Most runners who blow up in the second half of a 100-miler don't have a fitness problem. They trained their engine. Their VO2 max is fine. What fails is their legs. Months of long runs can't teach your quads to take 60,000 downhill steps without shutting down.
This is a braking problem. Scientists call it eccentric loading: the muscle gets longer while it soaks up force, instead of getting shorter to push. A 2025 study found a fix that works in 10 weeks, two days a week.1
Why Cardio Fitness Alone Is Not Enough for 100-Mile Races
Running works your muscles two ways. On push-off, the muscle gets shorter to make power. On landing, the muscle gets longer while it soaks up the hit. The landing is where the damage adds up. Every downhill step makes your quads brake your whole body against gravity. That braking force is several times your body weight on every stride. It causes tiny tears in the muscle.
Long runs build your fitness and get you used to hours on your feet. They don't get your legs ready for a long downhill at mile 60. A 2025 review cites a poll of 824 ultrarunners. 66% said muscle damage held them back most. Only 23% said stomach trouble.2
A 2025 study tested leg strength during a 156 km trail race. Runners did six laps of 26 km, and their thigh strength was tested after each lap. By the finish, it had dropped about 41%. The runners who quit after lap 5 had started the race with about 27% less leg strength than the finishers.4 This shows a link. It does not prove that weak legs made them quit.
A 2025 study in Medicine & Science in Sports & Exercise tested if strength training could help.1 Well-trained male runners did heavy lifting and jump training twice a week for 10 weeks, on top of their normal running. Then researchers tested them after 90 minutes of hard running, when their legs were already worn down. On a hard effort in that tired state, the lifting group lasted 35% longer than before. The group that only ran got 8% worse. The lifters also used less energy per step at the 90-minute mark. But the big finding was durability: they could keep going strong on tired legs.
What the research says: A 2025 review by Ramos-Campo and others pulled together 17 earlier reviews on strength training in distance athletes. Lifting helped running economy in every review that measured it. None of the reviews found a real change in VO2max. The authors warned that many of the studies were not high quality. (Ramos-Campo et al., Journal of Strength and Conditioning Research, 2025) Single trials don't always agree. In a 2026 trial, 25 recreational runners were split into two groups. One group lifted for 8 weeks. Their running economy did not change on flat ground or uphill.5
What this means for you: Lifting makes you a more efficient runner. You use less energy per mile at the same pace. It does not make your engine bigger. For ultra runners, efficiency matters more. Late in a race, when your legs are beat up, your efficiency is what slips. Lifting two days a week on top of your running is safe. It makes you more efficient without hurting your aerobic fitness.
The adaptation you need is not more aerobic capacity. It is the ability to absorb damage and keep producing force anyway. That is a different kind of training.
What Strength Training Should Ultramarathon Runners Do?
Two sessions per week, 20 to 30 minutes each. Sessions should be short and heavy. Endurance athletes respond to strength training when the load is high enough to recruit fast-twitch fibers. Three to four sets of four to six reps on compound movements is the target. If you can do 15 reps, you are training endurance, not strength.
If your schedule puts a strength session and a run on the same day, lift first. Korth does exactly this: heavy squats in the morning, long run in the afternoon. The soreness from a strength session is delayed by several hours, so it does not carry into the run. Run first, and the accumulated fatigue will blunt your strength output.
Timing note: Stop heavy strength work 10 to 14 days before race day. The goal in the final two weeks is to arrive fresh, not to squeeze out more adaptation. One light session in the final week keeps the pattern without adding fatigue.
Which Exercises Should Ultramarathon Runners Do?
Bulgarian Split Squat
The most direct transfer to downhill running of any gym exercise. Your rear foot is elevated, which forces the front leg to absorb all the load eccentrically as you lower. This is exactly the loading pattern your quad faces on a steep descent, with the added demand of controlling the movement through the full range. Heavy enough to require bracing, slow enough on the descent that you're working the eccentric, is the target.
Squat University on the Bulgarian split squat: setup, depth, and common errors
- Sets/reps: 3 sets of 5–6 per side
- Load: Heavy enough that the last rep is genuinely hard. If it isn't, add weight.
- Tempo: 3-second descent, pause at the bottom, drive up.
- Progression: Add weight before adding reps.
Single-Leg Romanian Deadlift
This works your hamstrings, glutes, and lower back. You bend forward at the hips on one leg, which is a lot like the motion of trail running. Balancing on one leg while you control the weight builds the steadiness you need on rocky trails. Go slow. Lowering the weight is what does the work, not standing back up.
Four single-leg RDL variations for runners, with coaching cues for form
- Sets/reps: 3 sets of 6–8 per side
- Load: A single dumbbell or kettlebell in the opposite hand from the standing leg.
- Tempo: 3–4 seconds down, controlled return.
- Cue: Think of pushing your heel behind you as you hinge, not bending forward.
Clamshells and Lateral Band Walks
A hip muscle called the gluteus medius keeps your pelvis steady every time your foot lands. When it gets tired, your knee caves in. That strains the outside of the knee and tightens the IT band. IT band pain is one of the most common injuries in ultra running. These moves are boring, but they target the most common way legs break down late in a race. Use a band that makes you work hard through the whole motion.
Hip stability exercises for runners: clamshells and lateral band monster walks
- Clamshells: 2–3 sets of 15–20 per side, medium band. Slow and controlled. Feel the glute working, not the hip flexor.
- Lateral band walks: 2–3 sets of 10 steps each direction. Stay low, keep tension in the band throughout.
- When to do them: These work well as a warmup before the main strength session, or on easy recovery days.
Eccentric Single-Leg Calf Raise
Your Achilles tendon takes a huge load on steep, rocky downhills. The best-studied way to protect it is a slow, one-leg calf raise where you focus on lowering down. It helps prevent Achilles injuries, and it toughens the tendon for long downhills. Stand on the edge of a step so your heel can drop below the step. The lowering is the point.
Eccentric calf raise technique for runners: setup, tempo, and progression
- Sets/reps: 3 sets of 10–15 per side
- Tempo: 3–4 seconds down. Rise on both feet, lower on one.
- Progression: Once 15 reps feel easy, add a loaded pack or hold a dumbbell.
- Note: Some soreness in the Achilles tendon area is normal in the first week. Back off if you feel sharp pain.
If You Only Have Time for Two Exercises
The Bulgarian split squat and the single-leg RDL work the main muscles you use to control downhills. If you're short on time, do those two. Add the calf work if you've ever had a tight Achilles or a calf strain. Make hip work your top priority if you've ever had IT band trouble or pain on the outside of your knee.
When your miles are high, drop the jump training first. Jumps help most in the Base phase, when your legs are fresh enough to make real power. They are riskier when your legs are tired from long runs. Never add jump training in the last six weeks before a race.
Strength Work Built Into Your Plan
The Iron Miles Training Planner puts strength sessions right into your weekly plan, around your long runs and key workouts. It changes how often you lift as you move from Base to Build to Taper.
Build My Training PlanWhat Elite Ultrarunners Actually Do
Kilian Korth won the Cocodona 250 men's race in May 2026 and set a course record. He does two leg workouts a week, each built around one big, heavy lift. One day is back squats. The other is deadlifts. He adds box step-ups, band work for his glutes, and single-leg moves around the main lift. He lifts in the morning and runs in the afternoon on the same day. His reason: soreness from lifting takes a few hours to show up, so it doesn't hurt the run that follows. If you run first, you bring that tiredness into the gym. His choices line up with the research: heavy leg lifts, one-leg balance work, and glute work.
Kilian Korth on why he incorporates strength training into ultra preparation
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Build My Plan- Zanini M, Folland JP, Wu H, Blagrove RC. "Strength Training Improves Running Economy Durability and Fatigued High-Intensity Performance in Well-Trained Male Runners: A Randomized Control Trial." Medicine & Science in Sports & Exercise, 57(7):1546–1558, 2025. doi:10.1249/MSS.0000000000003685. 28 well-trained male runners. Heavy strength plus plyometrics twice a week for 10 weeks. After a 90-minute run, time to exhaustion at high intensity rose 35% in the strength group and fell 8% in the running-only group.
- Tiller NB, Millet GY. "Decoding Ultramarathon: Muscle Damage as the Main Impediment to Performance." Sports Medicine, 55(3):535–543, 2025. doi:10.1007/s40279-024-02127-9. Review article. It cites a poll of 824 ultrarunners in which 66% named muscle damage as their main race limiter and 23% named GI distress.
- Ramos-Campo DJ, Andreu-Caravaca L, Clemente-Suárez VJ, Rubio-Arias JÁ. "The Effect of Strength Training on Endurance Performance Determinants in Middle- and Long-Distance Endurance Athletes: An Umbrella Review of Systematic Reviews and Meta-Analysis." Journal of Strength and Conditioning Research, 39(4):492–506, 2025. doi:10.1519/JSC.0000000000005056. Umbrella review of 17 reviews. Strength training improved running economy; no meta-analysis found a significant change in VO2max.
- Markov A, Edwards BJ, Goutorbe A, et al. "Profiling measures of muscle strength and power throughout a 156 km ultra-trail running event." Frontiers in Sports and Active Living, 7:1734785, 2025. doi:10.3389/fspor.2025.1734785. Observational study of 55 runners over six 26 km laps. Knee-extensor strength fell about 41% by the finish. Runners who withdrew at lap 5 had 27% lower baseline strength.
- Robinson RM, Hahn ME. "The Effect of Resistance Training on Uphill Running Economy and Biomechanics." Journal of Strength and Conditioning Research, 2026. doi:10.1519/jsc.0000000000005567. Randomized trial, 25 recreational runners, 8 weeks. Running economy did not change at 0, 5, or 10% grade.