| What it is | Unstructured 'speed play' surges |
| Structure | By feel or landmarks (loose) |
| Builds | Speed & aerobic fitness |
| Great for | Variable trail terrain |
Swedish for 'speed play'.
Fartlek — Swedish for ‘speed play’ — is an unstructured form of interval training in which a runner mixes faster bursts of varying length and intensity into an otherwise easy run, often by feel or using landmarks. Flexible and fun, it builds speed and aerobic fitness while suiting the variable terrain of trail running especially well.
Play with pace
The freeform cousin of structured intervals and the tempo run — great on rolling trails.
Frequently asked questions
What is fartlek training?
Fartlek, Swedish for 'speed play,' is a loosely structured workout where you blend faster surges of varying duration and intensity into an easy run, usually by feel rather than precise times. For example, you might pick up the pace to a distant landmark, then recover at an easy jog until you feel ready to surge again.
How is fartlek different from intervals?
Intervals are structured — set durations or distances at set intensities with planned recovery. Fartlek is freeform and intuitive, with surges and recoveries decided on the fly, often using terrain and landmarks. Fartlek is more playful and flexible, while intervals are more precise and measurable; both develop speed and fitness.
Why is fartlek good for trail running?
Because trails have constantly changing terrain — climbs, descents, technical sections — that lend themselves to running by effort and feel rather than rigid splits. Fartlek's flexible surges fit naturally with this variability, letting you push the runnable sections and recover on the rest while building speed and aerobic fitness.
Sources
- Fartlek training — American Council on Exercise
- Speed play on trails — American Trail Running Association