What is 'chronic training load' and why is it important to measure? I asked an expert to find out

Whatever your favorite sport, regular training puts a strain on your body. This strain is a good thing: it will make you fitter, healthier and, if that’s what you’re aiming for, faster. However, you have to apply the right amount of stress to your body: too little and you won’t get fitter, too much and you run the risk of exhausting yourself and becoming ill or injured.

Chronic training load is a measure that can help you assess your daily effort and over long training periods. It was popularized by the team behind the training platform TrainingPiekand is widely used around the world best sports watches And workout appsalbeit sometimes under a different name such as fitness.

To learn more about what chronic training load is, how it’s calculated, and how you can use it to get fitter, I spoke with Cody Stevenson, head of education at TrainingPeaks.

Cody Stephenson

Cody Stephenson is head of education at fitness platform TrainingPeaks. He is also a certified coach and cyclist with 30 years of experience. Stephenson has a bachelor’s degree in biochemistry from Fort Lewis College and a degree in health and exercise science from Colorado State University.

What is chronic training load?

“Chronic training load or CTL is a way to measure training volume in a way that also takes into account the intensity of individual workouts and how long workouts affect you,” says Stephenson. “In TrainingPeaks you also see fitness. As nice as it sounds and how much math goes into it, it’s really just your base number for how much you’ve trained.

Most fitness apps and wearables now include some measure of your CTL, although the exact term used and the length of time it covers may change. With the classic measure from TrainingPeaks, CTL covers a period of six weeks, but with Apple’s new training load feature for example, it is 28 days.

How is chronic training load calculated?

CTL is the cumulative effect of your training over the past six weeks. To get that number you must first calculate the training effect of each workout you do. This is measured via another metric with a three-letter acronym; your training stress score or TSS.

“This is basically the (exercise) intensity times the (exercise) duration with a few small adjustment factors to make it a nice number and not a crazy decimal,” says Stephenson. “Intensity times duration gives you that individual session number. A single workout in TrainingPeaks can be 60 TSS, 120 TSS or 300 TSS.

“To get from there to CTL, we weigh it, because a workout from six weeks ago affects you less than a workout from eight days ago. So we create an exponential weighted average of all those workouts, all those TSS scores over the last six weeks.

The result of all these TSS scores is a CTL score that can range from 30 to 200 and is comparable between athletes of all levels because your TSS is adjusted to your fitness and ability, with intensity measured based on factors such as strength when cycling, and pace and heart rate when running.

What is the difference between acute and chronic training loads?

a woman performing dumbbells raises near a swimming pool

(Image credit: Getty Images)

If CTL is your long-term training load, acute training load or ATL is your short-term training load, a period that covers the last seven days. Your CTL, or fitness level, is an indication of how much training you can do in the short term, your ATL.

“You really have to look at both together and look at the balance between the two, because if you have a very high acute training load, you are probably more tired than what your chronic training load says you can tolerate.” says Stephenson. “As those two go back and forth against each other, you’re either putting more pressure on your body or allowing it to recover.”

This balance between CTL and ATL is measured using another statistics, your training stress balance or TSB.

“TSB is the difference between chronic training load and acute training load,” says Stephenson. “If that number is zero, your acute training load and your chronic training load are the same and that means you are training as much as you are used to. If it becomes really negative, that means you’re training more than you’re used to, and if it’s really positive, that means you’re training less than you’re used to.

“Those are the three things we constantly look at as a coach; ‘Should we train more, less or the same?’ We do the same the least. You don’t gain much from it, but you still put a lot of effort into it.”

To get fitter you need to increase your ATL so that you train more than you have in the past, but if you take this too far you risk burnout or injury. Therefore, you use your CTL to judge how much to increase your training in the short term.

This increase is measured by another term: ramp rate (fortunately not a three-letter acronym this time). In general, people looking to increase their training can increase their CTL by 5-7 each week, although athletes in training camps can aim for higher numbers.

Can you use chronic training loads to prevent injuries?

There’s no way to completely eliminate the risk of injury when training, but you can use your ATL and CTL to reduce that risk by making sure you don’t overdo it by training more than your fitness level indicates you can handle .

“It’s that balance between ATL and CTL,” says Stephenson. “It makes sense, right? If you suddenly add more volume over the course of a week than you have become accustomed to over the past six weeks, there is a risk of injury. To what extent it correlates or can be quantified, I’m not sure, but it’s certainly something.”

Should you include activities outside training in the chronic training load?

A group of friends going for a walk together

(Image credit: Getty/Hinterhaus Productions)

Your workouts will be the most intense activity you do every day, but they won’t be the only activity you do, so I asked Stephenson if you should consider things like household chores or an active job.

“If they are shorter and low intensity, like a dog walk, you can probably ignore them,” says Stephenson. ‘But if you want to involve them, you can, as long as you are consistent. Every time you do it, record it or always ignore them, it will be accurate.

“But if you have a very active lifestyle, work in construction or something like that, then I would say that’s more something to think about with your recovery and how you handle the training load. You have a very active lifestyle, so your CTL should actually be lower and make sure you get enough sleep and recovery.”

How does recovery affect chronic training load?

CTL is not a metric that includes any form of recovery tracking, and while some wearables are now getting better at using things like heart rate variability to track the state of your body, and advising on how long it will take to recover after your workouts, this is is still an area that requires careful consideration.

“Anyone looking to start using training load metrics should remember that they will never be directly related to performance,” says Stephenson. “Because a CTL of 60 if you sleep well, don’t drink too much beer and eat well, is different from a CTL of 60 if all those things are opposite and you lead a stressful life.