Quads Gone by 30 km: Downhill Running and Eccentric Strength for the Ironman Marathon

Ask Ironman athletes what fails first in the marathon and many won't say their breathing or their heart rate. They'll say their legs: quads that feel bruised and stiff from around 25 or 30 km, each step down a kerb or off an aid-station ramp noticeably harder than the last.

TriForward's "Why the Ironman Marathon Falls Apart After 25 km" covers the big picture: heart-rate drift, running economy, pacing, heat and fuelling. This article focuses on one piece of that puzzle, muscle damage, and on the training response that's most often suggested for it: downhill running and eccentric strength.

What eccentric damage is

Muscles work in three ways:

• Concentric: the muscle shortens as it produces force, as when your quads straighten your knee when you climb.
• Isometric: the muscle produces force without changing length.
• Eccentric: the muscle lengthens while producing force, acting as a brake. Every time you land when running, your quads lengthen to control the bend in your knee.

Eccentric contractions produce high forces at a relatively low energy cost, but they are also the main cause of exercise-induced muscle damage. That damage shows up as soreness a day or two later, a temporary loss of strength, and a rise in blood markers such as creatine kinase (CK) and myoglobin, which leak from damaged muscle fibres.

Downhill running magnifies the eccentric load on the quads. But flat running is eccentric too. Over a marathon, especially after 180 km on the bike, the total dose adds up.

Does muscle damage explain slowing?

The marathon study

A 2013 study by Juan Del Coso and colleagues in PLoS ONE followed 40 amateur marathon runners, 34 men and 6 women, in a race at 27 °C.

• Average pace fell from 3.5 m/s in the first 5 km to 2.9 m/s in the last 5 km.
• Runners who slowed by more than 15% had higher post-race myoglobin (1,318 vs 623 µg/L) and lactate dehydrogenase, another damage marker, than runners who slowed less.
• CK was also higher (564 vs 363 U/L), though this difference didn't reach statistical significance (P = 0.07).
• Body-mass loss was similar in both groups (around 3%), as was body temperature.

The authors concluded that the drop in pace was related to muscle breakdown, but that more research was needed to understand the mechanisms. This is an observational study: it shows an association, not that muscle damage caused the slowing.

The half-iron study

A 2012 study by the same group, also in PLoS ONE, tested 25 trained triathletes, aged around 36, before and after a half-iron race (1.9 km swim, 75 km bike, 21.1 km run). The paper doesn't report how many were men or women.

• Countermovement jump height fell from 30.3 to 23.4 cm.
• Leg power fell from 25.6 to 20.7 W/kg.
• Handgrip strength didn't change (430 N before and after).
• The fall in jump height correlated with myoglobin (r = 0.65) and CK (r = 0.54), but not with dehydration (r = 0.16).

The authors concluded that leg fatigue tracked muscle damage more than dehydration, and they attributed the damage mainly to the run, because swimming and cycling aren't known to cause moderate to severe muscle damage. Inference: in a full Ironman, with a run twice as long, the damage is likely to be greater, though this study didn't measure it.

Cramping and strength training

A 2022 study by Martínez-Navarro and colleagues, which TriForward's "Cramping at Kona" covers in detail, followed 98 marathon runners, 83 men and 15 women. 24% of the runners analysed cramped.

• Crampers had higher CK after the race and 24 hours later than non-crampers.
• Crampers slowed more from around 21 to 25 km onward.
• 25% of crampers did strength training, compared with 47.6% of non-crampers. This difference didn't quite reach statistical significance (p = 0.074).

Inference: muscle damage, cramping and late-race slowing seem to cluster together, and strength training might be part of what separates runners, but this is observational and can't show that strength training prevents any of them.

What a downhill run does to your legs

A 2007 study by Chen, Nosaka and Tu in the Journal of Sports Sciences showed how much a single downhill run can disrupt running. 10 young men ran downhill at a −15% gradient for 30 minutes at 70% of their peak oxygen uptake.

• Knee-extensor strength fell by 7% to 21%.
• CK and myoglobin rose three- to six-fold.
• Muscle soreness lasted 4 days.
• The oxygen cost of running on the flat rose by 4% to 7% from immediately after to 3 days after. Stride length and knee and ankle range of motion fell, and stride frequency, heart rate and perceived effort rose.

The authors linked the poorer economy to altered running form and impaired muscle function. Inference: this is a small-scale version of what a long-course run does. Damaged quads change how you run and make each kilometre more expensive.

The repeated bout effect

Muscle adapts quickly to eccentric damage. After one damaging bout, a similar bout days or weeks later causes less soreness, strength loss and rise in CK. This is called the repeated bout effect.

A 2017 review by Hyldahl, Chen and Nosaka in Exercise and Sport Sciences Reviews described it as a protective adaptation whose mechanisms are still not fully understood. They proposed that four processes work together:

• Neural adaptations, in how the nervous system recruits muscle fibres.
• Changes in the muscle's mechanical properties.
• Remodelling of the connective tissue around muscle fibres.
• Biochemical signalling linked to inflammation and repair.

The practical idea follows naturally: if a race will damage your quads, train them with similar eccentric loads first, so race day is not the first time they meet that stress.

Training studies of downhill running

Ten sessions in recreational adults

A 2025 study by Bastien Bontemps and colleagues in the European Journal of Sport Science tested this directly. It involved 12 healthy adults, 5 women and 7 men, aged around 25, who were recreationally active but not used to downhill running.

• They did 10 downhill sessions in two blocks of five, at 60 to 65% of VO2max for 15 to 30 minutes, on slopes of −5% to −15%.
• Before training, and after 5 and 10 sessions, they did a standardised 15-minute downhill run.

The results:

• Quadriceps soreness after the standard run fell from 29.6 mm (on a 100 mm scale) at baseline to 8.7 mm after 10 sessions. After 5 sessions it was 15.4 mm, a smaller change that wasn't statistically significant.
• Strength loss after the standard run was similar at all three points (−13.9% at baseline and −9.3% after 10 sessions, with no significant difference).
• Central and peripheral fatigue didn't change.

So the training reduced soreness but didn't clearly change how much the standard run fatigued the muscles. The authors noted their standard run was short, at 15 minutes, and that a longer or harder bout might have shown a larger effect.

Five weeks in young men

A 2018 study by Toyomura and colleagues in Applied Physiology, Nutrition, and Metabolism compared downhill and level training in 18 healthy young men: 10 ran downhill at −10% and 8 ran on the level, for 20 minutes, 3 times a week for 5 weeks, at the same heart rate.

• The downhill group increased knee-extension strength by 9% to 24% at all speeds tested, and improved change-of-direction times by 2% to 3%.
• The level group improved VO2max by 5%, energy cost at lactate threshold by 3% and speed at VO2max by 7%. The downhill group didn't.

Inference: downhill running works more like an eccentric strength stimulus than an aerobic one. That's why it complements rather than replaces normal run training.

What the reviews say

A 2020 narrative review by Bontemps and colleagues in Sports Medicine pulled together downhill running research. Key points:

• The main drivers of damage are slope, duration and speed. In past studies, the median protocol was a −12% slope for 40 minutes at 11.3 km/h.
• People least accustomed to downhill running generally suffer the most damage.
• Evidence for prior exposure (the repeated bout effect) as a preventive strategy is growing.
• The benefits of compression garments and specific footwear aren't confirmed.
• Differences in protocols prevent firm conclusions.

What's missing for triathletes

The gap is large:

• No trial has tested downhill or eccentric pre-conditioning on marathon or Ironman performance. The trials measured soreness, strength and markers in short laboratory bouts.
• The training studies used untrained or recreationally active people, often young men. Trained triathletes already run regularly and may have less to gain.
• Most downhill research uses steep treadmill slopes, not the gentle descents of real courses.
• The damage in an Ironman comes after 180 km of cycling. No study has tested whether pre-conditioning helps in that context.

Putting it into practice

The following is inference from the research above, not a tested programme:

1. Start small. Begin with 5 to 10 minutes of gentle downhill running (a slope you can run relaxed) at the end of an easy run, once a week. Expect soreness after the first session.
2. Build gradually over several weeks. Add time before adding steepness or speed. The repeated bout effect means soreness should lessen with each session.
3. Keep it regular. The protective effect is thought to fade if the stimulus stops, so a small dose every week or two is likely better than an occasional big one.
4. Add eccentric strength work. Slow lowering in squats, step-downs and split squats loads the quads eccentrically. TriForward's "Two Sessions a Week" covers strength training for durability.
5. Stop new downhill work well before race day. Avoid a first-ever or unusually hard downhill session in the last 3 to 4 weeks.

Who is likely to benefit most

Inference, based on the finding that unaccustomed runners suffer the most damage:

• Athletes who train mostly on flat roads or a treadmill, and rarely run downhill.
• Athletes racing on hilly run courses, where descents add to the eccentric load.
• Athletes who came to triathlon from swimming or cycling, with a short running history.
• Athletes whose long runs are short, because of time, injury history or a bike-heavy plan.

Athletes who already run hilly routes every week probably have much of this protection already. For them, extra downhill work is likely to add little beyond what their normal training provides.

How long runs fit in

Long runs are already an eccentric stimulus. A long run that's 25 to 30 km in Ironman preparation repeatedly loads the quads in the way the race will. TriForward's "How Long Does Your Ironman Long Run Really Need to Be?" covers how long is useful without too much injury risk. Inference: downhill running is a way of getting a concentrated eccentric dose in less time, which may help triathletes who can't do many long runs.

A triathlon scenario

The following example is illustrative, not drawn from a study. A 39-year-old age-grouper's last two Ironman marathons fell apart at about 28 km. Their heart rate was under control and they fuelled to plan, but their quads felt shredded. They live somewhere flat and train mostly on a treadmill, which, as TriForward's "Treadmill Miles" notes, usually can't simulate descents.

Based on the evidence above, they might:

• Find a gentle hill or bridge ramp and add short downhill segments to an easy run once a week, starting 16 weeks before the race.
• Add slow eccentric squats and step-downs to their strength sessions.
• Extend their long runs gradually, and include some bricks with longer runs off the bike.
• Review pacing, because starting the run too fast increases the damage done in the first half.

Common mistakes

• Doing a big downhill session close to a race. The resulting damage can affect running for days.
• Going too steep, too soon. The first session causes the most damage. Build up.
• Treating downhill running as a speed session. The trials suggest its benefit is protection and strength, not aerobic fitness.
• Blaming everything on muscle damage. Fuelling, heat and pacing matter too. TriForward's "Why the Ironman Marathon Falls Apart After 25 km" covers them.
• Running through severe pain. Soreness is expected. Sharp, localised pain is not.

What we don't know

• Whether downhill or eccentric training improves Ironman marathon times.
• How much training is needed, and how long the protection lasts in trained athletes.
• Whether gentle real-world descents give the same protection as steep laboratory slopes.
• How the findings apply to women and masters athletes, who were few or absent in these studies.

TriForward's "Sore From the Gym, Flat on the Bike" covers how to manage soreness from strength training around key sessions.

References

• Del Coso J, Fernández D, Abián-Vicén J, Salinero JJ, González-Millán C, Areces F, et al. Running pace decrease during a marathon is positively related to blood markers of muscle damage. PLoS ONE, 2013;8(2):e57602. doi:10.1371/journal.pone.0057602
• Del Coso J, González-Millán C, Salinero JJ, Abián-Vicén J, Soriano L, Garde S, Pérez-González B. Muscle damage and its relationship with muscle fatigue during a half-iron triathlon. PLoS ONE, 2012;7(8):e43280. doi:10.1371/journal.pone.0043280
• Martínez-Navarro I, Montoya-Vieco A, Collado E, Hernando B, Panizo N, Hernando C. Muscle cramping in the marathon: dehydration and electrolyte depletion vs. muscle damage. Journal of Strength and Conditioning Research, 2022;36(6):1629–1635. doi:10.1519/JSC.0000000000003713
• Chen TC, Nosaka K, Tu JH. Changes in running economy following downhill running. Journal of Sports Sciences, 2007;25(1):55–63. doi:10.1080/02640410600718228
• Hyldahl RD, Chen TC, Nosaka K. Mechanisms and mediators of the skeletal muscle repeated bout effect. Exercise and Sport Sciences Reviews, 2017;45(1):24–33. doi:10.1249/JES.0000000000000095
• Bontemps B, Louis J, Owens DJ, Miríc S, Vercruyssen F, Gruet M, Erskine RM. Muscle soreness but not neuromuscular fatigue responses following downhill running differ according to the number of exercise bouts. European Journal of Sport Science, 2025:e12240. doi:10.1002/ejsc.12240
• Toyomura J, Mori H, Tayashiki K, Yamamoto M, Kanehisa H, Maeo S. Efficacy of downhill running training for improving muscular and aerobic performances. Applied Physiology, Nutrition, and Metabolism, 2018;43(4):403–410. doi:10.1139/apnm-2017-0538
• Bontemps B, Vercruyssen F, Gruet M, Louis J. Downhill running: what are the effects and how can we adapt? A narrative review. Sports Medicine, 2020;50(12):2083–2110. doi:10.1007/s40279-020-01355-z