While high-intensity interval training (HIIT) hogs the attention of the popular media, science has repeatedly shown that both elite and amateur athletes improve most when they spend about 80% of their weekly training time at low intensity (i.e., in Zone 2).
There’s a difference between knowing and doing, however. Research has demonstrated that, whereas virtually all elite endurance athletes adhere to an approximate 80/20 intensity balance, only a fraction of recreational athletes do so. That’s likely because they’re skeptical of the idea that slowing down in training will make them faster in races. This article will prove that low-intensity training really does work.
What is low-intensity training?
One reason so few recreational endurance athletes balance their training intensities optimally is that they don’t know what low intensity is. There is a growing consensus among exercise scientists that the dividing line between low and moderate intensities is set at the first ventilatory threshold (VT1), which falls between 77% and 81% of maximum heart rate in most trained individuals. Put into laymen’s terms, VT1 marks the upper boundary of heart rate Zone 2, which means that training at low intensity equates to training in Zone 2 or even lower.
Athletes who make the effort to determine their VT1 — which can be done simply by finding the highest speed, power, or heart rate at which you can speak comfortably in complete sentences — are often surprised to discover that staying below it in designated low-intensity (or Zone 2 training) sessions requires them to go significantly slower than they are accustomed to.
How close to VT1 should I train?
This is an interesting question. VT1 places a ceiling on low intensity, but there’s a lot of room underneath it. Should you consistently train just below your VT1, thereby making your low-intensity work as intense as allowable, or is it okay or even better to train well below it? Essentially, how slow is too slow?
Science has no clear answer to these questions as they relate specifically to competitive endurance athletes. The closest thing we’ve got is a collection of studies on the effects of low-intensity exercise on aerobic capacity (aka, VO2max) in nonathletes. A review of such research found that an exercise intensity equal to 30% of VO2 reserve — the difference between resting oxygen consumption and maximal oxygen consumption — was the minimum requirement for increasing VO2max in unfit individuals, while 45% of VO2 reserve was the minimum requirement in fitter individuals.
For the average trained athlete, 45% of VO2 reserve equates to roughly 68% of maximum heart rate, or the lower range of Zone 2. Therefore, you’ll need to stay within Zone 2 to increase your aerobic capacity.
Can I really get fitter by training at such a low intensity?
Yes and no. If you’re coming off a break in training and are therefore far below your peak fitness level, you can expect to gain aerobic fitness as long as you continue to increase the volume of training you do at this intensity. Once your training volume plateaus, however, so will your aerobic capacity.
If you’re already fit as a result of consistent and progressive training, then you’re not even relying on your low-intensity sessions to increase your fitness, so it doesn’t really matter. For athletes in this situation, fitness gains come mainly from the two or three key workouts that are included in each week’s planning: high-intensity interval sets, moderate-intensity tempo blocks, and long endurance workouts. Your easier low-intensity sessions serve merely as a foundation that supports these key fitness builders.
Further evidence that athletes need not worry about going too slow in their low-intensity sessions comes from a recent large-scale study that was published in Nature in 2020. Training and race data were collected from devices worn by more than 14,000 runners for a combined 1.6 million exercise sessions. An analysis of the data revealed that faster runners (as judged by race times) trained at a significantly lower average intensity than slower runners. In fact, Emig and Peltonen noted that runners whose VO2max pace was around 5:20 per mile did their easy runs at about the same pace as runners whose VO2max pace was around 6:40 per mile.
Training Like the Pros
In addition to looking at the relevant science, it’s always helpful to look at the habits of elite athletes when trying to answer training-related questions such as “How slow is too slow?”. If you do, what you will find is that most of them perform their low-intensity workouts by feel, eschewing numerical targets other than perhaps a heart-rate ceiling corresponding to their VT1, and letting their body dictate the effort. The result is a pattern of significant day-to-day fluctuation in pace or power in these training sessions. On days when they’re feeling fatigued from prior training, the elites go slower; on days when they feel fresh and strong, they go a little faster. The one thing that’s consistent is that they make sure these sessions always feel comfortable, whatever is required.
Rules for Successful Low-Intensity Training Sessions
- Don’t exceed the first ventilatory threshold (VT1), or the highest speed, power, or heart rate at which you can speak comfortably in complete sentences.
- Don’t dip below 45% of your VO2 reserve, which equates to roughly 68% of maximum heart rate (or Zone 2) — something you’re very unlikely to do anyway.
- Allow your specific pace or power to fluctuate between low-intensity sessions, always choosing an effort that feels comfortable in the moment.
In summary, while it is theoretically possible to train too slowly to gain any meaningful fitness benefit, it’s more likely that you need to go a little slower than you do currently in some of your designated low-intensity sessions to get the greatest possible benefit from them.
References
Emig, T. & Peltonen, J. (2020, October 6). Human running performance from real-world big data. Retrieved from https://www.nature.com/articles/s41467-020-18737-6
Swain, D.P. & Franklin, B.A. (2002, January). VO(2) reserve and the minimal intensity for improving cardiorespiratory fitness. Retrieved from https://pubmed.ncbi.nlm.nih.gov/11782661/
