Whatever your favorite sport is, training regularly puts strain on your body. This strain is a good thing — it’s how you get fitter, healthier and, if that’s what you’re aiming for, faster. However, you have to get the amount of strain you put on your body right — too little and you won’t get fitter, too much and you risk exhausting yourself and getting ill or injured.
Chronic training load is a metric that can help you judge your efforts each day, and over long periods of training. It has been popularized by the team behind the training platform TrainingPeaks, and is widely used across the best sports watches and workout apps, albeit sometimes under a different name like fitness.
For more information on what chronic training load is, how it’s calculated, and how you can use it to get fitter, I spoke to Cody Stevenson, head of education at TrainingPeaks.
What is chronic training load?
“Chronic training load or CTL is a way of measuring training volume in a way that also accounts for the intensity of individual workouts and how long workouts affect you for,” says Stephenson. “In TrainingPeaks you’ll also see it called fitness. For how fancy it sounds and how much math there is in it, it's really just your baseline number for how much you've been training.”
Most fitness apps and wearables now include some measure of your CTL, though the exact term used and how long the period it covers can change. With the classic measure from TrainingPeaks, CTL covers a six-week period, but with Apple’s new training load feature it’s 28 days, for example.
How is chronic training load calculated?
CTL is the accumulated effect of your training over the past six weeks, and to get that number you first have to calculate the training effect of each workout you do. This is measured through another metric with a three letter acronym; your training stress score or TSS.
“This is basically [workout] intensity times [workout] duration with a couple little adjustment factors in there 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 might be 60 TSS or 120 TSS or 300 TSS.
“Then to get from there to CTL we weight it, because a workout six weeks ago affects you less than a workout eight days ago. So we do an exponentially weighted average of all of those workouts, all of those TSS scores over the past six weeks.”
The result of all these TSS scores is a CTL score that might range from 30 to 200 and is comparable across athletes of all levels, because your TSS is adjusted in line with your fitness and ability, with the intensity being measured based on factors like power in cycling, and pace and heart rate in running.
What is the difference between acute and chronic training load?
If CTL is your long term training load, acute training load, or ATL, is your short term training load, a period that spans the last seven days. Your CTL, or fitness, is an indication of how much training you can do in the short term, your ATL.
“You really need to look at both together and look at the balance between those two, because if you have a really high acute training load, then you're probably more fatigued than what your chronic training load is saying that you can tolerate,” says Stephenson. “As those two go back and forth relative to each other, you're creating more strain on your body or allowing it to recover.”
This balance between CTL and ATL is measured using another metric, 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're training as much as you're used to. If it gets 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're looking at as a coach all the time; 'do we need to train more, less, or the same?'. The same is the one we do the least. You're not gaining a lot from that, but you're still spending a lot of effort.”
To get fitter, you need to increase your ATL so that you’re training more than you have done in the past, but pushing this too far can put you at risk of burning out or injury, so you use your CTL to judge how much to increase your training in the short term.
This increase is measured through another term — ramp rate (no three letter acronym this time, thankfully). In general, people looking to increase their training might increase their CTL by 5-7 each week, though athletes at training camps can push to higher numbers.
Can you use chronic training load to avoid injury?
There’s no way to eliminate injury risk entirely when you’re training, but you can use your ATL and CTL to reduce that risk by ensuring you’re not overdoing it by training more than your fitness suggests you can handle.
“It is that balance between ATL and CTL,” says Stephenson. “It makes sense, right? If all of a sudden over the course of one week, you add a load more volume than you become accustomed to over the previous six weeks, that's a risk for injury. How much it correlates or can it be quantified I'm not sure, but it's definitely a thing.”
Should you include activity outside workouts in chronic training load?
Your workouts will be the most intense activity you do each day, but they won’t be the only activity you do, so I asked Stephenson whether you need to factor in things like household chores or an active job.
“If they're shorter and low intensity, like a dog walk, then you can probably ignore them,” says Stephenson. “But if you want to include them, you can as long as you're consistent. Every time you do it, include it, or always ignore them, then it will be accurate.
“But if you have a very active lifestyle, you work construction or something, then I would say that's more something to think about with your recovery and how you manage training load. You have a really active lifestyle, so your CTL should actually be lower and make sure you're getting plenty of sleep and recovery.”
How does recovery factor into chronic training load?
CTL is not a metric that includes any kind of recovery tracking, and even though 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 takes recover after your workouts, this is still an area you need to consider carefully.
“Anybody who wants to start using training load metrics should remember that they aren't ever gonna be directly correlated to performance,” says Stephenson. “Because a CTL of 60 when you're sleeping well and not drinking too much beer and eating well is different than a CTL of 60 when all of those things are opposite and you're living a stressful life.
