Delayed onset muscle soreness in the lower calf (soleus) muscle is very common among new barefoot and Vibram Fivefingers runners. This video clearly shows the tightening of this muscle and the Achilles tendon upon footstrike in the Vibrams. Given the lack of a heel, stretching and eccentric contraction of these muscles is likely greater while running in Vibrams than in typical running shoes, and this may be why they are suggested to be better at strengthening the lower legs in addition to the feet (though no studies have been done to confirm this). Filmed at 300fps with a Casio Exilim EX-F1 digital camera, and slowed down further in Virtualdub. Courtesy of runblogger.com.