The training term for changing running speed and terrain in a “speed play” workout is Fartlek.
Fartlek comes from Swedish: fart means “speed” and lek means “play.” Swedish coach Gösta Holmér developed the method in the late 1930s for cross-country runners. His approach blended faster-than-race-pace efforts with sustained running and terrain changes, helping athletes develop speed and endurance in the same session.
Modern Fartlek workouts usually alternate faster and slower running, but the efforts are flexible rather than rigidly timed. A runner might accelerate between trees, surge up a hill, recover on a flat section, and then repeat the pattern according to terrain or perceived effort. This continuous, adaptable structure is the main difference from traditional interval training.
Fartlek is not synonymous with a tempo run, which generally involves a sustained effort at a controlled, challenging pace. Hill repeats are repeated uphill efforts with more fixed terrain and recovery. Fartlek sessions can train both aerobic and anaerobic systems and may be adapted for runners of many experience levels.