The High Load of Low Forces in Running

Abstract

0416 While horizontal ground forces are substantially lower than vertical ground forces during running, the metabolic cost of generating horizontal forces and thus moving forward is relatively high. Chang & Kram (1) showed that horizontal forces were an order of magnitude lower than vertical forces during running but the metabolic cost required to produce the horizontal forces was more than 33% of the total metabolic demand. We hypothesize the relatively low horizontal ground forces produce relatively high external torques applied onto the lower extremity segments. Thus high metabolic demand may be directly associated with external torques. PURPOSE: Compare external torques applied to the thigh, leg and foot by horizontal and vertical forces during running. METHODS: Video and ground force data were obtained during running at 3.83 m/s from 6 adults, (age 21 yr). Joint torques were partitioned into components due to horizontal and vertical forces and angular impulse (area under torque-time curve) was used to assess the segmental loads from each component. RESULTS: Magnitude of horizontal ground force was 12% of vertical force. Total torque on all segments due to the horizontal forces was 35% as large as torque from vertical forces and was 43% of the total external torque applied to the segments. Lever arms for the horizontal forces were 44% longer than for vertical forces. CONCLUSIONS: Relatively low horizontal ground forces apply relatively high torques on runners due to long lever arms which may account for their high metabolic cost.Table 1: Angular impulse means in Nms and ratios in %