Fartlek
Evidence: moderate
A flexible form of variable-intensity running. It shares the physiology of structured intervals, so the evidence rests on that; the format itself is coaching tradition with little dedicated study.
Fartlek, Swedish for “speed play”, is continuous running with surges of faster effort mixed in, usually by feel or against landmarks rather than to a stopwatch. The approach is credited to the Swedish coach Gösta Holmér in the 1930s. It sits between an easy run and a formal interval session: the hard bits raise the stimulus, but the structure is loose and the recoveries are run, not stood through.
How it differs from intervals
A track interval session fixes the rep distance, pace and recovery precisely. A fartlek leaves some or all of that open: surge to the next lamppost, push the uphills, run two minutes hard then two easy by feel. That flexibility is the point. It lets a runner train pace change and tolerate discomfort without the mental rigidity and pacing precision of the track, and it adapts naturally to undulating terrain and trails where fixed paces make no sense. The cost is exactly that looseness: it is easy to drift into a moderate slog, neither easy nor truly hard, which is the grey-zone trap.
How to use it
A simple fartlek
After a warm-up, alternate harder and easier blocks for 20 to 30 minutes: for instance 6 to 8 × (2 min at roughly 5 km to 10 km effort, 2 min easy jog), or the classic “by feel” version surging on the uphills and floating the downhills of a hilly route. Keep the hard bits controlled, not all-out, and let the easy bits genuinely recover.
Fartlek earns its place as an accessible introduction to faster running for newer runners, as a less mentally taxing alternative to the track for experienced ones, and as the natural way to inject intensity on varied terrain. The underlying adaptations are those of interval training; fartlek is one way to deliver them, not a separate physiology.