ages of 3–6 years old. Another difference includes the plantar contact area. Girls showed a smaller contact area in plantar loading patterns than boys in children with healthy feet. Sex differences There are sex differences in human gait patterns: female-assigned people tend to walk with smaller step width and more pelvic movement. Gait analysis generally takes biological sex into consideration. Sex differences in human gait can be explored using a demonstration created by the BioMotion Laboratory at York University in Toronto. Efficiency and evolutionary implications Even though plantigrade locomotion usually distributes more weight toward the end of the limb than digitigrade locomotion, which increases energy expenditure in most systems, studies have shown that humans are economical walkers, but not economical runners, which is said to be consistent with evolutionary specialization for both economical walking and endurance running. For the same distance, walking with a natural heel-first gait burns roughly 70% less energy than running. Differences of this magnitude are unusual in mammals. Kathyrn Knight of the Journal of Experimental Biology summarizes the findings of one study: "Landing heel first also allows us to transfer more energy from one step to the next to improve our efficiency, while placing the foot flat on the ground reduces the forces around the ankle (generated by the ground pushing against us), which our muscles have to counteract." According to David Carrier of the University of Utah, who helped perform the study, "Given the great distances hunter-gatherers travel, it is not surprising that humans are economical walkers." Key determinants of gait A normal gait pattern depends on a range of biomechanical features, controlled by the nervous system for increased energy conservation and balance. These biomechanical features of normal gait have been defined as key determinants of gait. It is therefore necessary for the refined neurological control and integration of these gait features for accuracy and precision with less energy expenditure. As a result, any abnormality of the neuro-musculo-skeletal system may lead to abnormality in gait and increased energy expenditure. The six kinematics or determinants of gait, described below, were introduced by Saunders et al. in 1953, and have been widely embraced with various refinements. Recent studies have suggested that the first three determinants might contribute less to reducing the vertical displacement of the center of mass (COM). These determinants of gait are known to ensure economical locomotion, by the reduction in vertical center of mass