August 6, 2025
EF stands for Efficiency Factor, a performance metric that can become a secret weapon in your training arsenal. For experienced runners eyeing big goals (say, a Boston Marathon qualifying time), understanding EF can help you train smarter. In this post, we’ll demystify Efficiency Factor, explain how it relates to your heart rate and pacing, and show how you can use it to gauge aerobic fitness progress. We’ll also see how concepts like Heart Rate Reserve (HRR) tie in, and how platforms like NXT RUN (and its AI coach, Brio) leverage EF data to fine-tune training. Let’s dive in with a conversational, no-nonsense approach – just like the title, we’re playfully curious but serious about performance!
Efficiency Factor (EF) is essentially a measure of how much speed or distance you get out of each heartbeat. In other words, it’s an indicator of how efficiently your body is turning heartbeats into forward motion. Mathematically, EF is defined as your running speed (pace) divided by your heart rate for a given run or segment. In practice, we usually use an adjusted speed called Normalized Graded Pace (NGP) – which accounts for hills – and your average heart rate over the run. So you might see it expressed as:
EF = Normalized Pace ÷ Average Heart Rate
For example, suppose you run at an adjusted pace of 7:30 per mile with an average heart rate of 150 bpm. Your EF would be about 1.56 (in units of say, meters per beat), meaning you cover roughly 1.56 yards per heartbeat in that effort. The higher this number, the more distance you’re traveling for each beat of your heart – which is a good thing! High EF = getting more out of every heartbeat.
Now, where do these numbers come from? Modern training platforms (like TrainingPeaks, NXT RUN, etc.) calculate EF automatically from your GPS and heart rate data. Your watch provides your pace and distance, and your heart rate monitor gives effort data. If your route has hills, the software will adjust your pace to a level “as if” you ran on flat ground – that’s the Normalized Graded Pace (NGP). It then converts NGP into a true speed (e.g. meters per minute) and divides by your heart rate. Voila – you have your Efficiency Factor for that run. Essentially, EF is telling you how many meters you ran per heart beat. It’s like a proxy for your running economy that you can measure on any given day.
Okay, so EF gives a number for how efficient you are on a run – but why should you care? The short answer: EF is a window into your aerobic fitness. In exercise physiology, a truly efficient runner uses less energy (and oxygen) to maintain a given pace than a less efficient runner. If you went to a lab, they’d measure your oxygen consumption while you run at a submaximal speed; the less oxygen you need, the more efficient (economical) you are. Elite runners, for instance, can hold a fast pace with remarkably low effort – they make it look easy because, in a way, it is easier for their highly trained bodies.
EF translates this lab concept into everyday training. Instead of oxygen, it uses heart rate as a stand-in for effort or “cost.” Heart rate correlates with how much energy your body is burning – as your speed increases, your heart rate climbs to supply more oxygen to muscles. By looking at how fast you run versus how high your heart rate gets, EF effectively measures your efficiency: how much performance (pace) you get for a given effort (heart rate). An increasing EF over time means you’re becoming more aerobically fit and efficient. You’re either able to run faster at the same heart rate or hold the same pace at a lower heart rate. Either scenario is a win for your training progress.
On the flip side, if EF is dropping for similar runs, it might indicate fatigue or a setback – you’re getting less speed per heart beat than before. That’s why coaches like Joe Friel suggest tracking EF for comparable aerobic workouts over the weeks. When conditions are kept similar (same route, similar effort level, decent weather, etc.), an upward trend in EF is a clear sign of improved aerobic capacity. It’s a bit like tracking your car’s fuel efficiency: if you’re going farther on each “gallon” of heartbeats, your engine (cardio system) is getting tuned up.
In the chart above, for instance, you can see a hypothetical runner’s EF rising about 7% over 8 weeks of base training – from roughly 1.50 to 1.60. That kind of improvement signifies a meaningful boost in aerobic fitness. Being able to sustain a higher EF means when race day comes, you can maintain your goal pace at a lower percentage of your max effort. It’s like having extra gears to spare, which is especially crucial in a marathon. This is why efficiency matters: it can translate to better endurance and performance.
Technical note: Make sure you compare EF only under aerobic conditions. EF is most insightful during steady, submaximal runs (below your lactate threshold). If you try to compare EF from an easy run to EF from a track interval session, it’s apples to oranges – higher intensity or interval workouts won’t follow the same efficiency rules because you’re tapping into anaerobic energy. So, keep your EF comparisons to similar-type runs (e.g., your weekly long aerobic run or a routine tempo at marathon effort) to truly gauge progress.
Now, let’s connect EF to Heart Rate Reserve (HRR) and the idea of pacing smartly by heart rate. HRR is another metric experienced runners often use – it’s basically the difference between your maximum heart rate and your resting heart rate. If you know these two numbers, you can express your exercise intensity as a percentage of your heart rate reserve. Many training plans use HRR (via the Karvonen formula) to set heart rate zones that are personalized to your fitness. For example, an easy aerobic run might target ~60–70% of your HRR, while a threshold workout might hit 85%+ of HRR. This method is considered more precise than just using a flat percentage of max HR, because it accounts for your resting heart rate (a lower resting HR typically means you have more “room” to work with, signifying better fitness).
So, how does this tie into EF? Smart pacing means using metrics like HRR to guide your effort during runs, rather than arbitrarily running at a certain pace that might be too hard or too easy. If your training schedule calls for an aerobic base run, you might aim to keep your heart rate around, say, 70% of your HRR. That ensures you’re squarely in the aerobic zone. While you’re doing that, EF is quietly measuring your output-input ratio: how much pace (output) you’re getting for that heart rate (input).
Over time, if you stick to these heart-rate guided intensities, you should see your pace at 70% HRR get faster as you get fitter – meaning your EF for those efforts will increase. For instance, maybe in week 1 of training, 70% HRR correlates to a 9:00/mile pace. A couple of months later, 70% HRR might yield 8:30/mile pace for you. Your EF at that effort has gone up, because for the same relative heart effort, you’re running 30 seconds per mile faster. This is exactly the kind of trend you want to see as an endurance athlete.
Heart Rate Reserve is also useful for ensuring consistency when tracking EF. If you always do your EF test or benchmark runs at a defined HRR range (like an “aerobic threshold run” around 75% HRR), you remove some variability. You know you’re comparing apples to apples – same relative effort level each time, so changes in EF aren’t due to you accidentally pushing harder, but due to actual fitness gains. It’s a smart pacing strategy to use HR (and HRR) as a governor on your pace, especially on easy days, so you don’t sabotage the efficiency metrics by going too hard.
Additionally, watching EF together with HRR can clue you in to when to adjust training. If you notice that your EF has plateaued even though you’re consistently training at a given HRR, it might be time to mix things up (perhaps add more mileage or intensity, or conversely, back off if you’re fatigued). Or if your resting heart rate drops over the training cycle (increasing your HRR because your max HR stays same but resting is lower), that’s a sign of improved fitness – and you might expect EF improvements to follow as your heart becomes more efficient.
One more concept related to EF and pacing is aerobic decoupling (also tracked in TrainingPeaks as Pa:HR or Pw:HR). In a well-paced long run, your EF should stay relatively steady; if your heart rate starts to drift significantly upward for the same pace (i.e., EF drops in the second half of a long run), it indicates your aerobic endurance might be breaking down . Keeping an eye on EF during long steady efforts is thus a way to ensure you’re not unknowingly redlining. In practice, you don’t need to calculate this on the fly – but after the run, you can see if EF in the first half vs second half changed. Ideally, it remains close (low decoupling), which shows good endurance. If not, that’s a signal to improve your aerobic base further.
Key takeaway: Use heart rate (especially HRR%) to guide your pacing – this keeps you in the right zone – and let EF data reveal how your speed at those efforts is changing. Together, HRR and EF help you train “smarter, not harder.” You’ll be running with intention, ensuring your easy runs are truly easy (but productive) and getting tangible feedback as your aerobic engine improves.
It’s one thing to know about EF and HRR in theory, but how do you apply it? This is where tools and platforms can add a ton of value. A great example is NXT RUN, an advanced running training app that treats performance metrics very seriously. Its built-in AI coach, Brio, crunches numbers like EF and heart rate data behind the scenes to personalize your training plan. The goal isn’t to overload you with stats, but to use those stats to make smarter decisions for you.
For instance, NXT RUN’s AI coach monitors your Efficiency Factor over time for your steady runs and long runs. If Brio notices your EF steadily improving, it might signal that your aerobic base training is paying off – maybe the AI will then adjust your paces a bit faster or introduce a slightly higher-intensity workout, knowing you can handle more now. On the other hand, if EF is stagnating or dropping despite consistent training, Brio might flag that you’re not recovering enough or that you need more low-intensity volume before piling on speed work. In this way, EF becomes an early indicator of whether your training is on the right track, and the AI coach responds to keep you progressing.
Similarly, Brio looks at your heart rate reserve and trends in your resting and max heart rate. Because NXT RUN asks for or helps estimate your HR max and resting HR, it can tailor your heart rate zones very precisely. The AI coach uses these zones (based on HRR) to guide your workouts – for example, ensuring your “easy” runs are truly in the easy zone (which many runners have a hard time doing on their own!). By doing so, it guarantees that EF measurements during those runs are valid indicators of aerobic fitness (since you’re not accidentally turning an easy run into a moderate one). Essentially, the platform automates the smart pacing strategy we talked about earlier. You train at the right intensities, and the app tracks how your pace/heart-rate relationship (your EF) is evolving.
What’s great is that all this happens without over-marketing or hype – you just see the results in your improved race predictions, or in the feedback Brio gives you after a run. NXT RUN treats metrics like EF, HRR, VO₂ max, etc., as serious inputs to coaching decisions, not just flashy numbers. This demonstrates the platform’s commitment to evidence-based training. As a runner, this builds trust – you know the guidance isn’t random but grounded in data from your own body. It’s like having a coach who’s deeply knowledgeable about sports science, but also watching your individual trends closely and adjusting on the fly.
So when you see Brio recommend, say, sticking to a 8:45/mile pace on your long run because “we want to keep this in Zone 2 to build efficiency,” you can nod and understand that means: let’s keep your heart rate in check so we can boost that EF! And when after a few weeks the app notes “Your aerobic efficiency is improving,” that’s EF at work, giving you a virtual high-five. By leveraging EF and HRR together, NXT RUN ensures that training is personalized, adaptive, and geared toward making you a more efficient runner – exactly what you need to reach tough goals like Boston qualifiers.
The phrase “what the ef is EF” might have started as a cheeky title, but by now you should have a solid grasp of Efficiency Factor and its role in run training. To recap a few key points:
In a friendly, conversational nutshell: Efficiency Factor is a big deal for those of us chasing performance. It distills complex physiology into a handy number you can track. Rather than getting lost in the weeds of VO₂ max tests or lactate thresholds, EF gives you a practical way to see if your aerobic engine is getting stronger. And with tools that automatically analyze it for you, you can focus on executing your training while still reaping the insights.
Next time you head out for a run with your heart rate monitor, think about EF. Keep the effort where it should be, and later check that EF score. Over weeks and months, watch that number (hopefully) climb. There are few things more satisfying in marathon training than seeing hard evidence of your improvements. It’s like getting a report card that says “Yep, you’re more efficient than you were last month.” That confidence boost is huge – and it’s backed by real data.
So, what the ef is EF? It’s your friend! Use it to train smarter, gauge your progress, and ultimately, toe the start line of your next race knowing you’ve become a more efficient running machine. When you’re 20 miles into the marathon and everyone around is fading, you’ll be glad you spent time building that efficiency. Every heartbeat counts – make them count for more.
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