What is Impact Asymmetry?

Impact Asymmetry summarizes the between-limb difference in mean average impact intensities over a period of time. Impact Asymmetry is an objective measure, describing how a person is loading one side of their body compared to the other, depending on the type of activity.

If an activity by nature is asymmetrical at the lower limbs, we would expect Impact Asymmetry to express that asymmetry (e.g. hopping). If we would expect an activity to be relatively symmetrical (e.g. straight-line running), and Impact Asymmetry yielded an asymmetry, then this can provide insight into how a person is distributing load between their lower limbs, allowing us to make better-informed decisions on aspects of their workload management such as technique, protocols, and alike.

Impact Load and Impact Asymmetry are two key metrics we provide to help quantify lower limb loading. Impact Load is our “movement volume” metric (i.e. how much work has been done), and Impact Asymmetry is our “movement quality” metric (i.e. how was that work split between the left and right limbs).

In other words:

• Impact Load is the sum of all impacts propagated into the lower limb.
• Impact Asymmetry describes the between-limb difference in mean average impact intensity.  

Impact Asymmetry is calculated from the following equation:

Impact Asymmetry=(max(μL,μR)− min(μL,μR))/sum(μL,μR)

where

μ = the mean average intensity of impacts that occurred.

As an example, if a person went for a run, landing on the ground with a mean average impact intensity of 10g on their right leg and 5g on their left leg, the Impact Asymmetry calculation would be:

Impact Asymmetry=(max(5,10)− min(5,10))/sum(5,10)    

Impact Asymmetry= (10 − 5)/(5+10)                                  

Impact Asymmetry=0.333=33.3% Right.        

How do I interpret Asymmetry?

Impact Asymmetry is a measure of between-limb difference with respect to the impacts propagated into the lower limbs. It is a way to compare the difference in mean average impact intensity between the left and right limbs, which can be useful for:

• Baselining athletes.
• Tracking asymmetry over time.
• Return-to-play monitoring.
• Identifying changes in movement quality when it comes to different movement activities.

Impact Asymmetry is:

• A single, numerical value, that is comparable over any time period where impacts have been detected e.g. minutes, footnotes, sessions, days, and weeks.
• A representation of whether one leg is experiencing more load on average than the other (e.g. 10% Left).

Impact Asymmetry is derived from the Avg. Impact Intensities of the left and right legs. If the left leg is landing on the ground with a substantially larger impact intensity than the right leg, then the Impact Asymmetry will be skewed more to the left.

When looking at the asymmetry tree in the IMU Step Dashboard, the Avg. Intensity is derived from the impacts detected during the time period selected.

Note that the difference in Left and Right Avg. Intensity is an absolute measure, whereas Impact Asymmetry is a relative measure. This means that at lower intensities (e.g. 5g), an average intensity difference (e.g. 2g) will yield a proportionally greater Impact Asymmetry than at higher intensities (e.g. 20g). Therefore, it’s important to look at the context with which Impact Asymmetry is measured, and consider all available pieces of information when interpreting the metric.

We recommend having at least 50 steps on each leg to ensure you have captured a broad range of the impacts on each leg during an activity, ensuring that the resultant asymmetry value is representative.

To learn more about why we recommend this, please follow this link to an explanation of the reasoning behind it.

Why use Impact Asymmetry?

IMU Step quantifies asymmetry at the lower limbs for any movement activity, boiled down to a single, simple metric - Impact Asymmetry. Impact Asymmetry enables you to track a subject’s loading response to training or rehab strategies over both short and longitudinal time periods, allowing you to paint a better picture of:
a) How a subject has responded to a training/rehab strategy over time
b) Whether that strategy should change given their response
Furthermore, by utilizing Vicon’s BlueTrident sensor, we can capture the largest possible range of impact loading and asymmetry patterns on the market.
Note that Impact Asymmetry should be considered alongside IMU Step metrics such as Impact Load, and other external data sources to provide even more context to the subjects movement quality.

More Impact Asymmetry information:

3 Ways to Use Impact Asymmetry - an article by James Grant, ATC

Practical Examples of Using IMU-Step to Modify Outcomes: Asymmetry and Impact Load in the Real World - an IMU webinar with Andrew Gray