Suppose you ran continuously for as long and as hard as you could for one minute; then, on another occasion, supposed you ran intermittently, running just as hard as you did continuously, but for only 10 seconds at a time with 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—but the degree of fatigue following intermittent running would be considerably less.
Why?... Comparatively, the energy supplied via anaerobic glycolysis will be less and that via the phosphagen system will be more in the intermittent runs. What this means is that there will be less lactic acid accumulated and thus less fatigue associated with the intermittent work. This will be true no matter how intense the intermittent work bouts or how long they last.
...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. It has been shown that an intermittent work level as much as two and one-half times the intensity of continuous level can be performed before blood lactic acid levels in each are comparable.
…Interval-training system…allows the stores of ATP+PC to be used over and over. This, in turn, promotes an adequate stimulus for promoting an increase in the energy capacity of this system and aids in delaying the onset of fatigue by not delving so deeply into anaerobic glycolysis.
