During the first post on the Olympics I discussed how many different injuries can occur when an athlete lands from a jump. Not only is the amount of force they have to absorb when landing a key factor, but the time over which that force is applied is also a very important factor. Impulse is the product of the average amount of force and the time over which that force is applied. In the graph below (Flannagan, S., 2014), the impulse for both the "stiff" landing and the "soft" landing are the same, but the amount of force that has to be absorbed by the jumper with a "stiff" landing is twice that of the force in the "soft" landing. So, what is a "stiff" landing and what is a "soft" landing? In a "stiff" landing, the joints of the lower extremity, primarily the hips, knees, and ankles, stay in a more extended position and do not give, or flex, as the person lands. This means that the ground reaction force is applied over a very short time period. In a "soft" landing, the person flexes their hips, knees, and ankles (technically dorsiflexion of the ankles) as they land to increase the amount of time over which the ground reaction force is applied. This leads to a lower amount of stress placed on the lower extremity. That's why it is important to teach young athletes to perform a soft landing by flexing the joints of the lower extremity when they land (second figure below).
Flannagan, S.P. (2014) Biomechanics: A case based approach (1st ed.), Jones & Bartlett.
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