An increasingly talked about area in sports science in regards to load management is the acute to chronic training load ratio. Traditional load management models centre around the increase in training load from week to week. In the running world you often hear about the 10 percent rule talked about, which basically means that we don’t want to increase our training load by more than 10% each week.
The alternative to that model which has increasing sport science support is the acute to chronic training load ratio. Acute load basically refers to a period of a week . Chronic load generally refers to a 4 week period, generally calculated on a 28 day rolling average of load.
The acute:chronic ratio is calculated by taking the acute load and dividing it by chronic load. Most sport science literature tells us that a acute:chronic ratio of less than 1.2 is ideal for minimising injury risk. In other words as long as our load for the week is not 20% more than our load over the past four weeks then we are minimising our injury risk.
Here’s an example based on calculating load from mileage (which isn’t the best way, i’ll explain why later). If four weeks of running training look like this: Week 1: 40km, Week 2: 45km, Week 3: 50km, Week 55km, Week 4: 60km. In this example our chronic load is 47.5km, our week 4 or acute load is 60km, therefore our acute:chronic ratio is 1.26. In other words we are at increased injury risk.
An element of load that a lot of people, particularly endurance athletes fail to consider is intensity. It’s very easy to just add up the mileage and see how many kms per week you do. However intensity is a key variable that needs to be included in the training load calculation.
When you have an easy week and run 5 runs all at 4/10 exertion for a cumulative mileage of 50km then the actual training load is going to be very different than if you do 4 of these runs at 8/10 exertion for a total milage of 50km.
The most common way to calculate training load is to take the duration (time) of the session and multiple by the exertion level (perceived intensity out of 10). Therefore if you train for 30mins at 8/10 exertion level then then the training load would be 30 x 8 = 240 units.
So that’s the theory behind the acute:chronic training load, but what does this tell us about our training?
We need to carefully consider how we respond to time away from training due to sickness or injury and start back gradually. When we race and have a taper period plus a rest period after the race this will usually decrease our chronic load and again means we have to be conservative when starting back.
I understand that this article is a little rich on science, and normally I try to simplify things however the more endurance athletes and injuries I see most of them can be prevented by appropriate load management.
The thing to be aware of is that people don’t necessarily get injured because of high training load but it is quite often things that cause a drop in chronic load like competition, illness, injury etc that then leads to the acute load becoming too high and then leading to injury.