The subject cycled at 400W for work:rest period of of 60:120s, and was exhausted at 24min. Blood lactate levels rose to 150mg/100ml
3. The subject cycled at 400W for work:rest periods of 10:20s, completing the total work requirement over 30min, with no increase in blood lactate
4. When the subject cycled at 400W for 30:60s on-off, an intermediate blood lactate result was obtained
Tip for strength and conditioning coaches working with supra-maximal aerobic loads: interval training is a very effective way of performing a lot of work that is broken up, and recent research in interval training has started to uncover the molecular mechanisms underpinning it. Have your athlete interval train between 30-45min with work intervals anywhere 10-30s, and they will enjoy the benefits. If your goal is to train your athlete aerobically & alactically, choose short work periods. If you’re looking for aerobic power with lactate tolerance, prolong the interval from 30s to 3min. Sports such as soccer, rugby and tennis all involve interval style efforts and the balance between alactic and lactic training dose, and match-play, should be planned for to the best extent. A mix of both alactic and lactic tolerance intervals are probably best. When multiple phases of play lasting 30-60s on the rugby pitch, or 30-40s duration rallies during clay-court tennis (which are more & more common) happen multiple times in a match, you don’t want to find your athlete fatiguing. Interval training is also an excellent way of maintaining aerobic power in-season when the majority of training is focused on speed, skill and tactical development. However, we also believe that interval training when performed correctly can be tough on the joints, is difficult to perform without a solid fitness base. Therefore it should always be planned for, and built on a solid foundation of strength and movement efficiency. Fitness coaches should be careful in pre-season training not front-load too much with interval training before proper running biomechanics and velocity specific, musculoskeletal and musculo-tendinous adaptations are in place.