Inertial sensor-based centripetal acceleration as a correlate for lateral margin of stability during walking and turning

Abstract

There is growing interest in using inertial sensors to continuously monitor gait during free-living mobility. Inertial sensors can provide many gait measures, but they struggle to capture the spatial stability of the center-of-mass due to limitations estimating sensor-to-sensor distance. While the margin of stability (MoS) is an established outcome describing the instantaneous mechanical stability of gait relating to fall-risk, methods to estimate the MoS from inertial sensors have been lacking. Here, we developed and tested a construct, based on centripetal acceleration, to estimate the lateral MoS using inertial sensors during walking and turning. Using three sensors located bilaterally on the feet and lumbar spine, the lateral MoS can be consistently and reliably estimated based on the average centripetal acceleration over the subsequent step. Relying only on a single sensor on the lumbar spine yielded similar results at the expense of identifying left versus right stance foot. Additionally, the centripetal acceleration estimate of lateral MoS demonstrates clear differences between walking and turning, inside and outside turning limbs, and speed. While limitations and assumptions need to be considered when implemented in practice, this method presents a novel, reliable way to estimate the lateral MoS during free-living community ambulation using inertial sensors.

Use of the margin of stability to quantify stability in pathologic gait – a qualitative systematic review

“…1 ) we can describe and predict stability, i.e. the systems instantaneous mechanical stability [ 9 ]. When the MoS is positive, the pendulum will not rotate over vertical, and will instead return back to its current position, which we consider to reflect a positive stability.…”

“…Since 2008, the MoS, sometimes termed the Dynamic Stability Margin among other similar terms, has been increasingly used by researchers in healthy and pathologic [ 10 – 14 ] populations, during straight line walking [ 15 ], turning [ 16 ], rehabilitation [ 17 ] and for perturbation response [ 18 ]. The MoS is most commonly measured using a kinematic gait laboratory, but options for measurement with wearable devices are emerging [ 9 , 19 , 20 ]. Throughout these studies, the calculations that contribute to the MoS have been interpreted differently or not explicitly described across the literature, making direct comparisons and interpretations between papers studying the same clinical population difficult for clinicians and researchers alike.…”

Moving on: measuring movement remotely after stroke

“…Although this is similar to the local foot frame defined by other studies (Fino et al, 2020;Rebula et al, 2013), ψ cs(k) is defined for each step. We define the X axis of this frame as positive in the forward direction, defined by the line between the beginning and end of a step.…”

“…• Employing correlation models: Finally, instead of building any of the above models, correlates for gait parameters of interest can be extracted from IMU data. For instance, Fino and colleagues use an average of the acceleration over each step as a correlate for lateral Margin of Stability (MoS) (Fino et al, 2020). Such an approach may be suitable when measuring complex measures using a limited sensor setup and limited processing.…”

“…However, when we study kinematics using IMUs, we have the opportunity to explore a bodyfixed reference frame, as the IMUs move along with the body. For instance, Fino and colleagues use the average inclination with respect to gravity to define a body-fixed frame (Fino et al, 2020). Rebula and colleagues estimate the average heading from a number of strides in order to define the body-fixed frame with respect to the foot movement (Rebula et al, 2013).…”