Get Faster With… Interval Training

Welcome back to the "Get Faster With…" series. So far we've covered a goofy piece of plastic (the snorkel) and the art of doing nothing (leisure time). This edition is about the single most valuable piece of equipment in swimming, and it costs you exactly zero dollars.

It's the pace clock. That big round clock with the sweeping hand, or the digital one with the giant red numbers, hanging on the wall of nearly every pool on earth.

I've been swimming in front of one since I was a kid. And I can tell you that this free, unglamorous clock has made more swimmers fast than every gadget in every triathlon expo combined. Because the pace clock is what makes interval training possible, and interval training is how swimmers get fast.

What Interval Training Actually Is

Interval training means swimming a set of repeats on a send-off. Something like 10x100 leaving every 1:45. You push off at the top of the clock, swim your 100, and whatever time is left before 1:45 is your rest. Then you go again.

That sounds simple, and it is. But look at everything it does for you.

It controls your effort. The send-off decides how hard you have to work and how much rest you get, so every rep has a purpose instead of being a guess.

It measures your progress. When 10x100 on 1:45 feels hard in January and easy in March, you know you got fitter. When you can drop to 1:40, you know you got faster. No guessing, no vibes, just numbers on a wall.

It teaches you pace. Every rep, you glance at the clock and see exactly how fast you swam. Do that hundreds of times and you develop the most underrated skill in triathlon swimming: knowing your speed by feel.

It lets the rest match the goal. Short rest builds endurance and threshold. Long rest lets you swim genuinely fast for speed work. The send-off is how you dial in which one you're training today.

The Part That Builds Your Engine

Here's the benefit almost nobody thinks about. All that starting and stopping is fantastic for your heart.

Every rep, your heart rate climbs. Every rest, it drops back down. Then you push off and do it again. That rhythm of working hard, recovering, and working hard again trains your heart to pump harder when you need it and to recover faster when you don't. And because the short rests let you keep going, you can spend far more total time at a high heart rate than you ever could in one long continuous swim. More time up there is exactly the signal your body needs to build a bigger, stronger engine.

That's why intervals don't just make you faster. They make you fitter, and they do it in less time. It's also why I keep telling you that your aerobic fitness doesn't only come from long, slow swimming. Hard, structured work builds it too.

The Science Behind The Clock

I don't want you to take my word for any of this, so let's look at what the research says. Sport scientists have been studying interval training for decades, and the findings line up with what swim coaches figured out on the pool deck a long time ago.

It lets you spend more time near your max. The stop-and-go structure of interval training lets athletes pile up more time at high work rates and near their maximal oxygen uptake (VO2max) than a continuous effort would allow. Researchers treat that time near VO2max as a key marker of how much adaptation a session can produce. Sport scientists have put real effort into designing sessions that keep athletes above 90% of VO2max for long stretches, because that's where the big training signal lives. Time spent at or near V̇O₂max during high-intensity interval training - a systematic review and meta-analysis +2

That signal builds a bigger engine. Time near VO2max places a heavy demand on the heart and muscles, which drives adaptations like better oxygen transport and use, a heart that pumps more blood per beat, more capillaries in the muscles, and more mitochondria. In plain English: a stronger heart, better plumbing, and more power plants in your muscles. fisiologiadelejercicio

It works on fit athletes, not just beginners. A 2023 meta-analysis of elite athletes found VO2max increased significantly more after interval training than after conventional training methods. Interestingly, the benefit showed up when the recovery between efforts lasted two minutes or longer, which is a great reminder that the rest matters as much as the work. A larger review of 51 studies and over 1,200 athletes found every type of interval training raised VO2max compared to conventional training, while continuous training produced a much smaller effect. Heliyon (Jun 2023) +2

It works for swimmers specifically. A 2026 meta-analysis of competitive swimmers found interval training significantly improved both VO2max and actual swimming performance. And swimmers doing ultra-short race-pace work held about 98.5% of their target speed with 24% lower lactate and 14% lower perceived effort than with traditional methods. Race speed, at a lower cost. An earlier review comparing low-volume, high-intensity work against higher-volume training found most studies showed real improvements, and none showed physiology or swimming performance getting worse with the lower volume. That should sound familiar if you've read anything else on this blog. The effects of high-intensity interval training on competitive swimmers: A systematic review and meta-analysis +2

It's time-efficient. In one well-known study from Martin Gibala's lab, two weeks of interval training produced gains that would have taken over 10 hours of continuous moderate cycling to match. In another, twelve weeks of short interval training improved fitness and muscle mitochondria about as much as traditional endurance training, with a five-fold lower exercise volume and time commitment. Those studies used non-swimmers, some of them sedentary, but the principle carries straight into the pool: quality work buys a lot of fitness per minute. medicalnewstodaydoaj

And here's the honest part. Interval training isn't magic, and it isn't the only thing you need. One meta-analysis of runners found interval training was better for running economy and delaying lactate buildup, while continuous training actually produced bigger VO2max gains in that group. That's why most researchers land on a blend, not an either-or. One proposed model for elite endurance athletes puts roughly 75% of training at low intensity and about 10 to 15% at very high intensity. johkphysoc

Which is exactly why every TRI Swim Club plan keeps at least one aerobic session a week, and why your bike and run carry most of your low-intensity volume. You're already doing the long, easy stuff on land. The pool is where you buy the high-intensity work that only intervals can give you, and the science says that's a very good trade.

Why Swimming Needs It More Than Any Other Sport

On the bike, you have a power meter. On the run, you have a GPS watch. In the water, you have almost nothing. You can't see your speed, and your effort feels about the same whether you're swimming well or flailing. The water hides everything.

The pace clock is your power meter. It's the only honest feedback you get in the pool. That's why every serious swim program in the world, from a ten-year-old club team to an Olympic training group, is built on intervals and send-offs. You will not find a fast swimmer anywhere who trains without a clock.

The :20 Rest Trap

So why do so many triathletes skip it? Look at a typical triathlon swim plan and you'll see the same thing over and over: 8x100 with :20 rest. 6x200 with :30 rest. Rest, rest, rest, the same fixed amount after everything.

It looks like interval training. It isn't.

With fixed rest, you get the same 20 seconds no matter how you swim. Swim it hard or swim it lazy, the reward is identical, so there's no standard to rise to. You also leave at a random spot on the clock every rep, so you're never reading a clean time, and you never learn your pace. And the rest has nothing to do with the goal of the set. It's just a number someone picked.

The send-off fixes all of it. Swim faster and you earn more rest. Swim slower and you get less. Every rep has a target, every rep gives you a time, and the whole set trains exactly what it's supposed to.

The Excuses

When I ask triathletes why they don't train on intervals, I get some interesting answers.

"I don't like the pressure of the clock."

"I can't do math while I'm swimming."

"My watch tracks everything anyway."

"I just swim by feel."

It's wrong, all of it.

The pressure of the clock is the point. That pressure is race day, and practicing under it is how you stay calm when the gun goes off. The math is easy and you'll pick it up in a week.

As for the watch, I know, you've got a Garmin and it gives you your splits. But swim watches aren't as accurate as people think. They don't count strokes and laps consistently, and if you're starting and stopping it each rep, you're pressing buttons before you touch the wall, which kills your momentum and throws your times off by a second or two every time. And even when it works perfectly, it only tells you what happened after the fact.

And "swimming by feel" is how most triathletes end up swimming every set at the same medium pace for ten years. Feel gets good after you've trained it against a clock, not instead of one.

Bonus: It Saves You Time

Here's a perk nobody talks about. Intervals keep your workout on schedule.

10x100 on 1:45 takes exactly 17 minutes and 30 seconds. Every time. You know when the set starts, you know when it ends, and you know when you'll be out of the pool and on your way home, or onto the bike.

Now watch the fixed-rest swimmer. He takes his 20 seconds, then 25 because he's adjusting his goggles, then 40 because his watch didn't start. His set drifts and stretches and he has no idea where the time went. For a time-crunched triathlete with a lunch-break swim, the send-off isn't just better training. It's how you get the whole workout done in the time you actually have.

How To Start

You don't need anything but a pool with a clock and the willingness to look at it. And if you're a TRI Swim Club member, you have a clock already built into every workout in the app.

Start by leaving on the top of the clock, when the hand hits the 60, or on the 30. It makes the math much easier. For an aerobic set, pick a send-off that gives you somewhere around 10 to 15 seconds of rest per 100 at a steady pace. If you're swimming 100s in about 1:35, try leaving on 1:50. For speed work, give yourself much more rest, so every rep can be genuinely fast.

If you share a lane with a training partner, even better. Leave five or ten seconds apart on the same send-off and you'll keep each other honest.

So here's your job this week. Take one set you'd normally swim on fixed rest and put it on a send-off instead. Just one. Look at the clock every rep. By the end of the set, you'll know more about your swimming than a month of swimming by feel ever told you.

The best equipment in the pool is already hanging on the wall. Start using it.

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