Should you create a pacing chart for your next ultra?
You’ve probably heard that training too much, too soon is one of the biggest risk factors for injury. But how do you actually measure “too much, too soon”? That’s where the acute:chronic workload ratio (ACWR) comes in.
What Is ACWR?
ACWR compares your short-term training load (acute) to your longer-term training load (chronic).
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Acute load: the average training volume or intensity from the past 7 days.
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Chronic load: the rolling average from the past 28 days.
Divide the acute load by the chronic load and you’ve got your ACWR.
Example: If you’ve run 50 miles this week (acute) and averaged 40 miles per week over the past 4 weeks (chronic), your ACWR is 1.25.
Why It Matters
Research across endurance sports and team sports shows that big spikes in ACWR increase the risk of injury. Your body adapts to stress gradually. Consistent loading strengthens muscles, tendons, and bones. Sudden jumps overload those tissues before they’ve adapted.
Ideal Ranges
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Sweet spot: 0.8 - 1.3. This suggests your training is balanced; enough stress to improve fitness without overload.
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High risk: Above 1.5. This means you’re doing ~50% more work than your body is used to.
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Too little: Below 0.8. Extended undertraining makes your chronic load drop, which ironically raises injury risk if you ramp back up.
When It’s Okay to Be Out of Range
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Tapering before a race: ACWR will dip because your acute load is lower. That’s fine, it’s part of peaking.
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After an off-season break: It may spike as you restart training, but you should plan this ramp carefully.
The key isn’t to avoid fluctuation altogether, it’s to manage how quickly you change.
Lessons From the NBA
Professional teams, especially in the NBA, have leaned heavily on ACWR to prevent injuries. This is one reason “load management” has become a buzzword. Teams strategically rest players, even healthy ones, to keep their ACWR within safe ranges. It’s controversial with fans, but it works: fewer soft tissue injuries, fresher legs for the playoffs.
Practical Implementation for Runners
You don’t need a sports science lab. Here’s how you can apply ACWR to your training:
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Track volume: weekly mileage, duration, or training stress score (if using heart rate or power).
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Calculate averages: 7-day acute / 28-day chronic.
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Adjust gradually: Keep increases under 10% per week to stay in the sweet spot.
How We Use It at Single Step Running
Our coaching team has recently adopted the Steady State training platform, which automatically calculates and tracks ACWR for our athletes. This saves the manual math and lets us spot risky spikes before they lead to setbacks. Athletes get feedback in real-time, and we can adjust training loads with precision.
The Bottom Line
ACWR gives you a framework to balance stress and recovery. It’s not about chasing a perfect number, but about respecting your body’s need for gradual adaptation. Professional athletes use it to stay in the game longer. As a runner, you can use it to stay healthy, consistent, and ready to line up on race day.
Jen and Ryan Wold are UESCA Certified Ultrarunning Coaches. Click here to learn more about coaching!
