FOOT FUNCTION AND SPRINTING
The dictionary definition of sprinting is: ‘running at full speed over a short distance (400m or less), but from a biomechanical and coaching perspective sprinting is more than just ‘fast running’. Sprinting is a distinct biomechanical behaviour from running and is adopted at speeds over 15-16mph (7m/s). The typical characteristics of a ‘sprint’ are: stride rate higher than 4 hz; ground contact time less than 150ms and; a distinct forefoot-strike pattern. These biomechanical changes in running technique are an attempt by the body to create a ‘stiffer spring’ to deal with the increased forces associated with faster speeds.
The reactive strength required in the lower extremity, hips and postural muscles to use this movement strategy and avoid injury is beyond the physical capabilities of the average recreational runner. Coaching in the technical and conditioning aspects of sprinting is strongly advised.
The key features required in a ‘sprinting shoe’ are:
Anatomical toe box (wide, flat and foot-shaped). The role of the toe flexors, extensors and intrinsic foot muscles is the same as in running, but they are required to perform this function with greater force and over a shorter period of time.
Lightweight and stiff. The lower leg must act as a ‘stiff spring’ during sprinting, achieved by minimising movement at the ankle and foot joints. Flexible and/or cushioned footwear increases the demand on the muscles of the lower leg as they attempt to ‘stiffen’ the joints.
Maximum traction. Ground contact time and traction are inversely related. There is intelligent biomechanical reasoning behind the design of ‘running spikes’.
The biomechanics of human locomotion can be explained by the physics of pendulums and springs. Humans have essentially four locomotive strategies available: walking, jogging, running, sprinting.
- These locomotive strategies become progressively less pendulum like and more spring like as speed and gravitational loading increase and contact time decreases (see diagram).
- Each locomotive strategy has energetic and biomechanical consequences i.e. metabolic cost and risk of injury.
- Skillful human movement is characterised by adopting the locomotive strategy for a given speed and terrain that maximises economy while minimising injury risk.
- Movement strategy selection is influenced by several factors including habit, conditioning and accurate sensory feedback about the external environment.
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