Interval training is the backbone of modern swim practice. The concept is simple: swim a set distance at a target pace, then rest for a precise amount of time before repeating. The execution requires understanding pace, recovery, and how to balance intensity across a practice week.
Why Intervals Work
Swimming at consistent, challenging paces teaches your body to clear lactate efficiently and sustain higher speeds than steady-state swimming ever could. When you rest for the right amount of time, you're giving your body just enough recovery to repeat the effort at or near your target pace — but not so much that the set becomes easy.
The work-to-rest ratio defines the training zone. A short rest (say, 5-10 seconds for a 100m repeat) keeps you in anaerobic territory. Longer rest (30-45 seconds) lets you hover in the aerobic threshold. Zero rest — or sending off immediately — is the most demanding way to swim a set.
Pacing Intervals Correctly
For distance swimmers doing threshold work, your interval should be your target pace per 100m plus 5-10 seconds. For example, if you want to hold 1:15/100m, send off at 1:20-1:25. This means your repeat times will gradually slip as fatigue accumulates — that's intentional. The challenge is managing that slip within a target range.
For sprinters, shorter rest intervals with faster paces build that explosive speed. Try 8x50m on 45 seconds holding a pace you could hold for 100m, then assess whether your times stayed consistent or drifted significantly.
Structuring a Week of Intervals
Most swimmers benefit from one sprint-focused day, one threshold day, and the rest aerobic or technique work. Intervals on back-to-back days rarely work well — you need at least 48 hours between hard interval sessions for full recovery of the nervous system.
Your best interval sessions should come when you're genuinely rested, typically mid-week after a recovery day or two. Monday after a weekend of aerobic swimming is a classic spot for hard intervals.