We thought that we’d step back in time this afternoon, to the 1960’s where some of the preliminary research on interval training was performed and the work:rest paradigm was experimented with. This is exercise physiology 101.
Astrand & Rodahl, were among the first to publish extensive physiological adaptations in humans to interval training (1960). Today, the term interval training is interchangeable with HIIT (high intensity interval training), HIT (high intensity training) or intermittent work, all of which are terms used to describe what is essentially work performed where the work:rest ratio’s are altered. Interval training is used to develop maximal aerobic 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 their 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 working at a sub-maxmial % of VO2max is sustainable for continuous periods.
2. When the same subject exercised again at 350W, but this time for a 3min:3min work:rest 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. So it is possible to work on the threshold of maximal capacity for up to an hour during interval training, versus continuous training where maximal capacity work can only be sustained for <10min.
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. Reducing the VO2 requirement during continuous work allows us to exercise for longer, basically because the aerobic system is a high capacity, low power system. However, 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. Broken set intervals are very popular in rowing where the standard 2000m is broken up into 250m, 500m, and 1000m broken sets are repeatedly performed above race pace. So interval is a very time efficient, and physiologically effective way to train, and we’ve been demonstrating this in the lab. for over 50 years.
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
How to analyse the results?
In terms of an aerobic power equivalent, the 400W was supra-maximal. Therefore the intervals performed here relate exclusively to supra-maximal aerobic loads. Working continuously at 400W caused him to work at >50-100W above his aerobic threshold. We don’t know the precise aerobic:anaerobic contribution to the exercise, but the inability to buffer lactate was perhaps one of many factors in the subjects inability to continue exercising (other factors beyond scope of this short post).
Interval training using a 1:2 work:rest ratio produces mixed results, depending on the work:rest times. The 10:20s work:rest ratio allowed for a continuation of supra-maximal aerobic exercise for 30min. It is likely that two metabolic processes were predominantly at work here: 1) solicitation of the ATP-PCr high powered system, particularly during the initial exercise period before the cardio-respiratory system could match local muscular demands 2) intramuscular oxygen stores (myoglobin) were used for ATP transfer throughout the 30min and continuously replaced once the cardio-respiratory system kicked in. It is also evident, that the demands for ATP were adequately met without the need for glycolysis.
The 30:60s interval also allowed for exercise to continue for 30min. This is despite an increase in blood lactate, which rises quickly in the first 10min but from there is continuously buffered. Therefore an even split between aerobic and anaerobic metabolism is present, and can be tolerated with sufficient motivation. So an increase in blood lactate is not the likely culprit when exercise can’t be continued. During the 60:120s interval, a complex mix of metabolic factors are suspected which in turn led to impaired cross-bridge functioning at the contractile filament level. A loss of motivation subsequent to this may have been present, but more on fatigue in later posts.
Tip for professional fitness 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 endurance. Fitness coaches should be careful in pre-season training not front-load too much with interval training before central cardio-respiratory adaptations or proper running biomechanics and velocity specific, musculoskeletal and musculo-tendinous adaptations are in place. Therefore an extensive low-intensity, high volume period is recommended initially. Concomitantly, interval training for anywhere between 3-5min on sub-maximal aerobic loads is favoured, where the work can be performed for anywhere from 90-120min.