Why Does My Heart Rate Drop During an Ultra?
- Henriette Albon
- Jun 18
- 6 min read
Updated: 14 hours ago
Have you ever looked back at your ultra race data and thought: “Why is my heart rate going down, even though it still feels just as hard?”
You're not alone. This “reverse cardiac drift” is surprisingly common in longer trail races. It's the opposite of what we often see in shorter road marathons, where heart rate tends to rise over time.
In this post, I’ll share insights from my recent race at Saint Jacques by UTMB. We will explore some of the key reasons behind this downward heart rate trend. I’ll also offer some practical tips on what to do with that data.
Understanding Reverse Cardiac Drift in Ultras
Case Study: Saint Jacques by UTMB
A quick breakdown of the race:
Course profile: Mostly uphill until ~55 km, followed by a long, gradual descent to the finish. Although described as “downhill,” it felt relatively flat. The technical terrain in the final stretch kept me on my toes.
Conditions: Hot and sunny! Just over 20°C at the start, quickly rising to around 30°C by midday.
Strategy: I took the lead about three hours in and ran relatively controlled from there, aiming to avoid cramping or gut issues amid the heat.
Pacing: My grade-adjusted pace (GAP) stayed fairly consistent around 5:00 min/km, apart from a few fast early kilometers and a slowdown in the final 10 km. This was due to the more technical terrain and hotter conditions.
Heart rate: I used an optical arm strap for more reliable readings than a wrist sensor. The majority of the race was in zone 2, but I spent a good chunk of time in zone 1. My heart rate drifted downwards in the final hours.
Despite what felt like a consistent effort (RPE), my heart rate gradually declined throughout the race. This pattern is something many ultra runners will recognize. Here's my take on why this happened.
Reasons Why Your Heart Rate Might Drop in a Long Ultra
1. Leg Fatigue and Trail Technicality Lowers Output
As your legs tire, your stride becomes less powerful and efficient. You're no longer pushing off the ground as strongly, which means your muscles require less oxygen. As a result, your heart doesn’t need to work as hard to support them. Even if your perceived effort stays the same, your actual mechanical output drops. In simpler terms, it feels similarly hard, but you're doing less.
This was exactly my experience at Saint Jacques. After a niggle in April sidelined me for two weeks, my Cardio Fitness was well ahead of my Running Impact Fitness. My longest run before the race was around 3.5 hours — much shorter than race duration.
As expected, my legs began to stiffen and lose fluidity around the 4-hour mark. I could feel myself moving less smoothly, especially on flat and rolling terrain. While the effort felt consistently high, my power output was dropping, which likely explains part of the downward trend in heart rate.
Looking closely at my grade-adjusted pace on Strava, this makes sense. There are definitely more “white bars” from 50 km and onward. This was partly due to increased trail technicality as well, contributing to an “inability to access higher power output.”
2. Downhill Terrain Reduces Cardio Load
After the halfway mark, the course trended downhill. While this sounds easier on paper, it changes the type of stress your body experiences. Running downhill typically places less demand on your cardiovascular system, especially compared to climbing.
Even if your legs are sore or your brain is tiring, your heart rate may drop. This happens not because you're easing off, but because the terrain requires less from your “engine.”
It’s also important to note that heart rate responds slowly to changes in effort. So even if the trail kicks uphill briefly, those short spikes in effort might not clearly show up in your heart rate data. In contrast, your perceived effort adjusts more quickly, which is why you might feel like you're pushing, even if your heart rate doesn’t reflect it.
3. Fuel and Fluid Depletion
Underfueling or underhydrating during an ultra slows down energy delivery and reduces muscle efficiency. The reality is that in a race lasting many hours, it’s nearly impossible to meet 100% of your energy and hydration needs. No matter how well you plan, the deficit builds over time, leading to noticeable effects.
In this race, I managed my carbs, fluids, and electrolytes fairly well. However, the heat and duration still wore me down. By the final third, I could feel the edge coming off: cramping felt imminent, and maintaining pace became a grind.
I believe this was not purely a fueling issue. More likely, it was a combination of accumulated depletion and impact fatigue from limited long-run prep. As mentioned earlier, these factors made it harder to keep running strong to the finish despite feeling a consistent effort.
4. Gut Discomfort
Even mild gastrointestinal issues can cause your brain to ease off the gas as a self-protection mechanism. In the final 10 km of Saint Jacques, I had that familiar ultra feeling: not actually being sick, but hesitant to go faster for fear of pushing too hard.
With a 20-minute lead, I allowed myself to back off slightly to avoid any significant gut issues.
5. Mental Fatigue
This factor is substantial. As hours pass, your mental energy depletes, reducing your willingness to push. You may stop attacking climbs, play it safer on descents, and default to survival mode—especially if you're comfortably placed or primarily focused on finishing.
Even if your body can give more, your brain might not let it. Jon aptly puts it: If a lion jumped out at you, you'd probably be able to go faster. That alone proves the potential within you, but late in ultras, your brain tends to rein things in to manage discomfort and risk.
The difference between RPE (how things “feel”) and heart rate data is significantly linked to this mental factor. As the race drags on, so does the mental fatigue. Eventually, even running your usual long run pace feels like a battle.
Training mental strategies can help maintain output when your mind wants to check out. Techniques like chunking the race, staying present, and practicing late-stage effort can be beneficial. I also find that some good tunes can help shift my mindset. In this race, with a comfortable lead, I didn’t feel pressured to push harder than necessary. Consequently, I settled into what felt relatively “comfortable”—and my heart rate data reflect this.
Is Downward HR Drift a Problem?
Not necessarily. If your pace or GAP remains stable, with a drop off of less than 20%, and your RPE does not increase dramatically, a slow heart rate drift can signal efficient pacing and solid endurance.
However, if your pace drops significantly along with heart rate—especially on runnable terrain—it might reveal a gap; your legs are tiring before your heart and lungs do. This is a clear sign to adjust your training and prioritize more specific running.
Cardio vs. Impact Fitness: Why It Matters
Many trail runners, myself included, enjoy building cardio fitness through cross-training. However, if your “running impact readiness” isn’t up to scratch, you’ll still slow down late in the race.
That’s exactly what I observed at Saint Jacques. As I prepare for UTMB, I’m shifting my focus to specific running impact. Long runs at race pace, downhill fatigue tolerance, and overall volume on my feet will take precedence over cross-training.
I’ll likely reduce my total training hours to allow for more recovery. The goal is clear: I need to adapt to running at race effort on terrain similar to race day. This means utilizing AlbonApp training plans to build up impact fitness while remaining cautious of high-risk areas.
For me, this involves:
Shortened base-building blocks focused on Build Impact and Build Impact (Long Run Focused) week types.
A sharpening phase as race day approaches. This will include about three very specific Race Simulation week types to ensure I get some extended long runs in.
Each block contributes a piece to the larger puzzle. By race day, I hope all the bricks will be in place, even if my long runs in training won't match race duration.
Training & Racing Smarter: Practical Tips
Don’t chase heart rate. Use it as feedback, not a target.
Train late-stage effort. Try progressive long runs where pace or RPE rises in the final hour.
Avoid chasing Cardio Fitness as race day nears. Focus on run-specific work to ensure your running impact is adequate.
Fuel early and often. Practice this in training, not just on race day. Without sufficient fuel, it's game over sooner rather than later.
Know your course. Even downhills can harm your legs if you're not prepared.
Respect fatigue signals. Learn the difference between true exhaustion and brain-protection mode.
Final Thoughts
A downward heart rate trend during an ultra isn’t something to fear; it’s useful feedback. It encapsulates a blend of factors: terrain, fueling, hydration, fatigue, mental load, and how well your legs are equipped to handle impact over time.
For me, it’s a window into how my body and brain coped on race day. It's already shaping my training approach moving forward.
The biggest takeaway? Time at race pace truly matters. Whether you're targeting short, long, fast, or steady ultras, there’s no substitute for teaching your body and mind to stay efficient and resilient. That’s where the real adaptations happen.
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