Accelerometer Artefacts from Body-Worn Sensors
Inertial sensors are commonly found in commercial products such as smartphones and sports bands. When these sensors are stationary, the variation in the recorded signal (noise level) is very small (approaching 0.1% of the gravitational acceleration). In many sports applications, the recorded acceleration has noise spikes related to the sensor itself and impulsive body movements. The wrist acceleration (100 samples/s) from a boxing jab (posterior–anterior) to a stationary bag using five subjects demonstrated that following the impact, the sensor oscillated in a manner dependent on the sensor weight. Mathematically, the integrated acceleration at the end point of the jab should be zero. This was used to remove the vibration artefacts and verified using a high-speed camera (2014 fps). A male participant with a rotator cuff shoulder reconstruction was asked to raise his arms (front lateral raise) individually from rest (vertical) to above the horizontal position carrying several different weights. The shoulder instability of the participant was evident in the accelerometer record as significantly larger anterior–posterior vibration at 9 Hz. This was not evident in the opposite shoulder which had no reported injury. This technique might prove a useful tool in quantifying shoulder instability over long periods of time.
KeywordsInertial sensors Accelerometers Boxing jab Shoulder stability Noise
The authors wish to thank the participants for their assistance and involvement in the project. Thanks also to Ray Leadbetter from Sabel labs for assistance with the instrumentation.
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