Muscular Endurance – Building the Chassis

I've always loved Speed Endurance training. Intervals, tempo runs, threshold sets - the kinds of workouts that sharpen you up, make you feel fast, and give you the confidence to hold a strong pace deep into a race. But I've never really seen myself as an ultra-endurance athlete. Anything over three hours feels like a different ball game.

I've always known that my muscular endurance was a key limiter in longer, more rugged events. But my experience this September at the ÖTILLÖ World Championship brought this weakness into sharp focus once again. With an injured partner, thirteen hours of running and swimming across the Stockholm archipelago left me in awe of its rugged beauty, but it also delivered a clear message. Even at these slower-than-normal speeds, my lungs were fine, my breathing under control… but my legs? My aerobic engine was still purring, but my chassis was in danger of falling apart.

A week later, I watched Tom Evans and Ruth Croft dominate the Ultra-Trail du Mont-Blanc (UTMB), the pinnacle of ultrarunning. Both are coached by Scott Johnston, and their performances made me curious about his training philosophy. Could there be lessons for those of us who aren't pure ultrarunners but still want to build resilience for long races?

The Engine vs. the Chassis

Think of endurance performance like a car. The engine is your aerobic system: VO₂ max, lactate threshold, and your capacity to produce energy. The chassis is your muscular endurance: the ability of your legs and connective tissues to keep firing for hours without buckling under the load.

Most of us spend our training time building the engine. But as Scott Johnston points out, the real limiter in long races often isn't the engine - it's the chassis. In his words: "The heart doesn’t get tired. The athlete is limited by fatigue out at the muscle."

This explains my ÖTILLÖ experiences. Aerobically, I felt fine. But each step on the rocky trails added tiny cracks to the chassis until, by hour eight or so, my legs didn’t want to deliver what my lungs still had in reserve.

What Is Muscular Endurance?

Muscular endurance is your muscles’ ability to repeatedly contract at a relatively high percentage of their max force output for an extended period. In simpler terms, it's how long your legs can keep pushing before they fail.

Traditional speed endurance training (intervals, tempos) recruits muscle fibers by running hard. That builds capacity but comes with heavy cardiovascular strain and whole-body fatigue.

Muscular endurance training recruits the same fibers differently, by adding resistance and creating local muscular fatigue without pushing the heart and lungs to their max. The result is legs that are more resilient - they keep firing late into a race, even when your engine could still go faster.

Scott Johnston’s Approach

Johnston, co-author of Training for the Uphill Athlete, has made muscular endurance the cornerstone of his coaching. He saw that many endurance athletes had strong engines but couldn't use them fully because their legs gave out first.

His solution: target the muscles directly in a way that burns the legs without exhausting the lungs.

The hallmark workout is the weighted uphill climb. Imagine fast-hiking a steep incline (10–30% gradient) with 10–15% of your body weight in a pack. The pace isn't fast, and your breathing is steady, but your quads and glutes are on fire. Evans and Croft did these sessions weekly, sometimes accumulating over 1,000m of climb loaded with water bottles in their packs before emptying them and running downhill at a strong pace.

Other variations include:

• Tire drags or resisted uphill walking

• Stair machine sessions with a weighted vest

• Gym circuits: step-ups, lunges, squat jumps, and high-rep movements targeting running muscles

The key principle: local muscular fatigue should be the limiter, not breathing. If your lungs are the bottleneck, you’re doing speed endurance. If your legs are the bottleneck, you’re training muscular endurance.

Where Do Heavy Lifts Fit In?

A natural question is whether Johnston’s athletes also spend time on heavy barbell lifts - deadlifts, squats, cleans. The short answer: yes, but sparingly and with purpose.

He sees traditional strength work as laying the foundation: general strength and stability, joint resilience, and connective tissue robustness. Early in a training cycle, or in the off-season, athletes may include squats, deadlifts, and Olympic lift variations to build that base. However, as the season progresses, Johnston shifts focus toward sport-specific muscular endurance rather than chasing higher one-rep maxes.

The heavy lifts are there to make the athlete more durable and balanced. The race-specific durability comes from muscular endurance sessions like weighted climbs and eccentric downhill running. In Johnston’s words, if you can squat twice your bodyweight but can’t run downhill for an hour without your quads failing, the strength isn’t translating. So he uses the gym, but always as a complement — never as a replacement for specific endurance strength.

For highly specialised elite ultrarunners, heavy lifting eventually becomes secondary. But for many recreational athletes, lifting heavy can be a very good starting point. Building a solid base of strength through compound lifts like squats and deadlifts improves robustness, posture, and injury resistance. Once that foundation is established, layering in muscular endurance work ensures that strength carries over into long runs and races.

Speed Endurance vs. Muscular Endurance

Both aim at the same ultimate goal: holding pace when fatigue sets in. But they go about it differently:

In practice, they complement each other. Muscular endurance work builds a stronger chassis so that when you do speed endurance sessions, your legs don’t fail before your engine.

How to Apply This to Your Training

You don't need to be prepping for UTMB to benefit. Even training 3–4 times a week, a single ME session can make a noticeable difference in how strong your legs feel late in a long run or race.

For trail runners & swimrunners:

• Add one weekly weighted uphill session. Start with 20–30 minutes total uphill time at a brisk hiking pace, with 10–15% of your body weight in a pack.

• Progress by increasing either the duration or the load—but not both at once.

• Plan for 48 hours of easier training afterward.

For road runners:

• Use gym-based sessions: circuits of lunges, step-ups, squat jumps, and split jumps. Six to eight sets of 8–12 reps per movement is a good start.

• Or incorporate hill circuits (short, steep reps at a hard hiking or running effort).

• Use ME training earlier in the season, then transition to race-pace work closer to your marathon.

For everyone:

• Keep your aerobic base and speed work. Muscular endurance doesn't replace the easy miles and speed work; it enhances them.

• Less is more. Once a week is plenty.

• Recovery matters. Expect DOMS; don’t pair ME with your hardest intervals or longest run.

ÖTILLÖ vs. Marathon: Different Demands, Same Lesson

After eight-plus hours of running in wet shoes over rocks and trails at ÖTILLÖ, it wasn’t aerobic fitness that was a constraint - it was muscle breakdown. In races this long and rugged, muscular endurance is arguably the #1 limiter.

Now I’m switching gears. My next big goal is the Valencia Marathon on December 7th. A marathon is a very different event: flatter, faster, and much shorter than ÖTILLÖ. Aerobic capacity, pacing, and fueling are front and center. But even at 42.2 km on the road, the same truth applies: most runners don’t slow down because they can’t breathe - they slow down because their legs fall apart.

That’s why I’ll be keeping muscular endurance in my program as I train for Valencia. Not with a weighted pack up a thousand meters of climb, but with gym-based sessions (lunges, step-ups, squat jumps) and strategic hill work. The aim is simple: to reach 35 km in Valencia with a chassis strong enough to let the engine do its job.

Closing Takeaway

Scott Johnston’s UTMB champions didn’t just have big engines; they had chassis built to last.

That’s the lesson I’m taking forward: it’s not enough to train the lungs and heart. We also need to train the legs’ ability to resist fatigue. For me, Speed Endurance will always have its place, but muscular endurance is the missing piece that keeps the whole system working when the race gets long.

If you’ve ever finished a race with air in the tank but legs that wouldn’t turn over, you know exactly what I mean.

Time to strengthen the chassis!