One of the most recognisable stereotypes in our business is the “New Year’s Resolutioner”. They go from no training to 3-5 times per week overnight. They make lots of progress in a short space of time. Just as they’re feeling back to their old selves, some injury (often a tendon, ligament or stress fracture) stops their progress in its tracks. What they really struggle with is understanding why – from their perspective, they’d done the hard work and were already fit.

So, what’s the best way to prevent these and similar injuries? It might surprise you to find out that we don’t actually know a lot about that. A lot of common sense practices – which may well be effective – haven’t yet been studied.

What we know

A sports specific warm-up like World Rugby’s Activate or FIFA’s 11+ programme can help to prepare your body for the activity to come. We teach safe contact techniques in collision sports like rugby with the hope of reducing injury, but finding research articles that demonstrate its effectiveness is not easy. This does not mean teaching tackle technique is not effective. Some things are just hard to research.

Many "over-use" injuries are preventable with proper management

This is especially true in areas like gym-based exercise classes or self-directed “keep-fit” – running, home workouts and so on. We’ve simply never measured on a large scale what works for injury prevention in these groups of people. Instead, we’re going to show you how the pros do it, and how you can manage yourself to keep your injury risk as low as possible.

The only consistent variable that tracks with injury rates is load – how much stress the body is put under in a given amount of time. How we measure it and how exactly it impacts our injury risk are different from what you may think though.

Measuring Load

If we run for 45 minutes, three times a week, this is easy to keep track of. But what if one run was a slow recovery jog, or an interval training run? How would you add in a weights session or circuit class? Measuring load across different activities quickly becomes complicated. This is one of the key reasons the Arbitrary Unit (AU) was developed. In sports like football where lots of varied exercise goes into training, it gives us a good approximation of the load our bodies are put through.

To calculate how many AUs of load you accumulate with a given exercise, you use the following calculation:

Time in minutes x Rate of Perceived Exertion (RPE) = Arbitrary Units (AUs)

Your RPE is calculated based on the scale below. So for our example above, if we maintained a steady run (e.g RPE of 4) for 45 minutes we would have 45 x 4 = 180 AUs per outing. Three runs in a week would give us a total of 540 AUs for the week.

Using load calculations

You might think that we use these numbers to stay below some sort of threshold, and you’d be right… Sort of. This study looked for correlations of load and injury in elite rugby players. They found that injury rates increased if an athlete’s four week AU total was especially high, or especially low. This may surprise you if you thought low amounts of training helped to keep players fresh.

In fact it is the relationship between how much you’re doing at the moment (acute) and how much your body is accustomed to doing (chronic) that predicts injury. This paper calls it the acute:chronic workload ratio.

This means that if your body is accustomed to low levels of exercise, you risk injury when you do lots of exercise in a short amount of time. Our “New Year’s Resolutioner” at the start of the article all but guaranteed they would succumb to injury sooner or later, because they were consistently putting themselves at risk with their training load. Risk will get you in the end - you can’t win coin flips forever.

It goes without saying that you shouldn’t jump off the couch and try to run a marathon with no preparation. But if you’re worried about adding a circuit training class or ten miles a week to your running schedule, is it wise? Luckily the research gives us some guidelines.

How to compare acute and chronic workloads

The authors of this study used a player’s 4-week average as their “chronic” calculation, and their current one-week total as their “acute” calculation. Here’s what they found.

• The lowest possible injury risk came where the acute load was exactly the same as the chronic load (100%).

• The “sweet spot” for training was maintaining an acute training load somewhere between 80% and 120% of the chronic load.

• Injury risk increases substantially when acute training load exceeds 150% of the chronic load.

This means that our runner who regularly gets through 540 AUs per week can expect to train safely up to about 650 AUs. Risk of injury would become significant at 810 AUs in a week.

There are research articles supporting the idea that change in load may be an important factor in injury risk in soccer and the NFL too.

Why bother?

This may seem like an overly-complicated way of measuring exercise. It is certainly easier just to measure mileage if you are running, or even just “listening to your body” if you do lots of different activity.

Both of those come with problems too though. Mileage alone does not account for RPE. A common guideline is to increase mileage by no more than 10% a week. This will work well for the majority of people, but if you are significantly fatigued by the extra effort your RPE can shoot up and significantly increase your load. Listening to your body is something we generally encourage but novice and experienced athletes alike can struggle to differentiate between a developing injury and the inevitable aches and pains that accompany gains. Load calculations help to give us context, more information to make a better decision.

Normally, we try to boil down our articles to a simple take-home.  However this one is not easily distilled. It’s one of the hardest questions to answer with patients; precisely because:

• Measuring exercise accurately can be fiendishly difficult, as demonstrated above.

• Injury risk is never zero; we can’t eliminate injuries.

• Other (more) fun injury prevention interventions may work. But we can’t enthusiastically recommend them until we have the evidence.

So, if you want to use everything at your disposal to manage your injury risk, the pros trust the acute:chronic workload, measured in AUs. And if nothing else, at least you understand that sentence now.