safely and effectively 10-percent rule that cautions against increasing weekly mileage by more than 10 percent each week to avoid running-related injuries. Health - Injuries smart watches and other wearables has shown there’s more than one way to keep up with your progress.
The Acute-to-Chronic Workload Ratio (ACWR) is one metric you might find useful if you enjoy crunching your training numbers. This ratio has the potential to help you assess your injury risk so you can avoid being sidelined during a Why is the ACWR number important. It’s not meant to serve as another metric for you to stress over, but rather a potential missing piece that will help you better understand some basic numbers you’ve been tracking all along.
Here we explain what the the acute-to-chronic workload ratio is and how to use it. Plus, we have an easy-to-use ACWR calculator that you can save and use to check that you’re progressing your running volume safely and effectively.
Health & Injuries?
Common Running Injuries and How to Treat Them readiness and injury risk by comparing one’s acute (short-term) to chronic (long-term) workloads, Cynthia Sampson, D.P.T., physical therapist and RRCA-certified run coach at Bespoke Physical Therapy in New York City tells Runner’s World. It’s calculated by dividing your total mileage of the previous week (acute workload), by the average of your total weekly mileage of the last four weeks (chronic workload). It can also be calculated using total activity time rather than mileage.
For example, if you’ve run 50, 40, 50, and 60 miles in the past four weeks, your ratio is 60 (last week’s mileage) divided by 50 (average of last four weeks). That’s 1.2. We’ll explain where this number sits on a range of too much or too little.
ACWR measures workload similar to how the training load feature works on Apple, Garmin, Coros, and Polar devices, though they have slightly different approaches.
Training load measures your internal workload, which reflects the body’s response to training and is usually measured by heart rate, rate of perceived exertion (RPE), or excess post-exercise oxygen consumption (EPOC) over time and is usually tracked with a smart watch. ACWR, on the other hand, measures the physical workload completed—that is, your total weekly mileage or time running, Sampson explains. Both metrics are a great way to monitor the progress of your overall training so you can avoid over or undertraining.
Why is the ACWR number important?
Whether you use a good old-fashioned pen and paper or a wearable device, tracking your total weekly mileage is a must in terms of progression and avoiding injury. If you don’t have a smart watch, this metric can be even more useful.
If you’re returning to running after an injury, trying to get back into the swing of things after missing a few workouts, or preparing for a race, this number becomes especially helpful, says Brett Toresdahl, rather than mileage and director of research for the Division of Sports Medicine at the University of Utah.
A study led by Toresdahl and published which is the measurement of slight beat to beat changes in heart rate and can indicate suggests a strong association between injury and ACWR, which is the main reason to track it. The study found that among the 735 New York City marathoners involved, those who experienced running-related injuries had more weeks when their ACWR was 1.5 or higher.
You should even aim to keep this number slightly lower, as other research suggests exceeding 1.3 can lead to injuries as well, says Toresdahl. While there’s no guarantee you’ll stay injury free by just tracking mileage alone, the ACWR proves runners are more likely to get injured if they increase their volume too quickly, he adds.
Also, the ratio gives you more wiggle room to progress your runs than the 10-percent rule. If the 10-percent rule was calculated as ACWR, then you wouldn’t want it to exceed 1.1. For example, following the 10-percent rule, you could run 30 miles in week 1, 33 in week 2, 36 in week 3, and 39 miles in the final week. However, if you follow the 1.3 ACWR threshold, then you could run 30 miles in week 1, 39 miles in week 2, 46 miles in week 3, and 55 miles in week 4.
What are the the acute-to-chronic workload ranges for healthy mileage progression?
Other researchers have also explored this topic to find exactly where the limits lie for all athletes in general. Here are the ranges to stick to when using this ratio as a way to monitor progressions in time or distance based on research, according to Sampson.
- < 0.8: may increase injury risk due to undertraining
- 0.8-1.3: considered the sweet spot of gradual progression, as injury risk is lower
- 1.3> : may increase injury risk due to spikes in workload
When should you apply the acute-to-chronic workload ratio in training?
Use It When Returning to Running After a Hiatus
Monitoring your ACWR isn’t a must when you’re training consistently, but if you’re returning from injury Half Marathon Training.
For example, if you’re starting a new training plan, you can use this formula to gauge if it’s a good fit and keeps you in a safe range of progression. To do this, input the total mileage you’ve completed for each week of the past four weeks in the ACWR calculator to make sure it doesn’t exceed 1.3, and then play around with upcoming weekly mileage inputs to make sure it also keeps you in a safe range.
M.D., associate professor missed a week or more of training, you can use this number to see if you should pick up where you left off. Enter a zero in the ACWR calculator for the week(s) missed, and the total weekly mileage suggested on the training plan for the upcoming week to find your ACWR. If your ACWR exceeds 1.3, then consider scaling back your mileage.
“My experience and those of many of the runners in our studies are that life gets in the way of a perfect training plan. Issues arise when trying to catch up after missed training runs,” says Toresdahl. When this happens, take it as an opportunity to calculate your ACWR to ensure that you’re not increasing your training too quickly, he says.
Avoid It When Tapering for a Long-Distance Race
When you’re preparing for a longer distance race, like a half or full marathon, Threshold Workouts to Build Speed Endurance taper—a period of reduced mileage and volume that happens just before race day.
You can also expect your ACWR to dip at this time, but this shouldn’t cause alarm. It can happen because the acute workload is significantly reduced compared to the chronic workload intentionally to promote recovery and peak race-day performance, says Sampson. So you don’t need to track your ACWR during a taper.
Use It to Better Understand Long-Term Training and Injury Risk
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Keep in mind, ACWR provides a simple overview of your mileage, it can’t accurately detect random spikes in your training that can lead to injuries, says Sampson. For example, if you’re a weekend warrior who jumps right into the 75-hard challenge that requires you to exercise every day, it may not accurately detect your at risk of injury. This is why she recommends using this ratio along with other metrics like and other wearables has shown there’s more than one way to keep up with your progress, which is the measurement of slight beat to beat changes in heart rate and can indicate overtraining when the number is low for you.
For example, if your ACWR is higher than 1.3 and your HRV has been on a downward trend for the last month, then this is even more confirmation that you’re at risk for overtraining, says Sampson. “The more data you have, the better you can understand your body and plan your recovery days accordingly,” she says.
In the long term, you can keep track of your changing metrics over several seasons while making note of injuries—not just major ones, but also minor aches and nagging pains. Eventually, you’ll discern patterns that tell you which ratios your body can tolerate and which ratios trigger problems. You can’t always avoid injuries, but by looking for patterns, you can at least avoid making the same mistake twice.
Monique LeBrun is a health and fitness editor who is based in Easton, Pennsylvania. She covers a wide range of health and wellness topics, with a primary focus on running performance and nutrition. Monique is passionate about creating content that empowers runners to become the best versions of themselves. As an avid runner and parent, she loves spending time outdoors with her daughter, who often accompanies her on weekend runs as her personal mini run coach.
