Effects of load carriage on lateral stepping regulation during forced-marching

Forced-marching with heavy loads is a typical task for warfighters. Unfortunately, injury incidence is high during these tasks with the most common mechanism of injury being a slip, trip, or fall. Given that individuals are less stable in the mediolateral plane we sought to understand how load carriage, sex and task completion impacts parameters of mediolateral stability during forced-marching. Twenty-four (12F, 12 M) recruit-aged adults (18–35 years) completed trials of forced-marching unloaded and with a load 55 % of bodyweight. Mean and standard deviation of center of mass position (z_B), heading ($\Delta$ z_B), step-width (w), and trunk impulse (J_Trunk) were calculated. Detrended fluctuation analysis and direct control analysis were performed on time-series of each parameter. Only J_Trunk mean and variability changed between conditions, increasing with load (p = 0.006; p < 0.001). Load had no effect on z_B and $\Delta$ z_B control with the former being minimally controlled and the latter tightly regulated. The addition of load decreased (p = 0.004) control of w and J_Trunk but the latter was still tightly regulated. There were no effects of sex but individuals who failed to complete the loaded trial had greater (p = 0.038) J_Trunk compared to those who did. The continued strict regulation of J_Trunk when loaded suggests an attempt to reduce large magnitude deviations. Larger J_Trunk of individuals unable to complete the task may indicate limited trunk muscle strength/endurance to restrict impulse. Alternatively, these individuals may utilize a coordinative pattern poorly suited for constraining trunk movement. Therefore, J_Trunk may represent an important parameter of assessment to identify individuals of greater risk.