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# Mechanical Determinants of Running Speed

### This is an excerpt from Developing Speed-2nd Edition by NSCA -National Strength & Conditioning Association & Ian Jeffreys.

Movement is clearly driven by the application of forces, so how are these concepts applied in the generation of speed? Classically, running speed has been defined as the product of stride rate (also called stride cadence or stride frequency) and stride length (6). Stride rate refers to the number of strides taken per second, and stride length refers to the distance traveled by each stride in yards or meters. The product of these variables (i.e., stride rate × stride length) gives a mathematically accurate calculation of running speed. Therefore, the focus of speed training has been on improving stride rate, improving stride length, or improving both. However, recent research suggests that while improving these factors plays a role in determining running speed, they may provide misguided advice when developing speed training programs (6).

In particular, the concept of stride length, traditionally measured as the distance between each successive foot contact, can be problematic. Too much focus on artificially lengthening an athlete’s stride can result in placing the foot ahead of the athlete’s center of mass. This position compromises the athlete’s ability to generate force and ultimately slows running speed. Instead, an effective stride length should be the focus. This is the distance traveled by the athlete’s center of gravity per stride. An effective stride length is generated by applying a force into the ground (pushing off the ground) and propelling the athlete forward rather than reaching forward with the legs in an attempt to pull the athlete forward. The athlete’s force-producing capacities are fundamental to achieving optimal stride and length and maximal speed.

Stride rate is a function of contact time (the time spent on the ground with each stride) and flight time (the time spent in the air on each stride). Research has shown little variation in flight time between runners of different speeds, and the greatest variations in stride rate are a result of differences in ground contact time (12). Therefore, efforts to improve stride rate should predominantly focus on shortening ground-contact times rather than focusing on cycling the legs faster, unless there is a clear issue that limits performance. However, here again a challenge may present itself where an overemphasis on producing a faster stride rate may result in reduced ground force application and thus a reduced stride length (5). Finding the optimal balance will always be important.

Stride length is largely a function of the impulse and velocity generated at toe-off. The velocity of the athlete’s center of gravity, which is a key factor in dictating stride length, does not alter between successive steps. Instead, it is generated by the impulse applied during the time the athlete’s foot is in contact with the ground (the stance phase). Therefore, efforts to enhance stride length by technical means during the flight phase, the time the body is not in contact with the ground, are limited and should instead focus on applying impulse and generating velocity during the time the athlete is in contact with the ground.

More Excerpts From Developing Speed 2nd Edition

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