Fartlek means "speed play" in Swedish. In the pool, that's exactly what it is — a steady aerobic swim broken up by surges of harder effort whenever the swimmer feels like pushing. Some surges are short and sharp, others longer and more sustained. The pattern is loose by design.
Where the CSS swim drills pace discipline and the sprint set chases the aerobic ceiling, fartlek swim teaches gear-changing. The ability to lift the effort, hold it, and bring it back down without losing the stroke. For a marathon runner who'll need to change gears mid-race — surges, hills, climbing terrain — that flexibility is a real skill.
What Fartlek Effort Feels Like
During surges: Comfortably hard to hard. RPE 6 — somewhere between CSS and a sprint set. Each surge is its own micro-effort with its own length; some last 30 seconds, others a full minute and a half. Variety is the point.
Between surges: Easy swim, RPE 3. Long enough to let the breath settle, short enough that the next surge doesn't feel like a fresh start.
Across the session: Playful. Fartlek is the one swim where the structure deliberately bends. The swimmer picks the moment, picks the length, picks the next surge. The goal is variety, not a fixed prescription.
Session Structure
Warmup (10–15 minutes). Easy swimming. The last 100m can include a couple of build strokes to wake the system up.
Surges block (6–10 surges of 30s–90s within continuous swimming). Steady aerobic base with surges sprinkled in. Each surge at RPE 6 with 1–2 minutes of easy swimming between. Vary the length deliberately — short and sharp, then medium, then longer. No two surges need to look alike.
Cooldown (10 minutes). Easy swimming. Bring it back down gently.
Why Fartlek Swim Matters
Fartlek work develops something the structured sessions can't: the feel of changing gears under aerobic effort. Most quality swims live at one pace — CSS holds CSS, sprint sets hit sprint pace. Fartlek demands the swimmer move between paces fluidly, lifting and holding without thinking about it.
The session is also less psychologically heavy than a structured CSS or sprint set. For a build phase that's already running-heavy, a fartlek swim delivers real quality work without the mental weight of a fixed prescription.
Practical Guidelines
- Duration: 35 to 50 minutes total.
- Surges: 6 to 10 within the session. Vary length from 30 seconds to 90 seconds.
- Effort: RPE 6 during surges. RPE 3 between.
- Stroke: Freestyle for the surges. Easy stroke variety between is fine.
- Frequency: Once per week at most. Often a good substitute for CSS work when the swimmer wants something less structured.
- Drills: Pair as a warm-up — fartlek starts fresh, which is when technique work lands best.