Dynamics of carrying a load with a handle suspension.

Carrying loads with a compliant pole or backpack suspension can reduce the peak forces of the load acting on the body when the suspension natural frequency is tuned below the stepping frequency. Here we investigate a novel application for a load suspension that could be used to carry a load by hand, which is a common yet difficult method of load carriage and results in inherently asymmetric dynamics during load carriage. We hypothesize that the asymmetric dynamics of carrying a load in one hand will result in multiple locomotion frequency modes which can affect the forces of carrying a load with a handle suspension. We tested an adjustable-stiffness hand-held load suspension with four different natural frequency values while walking and running compared to a rigid handle. As expected, the peak forces acting on the body decrease compared to a rigid handle as the effective suspension stiffness decreases below the stepping frequency. However, the asymmetric dynamics of carrying a load with one hand introduce another frequency mode at half the stepping frequency which increases the peak forces acting on the body when the natural frequency of the handle is tuned near this frequency. We conclude that hand-held load suspensions should be designed to have a natural frequency below the half-stepping frequency of walking to minimize the peak forces and the musculoskeletal stress on the human body while carrying loads with one hand.