It has been reported frequently that human locomotion is self-optimizing; the natural gait pattern results in the most favorable ratio of metabolic cost to work output. This often does not appear to be the case in laboratory settings. It has been shown, however, that locomotor economy improves when stride frequency is matched to the resonant frequency determined by a leg-pendulum model or when performers are placed under some form of stress. Load carrying has become commonplace in many occupations, particularly in the military where heavy loads are carried for long distances in less than ideal conditions. Soldier survivability is often dependent on the ability to perform tasks under fire, thus economy becomes a life or death factor. The purpose of this project was to create a demanding walking condition intended to serve as a natural impetus to draw out optimized gait patterns. Thirty adult volunteers participated. Anthropometric measures were taken for pendulum modeling of the legs' resonant frequency and participants were fit with a heart rate monitor. A total of 4 treadmill walking trials at self-selected speeds were gathered: (1) No load, free frequency; (2) No load, resonant frequency; (3) Loaded, free frequency; and (4) Loaded, resonant frequency. The load consisted of 40 lbs carried in a standard issue internal frame rucksack. Resonant frequency test conditions were paced by an audible signal from a metronome. Load and frequency effects on stride characteristics and economy were examined via repeated measures ANOVA. With load carrying, velocity decreased, heart rate increased, and economy decreased. With the ideal frequency, velocity increased, heart rate decreased, and economy increased. The importance of resonance as the physical basis supporting locomotor optimization was shown again. The increased cost and decrease in economy for load carrying emphasizes the importance of understanding and applying such knowledge. Keyword(s): exercise/fitness/physical activity, research