So what's your method... and can you cite scientific studies? It seems like everyone's in agreement on the benefits of intervals, but everyone has a different way of doing them. And, do you do intervals differently on the bike vs. running vs. any other sport?

- Intensity of intervals: definitely anaerobic, but how hard? Current race pace? Target race pace? All-out sprint? Something in between?

- Duration of intervals: mostly 30 seconds to 2 minutes, but I think I've seen people talk about "intervals" up to 10 minutes.

- Intensity of recovery periods: this is mostly an issue for me running, because there's a minimum intensity needed to keep up even a slow jog, and when I'm in deep oxygen debt from a sprint interval, my heart rate may only drop by 15 bpm during recovery. Should I try to maintain a heart rate in, say, zone 2 or lower, even if it means walking? Or just recover by maintaining an easy pace, even if my heart rate stays high?

- Duration of recovery periods: anything from half the duration of the interval, to three times as long?

- And one thing I don't think I've ever really seen discussed: is there a certain number of intervals one should aim for? Do you progress by increasing the intensity, increasing the duration, shortening the recovery, or adding more intervals?