Joint moments and muscle excitations increase with body-mass normalized backpacks across walking slopes

Human locomotion is characterized by joint moments and muscle excitations of the leg, which have quantified the demands of backpack loads and sloped walking. However, the combined impact of backpack loads and sloped walking remains poorly characterized, particularly during downhill walking. This study characterized 3D hip and sagittal knee and ankle moments and integrated muscle excitations when walking downhill, level, and uphill while carrying a body-mass normalized backpack load.

Fourteen healthy, active-duty military service members were enrolled in this study. Participants walked on a treadmill under six walking conditions - uphill, level, and downhill (1) with a baseline load, and (2) with a heavy backpack load. Surface EMG, full body motion capture, and ground reaction forces were captured, and lower body moments and muscle excitations were examined. Fixed effects of slope, pack, and the slope-by-pack interaction were evaluated using linear mixed effects models RESULTS: The addition of backpack loads generally resulted in greater joint moments and muscle excitations on all slopes; however, several interaction effects highlight the compounding influence of walking slopes and backpacks. The effect of added backpack mass was greatest during uphill for hip extension and ankle plantarflexion moments. In addition, backpack loads had a greater knee extension moment throughout stance when walking downhill, but level and uphill walking had a knee flexion moment in terminal stance that was not affected by the backpack.

Our overall findings define the biomechanical task demands for sloped walking with a backpack load. These results provide a mechanical context that may be useful for understanding musculoskeletal overuse injury attributed to load carriage.