Most runners who blow up in the second half of a 100-mile don't have an aerobic problem. They trained their engine. Their VO2 max is fine. What fails them is something months of long runs can't develop: the capacity to absorb 60,000 downhill footstrikes without their quads shutting down.
This is an eccentric loading problem — eccentric meaning the muscle is lengthening and absorbing force, rather than shortening to generate it. And a 2025 study found a specific fix that works in 10 weeks, two days per week.1
The Problem Fitness Training Doesn't Solve
Running loads your muscles in two ways: concentrically (the push-off, when the muscle shortens to generate power) and eccentrically (the landing, when the muscle lengthens under load to absorb impact). The eccentric phase is where damage accumulates. Every downhill step forces your quads to brake your body weight against gravity. That braking force is several times your body weight per stride, and it causes microscopic tears in the muscle fibers.
Long runs build your aerobic system and condition you to be on your feet for hours. They do not adequately prepare your legs for the specific eccentric demand of a long downhill at mile 60. Survey research published in 2025 found that 66% of 824 ultramarathon finishers identified muscle breakdown as their primary race limiter, ranking above aerobic capacity, pacing errors, and GI distress.2
A 2025 study in Medicine & Science in Sports & Exercise tested whether strength training could change this.1 Trained endurance runners completed 10 weeks of twice-weekly heavy strength and plyometric training alongside their normal run training. Researchers tested running economy in a fatigued state, specifically after 90 minutes of hard running when leg muscles were already degraded. The strength-trained group improved high-intensity performance in that fatigued condition by 35% compared to the running-only control. Running economy in the fresh state improved too, but the key finding was durability: the ability to sustain output when already broken down.
What the research says: A 2025 study by Ramos-Campo et al. in the Journal of Strength and Conditioning Research found that a 12-week concurrent strength and endurance training program improved running economy by 4.8 percent in recreational ultramarathon runners but did not significantly change VO2max. (Ramos-Campo et al., Journal of Strength and Conditioning Research, 2025)
What this means for you: Lifting weights makes you a more efficient runner — you use less energy per mile at the same pace. But it does not raise your aerobic ceiling. The gain from strength training is in efficiency, not raw engine size. This matters for ultramarathon runners because running economy, not VO2max, is what erodes in the late miles when the legs have been beaten down. Concurrent strength and endurance training — two lifting days per week alongside normal run training — is safe and produces measurable efficiency gains without interfering with aerobic development.
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.
The Protocol
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.
The Exercises
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
Trains the hamstrings, glutes, and posterior chain in a single-leg hip hinge that closely matches the mechanics of trail running. The balance demand — stabilizing on one leg while controlling a loaded hinge — directly trains the stability you need on technical terrain. Keep the movement slow. The eccentric phase as you lower is the training stimulus, not the return.
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
The gluteus medius stabilizes your pelvis every time your foot hits the ground. When it fatigues, your knee drops inward, which creates the lateral knee stress and IT band tightness that sideline more ultrarunners than almost any other injury. These exercises are not glamorous, but they address the single most common late-race breakdown pattern. Use a resistance band that creates genuine effort through the full range of 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
The Achilles tendon absorbs enormous load on technical descents. Eccentric calf loading, the controlled lowering phase of a calf raise done on a single leg, is the most evidence-based intervention both for preventing Achilles tendinopathy and for building the tissue resilience to handle sustained downhill running. Use a step or stair edge so you can lower below neutral. The descent 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 together cover the primary movers for downhill control. If your training week is compressed, do those two. Add the calf work if you have any history of Achilles tightness or calf strains. Hip stability work is highest priority if you have ever dealt with IT band syndrome or lateral knee pain.
Drop plyometrics first when volume is high. They are most useful in the Base phase, when your legs are fresh enough to generate real power output, and they carry a higher injury risk when you are already carrying fatigue from long run training. Never add plyometrics in the final six weeks before a race.
Strength Work Built Into Your Plan
The Iron Miles Training Planner incorporates strength sessions directly into your weekly schedule, timed around your long runs and key workouts. The plan automatically adjusts strength frequency as you move from Base through Build and Taper.
Build My Training PlanWhat Elite Ultrarunners Actually Do
Kilian Korth, who won the Cocodona 250 men's race in a course record in May 2026, runs two lower-body sessions per week built around one heavy compound lift per day: back squats on one day, deadlifts on the other, with box step-ups, band work for the glutes, and single-leg exercises layered in around the main lift. He lifts in the morning and runs in the afternoon on the same day. His reasoning: muscle soreness from a strength session is delayed by several hours, so it does not affect the run that follows. Run first and you carry the fatigue into the gym. The exercise choices map closely to what the research supports — heavy compound lower body, single-leg stability, glute-specific loading.
Kilian Korth on why he incorporates strength training into ultra preparation
- Zanini A, et al. "Combined Strength and Plyometric Training Improves Running Economy and Fatigued High-Intensity Performance in Trained Endurance Runners." Medicine & Science in Sports & Exercise. 2025. A 10-week combined strength and plyometric training program (2x/week) improved running economy in fatigued conditions and boosted high-intensity time-to-exhaustion by 35% compared to running-only controls, measured after 90 minutes of effort.
- Tiller NB, Millet GY. "Decoding Ultramarathon: Muscle Damage as the Main Impediment to Performance." Sports Medicine, 55(3):535–543, 2025. doi:10.1007/s40279-024-02125-z. Survey of 824 ultramarathon finishers; 66% identified muscle breakdown as their primary race impediment, ahead of aerobic capacity, running economy, and GI distress.
- Ramos-Campo DJ, et al. "Effects of concurrent strength and endurance training on running economy and maximal oxygen uptake in recreational ultramarathon runners." Journal of Strength and Conditioning Research. 2025. A 12-week concurrent training program improved running economy by 4.8% without significantly changing VO2max, confirming that strength training gains for ultramarathon runners operate through efficiency, not aerobic ceiling expansion.