BIOMECHANICAL EFFECTS OF CARRYING A UNILATERAL LOAD ON HUMAN BIPEDALISM AS INDICATED BY FOOTPRINT TRAIL PARAMETERS

The evolution of bipedalism allowed hominins to carry tools, food and infants more easily. Now, humans carry suitcases, grocery bags and toolboxes that sometimes comprise a large percentage of body weight. Preliminary research suggested that when humans carry a heavy, unilateral load we in-toe on the side opposite the load and narrow step width. This project was undertaken to test the idea that both of these changes in foot placement help compensate for the imbalance produced while carrying a load in one hand. If so, a greater load should produce greater effects. Thirty subjects of both sexes were asked to walk on a paper runner, while wearing paint-soaked socks and carrying a canvas bag with various loads: empty; 7% of body weight; 14%; and 21%. Foot angle, medio-lateral foot placement, step width and step length were recorded and analyzed. Cadence, speed and duty factor were calculated from videos made during the footprinting procedure. As load increased, foot angle decreased (intoeing), as did step width. Also, step length decreased and the variability of all footprint trail parameters increased with increasing load. The increased variability in foot placement was taken as an indication of "staggering" and partially explains the shorter steps. Cadence, speed and duty factor did not vary significantly with different loads, suggesting that changes in temporal characteristics of gait did not account for the changes in footprint trail parameters. Studies of modern footprint trails are the best way to understand ancient footprint trails. The results here suggest that the unusual foot angle and step length of one of the Laetoli footprint trails are best understood as resulting from pathologj, rather than carrying or species differences.