Fixing the Weak Leg: Can a Winter Swim-Focus Block Shift a Slow Swimmer?

Most triathletes have a weak leg, and for adult-onset swimmers it's usually the swim. Racing has a way of reinforcing it: you exit the water further back than you'd like, spend energy fighting the swim and watch people you can outride disappear up the road. Winter looks like the time to fix it, with three or four swims a week while the bike and run tick over.

This article looks at whether that's worth doing. It covers how much the swim actually matters to your result, how quickly swimming improves, why swim fitness doesn't help the other two sports, and what a block costs your bike and run. There are no trials of a winter swim-focus block in age-group triathletes, so the plan near the end is labelled inference.

TriForward's "Where Should Your Next Training Hour Go?" covers the race value of swim fitness in detail. This piece is about the block: whether to do one and how to keep it from costing more than it gives.

How much does the swim matter?

Age-group 70.3 data

The largest relevant dataset is a 2023 study by Nikolaidis and colleagues in Scientific Reports. They analysed 823,459 age-group results (198,066 women and 625,393 men) from 787 Ironman 70.3 races between 2004 and 2020, in age groups from 18 to 75 and over.

• Bike and run splits correlated more strongly with overall time than swim splits did.
• Running was the strongest predictor of overall performance in every age group.

This is observational race data. It shows which splits vary most with finishing time, not what would happen if you improved one of them.

Elite data across distances

A 2021 study by Sousa and colleagues in Frontiers in Physiology looked at 16,667 elite triathletes across sprint, Olympic, 70.3 and full-distance races from 2015 to 2020.

• Swimming was the strongest predictor in Olympic-distance races, where drafting on the bike makes swim position decisive.
• Running was the strongest predictor in full-distance races, and its influence grew as races got longer.

These were elite athletes, many in draft-legal racing, so the Olympic-distance result doesn't transfer directly to non-drafting age-group racing. But the overall pattern is consistent: the longer the race, the smaller the swim's share of the result.

A worked example

Inference, using made-up but realistic numbers: a 70.3 swimmer who improves from 2:10 to 2:00 per 100 m saves about 10 seconds every 100 m, or roughly 3 minutes over 1,900 m. That's worthwhile, but similar time is often available on a 90 km bike or 21 km run. The case for a swim block is stronger when:

• Your swim is very slow relative to your bike and run, so the gains available are large.
• The swim costs you energy, so you start the bike tired or anxious.
• You race shorter distances, where the swim is a bigger share of total time.
• You're nervous in open water, where confidence and skill matter as much as fitness.

Why slow swimmers can improve quickly

Efficiency, not fitness

A classic 1990 study by Toussaint in Medicine and Science in Sports and Exercise compared 6 competitive swimmers and 5 triathletes at the same power input. Propelling efficiency, the share of power that actually moved them forward, averaged 61% in the swimmers and only 44% in the triathletes. The triathletes weren't less fit. They wasted more of their effort.

That matters for a swim block. A fit cyclist or runner who swims slowly doesn't need much more aerobic fitness. They need to waste less of what they have, which is why technique work should be a large part of any block. TriForward's "The Winter Technique Block" covers how to make technique changes stick.

Gains in four weeks

How quickly can amateur triathletes improve? A 2019 study by Clemente-Suárez and colleagues in Physiology and Behavior randomised 32 amateur triathletes (20 men, 12 women) to four weeks of volume-focused training, intensity-focused training or an unstructured control.

• Both training groups improved their 50 m and 400 m swim times.
• There were no differences between volume and intensity groups in running, jump or other measures.

The abstract doesn't report the size of the improvements, the training was across all three sports rather than a swim-only block, and four weeks is short. But it shows that amateur triathletes can improve swim performance quickly with structured training.

Volume isn't the only lever

Many triathletes assume more kilometres is the answer. A 2008 crossover study by Faude and colleagues in the International Journal of Sports Medicine had 10 competitive swimmers do four weeks of high-volume, low-intensity training and four weeks of low-volume, high-intensity training.

• 100 m and 400 m times didn't change significantly after either block.
• High volume gave no advantage over lower volume with more intensity.

These were competitive swimmers, not triathletes, over short blocks. TriForward's "Six Months, Same Pace" and "Critical Swim Speed" cover intensity and threshold work in more detail. Inference: for a time-limited triathlete, more swims of good quality probably matter more than very long swims.

Why swimming won't help your bike and run

Swim fitness is unusually specific. A 2002 study by Millet and colleagues in the International Journal of Sports Medicine tracked 4 elite triathletes for 40 weeks, modelling how training in each sport related to performance.

• Swim training related to swim performance (r = 0.37), and run training to run performance (r = 0.74).
• Cycling training transferred to running (r = 0.56).
• Swimming didn't transfer to or from cycling or running.

Four athletes is a tiny sample and the method is a mathematical model, not a trial. But the finding fits with the wider cross-training evidence and with experience: strong cyclists and runners often swim slowly, and lots of swimming does little for a weak run. The practical implications cut both ways:

• Extra swimming won't maintain bike or run fitness. You need to keep some of both.
• Bike and run training won't maintain your swim. That's why swim fitness fades quickly when you stop swimming.

What a swim block costs the bike and run

Intensity keeps fitness

The good news is that maintaining fitness takes much less training than building it. A 2021 review by Spiering and colleagues in the Journal of Strength and Conditioning Research concluded that, in general populations:

• Endurance can be maintained for up to 15 weeks with as few as two sessions a week.
• Volume can fall by 33 to 66% without losing endurance, as long as intensity is maintained.
• Intensity appears to be the key factor for maintaining performance when frequency and volume drop.

The authors stress that most of this evidence comes from general populations rather than athletes.

What happens when intensity drops

A 1985 study by Hickson and colleagues in the Journal of Applied Physiology showed what happens when intensity is the thing that's cut. 12 people trained 40 minutes a day, six days a week, cycling and running for 10 weeks, raising VO2max by 11 to 20%. They then continued for 15 weeks with the same frequency and duration, but at a third or two-thirds lower intensity.

• VO2max fell in both groups but stayed above pre-training levels, with a larger fall in the two-thirds group.
• Long-term endurance fell significantly, by 21% in the one-third group and 30% in the two-thirds group.

This was a small study in previously untrained people, and the sex split isn't given in the abstract. Combined with the Spiering review, it supports the same rule: when you cut bike and run training to make room for swimming, cut volume, not all of the intensity.

Building a swim-focus block

The following plan combines the studies above. It's inference for age-group triathletes, not a tested programme.

1. Decide whether you need it. A block makes most sense if your swim is very slow relative to your bike and run, costs you energy, or makes you anxious. If your swim is already respectable, a smaller adjustment may be better value.
2. Choose a length. Ten to 12 weeks over winter is a common choice, long enough for technique and fitness changes and short enough to rebuild bike and run before spring. There's no research on the ideal length.
3. Add frequency rather than very long swims. Going from two to three or four swims a week gives more practice of the skill. Raise swim volume gradually to protect your shoulders.
4. Make technique a large part of it, with filming at the start and every four to six weeks.
5. Keep some intensity in the pool. Threshold sets based on critical swim speed give a fitness stimulus without huge volume.
6. Keep two or three quality bike and run sessions a week. Cut volume rather than intensity. For example, one shorter threshold or tempo session on the bike and one on the run, plus an easy ride or run if time allows.
7. Plan the hand-back. In the last few weeks, start rebuilding bike and run volume while keeping two or three swims a week, so the swim gains don't fade.

How to tell whether it's working

A swim block needs a way to judge progress that isn't just how sessions feel. Some practical options, none of them tested as markers of a block's success:

• A repeatable time trial. A 400 m or a critical swim speed test (a 400 m and a 200 m on the same day) at the start, middle and end of the block, in the same pool, gives you a pace to compare.
• Stroke count at a set pace. If you need fewer strokes per length at the same pace, or swim faster at the same count, efficiency is probably improving, which fits the Toussaint finding that efficiency is the triathlete's main gap.
• Video at race pace. Compare clips from the start and end of the block, filmed during a hard set rather than a drill.
• How the next session feels after a hard swim. If you're arriving at bike and run sessions flat, the extra swimming may be costing more than planned.

Inference: if none of these has moved after six to eight weeks, the block probably needs a different emphasis, usually more technique, rather than more kilometres.

A triathlon scenario

The following example is illustrative, not drawn from a study. A 38-year-old woman training for her second 70.3 swims 1,900 m in 45 minutes but rides and runs in the top third of her age group. She has about eight hours a week.

• Before (two swims a week): two swims, three rides and three runs.
• The block (December to February): four swims of 45 to 60 minutes, two with technique focus and drill-to-full-stroke work and two with threshold sets; one indoor bike session with threshold intervals and one easy ride; one run with tempo work and one easy run.
• March: back to three swims a week, keeping one threshold set, while bike and run volume returns over about four weeks.

Inference: she won't lose much bike or run fitness if she keeps some intensity, and her swim has the most room to move. Whether she gains more than 3 or 4 minutes depends mainly on how much her technique improves.

Common mistakes

• Swimming more of the same. Extra kilometres at the same pace with the same stroke rarely change much. Technique and some intensity matter.
• Dropping bike and run intensity completely. Easy-only bike and run sessions lose fitness faster than shorter sessions with some quality.
• Increasing swim volume too fast. Going from two to five swims a week in a fortnight is a recipe for a sore shoulder.
• Stopping the swim focus abruptly in spring. Swim fitness fades quickly if swimming falls back to once a week.
• Doing a block when the swim isn't the problem. If a strong swim is followed by a slow run, the run is where the time is.
• Never swimming in open water. Pool gains don't always show in open water. TriForward's "Learning to Draft in the Pool" covers practising open-water skills over winter.

Women, masters and adult-onset swimmers

The Clemente-Suárez study included 12 women among 32 amateurs. The Nikolaidis data showed that the gap between men and women narrowed in athletes aged 50 and over. Most of the other studies here were small and didn't report the sex split. Nothing in the evidence suggests that a swim block works differently for women or older athletes, but it hasn't been tested. Older athletes may need more recovery when adding swims, and adult-onset swimmers of any age probably have the most to gain from the technique part.

What we don't know

• Whether a swim-focus block works in age-group triathletes. No trial has tested one.
• How large the swim improvement is over 10 to 12 weeks, and how much survives the spring.
• The best swim frequency for adult-onset swimmers. Three or four swims a week is a coaching convention, not a research finding.
• How much bike and run fitness is lost in trained triathletes during a block. The maintenance evidence comes largely from general populations.
• Whether race-day gains justify the trade at different race distances.

References

• Nikolaidis PT, Valero D, Weiss K, Villiger E, Thuany M, Sousa CV, Andrade MS, Knechtle B. Predicting overall performance in Ironman 70.3 age group triathletes through split disciplines. Scientific Reports, 2023;13(1):11492. doi:10.1038/s41598-023-38181-y
• Sousa CV, Aguiar SS, Olher RR, Cunha RR, Nikolaidis PT, Villiger E, Rosemann T, Knechtle B. What is the best discipline to predict overall triathlon performance? An analysis of sprint, Olympic, Ironman 70.3, and Ironman 140.6. Frontiers in Physiology, 2021;12:654552. doi:10.3389/fphys.2021.654552
• Toussaint HM. Differences in propelling efficiency between competitive and triathlon swimmers. Medicine and Science in Sports and Exercise, 1990;22(3):409–415. doi:10.1249/00005768-199006000-00020
• Clemente-Suárez VJ, Delgado-Moreno R, González B, Ortega J, Ramos-Campo DJ. Amateur endurance triathletes' performance is improved independently of volume or intensity based training. Physiology and Behavior, 2019;205:2–8. doi:10.1016/j.physbeh.2018.04.014
• Faude O, Meyer T, Scharhag J, Weins F, Urhausen A, Kindermann W. Volume vs. intensity in the training of competitive swimmers. International Journal of Sports Medicine, 2008;29(11):906–912. doi:10.1055/s-2008-1038377
• Millet GP, Candau RB, Barbier B, Busso T, Rouillon JD, Chatard JC. Modelling the transfers of training effects on performance in elite triathletes. International Journal of Sports Medicine, 2002;23(1):55–63. doi:10.1055/s-2002-19276
• Spiering BA, Mujika I, Sharp MA, Foulis SA. Maintaining physical performance: the minimal dose of exercise needed to preserve endurance and strength over time. Journal of Strength and Conditioning Research, 2021;35(5):1449–1458. doi:10.1519/JSC.0000000000003964
• Hickson RC, Foster C, Pollock ML, Galassi TM, Rich S. Reduced training intensities and loss of aerobic power, endurance, and cardiac growth. Journal of Applied Physiology, 1985;58(2):492–499. doi:10.1152/jappl.1985.58.2.492