Why Heart Rate Isn’t Always the Best Guide for Running Performance

I get more questions about heart rate than almost anything else when it comes to running. The endurance world is filled with thresholds, zones, and heart rate-based prescriptions. It feels measurable, controlled, and, at first glance, safe.

And it does work sometimes.

For low-intensity runs, heart rate can be a useful guide. Below your second threshold, the relationship between pace and heart rate is relatively stable and proportional. As pace increases, heart rate rises in a predictable way. For easy runs and building aerobic volume, heart rate gives you a reasonable idea of effort.

The problem starts when you try to use heart rate where it no longer applies.

Once you spend real time doing hard intervals or threshold work, the cracks begin to show. Some days, your heart rate refuses to climb no matter how hard you push, and you feel awful anyway. Other days, it spikes higher than expected at paces that normally feel controlled. The signal becomes inconsistent and frustrating.

Why? Once you cross your second threshold, the relationship between pace and heart rate breaks down. Speed can increase without a matching rise in heart rate, or heart rate can spike disproportionately relative to pace. Fatigue, heat, sleep, stress, hydration, and accumulated load all interfere with the signal.

At higher intensities, heart rate becomes unreliable. If you let it dictate your training here, you are reacting to noise instead of training the system that actually drives performance.

So what should you focus on instead? Pace and RPE, and power if you have access to it. These metrics show what actually matters: what you are producing and what the effort feels like.

If your goal is to get faster, more durable, and more capable under pressure, your focus has to shift outward. Internal load will not tell you whether you can adapt to external demands. Pace and time will.

Stop worrying about what your heart rate says and start paying attention to your performance. Are you running faster? Are you covering more ground in the same amount of time? Are you becoming harder to break?

For a deeper look at how to structure intervals, apply intensity correctly, and actually train for speed, check out my free Running Interval System Guide. It lays out exactly how I program faster work so effort turns into adaptation instead of confusion.

Download your free Running Interval System Guide!

Never say uncle.

-Cody Miller