FTP vs Threshold Pace vs CSS: How Triathlon's Three Thresholds Relate

Triathletes juggle three different “threshold” numbers — FTP on the bike, threshold pace on the run, and CSS in the pool — and it’s tempting to assume fitness in one predicts fitness in the others. It doesn’t, and understanding why is the difference between training zones that actually fit you and zones borrowed from the wrong sport.

FTP: Functional Threshold Power (Bike)

FTP is the highest average power, in watts, you can sustain for roughly an hour. It’s typically estimated from a 20-minute all-out effort (take 95% of that average power) or a structured ramp test that raises power in steps until you can’t hold the pace. Every cycling training zone — endurance, tempo, sweet spot, threshold, VO2 max — is defined as a percentage of FTP, which is why retesting matters: train on a stale FTP and every zone underneath it is wrong too.

Threshold Pace (Run)

Run threshold pace — sometimes called T-pace or lactate threshold pace — is roughly the fastest pace you can hold for about an hour, similar in concept to FTP but measured in minutes-per-kilometre rather than watts. It’s commonly estimated from a recent 10K or half-marathon time, or a dedicated 20-30 minute time trial. Because running is weight-bearing and pace is affected by terrain, heat and fatigue in a way power isn’t, threshold pace tends to drift more session-to-session than FTP does.

CSS: Critical Swim Speed (Swim)

CSS is calculated rather than tested directly in one effort: swim a timed 400m and a timed 200m (ideally on the same session, well-rested), then divide the difference in distance by the difference in time. The result is your pace per 100m at critical swim speed — roughly the pace you could hold for 15-30 minutes of continuous swimming. It’s the pool’s equivalent of a threshold effort, used to set pace targets for everything from easy aerobic swims to short, sharp interval sets.

Why They Don’t Transfer Between Disciplines

Each threshold reflects a different combination of cardiovascular fitness, muscular endurance and sport-specific neuromuscular efficiency. You can have a strong FTP built from years of cycling and a mediocre CSS if your swim technique is inefficient — the aerobic engine is willing, but the stroke wastes energy the legs never had to think about. This is also why using running heart rate zones to set your bike zones (or vice versa) is a common and avoidable mistake: heart rate at a given effort genuinely differs by discipline, so each sport needs its own baseline test.

How Often to Retest

Every 6-8 weeks is a reasonable default during a structured training block, and always after a significant fitness shift — returning from injury, finishing a big training block, or noticing sessions that used to feel hard now feel easy (or the reverse). Testing all three in the same week isn’t necessary or even advisable; space them out so fatigue from one test doesn’t distort the next.