Running is thought to be efficient largely due to elastic energy storage in muscles and tendons, particularly the plantar flexor muscles and the Achilles tendon. Although plantar flexor muscle mechanics have been explored during rearfoot striking, little is known about how converting to a forefoot striking running pattern affects energy storage in the Achilles tendon or alters demands placed on the plantar flexor muscles. This study examines how plantar flexor muscle-tendon mechanics during running differ between rearfoot and forefoot striking. Plantar flexor mechanics were estimated using musculoskeletal simulations driven by joint angles and electromyography recorded from runners using both rearfoot and forefoot striking running patterns. The simulations revealed that foot strike pattern affected the gastrocnemius and the soleus differently. For the gastrocnemius, forefoot striking resulted in greater force generation ability (i.e. the force generated per unit of activation) and increased negative fiber work compared to rearfoot striking. For the soleus, forefoot striking resulted in decreased energy storage in the tendon and decreased positive fiber work compared to rearfoot striking. Forefoot striking appears to place greater demands on the gastrocnemius and take advantage of its improved force generation ability. Based on increased activation and negative fiber work during early stance, runners interested in altering their foot strike pattern should be mindful of the increased demands on the gastrocnemius when converting to forefoot striking.