Peak Muscle and Joint Contact Forces of Running with Increased Duty Factors

PURPOSE: Running with increased duty factors (DF) has been shown to effectively reduce external forces during running. In this study, we investigated whether running with increased DF (INCR) also reduces internal musculoskeletal loading measures, defined as peak muscle forces, muscle force impulses, and peak joint contact forces compared with a runners' preferred running pattern (PREF). METHOD: Ten subjects were instructed to run with increased DF at 2.1 m·s -1 . Ground reaction forces and three-dimensional kinematics were simultaneously measured. A musculoskeletal model was used to estimate muscle forces based on a dynamic optimization approach, which in turn were used to calculate muscle force impulses and (resultant and three-dimensional) joint contact forces of the ankle, knee, and hip joint during stance. RESULTS: Runners successfully increased their DF from 40.6% to 49.2% on average. This reduced peak muscle forces of muscles that contribute to support during running, i.e., the ankle plantar flexors (-19%), knee extensors (-18%), and hip extensors (-15%). As a consequence, peak joint contact forces of the ankle, knee, and hip joint reduced in the INCR condition. However, several hip flexors generated higher peak muscle forces near the end of stance. CONCLUSIONS: Running with increased DF lowers internal loading measures related to support during stance. Although some swing-related muscles generated higher forces near the end of stance, running with increased DF can be considered as a preventive strategy to reduce the occurrence of running-related injuries, especially in running populations that are prone to overuse injuries.