Prolonged Load Carriage With Soldiers Increases The Uncertainty Of Movement Dynamics

INTRODUCTION: Soldiers often perform extreme physical tasks over multiple days. A key example of intense military tasks is the ruck march that involves prolonged carriage of up to 50% body mass (50%BM). Load carriage has received considerable attention in the context of movements such as sprints, vertical jumps, maneuverability and agility tasks, quiet standing, and simple walking. However, musculoskeletal injuries caused by both acute incidents and chronic repeated bodily stress remain a significant health issue for the U.S. military and other physically demanding vocations. Most of the known characteristics of load carriage concern linear measures of movement. Studies investigating the nonlinear changes in movement dynamics over multiple days are lacking. PURPOSE: To determine the effect of prolonged load carriage on nonlinear movement dynamics of walking. METHODS: Seven male Soldiers (24.3 ± 3.6y, BMI: 26.8 ± 3.2, Body fat: 11.1 ± 2.5% BF) completed a 4 consecutive day protocol to simulate activities experienced during a 3-day mission and 1 day post mission: 3 mission (MIS1-3) and 1 recovery (REC) days. Data are presented for MIS1 and MIS3 days, where Soldiers performed a 2-hour ruck march on a treadmill with 50%BM added in a weight vest. The treadmill slope was manipulated throughout the trial, comprising of a 10 min 0% grade, 50 min 3% grade, 50 min -3% grade, and 10 min 0%grade. Sample Entropy (SampEn) was calculated from stride interval time series. Multilevel models (MLMs) were used to determine the rate of change in SampEn during load carriage tasks. RESULTS: MLMs revealed that SampEn increased by .12 SDs/10 min during MIS1 load carriage, t(71.20) = 4.53,p < .001. Similarly, SampEn increased by .11 SDs/10 min during MIS3 load carriage, t(74.02) = 5.80,p < .001. CONCLUSIONS: The movement patterns during prolonged bouts of loaded walking become more unpredictable with time. Results cast SampEn as a potential movement-based biomarker of fatigue during load-carriage. Supported by US Army MASTR-E Program and National Strategic Research Institute