Laboratory Evaluation of the gForce Tracker™, a Head Impact Kinematic Measuring Device for Use in Football Helmets.

This study sought to compare a new head impact-monitoring device, which is not limited to specific helmet styles, against reference accelerometer measurements. Laboratory controlled impacts were delivered using a linear pneumatic impactor to a Hybrid III headform (HIII) fitted with a football helmet and the impact monitoring device (gForce Tracker-GFT) affixed to the inside of the helmet. Linear regression analyses and absolute mean percent error (MAPE) were used to compare the head impact kinematics measured by the GFT to a reference accelerometer located at the HIII's center of mass. The coefficients of determination were strong for the peak linear acceleration, peak rotational velocity, and HIC15 across all impact testing locations (r(2) = 0.82, 0.94, and 0.70, respectively), but there were large MAPE for the peak linear acceleration and HIC15 (MAPE = 49 ± 21% and 108 ± 58%). The raw GFT was accurate at measuring the peak rotational velocity at the center of mass of the HIII (MAPE = 9%). Results from the impact testing were used to develop a correction algorithm. The coefficients of determination for all impact parameters improved using the correction algorithm for the GFT (r(2) > 0.97), and the MAPE were less than 14%. The GFT appears to be a suitable impact-monitoring device that is not limited to specific styles of football helmets, however, correction algorithms will need to be developed for each helmet style.

The influence of ankle muscle activation on postural sway during quiet stance.

Although balance during quiet standing is postulated to be influenced by multiple factors, including ankle stiffness, it is unclear how different mechanisms underlying increases in stiffness affect balance control. Accordingly, this study examined the influence of muscle activation and passive ankle stiffness increases on the magnitude and frequency of postural sway. Sixteen young adults participated in six quiet stance conditions including: relaxed standing, four muscle active conditions (10%, 20%, 30% and 40% maximum voluntary contraction (MVC)), and one passive condition wearing an ankle foot orthotic (AFO). Kinetics were collected from a force plate, while whole-body kinematics were collected with a 12-sensor motion capture system. Bilateral electromyographic signals were recorded from the tibialis anterior and medial gastrocnemius muscles. Quiet stance sway amplitude (range and root mean square) and frequency (mean frequency and velocity) in the sagittal plane were calculated from time-varying centre of gravity (COG) and centre of pressure (COP) data. Compared to the relaxed standing condition, metrics of sway amplitude were significantly increased (between 37.5 and 63.2%) at muscle activation levels of 30% and 40% MVC. Similarly, frequency measures increased between 30.5 and 154.2% in the 20-40% MVC conditions. In contrast, passive ankle stiffness, induced through the AFO, significantly decreased sway amplitude (by 23-26%), decreased COG velocity by 13.8%, and increased mean COP frequency by 24.9%. These results demonstrate that active co-contraction of ankle musculature (common in Parkinson's Disease patients) may have differential effects on quiet stance balance control compared to the use of an ankle foot orthotic (common for those recovering from stroke).