What’s the key physiological difference between short and long intervals?

When working continuously at intensities above the lactate threshold for prolonged durations, as you do during long intervals (LI) (>2min), small disturbances of the homeostasis gradually leads to muscular fatigue, in time reducing your ability to contract during the interval bout. That fatigue is important for mimicking a race scenario where you’re running under immense fatigue. Running intervals with a degree of metabolic load, or anaerobic lactate production, can lead to an increase in your anaerobic threshold.

In short intervals (SI) (<2min), homeostasis is quickly restored during the more frequent recovery periods. One of a number of important differences between SI and LI is explained by an interesting mechanism described in 1960 by Åstrand and colleagues. Myoglobin (the oxygen carrier inside the muscle cells), functions as an O2-store at the beginning of each short work interval. During the recovery periods of SI, myoglobin is reloaded with O2, thereby minimising the anaerobic contribution in the following work interval. SI leads to a high average oxygen uptake and power output during many SI formats, which cannot be maintained during LI, due to the gradually increasing disturbance of homeostasis (lactate build-up) leading to fatigue.

Short intervals a great for building aerobic capacity, increasing speed, and keeping training exciting. Although they have similar benefits to your standard aerobic run, they are more demanding and shouldn’t add as a direct replacement for aerobic runs.