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In running based sports, we know that the majority of injuries tend to occur in the lower limbs. For example, among adults aged 25 to 40, the most common injuries in basketball, soccer, baseball, and softball were fractured or sprained ankles and knees.

Figure 1: Injury stats in the NBA 2014-2015 Regular Season from GE Healthcare.

IMU Step aims to mitigate the risk of lower limb injuries sustained in running sports by measuring lower limb load.

3 key metrics are derived from 2 small, synchronized sensors which are placed on the left and right lower medial tibia to monitor the load of each step.

1. Cumulative Bone Stimulus

To start, it may be easier to define bone stimulus by defining what it isn't.

What it's not: Bone Stimulus is not an external workload metric for the lower limb, nor does it have anything to do with ground reaction forces.

Bone stimulus approximates tibial shock and then feeds that into a bone response model.

In short, Bone Stimulus is an estimate of the mechanical stimulus that would cause the bone to respond and remodel.

We break down that definition a little more and explore both the stimulus applied to the bone and the corresponding response by that bone in our Bone Stimulus Guide

Figure 2: Graph showing Cumulative Bone Load and High Intensity Steps. The Cumulative Bone Load corresponds to the line graph.

For more detail on Cumulative Bone Stimulus watch this short 5 part video series by our Chief Scientist Dr. Thor Besier - part 3 refers specifically to Bone Load. This whitepaper describes Cumulative Bone Stimulus in more detail.

2. High-Intensity Steps

IMU Step categorizes the "intensity" of each foot strike into "Low Intensity" (0g - 6g), "Medium Intensity" (6g - 21g) and "High Intensity" (21g+).

A larger proportion of "High Intensity" strikes, and indeed a high relative number of high-intensity steps, often correlates to a high-intensity session, which in turn indicates a greater athlete workload/energy expenditure for that training session. Over time coaches start to understand how many and what percentage intensity steps constitutes a hard training session for different athletes and positions. Therefore, it's possible to use the % High-Intensity Strikes metric to categorize sessions using

• % of high-intensity steps

• Total number of steps

• Cumulative bone load

These customized categories can then be used to manage player workloads over sessions/weeks/months/years.

3. Asymmetry

The asymmetry view looks at the symmetry of intensities (load) between the left and right leg. Each leg has a total number of steps recorded, with each step having an intensity level associated with it (e.g. 16g). If there is a difference in step count between the left and right legs for this intensity, there is an asymmetry present i.e. the left leg may have a greater number of 16g intensity steps than the right leg, therefore they are asymmetrical. There are two important views that that coaches, athletic trainers, physios and athletes are interested in:

1. How did the distribution of step loads look for a single session (e.g. today’s training session), for each leg as well as between leg comparison?

2. How did the asymmetry compare to that particular athlete’s historical ‘profile’ over time?

Figure 3 shows example data from a single session. Every individual step is categorized by step intensity (measured in resultant g’s) into it’s respective ‘intensity bin’, where 1g is the smallest bin, and 26g is the largest bin. Each ‘intensity bin’ has a number of steps for both left and right legs. E.g. from Figure 3 we can see that in the 26g bin there were 15 steps measured on the right leg, and 29 steps measured on the left leg. Furthermore, in bin 25, there were 15 steps measured on both the right and left legs.

Figure 3: Single session asymmetry between left and right legs, across all intensity bins.

Figure 4 shows the asymmetry from each day viewed over time. Time is on the x-axis with the most recent day on the far right. ‘Intensity bins’ is on the y-axis, with increasing intensity going up the y-axis (i.e. the smallest intensity bin at the bottom and the highest intensity bin at the top).

Figure 4: Multiple session asymmetry between left and right legs, across all intensity bins.

Step load asymmetry is visualized over time by color grading. The more right dominant the intensity bin is, the darker orange the square will be. The more left dominant the intensity bin is, the darker blue the square will be. If there are zero asymmetries in the intensity bin, the square will be white (i.e. the same number of strikes on the right compared to the left). We can see in Figure 4 that the subject is mainly right foot dominant in the 14g -19g bins, and mainly left foot dominant in the 20g – 26g bins.