Why do we get injured?
Soft tissue injuries occur when body tissue such as muscles, tendons, or ligaments experience a stress or load that is higher than the tissue is accustomed to. When undertaking training sessions, it is important to consider the stress on our tissues as this can either increase or decrease injury risk. The impact on our tissues as a result of training is associated with four main training patterns:
1. Insufficient challenge:
When we don’t stress tissues enough they will ultimately undergo a detraining effect and degrade. This can result in more injuries, as our bodies lose the tissue resilience that protects us from injury. We see this happen when people reduce their activity as they age, or even through a lazy sporting off-season.
When we rapidly increase the stress on our body tissues beyond the level of what can be accommodated, the exposed tissues do not have the time, capacity or injury resilience to adapt to or tolerate the imposed challenges. This results in an increased risk of injury, decreased performance, and decreased tissue resilience. A rapid increase in load is often seen during an aggressive pre-season training regime, or those times where we decide to train for the City to Surf or a half-marathon at the last minute.
3. Status Quo Maintenance:
If we maintain the exercise (tissue load) status quo, the tissue generation and degeneration processes are balanced and, all other things being equal, tissue performance remains the same (for example, the weight that can be lifted or the speed a runner can reach would stay the same). People who do the same exercise week in/week out, or those in the last half of a sports season are often in this stage of equilibrium where performance is consistent and injury risk low.
4. Optimal training:
If we stress body tissues at the optimal amount (a little bit more than they are accustomed to, but not too much) we see the desired training effect. This is an increase in tissue resilience to stress (lower risk of injury) as well as an improvement in performance-related measures. This is obviously the “sweet spot” and where we want to be, whether we are training for a marathon, returning from injury, or aiming to be able to walk 10km. For the physiologists among you, the 4 processes are summarised in Figure 1:
How do we stay in the ‘sweet spot’ and improve performance without increasing injury risk?
The most effective way to improve performance and prevent injury is by monitoring your training workload and comparing what you are doing now (acute workload) with what you have done in the past (chronic workload i.e. what your body has become accustomed/adapted to). We can use these two workloads to calculate our “sweet spot” to allow us to train in the most effective state for tissue adaptation and improving performance. Most commonly:
- Acute workload is based on one week
- Chronic workload is based over 4 weeks
How to measure your workload?
Measuring your workload can be as simple or complicated as you wish, but the key is choosing a measurement system that fits with your exercise routine and doesn’t require a supercomputer, and sticking with it.
At the most basic level, you might calculate workload by adding up your minutes of exercise in a week.
Next, you might account for intensity by multiplying the minutes trained by how hard the session was (out of 10) = workload units. For most recreational athletes, this is convenient and has been shown to be one of the most valid methods of workload monitoring. If you have a GPS watch then you could use distance as well. Professional teams will use a range of measures including time running in different speed zones among many others.
What you decide to measure is dependent on activity. A runner with a GPS watch could use running distance OR they could use Time x Difficulty (how hard it was and for how long). A swimmer could use swimming distance. A walker, their walking distance. A bowler, balls bowled.
What do we do with the data?
We can use the data to calculate our ACWR (Acute:Chronic Workload Ratio). Research has shown that if we keep our ACWR in the “sweet spot” range of 0.8 – 1.3 injury risk is reduced but training benefits are maintained. We can also see that our risk of injury can increase as our workloads spike.
How do you calculate your ACWR?
Let’s use running for example. We need to compare what our body is used to and what we are exposing it to in the acute timeframe.
For example: John has run over the past 4 weeks – 3.5, 5.0, 4.0 and 5.0 kms. This is an average of 4.375km per week and this is John’s chronic training workload. John’s acute workload is 5km as this is what he has run in the past week. We then compare his acute and chronic workload to get a workload ratio.
5 / 4.375 = 1.14
This means that John’s acute workload is a 14% increase on his past month’s average distance. The same process could be followed for using workload units (time*difficulty).
Let’s assume John is feeling good and is not having any issues. This is a good sign his body is dealing with the work he doing. But we can also check that his acute to chronic workload ratio is also in a “sweet spot”.
The fact that John is in the green zone does not guarantee he will remain injury-free – simply that his chance of injury is significantly lower. Similarly, being in the red zone does not imply injury is certain. There is a time and place for specific training strategies where training is performed above the recommended ratio but this is beyond the scope of this blog.
This concept can be used effectively to plan future weekly training workloads for recreational athletes to improve performance and reduce injury risk.
For people with an aversion to maths, there is also some less reliable research that simplifies the ‘safe zone’ based solely on what was done the week before (see Figure 3). Weekly changes in volume correlate with different injury rates. An increase of 15% or more on the week before is associated with a sharp increase in injury risk. This measure assumes a constant level of activity beforehand, which for many sportspeople is not realistic – we all have weeks where games are washed out, or we miss a training session, have a bye, get substituted, or play the easy beats of the competition. These sequences create natural variation in our weekly exercise load and make the ACWR a better measure.
Like all rules of thumb, the information above will not apply perfectly to everyone. Keep in mind there are some people (a fortunate minority) out there who can tolerate large changes in training volume without developing any injuries. By the same token there are people with genetic factors that will increase their likelihood of injury. There are consistent factors such as strength and aerobic fitness, which improve our ability to tolerate spikes in training load. Use this information as a general guide and consult your physiotherapist or exercise physiologist for further information and guidance.