Interval training

Interval training splits a session into repeated work bouts separated by recovery. The four variables — work duration, work intensity, recovery duration and number of repetitions — define what the session actually trains, from short sharp sprint repeats to long threshold intervals.

It matters because intervals let you accumulate time at intensities you could not hold continuously. That is how the central adaptations that raise VO2 max are driven: time spent near maximum oxygen uptake is the stimulus, and only broken efforts allow much of it.

In practice intervals are the smaller part of a well-built endurance programme, not the larger. They carry a high recovery cost, so most models put the bulk of training at easy intensity and reserve one or two hard sessions a week for this.