Late-race slowdown isn’t about luck or willpower—it’s measurable changes in your body. This article explains three science-backed signals: Decoupling (heart rate rising relative to speed), Cardiac Cost (how much your heart must work to hold a given speed), and Cadence decline. We translate these changes into plain language so you can recognize when things start to slip during races and long runs, and adjust pacing and form with confidence.
1. Decoupling (heart rate rises relative to speed)#
What’s happening#
You keep the same pace, but your heart rate creeps up. That means your body now needs more cardiac work for the same output. With rising carbohydrate reliance and thermoregulatory strain, the earlier proportional relationship between heart rate and speed breaks down. For many runners, this shows up around 25 km: the feeling of “cruising comfortably” fades. Decoupling quietly signals a drop in efficiency—often before you consciously feel it.
What research shows#
In an analysis of 82,303 marathoners, a typical pattern looks like this: a runner starts at 4:30/km with HR 150, and by 30 km still holds 4:30/km but HR climbs to 174—roughly a 16% increase. Classifying by this rise: under 10% (Low) tends to preserve efficiency; 10–20% (Moderate) shows mild fade; 20%+ (High) flags strong fatigue. For most, the rise begins around 25 km and strongly relates to late-race resilience.
Implications for runners#
When decoupling rises, don’t force the same pace at any cost—small pacing adjustments can save your finish.
References
- Smyth et al. (2022), Sports Medicine, n=82,303
2. Cardiac Cost (heart work required for a given speed)#
What’s happening#
Cardiac Cost captures how much your heart must work to maintain a set speed. As economy drops, the same pace demands a disproportionately higher heart rate. Subtle form drift, longer ground contact time, and reduced muscle-pump efficiency add up. You might feel you’re working harder for less pace—Cardiac Cost puts a number on that shift.
What research shows#
In a 14-runner study, two runners both hold 5:00/km: Runner A (HR 145) finishes in 3:30, Runner B (HR 165) in 4:00. Lower heart-rate-per-speed—better “fuel economy”—corresponds to faster finishes. A 280-runner study confirmed the same trend. In marathons, running the same speed at a lower heart rate is a late-race advantage.
Implications for runners#
Looking at heart rate or speed alone can mislead; watch the relationship between the two to avoid late-race fade.
References
- Billat et al. (2012); Billat et al. (2019)
3. Cadence decline (changes in step rhythm)#
What’s happening#
In the second half, step rate (cadence) tends to dip and ground contact time lengthens. That increases braking and reduces propulsion, so the same effort feels slower. This often becomes evident after 20 km, alongside more unstable pelvic tilt and general rhythm drift. Sensations like “contact lingers behind” or “push-off feels heavy” match these changes.
What research shows#
In race data from 1,437 runners, movement patterns shift around 20–22 km. Early race (0–20 km): cadence ~180 spm and ground contact ~0.22 s. Later (25–30 km): ~174 spm and ~0.26 s—about a 3% cadence drop and 18% longer contact, making each step heavier and longer.
Implications for runners#
Cadence dips are an early sign of form deterioration; restoring rhythm (not force) often recovers efficiency sooner.
References
- Miyazaki et al. (2025), n=1,437
Takeaways#
These three signals often appear together. Rhythm and contact begin to drift, efficiency drops, heart workload rises, and finally the heart rate–speed relationship breaks down. By tracking these shifts objectively, you can spot the turning point early. In the next article, we’ll dig deeper into measurement and interpretation tips, with practical examples for training and race day.