Trail Tips

When (and Why) to Train Below Aerobic Threshold

Run too fast some of the time, and you’ll probably get slower all the time.

That is the running-training paradox— a paradox that has led to the downfall of countless motivated trail runners through injury, burnout and reduced aerobic efficiency.

How It Unfolds

The story plays out so often that it’s like a horror movie running on a loop on a dysfunctional DVD player. A trail runner has success through consistent, moderately hard running. That trail runner thinks that his/her success comes from the moderately hard part, rather than the consistency. At that point, coaches and experienced runners usually start screaming “Don’t run too hard!” and “Slow down!” just like a horror-movie watcher will scream “Don’t go into that abandoned chainsaw warehouse!” But the trail runner keeps on pushing, venturing into the foreboding warehouse, oblivious to the chainsaws around the corner.

Often, that training horror movie does not have a happy ending. In the best cases, the trail runner starts stagnating. In the worst cases, the trail runner starts getting stress injuries or experiencing symptoms of overtraining syndrome.

Many times when professional runners reach out for coaching, they are at the inflection point, starting to feel run down and injury-prone from running a bit too hard, too often. But it’s not just pros. Most trail runners seem to struggle with the running training paradox, especially early on in the running journey.

Aerobic Threshold Defined

The most important physiological variable to understand to avoid being the star of your own training horror story is aerobic threshold. Aerobic threshold is the intensity range at which the body switches from primarily relying on fat oxidation for fuel to primarily relying on carbohydrates. Below aerobic threshold, the body has enough oxygen to function without producing significant amounts of lactate and other associated byproducts that build up with harder exercise. Above aerobic threshold, breathing rate increases and lactate levels begin to build up, plus there may be a bit more muscle damage. That is: above-aerobic-threshold training takes longer to recover from.

At even harder efforts, your body produces more lactate than it can use and waste products accumulate without being cleared. That tipping point is called Lactate Threshold.

For training purposes, it’s not helpful to think of aerobic threshold as a specific point. Instead, think of it as a range of intensities that vary slightly over time, depending on age, psychological stress, weather and many other variables. Bottom line: it’s when you transition from easy to moderate exertion, to a slightly harder effort with deeper breathing and a less-sustainable pace.

How to Figure Out Your Aerobic Threshold

The best way to calculate aerobic threshold is with a metabolic test in a lab. But there are a few other ways to approximate it. Coach Joe Friel estimates that aerobic threshold occurs at a heart rate of around 20 beats per minute below lactate threshold heart rate, which can be estimated easily (in my coaching experience, aerobic threshold is usually around 85 percent of LTHR, or 25 to 35 beats away, with the exception of very highly trained athletes or those 50+ years old, as discussed by Coach Gordo Byrn).

Dr. Phil Maffetone developed the famous (or infamous, depending on your perspective) maximum aerobic function (MAF) formula, which ballparks “MAF” heart rate at 180 minus your age. This often gets close to aerobic threshold. The exact heart-rate number is less important; what’s important is figuring out what truly constitutes an easy or hard effort for your physiology and background. Aerobic threshold should be a five or six perceived exertion on a scale of 1 to 10, or a mostly conversational effort you could hold for an extended period of time.

Why Aerobic Threshold Matters

Training too much above aerobic threshold can be a ticking time bomb for health and long-term aerobic development. First, the increased stress of training above aerobic threshold often causes an increase in the stress hormone cortisol; second, it can cause higher levels of muscle breakdown and forces your body to absorb more impact forces.

If your stress levels—physical or psychological—are higher than they should be on easy days, the body won’t be able to recover from hard days. If stress keeps piling up without adequate recovery, the body often breaks down through fatigue and injury.

Even if break-down doesn’t happen, too much running above aerobic threshold can actually cause you to get slower. High-volume aerobic training leads to increased aerobic enzyme activity; it also spurs your body to grow more capillaries to transport oxygen and fuel to working muscles, and more mitochondria, the cellular power centers that convert oxygen and nutrients to energy.

Train too hard too often, and you’re neglecting the aerobic base necessary for reaching your performance potential at all distances, including shorter trail races. To run fast when it counts, you need to get comfortable running slowly.

There’s an added peril for trail runners: uphills. Most runners find it difficult to stay below aerobic threshold on climbs. As a result, runners who spend lots of time on steep or mountainous terrain might break down sooner than someone training without big climbs, if they aren’t careful to take some days easier.

Think of training like a big salad, with time below aerobic threshold being the greens, time closer to lactate threshold being the dressing and time above lactate threshold and VO2 max being the bacon. A salad with just greens isn’t going to get a five-star yelp review; likewise, a cup of dressing and bacon might taste okay, but it’s not a well-balanced meal.

3 Tips for Putting Your Aerobic Threshold to Work

1. Get an understanding of what your aerobic threshold feels like.

You can approximate your aerobic threshold using a heart-rate monitor or perceived exertion. The MAF test works for many runners, though it can be off for physiologies that vary from average. I often coach the athletes I work with to do a Friel Lactate Threshold Heart Rate test—a 30-minute time trial, averaging your heart rate over the final 20 minutes to get LTHR. Multiply that number by 85 percent to roughly approximate an effort cap for easy runs (though adjusting to a different percentage if you feel like that’s a bit too hard or too easy).

You can also use perceived exertion to determine what your “easy” should be. Remember, “easy” does not mean the fastest pace you can go while finishing your run intact; it means truly relaxed and comfortable, talking mostly in complete sentences, with no urge to stop.

The goal isn’t to run at your aerobic threshold all the time, but to use it as a general guideline to prevent you from going too hard. It’s okay (and recommended) to do easy runs well below that number; it’s also OK to go above that number sometimes. But you shouldn’t spend lots of time above aerobic threshold without a training goal in mind.

2. Hold yourself accountable to aerobic threshold for most of your running.

Most of your training should be below aerobic threshold. A typical week for an athlete I coach is:

Monday: rest
Tuesday: Run below aerobic threshold (often with short, fast strides)
Wednesday: Interval workout (with warm-up and cool-down below aerobic threshold)
Thursday: Run below aerobic threshold (sometimes with faster finishes)
Friday: rest or run below aerobic threshold
Saturday: Long run or workout with time spent above aerobic threshold
Sunday: Run below aerobic threshold (often with short, harder strides or faster finishes)

The two days in bold are the days to play with truly hard efforts. All the other days are below aerobic threshold, with variation depending on the athlete. This approach lets you build aerobic volume while minimizing injury risk.

3. Listen to your body.

The body sends stress signals in a lot of different ways, from persistent fatigue, injury or insomnia to abnormal changes in sexual function. If you notice yourself feeling a bit worn down, consider spending more time below aerobic threshold to allow your body to recover.

David Roche runs for HOKA One One and NATHAN, and works with runners of all abilities through his coaching service, Some Work, All Play.