How Quickly Do We Lose Fitness?
The Real Timeline of Detraining
By Alicja Pilarska · Training
We all face setbacks in training for different reasons. An injury, illness, family commitments, work deadlines, or simply the chaos of everyday life can suddenly pull us away from the trails. One minute you’re logging steady miles and feeling strong on the climbs; the next, you’re sidelined and wondering how quickly your hard-earned fitness is slipping away. Understanding exactly how and when we lose fitness takes the fear out of the situation and gives us clear, practical tools to protect what we’ve built and return stronger.
What Is VO₂ Max and Why It Matters on the Trails
VO₂ max is your body’s maximum rate of oxygen consumption during intense exercise. Think of it as the size of your aerobic engine — the single best measure of cardiovascular fitness. It tells us how efficiently your heart, lungs, blood vessels, and working muscles deliver and use oxygen.
VO₂ max is typically expressed in millilitres of oxygen per kilogram of body weight per minute (ml/kg/min). For recreational trail runners it often sits between 40–55 ml/kg/min, while well-trained club runners commonly reach 55–65 ml/kg/min and elite ultra runners can exceed 70 ml/kg/min. It is usually measured in a lab on a treadmill while wearing a mask that analyses the air you breathe as the speed and incline gradually increase until exhaustion. While genetics set an upper limit on your potential, VO₂ max is highly trainable — consistent endurance running combined with targeted interval sessions can produce significant gains even in experienced athletes.
For trail runners, a strong VO₂ max translates into greater climbing power on steep, technical ascents, better ability to hold a steady pace over long distances, faster recovery between efforts, and improved resilience when the terrain turns tough.