Individualization: The Last Frontier of Endurance Training?


, by Matt Fitzgerald

Photograph: AngrySun / Adobe

Earlier this year, the International Journal of Sports Physiology and Performance published a paper that was catnip to exercise science geeks like me. Twenty-five of the world’s most respected experts on endurance sports training were invited to share their views on the most important recent advances in their field and the most likely future advances. Common themes were extracted from the responses received and summarized by the study’s organizers.

I’d like to focus here on the forward-looking part of the study, so I’ll skip over the retrospective part except to note that there was widespread agreement that recent advances in sports science had made a significant positive impact on endurance training, especially at the elite level. As for the future, the experts predicted further progress in four key areas: heavier use of technology for training prescription and monitoring; more precise use of heat exposure, altitude training, and nutritional interventions to improve fitness; a better understanding of the relationship between athletes and their equipment; and a greater focus on preventing injury and illness.

What stood out to me in reading this paper was a certain throughline linking all of these predictions. The words “individualized” and “individualization” appear seven times in three pages, suggesting that the common purpose of the innovations foreseen by scientists is to better meet athletes’ individual needs. While nobody asked for my opinion, I happen to concur. As a coach and an exercise science geek who has closely followed advances in endurance training for more than 25 years, I feel we’ve gotten a pretty good handle on which methods work best for athletes generally, but there’s work to be done in delivering optimized training prescriptions to individual athletes, who aren’t all the same.

MORE FROM MATT: Effort, Attitude & Judgment: Your Benchmarks of Success

While we wait for the experts to make new discoveries in this area, there’s a lot we can do right now to customize training to each athlete’s unique physiology, and it doesn’t take a PhD to do it. In my one-on-one coaching work, I pay attention to three key features of individual athletes’ response to training—aptitude, responsiveness, and tolerance—and then adjust their future training based on what they learn.

I pay attention to three key features of individual athletes’ response to training—aptitude, responsiveness, and tolerance—and then adjust their future training based on what they learn.


“Aptitude” refers to an athlete’s relative strength or weakness in the areas of speed (maximal intensity), aerobic capacity (high intensity), lactate threshold (moderate intensity), and endurance (low intensity). Consider two runners who run the same time in a 10K race. Even though they are equals by this measure, one runner might tend to perform better than the other in short sprints, longer intervals, tempo runs, or long runs based on natural aptitude.

A simple way to determine your strengths and weaknesses as a runner is to enter a personal-best race time in an online calculator such as the Strava Running Pace Calculator and look at the resulting pace guidelines for various workout types. If the paces seem easy for a particular type of run, you probably have a natural aptitude for the component of running fitness targeted by that workout, and if the paces seem too hard, it’s likely you have a lower aptitude in this area compared to runners of similar ability.

Want to calculate your target pace? Check out Strava's Run Pace Calculator

There’s a tendency among runners to skew their training in the direction of their aptitudes. This is a mistake. Your training should be balanced in a way that fits the demands of your next important race, regardless of how well these demands match up with your natural strengths. However, it is advisable to consider your strengths and weaknesses in setting pace targets for different workout types, allowing yourself to go a little faster than the calculators advise in those you’re good at and contenting yourself with going a bit slower in those that challenge your weaknesses.


If aptitude determines how well athletes perform in different types of workouts independent of training, responsiveness determines how quickly athletes improve when exposed to various training stimuli. In general, it takes longer to improve endurance lactate threshold and longer to improve aerobic capacity than speed, but there’s a high degree of variance between individuals.

Photograph: PAstudio / Adobe

If you notice that you need a lot of exposure to a certain type of workout to see meaningful improvement, plan your training in a way that gives you enough time to get where you want to be for your next important race. And if you notice that you gain fitness quickly in response to a certain training stimulus, take advantage of this by planning in a way that minimizes risk. For example, if your aerobic capacity increases quickly in response to longer high-intensity intervals, introduce this type of workout later in the process, thereby minimizing the risk of getting “too much of a good thing.”


As I’ve just indicated, training carries a certain amount of risk. Each type of workout imposes a certain amount of stress on the body, and with stress comes fatigue and tissue damage—and with fatigue and tissue damage comes the possibility of overreaching (a kind of physical burnout) and injury, respectively.

LOOKING FOR TIPS & ADVICE? There's more here

Tolerance refers to an athlete’s capacity to handle various training stimuli without overreaching or getting injured. Some athletes are fortunate to have a high general training tolerance, but most athletes have a higher tolerance for certain training stimuli than they do for others. The way to determine your training tolerance is to pay attention to your recovery after different types of workouts. How long after your last speed workout, VO2max interval set, tempo session, or long run or ride does it take before you’re ready for the next hard workout (of any type)?

The common-sense (and correct) response to discovering a lower tolerance for a particular type of workout is to be judicious in your planning, incorporating less of this stimulus in your program than another athlete of similar overall ability might do. Likewise, if you discover a high tolerance for a certain workout type, you have a green light to go a little heavier with this stimulus in your planning.

An Experimental Approach

It’s nearly impossible to predict how a given athlete will react to endurance training in the dimensions of aptitude, responsiveness, and tolerance. Instead of allowing yourself to be intimidated by this uncertainty, have fun with it, approaching the process as an ongoing experiment that is periodically rewarded by useful discoveries.