Modulation of force transmission to the head while carrying a backpack load at different walking speeds.

This study was designed to investigate the capability of the joints and segments to reduce transmission of forces during load carriage. Eleven subjects were required to carry a backpack loaded with 40% of their body weight and to walk at 6 speeds increasing from 0.6 to 1.6 ms(-1) in increments of 0.2 ms(-1), and then decreasing in the same manner. Subjects were filmed in 3-dimensions, but analysis of shock transmission ratio (TR) was limited to the sagittal plane. Shock transmission was measured as the ratio of peak vertical accelerations (ankle:head, ankle:knee, and knee:head) measured immediately following foot strike. TR for all ratios increased significantly as a function of increasing speed. TR from the ankle to the head showed no significant increase as a function of load carriage, but did increase as a function of load in transmission from knee to head. A significant interaction effect revealed that during load carriage at the higher speeds the acceleration of the ankle and knee decreased below that for the unloaded conditions. These findings suggest that the potentially injurious effects of previously observed increased ground reaction forces and increased joint stiffness while walking with loads are offset by adaptations in the gait pattern that maintain force transmission at acceptable levels. Increased variability in the acceleration of the head and in the transmission ratios suggest a potentially destabilizing effect of load carriage on the head trajectory.