30Forefoot running is advocated to improve running economy because of increased elastic energy 31 storage than rearfoot running. This claim has not been assessed with methods that predict the 32 elastic energy contribution to positive work or estimate muscle metabolic cost. The purpose of 33 this study was to compare the mechanical work and metabolic cost of the gastrocnemius and 34 soleus between rearfoot and forefoot running. Seventeen rearfoot and seventeen forefoot runners 35 ran over-ground with their habitual footfall pattern (3.33-3.68m•sP -1 P) while collecting motion 36 capture and ground reaction force data. Ankle and knee joint angles and ankle joint moments 37 served as inputs into a musculoskeletal model that calculated the mechanical work and metabolic 38 energy expenditure of each muscle using Hill-based muscle models with contractile (CE) and 39 series elastic (SEE) elements. A mixed-factor ANOVA assessed the difference between footfall 40 patterns and groups (α=0.05). Forefoot running resulted in greater SEE mechanical work in the 41 gastrocnemius than rearfoot running but no differences were found in CE mechanical work or 42 CE metabolic energy expenditure. Forefoot running resulted in greater soleus SEE and CE 43 mechanical work and CE metabolic energy expenditure than rearfoot running. The metabolic 44 cost associated with greater CE velocity, force production, and activation during forefoot running 45 may outweigh any metabolic energy savings associated with greater SEE mechanical work. 46