Did you know there is a collection of research that seems to indicate that sprinters do not want big quads? This goes along with conventional thought that there is not a lot of demand on the quads during upright sprinting because (1) the knee does not bend very deep during stance and (2) the sprinter is hoping to not be braking very much. Check out four studies below. What's the deal here? Should sprinters not develop quad strength? Are they not strong squatters?
MORE TO THE PICTURE
As a counterpoint to the research above, every athlete I have worked with who particularly excels at max velocity has also been naturally good at upright squatting, which indicates they have relatively strong knee extension. Knee extension is of course the joint action performed by the quads. The video below shows three examples. These athletes have balanced or even somewhat knee dominant squat mechanics. As an example of what I mean by "particularly excels at max velocity," one of the athletes in the video below ran a 21.6 indoor 200m but his broad jump was only about 8 feet. He was much better at max velocity than at other athletic movements; he was also knee dominant. Now obviously I have not coached every athlete in the world, and there is tons of variety among humans, but I really do not believe that being weak at the knee is a common trait among good sprinters. So how does this fit with the reasearch on quad size and speed?
THE MISSING LINK
A key piece of this puzzle is found in this study: "The knee extensor moment arm is associated with performance in male sprinters." If you are not familiar with the term moment arm, it's just a measurement of how much leverage a force has to rotate a lever. In this case, the force is quadriceps tension. The study found that faster people tend to have a longer knee extensor moment arm, which means the quads have more leverage to extend the knee. This explains how fast sprinters can be strong at the knee without having big quads and maybe without doing a lot of hard quad strength work.
Considering this research evidence and my coaching experience, I believe sprinters absolutely should develop knee extension strength and elasticity. It is a requirement for getting off the ground fast. Particularly if you do not possess it naturally, you will have to work hard on plyos and strength exercises to get it. Some examples of strength exercises that can help with this are upright back squats, pistol squats, knee forward split squats, and hack squats. In the plyometric realm some examples that help develop explosiveness and elasticity in the quads are running jumps off one leg, hopping on one leg, and hard deceleration.