In this episode, we explore why many trail and ultra runners feel that their breathing is still under control, but their legs simply stop working.

The main idea is that ultra running is not limited only by cardiovascular fitness. It is also limited by local muscular endurance: the ability of the quads, calves, glutes and stabilising muscles to keep producing force for hours.

Climbing, descending and technical terrain all place specific demands on the legs. Uphills require repeated force production, downhills create muscle damage through braking and impact, and trails constantly challenge stability and control. This is why a runner can still have a strong aerobic engine but lose performance when the legs are no longer able to absorb, stabilise and push.

The episode explains that the solution is not simply more mileage. Runners need race-specific preparation: uphill muscular endurance, downhill conditioning, purposeful hiking, and consistent strength training.

Key message: Train the engine, but build the chassis. In ultras, strong legs protect good fitness.

Key references:

• Millet et al. 2011 — neuromuscular fatigue in mountain ultras.
• Bontemps et al. 2020 — downhill running and eccentric damage.
• Pradas et al. 2021 — ultratrail racing, muscle damage and neuromuscular function.
• Balsalobre-Fernández et al. 2016 — strength training and running economy.
• Llanos-Lagos et al. 2024 — updated systematic review on strength training and running economy.