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validation of trunk mounted inertial sensors for analysing running biomechanics under field conditions using synchronously collected foot contact data

Reference:  AJ Wixted, DC Billing, DA JamesValidation of Trunk Mounted Inertial Sensors for Analysing Running
Biomechanics under field conditions, using synchronously collected foot contact data, Sports Engineering 12 (4), 207-212

Abstract: The biomechanical evaluation of elite athletes often requires the use of sophisticated laboratory-based
equipment that is restrictive, cumbersome, and often unsuitable for use in a training and competition environment.
Small, low-mass unobtrusive centre-of-mass triaxial accelerometers can be used to collect data but may not
reveal all the information of interest. This validation of centre-of-mass triaxial accelerometry uses previously
reported synchronously collected foot-contact information from in-shoe pressure sensors. A qualitative assessment of
the system output indicates that the centre-of-mass acceleration provides valuable insight into the use of accelerometers
for investigating the biomechanics of, in this case, middle distance runners.

A notable result was the absence in the acceleration data of any significant curve running versus straight running
identifying signature, in either the extracted orientation data or the dynamic running data. This is demonstrated in
Figs. 2 and 3 where the six traces for the race sample points cluster together making the data for each sensor
appear as either a thick dark line or a tightly interwoven mix of lines. The signals for each acceleration axis and
the in-sole sensors maintained an ongoing consistency across the sampled race points. The left hallux in-sole
pressure increased during running on the curve (for both athletes).

Fig. 2 Subject A, comparison of representative accelerometer and in-sole pressure data from six race points

Fig. 3 Subject B, comparison of representative accelerometer and in-sole pressure data from six race points 

Comparing in-sole pressure of subject A (Fig. 2) with subject B (Fig. 3) showed that subject A had a relatively
small heel strike, with the majority of the pressure initially occurring on the third MTH sensor and then moving to the
first MTH sensor and finally to the hallux sensor. Subject B had a comparatively large heel pressure with the pressure
then moving to the third and first MTH sensors. Relative to the first MTH pressure the hallux pressure was quite small.
This suggested that subject A landed with a flatter foot contact and then rolled forward to give the bulk of the final
propulsion from a combination of the area around the first MTH and hallux. Subject B landed with the heel striking
first and then obtained maximum pressure from the area of the first and third MTHs (1st MTH area predominated).

When comparing the in-sole signal to the accelerometer signal, the anterior–posterior acceleration had a significant
negative phase (braking) occurring at approximately the same time as the heel strike. This occurred for both athletes
but was far more noticeable for subject B, who also had the much larger heel pressure. For both athletes the vertical
acceleration went to zero at approximately the same time as foot contact ceased. Separate data collected from a
subject on a treadmill showed the toe-off point moving as a function of speed, with the toe-off occurring later with
respect to the acceleration data zero crossing as running speed decreased.

For these athletes, the orientation of the sensor extracted from the low-pass filter was reasonably stable for the
duration of the race. For subject A, the rotation about the AP axis (sideways tilting) was\1.5. and the forward lean
of the sensor was \3.. For subject B the corresponding values were\1. and around 11.. For subject B these values
made a noticeable difference in the relative size of the peak V and AP acceleration during contact when comparing the
rotated acceleration (Fig. 3) and the un-rotated acceleration (Fig. 4).

Fig. 4 Subject B, un-rotated accelerometer data corresponding to Fig. 3

Although this note only gives a brief qualitative analysis of the collected data, it appeared that the in-shoe pressure
sensors allowed the framing of the triaxial centre-of-mass accelerometry such that the accelerometry could be used on
its own to provide useful insight into the running technique of an athlete. The initial foot contact and final contact
appeared to be discernable in the acceleration data, as did the effective application of contact forces.

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