Foot Acceleration Attenuation Reduces During Military Load Carriage

Abstract

Military personnel are at risk of stress fracture injuries, especially those in load carriage-based military occupational specialty, such as infantry. Recently, wearable inertial measurement unit (IMU)-based accelerometry has become a useful tool for identifying markers of lower extremity musculoskeletal injury risk in soldiers in field settings. PURPOSE: To compare differences in accelerometry between non-dominant and dominant foot using foot-worn IMU sensors during a 2km best effort run with heavy (20kg) load carriage. METHODS: Acceleration data from six healthy participants (3 male: 30.33±6.7 y, 1.82±0.01 m, 77.80±11.0 kg and 3 female: 21.0±2.6 y, 1.66±0.1 m, 64.62±13.5 kg) were recorded using tri-axial IMU affixed to the dorsum of each foot. Participant performed a 2km best effort march (run and walk) across grass carrying 20 kg on their back. Data were divided into 200m +/- blocks from the beginning, middle and end of the exercise for analysis. The acceleration amplitudes from each trial were expressed as the root mean square (GRMS), calculated as the average of the square of the acceleration over time, and were used to quantify the accelerations attenuation. The magnitude of the resultant acceleration signal Accr, referred to as the “composite acceleration signal”, was computed as: sqrt ( accx+accy+accz), where accx, accy and accz are obtained from each individual axis of the tri-axial accelerometer. RESULTS: The mean GRMS values at the three phases obtained for the dominant and non-dominant feet of the men were 4.72, 5.15, and 5.23, and 4.93, 4.75, and 4.41 m/s2, respectively whereas those obtained for the dominant and non-dominant feet of the women were 3.91, 4.32, and 4.78, and 3.87, 4.42, and 4.71 m/s2, respectively. These findings revealed that the GRMS values of the feet obtained for both sexes gradually increased during the load carriage task, except for the non-dominant foot of the men, which decreased. CONCLUSION: Increases in GRMS during loaded marching suggest non-linear increases in culminative mechanical stress exposure as distance increases. Foot worn IMU-based measurement systems may provide means to accurately assess injury risk in real time. Supported by UK Ministry of Defence (WGCC 5.5.6-Task 0107) and US Dept. of Defense (W81XWH-17-2-0070).