Doubling your weekly yardage probably won’t make you faster. In swimmers who tried it, the extra miles bought nothing. What helps is knowing what each set is for, and that is the case for CSS training.
You may already know your critical swim speed (CSS), the pace you can hold at the edge of steady effort. If not, here’s how CSS works and how to test it. This post covers a different question. Should you chase yards, or should you build your week around pace zones?
One honest note up front. No one has run a controlled trial of CSS training in triathletes. We found none. The argument below is indirect, and we’ll say where it gets thin.
What the Volume Studies Show
Start with the old, famous swim studies. They tested the “more is better” idea directly.
In 1988, Kirwan and colleagues doubled the daily distance of competitive swimmers at constant intensity for 10 days. Physiological tests and swim performance didn’t improve. Their conclusion: past an optimal volume, more gains need higher intensity.
In 1991, Costill’s group ran a 25-week study. One group added a second daily session in weeks 5 to 11, going from 1.5 hours a day to 3 hours. Their sprint velocity fell while the control group’s rose. Over 25 weeks, the groups didn’t differ.
In short: twice the pool time gave the same result.
Volume vs Intensity, Head to Head
Those two studies show extra volume doesn’t help. A few others compared volume against intensity directly.
| Study | Who | What happened |
|---|---|---|
| Kirwan 1988 | Competitive swimmers, 10 days | Double distance, same intensity: no change |
| Costill 1991 | Competitive swimmers, 25 weeks | 3 h/day vs 1.5 h/day: no benefit |
| Faude 2008 | 10 swimmers, 4 weeks per block | High volume had no edge over lower volume, higher intensity |
| Nugent 2019 | National-level youth swimmers, 7 weeks | Cut easy volume; reportedly about half the distance with similar results |
| Sperlich 2009 | 27 kids aged 9 to 12, 6 weeks | Competition points rose 14.3% with higher intensity vs 9% with higher volume (conference abstract) |
The best of these is Faude’s crossover trial. Ten swimmers did a 4-week high-volume block and a 4-week lower-volume, high-intensity block, each followed by the same taper. Times for 100 m and 400 m weren’t significantly different between blocks. The authors said high volumes had no advantage.
Two caveats. The Nugent numbers come from a secondary summary, so treat the “half the distance” detail as likely, not certain. The Sperlich study is a conference abstract, not peer reviewed, and the kids were 9 to 12. These are competitive swimmers and kids, not 42-year-old age-groupers.
Still, the pattern repeats. Volume alone stalls.
Why CSS Training Isn’t Just “Easy Laps”
So what replaces yardage? Quality sets at known paces. This is where CSS training earns its place.
CSS isn’t a gentle pace. Martin and Whyte tested 8 elite triathletes. Critical swim velocity was 1.23 m/s, while the speed at lactate threshold was 1.15 m/s. Lactate was 3.0 mM at CSS and 1.9 mM at threshold. We could only see the abstract snippet, so quote it loosely.
Translation: CSS is a few percent faster than your comfortable steady pace. Sets there feel like work, not warm-up.
Think of a car. Yardage is the odometer. It tells you how far the car drove, not how hard the engine worked. Pace zones are the tachometer. They tell you which gear the engine was in. Ten thousand miles in the slow lane builds a very different car than a few hard laps.
This also tells you what a week needs. You want some CSS-pace work, some easy swimming, and some technique work. Don’t let CSS sets swallow the whole week.
Same Hours, Two Ways to Spend Them
Here’s an illustrative 4.5-hour pool week. Same time, different structure. These are example sets, not research-backed prescriptions.
| Session | Yardage-only week | CSS-structured week |
|---|---|---|
| Mon (90 min) | Steady laps to hit the weekly goal | Drills, then 10 x 100 at CSS pace |
| Wed (90 min) | More steady laps | Technique block, then easy aerobic swim |
| Sat (90 min) | Long continuous swim | 3 x 500 near CSS, short rest, easy cooldown |
The 10 x 100 and 3 x 500 ideas come from a masters coach’s CSS progression, not from a study. His plan builds to 10 x 100 by adding one rep a week over 10 weeks, then retests. That’s convention, but it’s a clear structure.
Every set has a job. Nobody is swimming yards just to fill a number.
Swim Is a Small Slice of the Race
Here’s the part many triathletes skip. In the case above, the swim was 7.9% of the race. A 10% faster swim saves about 5.9 minutes. That’s under 1% of a 747-minute day.
A large 70.3 study of 823,459 age-group finishers shows the same shape. Run and bike splits tracked finish time closely (run r = 0.89 for women and 0.90 for men, bike r = 0.87 and 0.86). The swim correlation was lower, though the paper shows it only in a figure.
In short: the swim won’t win your race, but it can wreck it.
That’s why pool hours need to be efficient. Spend extra hours on yardage and you steal them from the bike and run, where more of the race happens.
Race-Day Intensity: Go Under, Not Over
Training zones also tell you how hard to swim on race day. Peeling’s team tested 9 trained men in sprint triathlons. Swimming at 80 to 85% of time-trial speed led to faster cycling and a faster overall time than swimming at 98 to 102%. We could see only the summary, and a smaller study found 90% worked best. So the best level may depend on your fitness.
Start the bike fresh, and you gain more than you lost in the water. Know your CSS, and you know what “a bit under” feels like.
A Case Study: Marcus
Picture Marcus, 43, training for his first 70.3. He swims 3 times a week and counted yards. His goal was 9,000 a week. By the end of each month he’d hit the number, but his 400 time hadn’t moved. These numbers are illustrative.
He swapped the goal for a structure. One session of drills, one of CSS sets, one of easy aerobic swimming. His total pool time dropped by 30 minutes a week. He used that time for a second strength session.
Eight weeks later he retested in the same pool. His CSS pace was 4 seconds faster per 100. Over a 1.2-mile swim, about 1,900 m, that’s roughly 76 seconds. Not huge. But he came out of the water fresher and rode better.
Hit the pace. Skip the padding.
Honest Limits of the Evidence
Here’s what we can’t claim. No trial compares CSS-structured weeks to yardage weeks in triathletes. Most swim studies use competitive or young swimmers. Many triathletes learned to swim as adults, and technique can limit them more than fitness does. One thesis found competitive swimmers had longer strokes and better force use than triathletes, but we couldn’t verify the numbers.
So treat this as a reasoned bet, not a proven fact. The bet: if extra volume stalls, and CSS sits above threshold, and the swim is a small slice of race time, then quality sets by zone beat chasing yards. If you’re a beginner with poor technique, drills will likely beat any pace work.
For the other two sports, Zone 2 work builds the aerobic base, and aerobic decoupling shows if that base is working. You can also check what a good FTP looks like.
AthleteOS stores your CSS, turns it into swim pace zones, and prescribes swim sessions by zone, so each set has a job beyond adding to your yardage total. You can start a free trial at AthleteOS.
Swim smarter. Not further.