Stiffness of joints

When doing sprints and plyos, should a person consciously be thinking about having as little give in the knees and ankles as possible?

When I coach plyos I have the athlete think of a cue that will result in stiffness as a byproduct. For example a cue for hurdle hops would be “quick as possible off the ground”, by emphasizing this the stiffness will occur as needed.

http://www.youtube.com/watch?v=vH-ol7Xf-D8

This is me doing some sprints, as well as other stuff for my workout the other day. Do you mind commenting and giving critiques?