the study of the kicking components within sports

Monday, May 10, 2010

Leg Speed

"Big leg". "Strong leg". It is not uncommon to see and hear these descriptions used regarding a kicker that gets good distance on the ball. But as any kicker or their coach will tell you, "fast leg" is more accurate. Chargers kicker Nick Novak references speed in his kicking tips:
The lunge step: This is the final step into the plant spot. Your kicking leg is in the back swing. The farther you take your leg back, the more force and leg speed you can potentially generate. Remember to take the hip back as well. The entire body must be engaged, not just the leg.
More specific to strength vs. speed, Seahawks kicker Olindo Mare commented:
If you went by sheer leg strength, then Patrick [Kerney, defensive end] should be kicking off. I'm never going to be 200 pounds. I've tried that. I'm a gym rat and I love it, but I'm never going to be big. So I've got to go with what I've got.... Tiger Woods. Sergio (Garcia). I met Sergio, and he's smaller than I am. It's leg speed. Club speed. Racket speed. I'm sure those lightweight boxers can bring it because it's how fast you can bring your leg to the ball. That's why I play soccer and work on sprinting and running more than squatting and all that heavy stuff.
As we previously wrote, Steelers kicker Jeff Reed also discussed the topic of training for speed:
You hear the comment ‘you are just a kicker’ and that gets frustrating. I wouldn’t say anything to the person who said that to me because I have heard it a million times in my life. I work out just as hard as the (other) guys and I train just as much as the guys. I don’t train for speed, I train for leg speed. I am not going to beat Ike Taylor in a 40-yard dash, I’m not going to out-lift James Harrison and I am not going to out-throw Ben (Roethlisberger), but I work out just as hard as those guys because I realize it is hard to get to the NFL, and it is hard to stay here.
Getting especially specific, Dr. Timothy Gay discussed the physics of leg speed in his book The Physics of Football, 2nd edition:
Video analysis of typical leg speed in quasi-stationary kicks, whether they are done conventionally or soccer-style, shows that the end of the extended kicking leg is moving at a speed of approximately 60 feet per second relative to the hip joint just before contact with the ball. (This is essentially the kicker's ankle speed; foot speed is a less reliable measure of the leg's angular velocity because it is often rotating about the ankle before and during contact.) For a leg length of 3 feet, this corresponds to an angular velocity of 20 radians per second. Plugging this value of w into our launch speed equation yields v' = 109 feet per second for stationary kicker, or 74 miles per hour. This corresponds roughly to a punt, where the ball is not moving very rapidly in relation to the punter's hip joint when his foot makes contact with the ball. (The ball has about the same horizontal speed as the punter does, since he was carrying it before he released it.) If the kicker runs up to the ball from 10 yards away before booting it, as with a typical kickoff, he'll have a speed (v) of about 15 feet per second. This corresponds to a launch speed of 126 feet per second, or 93 miles per hour. The field-goal launch speed values will be somewhere in between those for punts and for kickoffs, because the approach distance is less than that for a kickoff.

How well do these numbers correspond to what we know happens in an actual football game? Given our assumption of conservation of energy, we might expect our model estimates to be a bit on the high side, since in reality some energy is always lost in these collisions. On the other hand, we have, for reasons of simplicity, neglected the effect of the lower leg "whipping" about the knee joint, which would increase the speed. Those two factors tend to cancel each other out, so we might expect our calculated speeds to be a reasonable estimation of the true launch speeds of the ball.

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