The “I’m Fine” Injury
A guy I coach, Brenton, was coming back from a 4-week break after 10 months of training for 3 marathons. He’d run 80km (50 mile) weeks before, no problem. So when he jumped back to 70km (42 miles) in week one, he shrugged and said, “I’ve done this before—I’m fine.”
10 days back, right after a Vo2 Max interval session, he felt that weird sesnation in his hamstring. Now he has to take off another week.
Was it bad luck, bad genetics, that workout, something else entirely?
This wasn’t random because there’s a pattern to these kinds of injuries. It’s more of a rule: The chronic vs acute training load ratio. It’s a boring, unsexy, barely-talked-about rule that could’ve saved him.
What Is Training Load? (Spoiler: It’s Not Just Distance)
Let’s define this without getting too deep in the weeds.
Training load = volume × intensity. A chill 10K (6-mile) jog is not the same as a 10K (6-mile) tempo between your half-marathon and 10k race pace. Your body knows the difference, even if smartwatch tech doesn’t.
Now break load into two timeframes:
- Acute load = what you’ve done recently (this week)
- Chronic load = what your body has adapted to (your 4-week average)
Think of your body like a bank account:
- Chronic load = your average balance
- Acute load = this week’s spending
Spend a little too much and you’re fine. But if you spend 50% more than usual, you’ll overdraft. In running terms, that overdraft is called an injury, burnout, sickness or all three at the same time.
And on the flip side, if you spend under your average for weeks going into the next week? You get slightly unfit and your body can’t handle if you push the load more the following week. It’s all connected.
The 20% Rule (aka Your Comeback Insurance)
While exercise science calls this the Acute-to-Chronic Training Ratio, you can call it: Don’t spike your training load more than 15-20% a week.
This has been proven with a new study that has finally concluded what all experienced runners and coaches know.
This is different than the 10% rule that most new runners learn about when progressing their training.
Why it matters:
- Spike too high (more than 20% and into the 30-50% zone) and the risk of injury goes sky high.
- Dip too low, and you get deconditioned; then try to come back too fast, and you get injured. Because what’s an extra 10km (6 miles) that you easily did 2 months ago?
It goes both ways up and down. This is why consistency is so key with running.
Your brain remembers the 80k weeks, but your tendons don’t.
How To Use This in Your Training
Training Calendar Hack:
- Add up your last 4 weeks, then divide by 4. That’s your chronic load
- Keep next week’s training within 20% of that.
Week 1 back = 40–50% of your old volume from when you were at peak fitness.
Weeks 2–4 = add 10–15% of the previous week, not your old peak.
Example:
- Week 1: 35k
- Week 2: 40k (+14%)
- Week 3: 45k (+12%)
- Week 4: 50k (+11%)
Brenton’s mistake?
- Chronic load = 25k (15 mile) week and no outside running (just elliptical cross training, which means his tendons and ligaments weren’t ready for the pounding of the road)
- Week 1 = 81k (50 miles) of all outdoor pavement pounding
- That’s a 68% spike and a shock to your body. That’s also not a comeback but more like Russian roulette.
The 1% Rule That Actually Works
Even elites take 4–6 weeks to come back safely. You’re not weak. You’re smart.
Consistency + Patience + Small Progress = You get to stay in the game. And that’s how you get faster. Not by forcing a comeback. But by respecting the math.
I talk more about consistency in my upcoming book, The 1% Better Runner. More on that in the near future – Hit me up (reply to this email) if you want to be a test (beta) reader!
