ACWR for runners: Which injuries it can (and can’t!) predict

Somewhere in the last fifty years, running acquired a rule: “Don’t increase your weekly mileage by more than 10 %“. The 10 % rule gets mentioned all the time, but nobody is quite sure where it came from. It sounds sensible, easy to remember and to calculate. But the 10 % rule has 0 % scientific evidence going for it, quite worse in fact: almost 20 years ago a randomised control trial concluded that the 10 % rule makes no difference whatsoever to prevent running injuries (Buist et al. 2008).

So when something better arrived, the running world was ready for it. Blanch & Gabbett proposed the Acute:Chronic Workload Ratio (ACWR) in a series of 2015 papers, and it was almost immediately incorporated into an International Olympic Committee consensus statement about training load management and risk of injury (Soligard et al. 2016). Millions of athletes have now had training decisions made for them on the strength of that number.

The ACWR compares your recent running against what you’re used to. Quick example, let’s say you’ve been running 20 km per week for a while, and decide to add one more 5k run this week. That would look like:

Week 1 (this week)	Week 2 (last week)	Week 3	Week 4
25 km	20 km	20 km	20 km

To work out your ACWR:

1. Add up your last 7 days of running, that’s your Acute Training Load (ATL)
2. Average your last 4 weeks, that’s your Chronic Training Load (CTL)
3. Divide ATL by CTL to get your Acute:Chronic Workload Ratio (ACWR)

From this example we get:
ATL = 25 km
CTL = (25 +20 +20 +20) / 4 = 21.25 km / week
ACWR = 25 / 21.25 = 1.18

The appeal is obvious: it accounts for fitness accumulated over several weeks, not just last week, and a body of research put injury numbers on it:

• ACWR below 0.8 you may be undertrained (or tapering for a race),
• between 0.8 and 1.3 you are in the sweet spot,
• above 1.5 your risk climbs sharply

Professional football, rugby, cricket, athletics adopted it. It’s been in myTF since 2017 as your green zone:

Happy days: runners now have a science-backed number to tell them when they’re doing too much too soon.

But wait, where did those numbers come from: 7 days and 4 weeks? There are plausible answers: muscle glycogen replenishes over days; connective tissue remodels over weeks. Perhaps 7 days and 4 weeks land somewhere biologically meaningful, catching acute fatigue and structural adaptation?

The paper that lays the groundwork for the ACWR methodology answers this directly: “this arbitrary method is the most convenient way for coaching, conditioning and sports medicine staff” (Hulin, Gabbett, Blanch et al. 2014). You read that right, “arbitrary” is the answer! The authors acknowledge that the most appropriate method of evaluating training/stress balance is still unknown, then note that periodisation has traditionally used one-week microcycles and four-week mesocycles. So the ACWR’s 7:28 day ratio was chosen because coaches plan in weeks and blocks, and those numbers fit nicely in their diaries. Nothing to do with the biology or adaptation of our muscles / tendons / bones to training, and the authors are refreshingly honest about it.

I find this genuinely admirable and slightly horrifying in equal measure. A convenient assumption, clearly labelled as one in the original paper, hardened into a global standard within about two years, and then applied to millions of athletes globally. But the thing is that it works. Not always, not for everyone, not for every injury, but the association is real and has been replicated by scientists. Something about comparing the last week to the last month captures something true. The interesting question is what?

So let’s look at it from another direction, forget the ratio and ask: What happens when we run too much too soon? Broadly, four things, and they run on wildly different clocks:

1. Muscle strain: Within days. Your calf or hamstring fails, you feel it straight away, usually during something quick or at the end of a long run. A session or two that are way harder or longer than you have adapted for, and muscles can fail in that same week.
2. Reactive tendinopathy: Days to a couple of weeks. This is the sore Achilles that appears after a fortnight of new hills or new high-speed sessions. It’s a short-term response to a sudden spike in load, in which the tendon thickens to spread the stress (Cook & Purdam, 2009). Pushing through reactive tendinopathy is a bad idea, and can turn into tendon dysrepair.
3. Degenerative tendinopathy: Weeks to couple of months. It’s the same tendon tissue but at the other end of that continuum, and a completely different animal. Chronic Achilles or patellar tendon problems that have been building for a month or two, involving disorganised collagen and failed healing rather than a reaction to any particular week. Tendon collagen turns over slowly, so this one accumulates across a season, and becomes particularly common as we age.
4. Bone stress: Three weeks to months. That’s the shin splints and tibial stress reaction. Bone adapts by remodelling, and that means removing old bone tissue before laying down some new, stronger bone. So for some weeks in the middle of a build, the bone is temporarily weaker than when we started, precisely as we continue ramping up the miles and sessions… Bone stress reactions typically happen after several continuous weeks of excessive overload.

Now back to our Acute:Chronic ratio: a 7-day window against a 4-week baseline is a reasonable model of a tissue that turns over in days. For muscle strains it’s asking exactly the right question, and for reactive tendinopathy it’s close enough to be useful: both are problems of doing too much this week than your body has been prepared for.

For degenerative tendinopathies and bone stress reactions, the ACWR timescale is looking in the wrong place entirely: the overloading could have taken a month or two, the past week doesn’t look excessive at all and the ACWR might be in the normal range.

So the ACWR was never built as a general injury model. It is a good instrument for fast injuries but a poor one for slow, gradual onset injuries. Because almost no study has ever separated those categories, the research has spent a decade arguing about whether it works without first agreeing on which injuries it is supposed to help avoid in the first place…

Running injuries are complex and multi-faceted, so we shouldn’t expect a single number to predict every injury. This is exactly why myTF includes 7 different injury metrics, including a new “Cumulative Overload” model I recently developed specifically for gradual onset running injuries that the ACWR can miss, but that’s a story for the next post!

References

Blanch P, Gabbett TJ, 2016. Has the athlete trained enough to return to play safely? The acute: chronic workload ratio permits clinicians to quantify a player’s risk of subsequent injury. British journal of sports medicine. 1;50(8):471-5. DOI: 10.1136/bjsports-2015-095445

Buist I, Bredeweg SW, Van Mechelen W, Lemmink KA, Pepping GJ, Diercks RL, 2008. No effect of a graded training program on the number of running-related injuries in novice runners: a randomized controlled trial. The American journal of sports medicine. 36(1):33-9. DOI: 10.1177/0363546507307505

Cook JL, Purdam CR, 2009. Is tendon pathology a continuum? A pathology model to explain the clinical presentation of load-induced tendinopathy. British journal of sports medicine. 43(6):409-16. DOI: 10.1136/bjsm.2008.051193

Hulin BT, Gabbett TJ, Blanch P, Chapman P, Bailey D, Orchard JW, 2014. Spikes in acute workload are associated with increased injury risk in elite cricket fast bowlers. British journal of sports medicine. 48(8):708-12. DOI: 10.1136/bjsports-2013-092524

Soligard T, Schwellnus M, Alonso JM, Bahr R, Clarsen B, Dijkstra HP, Gabbett T, Gleeson M, Hägglund M, Hutchinson MR, Van Rensburg CJ, 2016. How much is too much?(Part 1) International Olympic Committee consensus statement on load in sport and risk of injury. British journal of sports medicine. 50(17):1030-41. DOI: 10.1136/bjsports-2016-096581