Interval training, as the name implies, is a series of repeated bouts of exercise alternated with periods of relief. Light or mild exercise usually constitutes this relief period. To understand why this method of training has been so successful, I will start a discussion of energy production and fatigue during intermitent work
Energy Production and Fatique During Intermittent Work
To illustrate this difference, suppose you ran continuously for as long and as hard as you could for one minute; then, on another occasion, supose you ran intermittingly, running just as hard as you did continuously, but for only 10 seconds at a time within 30 seconds of rest between each run. If you repeated this six times, you would have performed the same amount of work at the same intensity intermittently as you did continuously (i.e., six runs at 10 seconds each equals one minute of running), but the degree of fatigue following intermittent running would be considerably less.
The reason for this can be explained physiologically. The answer lies in the different interaction between the phosphagen (ATP-CP) system and anaerobic glycolysis (LA system) during intermittent as compared with continuous running. Comparatively, the energy suppled via anaerobic glycolysis (LA system) will be less and that via the phosphagen (ATP-CP) system will be more in the intermittent runs. This means that there will be less lactic acid accumulated and thus less fatigue associated with the intermittent work. This will be true regardless of the intensity of the intermittent work bouts or how long they last.
Replenishing ATP and PC.
How is it possible that the ATP-CP system can supply more ATP and the LA system less ATP during the series of intermittent runs as compared with the continuous run? Didn’t I already indicate that the stores of ATP + PC are exhausted after only a few seconds of all-out running? Yes, I did, however, remember that between intermittent run there is a period of relief. Therefore, the question that needs answering, What, in terms of energy production, is occurring during the relef intervals?
During the relief intervals, a portion of the muscular stores of ATP and PC that were depleted during the preceding work intervals will be replenished via the aerobic system. In other words, during the relief intervals, part of the rapid recovery phase is complete. In addtion, a portion of the O2-myoglobin stores will also be replenished. Thus, during each run that follows a relief interval, the replenished ATP and PC and O2 myoglobin stores will again be available as an energy source. Consequently, energy from the LA system will be “spared,” so to speak, by that amount, and lactic acid will not accumulate as rapidly or as great an extent. In contrast, during a continuous run the phosphocreatine (PC) in the ATP-PC system will be exhausted within a matter of several seconds and will not be replenished until the work is termintated. In this case, energy in the form of ATP from the LA system will be called on early in the run, and LA will rapidly accumulate to high levels.
All this has real meaning when applied to training, for the savings in fatigue accompanying intermittent work can be converted to an increase in the intensity of work performed. This is the single most important feature of intermittent work and as such is the key to the interval training system. An intermittent work level as much as two and one-half times the intensity of the continuous level can be performed before blood lactate acid levels in each are comparable. The interaction between the ATP-CP and the LA systems during intermittent work also varies slightly according to the type of activity level of the relief interval used. This discussion has been centered on intermittent work using complete rest intervals. During actual interval training, the relief intervals should consist of either light or mild work. Later, you will learn when it is best to use one or the other. Right now, the primary difference between intermittent work of the same intensity and duration performed with complet