Only accelerations in the anterior-posterior (AP) plane are strongly associated with running economy. The runners with the smallest accelerations in this plane are the most efficient, regardless of their degree of bounciness or whatever else is going on with their stride. This makes sense when you consider that a runner who exhibits large accelerations in the AP plane is a runner who slows down and speeds up quite a bit from stride to stride, even while trying to hold a steady pace. The greatest source of energy waste in running is the braking that occurs when a foot makes contact with the
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