What the acute:chronic ratio can and can't tell you

The acute:chronic workload ratio has an appealing story: compare this week with what you are used to, and avoid sharp spikes. If you have read ACWR explained, you know how it works. But in recent years researchers have raised serious questions about it, and an honest training app should tell you about them.

What the critics say

The two halves are coupled

In the most common way of calculating the ratio, the current week is also part of the chronic average. That means the numerator and denominator are not independent. Lolli and colleagues (2019) pointed out that this mathematical coupling can create apparent relationships that partly reflect the arithmetic rather than anything about the athlete. Suggested fixes, such as leaving the current week out of the chronic average, change the numbers but do not resolve every concern.

Ratios behave oddly

Ratios are sensitive to small denominators. If your chronic load is low, for example after a quiet month, a modest session can produce a dramatic ratio. The same absolute increase looks frightening for a beginner and harmless for someone with a large training base.

The injury link is less solid than it first looked

Impellizzeri and colleagues (2020) published a detailed critique arguing that the evidence linking the ratio to injury risk has methodological problems, and that it should not be used as a tool to predict injuries in individual athletes. They also questioned whether the ratio adds anything beyond simply looking at changes in load. Other researchers have defended its use as a practical monitoring prompt. The debate is ongoing, and it is fair to say the evidence is mixed.

Climbing has little direct data

Most of the research comes from team sports such as rugby, cricket and football. Climbing loads are different: the limiting tissues are often small, finger loading is highly specific, and session-RPE does not distinguish crimping from easy mileage. Applying team-sport thresholds to climbers involves an assumption.

No single number can predict an injury. The ratio is a conversation starter, not a crystal ball.

What still holds up

The criticisms target the ratio as a precise statistical predictor. The underlying coaching principle is much less controversial:

• tissues adapt to the loads they are regularly exposed to
• large, sudden increases are worth avoiding, especially for fingers
• athletes with a solid base of training generally tolerate hard work better

Gabbett (2016) made the same broad point: well-built, progressive training is protective, while spikes are risky. You do not need a perfect statistic to act on that.

How Train Climb uses the ratio

We kept the ratio because it is easy to understand and it nudges people to look at their recent history. But we designed around its weaknesses:

1. Broad zones, not sharp cut-offs. Up to 1.5 is OK, 1.5 to 2.5 is Watch, above 2.5 is High. We do not pretend 1.49 is safe and 1.51 is dangerous.
2. A minimum history. The ratio only appears once there are 2 earlier weeks of training, which reduces the wild values you get from tiny baselines.
3. Shown with absolute load. The Progress page shows weekly load alongside the ratio, so you can see whether a High reading reflects a big week or just a quiet previous month.
4. One signal among several. The injury-risk light combines load with morning check-ins (sleep, resting heart rate, HRV, fatigue, soreness) and skipped sessions. A spike in load while sleeping well and feeling fresh is a different story from a spike while exhausted and sore.
5. Coaches decide. The app flags; a human interprets.

Reading your ratio sensibly

• Look at the trend, not one day. A single reading tells you little.
• Ask why. Was the spike planned, like a trip? Or did it creep in?
• Check the absolute numbers. A ratio of 2.0 from a very low base may be a small real change.
• Listen to your body. Persistent finger soreness matters regardless of the ratio.

In Train Climb

• Use the Progress page to view the ratio and weekly load together.
• Do a quick morning check-in so the injury-risk light has more than load to work with. See Readiness check-ins.
• Coaches: treat Watch and High as prompts to message the athlete, not as automatic reasons to cancel training.