Where the Knee Goes

Vern,
My first thought and perhaps that of others was that you were using this picture as a justification for training this athlete with a lunging position that mimicks the above position. I assumed that because “knee beyond toe” adversaries typically denounce the position during squating and lunging. Additionally, the above looks like a deep lunge and specificity of training states that we should train the positions we play. I realized my assumptions may be incorrect. I also realize that we may both be thinking along similar lines. Therefore, I wanted to expand on this for other readers.
I agree with the main sentiments of your post: specificity of training, movement based strength training, etc. Additionally, I also agree that the knee needs to go beyond the toe during training because we assume this position during game-play and everyday life. I think this picture is a great example of why a knee beyond the toe position is acceptable. However, I do not feel as though this particular image is a very good example of loads at the knee or that it should be extrapolated to a squatting or lunging position.
At first glance the above pictures resembles a lunge. Looking closer though, I wouldn’t classify this as a lunge. Lets do a mental free-body diagram / reconstruction of the scene:
1.It looks as though he is traveling from lookers right to lookers left.
2. He is going to take another step
2a. Judging from the shuttlecock’s orientation to the camera, I’d say it is also traveling from lookers right to lookers left which is away from him.
2b. I don’t think he could get a solid hit in where he is currently, so he is going to continue moving forward to make his shot.
3.I’ll confirm to myself that he’s at least on his second step in that direction because his feet are parallel in the direction of travel. IE. He isn’t in a right-lunge position from a static athlete position
4. His center of mass is traveling in the direction he is facing and with decent velocity because it is at least step 2.
Therefore, If we continue with the force diagram, the ground reaction force is not vertical as it would be in a squat or mostly vertical / retro like it would be in a lunge. In this instance the GRF is a vector more similar to the angle of his lower leg.
So lets say that we wanted to train this athlete. We could hook him up to a sled and have him pull something. He’d able to achieve a similar position as he pulls against an external load. At some point we’d probably want to be able to take him into the gym. Would you have him do squats with knees in front of toes? Extremely deep knee beyond toe Lunges that mimic the pictured position? I wouldn’t; the GRF would be in the wrong direction. We need to orient the GRF to gravity, which is our resistance. Conveniently for me, I’m on a laptop although others can tilt their head. When we align the lower leg (and I understand the vector probably isn’t exactly the same, but its a decent approximation) vertically with gravity, we see something interesting. That looks to me like a really tall step up onto a box. Additionally, in that position the knee is no longer over the toe. I would suggest that the force at the knee due to quad extension is not very high in this instance, that the main force is of extension about the hip.
Therefore this is a great example to not follow the knees / toes mantra because it illustrates that just because the knee is in front of the toe, it does not mean the knee is in trouble.
Trouble arises, however, when we attempt to apply this image to the extreme lunge that it seems to represent. The lunge is about a change in direction. It is not a pure acceleration like what is pictured. It is a deceleration with a re-acceleration in an opposite direction.
If our player is attempting to decelerate in this particular frame two things may happen:
1. He may fall over because of inertia, the position of his COM and of his foot.
2. The required knee extension / quad activation so that his leg doesn’t crumble with the changed force vectors would load the knee to an unfriendly level.
He’d be putting himself into a bad situation. Instead if he were changing direction he should do so with his lead foot well in front of where it is. This puts his COM safely inside his base of support and creates an obtuse angle at the knee.
Example: http://www.secsportsfan.com/images/college-tennis.jpg (credit: ukathletics via http://www.secsportsfan.com)
I would advise coaches that this position is safe if it involves a forward acceleration but that a decelerating knee beyond toe lunge this extreme is perhaps too much for athletes to deal with in a controllable manner.