Changing the horizontal position of a fixed backpack load: The effect on postural stability in young adults.

Background: Modifying the horizontal position of the load in a backpack will change the size of the external torque it creates on the wearer but the effect on postural stability is unclear.

Objective: To determine if changing the horizontal position of a fixed backpack load affects postural stability in young adults.

Methods: A backpack was attached to a steel frame with a bar protruding posteriorly. A fixed load (5% body mass) was placed at three distances along the bar - 0 m, 0.20 m, and 0.40 m. Centre of pressure (CoP) derived measurements were recorded from a force platform sampling at 100 Hz. For each condition participants performed three 90s narrow stance trials with their eyes closed whilst standing on a firm surface. A comparison was made across unloaded (no backpack) and loaded conditions.

Results: There was an immediate decrease in postural stability when a loaded backpack was worn. Only two of the CoP derived measures (Total Excursion - TEx, and Mean Velocity Total Excursion - MVel TEx) differed between the loaded at 0.20 m and loaded at 0 m conditions. All CoP derived measures differed between the loaded at 0.40 m and loaded at 0 m conditions. Furthermore, three of the CoP derived measures (Anterior/Posterior Root Mean Square - A/P RMSq, TEx, and MVel TEx) differed between the loaded at 0.40 m and loaded at 0.20 m conditions.

Conclusion: The distribution of a load within a backpack must be carefully considered. The findings for the 0.40 m condition are important for the use and design of large backpacks used by multi-day hikers, travellers, and the military.