- The leg is prevented from functioning like a spring by storing and transferring energy from one stride to the next
- Energy is lost to greater vertical displacement.
|An over striding runner at left contrasted with an optimal stride at right. Greater vertical displacement of an overstride can be seen by the relative magnitude of the arrows.|
An over stride occurs when the center of mass is behind the knee when the foot strikes. In extreme examples the leg is straight when the foot strikes as seen displayed by the runner at the left. An optimal stride is one that places the center of mass directly over a slightly bent knee when the foot strikes as seen displayed by the runner on the right.
Over stride and efficiency are directly related to cadence. Cadence being defined as the number of right or left foot strikes in a 60 second period. An over stride / reduction in efficiency occurs at any cadence less than 90 as cited in numerous studies and is easily verifiable by an increase in perceived effort, heart rate, and calorie burn. An optimal cadence of 90 is employed for much of the human endurance range of paces. Even faster paces require higher cadences as stride length has a maximum.
|My personal pace and cadence curve. The endurance range of paces is a constant 90. Outside of the endurance range speed is increased by increasing cadence.|
|Personal cadence made possible by ANT+ foot pod and compatible watch. . A foot pod is an underrated but indispensible training tool.|
The effect of even a slight over stride on race times of all distances is profound. Much of the year I employed a cadence of 86 preventing me from realizing my full potential. A cadence of 90 could have shaved 56 seconds from a 5k, and close to 5 minutes from a half marathon for the paces in the table. For the most severe over stride the potential is even greater at 3 minutes for a 5k and close to 17 minutes for a half marathon. As paces slow efficiency decreases when over striding
The potential pace in the table indicates what pace could be realized by employing an optimal cadence of 90 if the actual pace were ran in the time listed at the cadence listed. For a cadence of 90 no improvment is possible and the potential pace equals the actual pace. To further explain the mechanisms mentioned at the beginning of the post:
A running human creates a good amount of kinetic energy. Running is the closest to flying our bodies get and each stride does include an airborne portion. The leg can capture much of the energy (63% at 90 cadence) from the vertical displacement that accompanies coming back to earth and apply that energy to the next stride. The bones and tissue of the human foot and leg form a very functional and efficient spring.
An over striding runner will not fully benefit from the legs ability to function as a spring. Instead of the leg capturing and transferring this energy to the next stride some or all of that energy will be violently dissipated in the form of noise and vibration/deformation of the shoes, feet, muscle, and bones. Run quietly is apt advice. The next stride will need to generate this energy again requiring additional effort (0 to 16.5% + of total energy expenditure).
If the spring effect were not offensive enough over striding also requires greater vertical displacement, which sub sequentially requires greater effort to accomplish. Coming from greater heights also leads to greater impact forces. It is often repeated that races are a straight line, not up and down, thus in the interest of speed it is best to minimize vertical movement. This effect is secondary as it causes much less energy loss than losing out on the previously mentioned spring effect. A small amount of energy is saved from striding less but is does not change the net result.
The popularity of Run Faster: Shoe Weight and Performance helped inspire this post. If you enjoyed this post +1, forward it to a friend, or leave a comment. Feedback keeps me motivated. Additionally the topic is rich and I am happy to discuss. If you hated it or see inaccuracies, let me know too. This post was made possible by way of mathematical analysis utilizing basic physics and efficient runner model rev 1.0. A follow up post will contain the analysis for the select few. If you are interested in reading more on the topic this link contains great material. Thanks for reading.
NOTE: An optimal cadence of 180 refers to both the left and right foot strikes in a 60 second period. An optimal cadence of 90 refers to just the left or just the right foot strikes in a 60 second period. Different device manufacturers and authors use either or.