Dynamic Soft Tissue Artifacts during Impulsive Loads: Tuning Measurement Errors with Wearable Inertial Measurement Unit Sensor Design

Abstract

Abstract Purpose As inertial measurement unit (IMU) sensors become more ubiquitous to measure human motion in real-world environments, it is important to understand the errors that arise when using these sensors. We characterized measurement errors due to transient dynamic soft tissue artifacts that are excited by impulsive loads to the body, such as foot strikes during running and jumping. Methods We instrumented 10 participants (5 female, 5 male) with IMUs on the dominant leg. An ankle IMU measured ground truth vertical accelerations during impulsive loads and was cross-validated against a force plate. These were compared against measurements from two IMUs on the posterior shank and anterior shank to characterize errors caused by dynamic soft tissue artifacts. The shank sensors’ masses were varied to explore their effect on dynamic soft tissue artifacts. Results Both the posterior IMU and anterior IMU overestimated peak vertical accelerations during the impulsive load (gain of 2.18 ± 0.63 and 1.55 ± 0.35 respectively) and exhibited post-impulsive load oscillations lasting 326 ± 75ms and 151 ± 50ms respectively with a natural frequency of 9.79 ± 2.68Hz and 18.22 ± 12.10Hz respectively. The oscillation duration and natural frequency varied with sensor mass in accordance with an underdamped second-order system. Low-pass filtering reduced overestimation of peak vertical accelerations, but also attenuated the ground truth signal. Conclusion Our study suggests dynamic soft tissue artifacts result in transient, but substantial measurement errors that may not be appropriately mitigated through low-pass filtering alone. However, the measurement errors predictably varied with sensor mass, allowing for optimization in specific applications for improved kinematics measurement.