Naw, go for the knee. A guy can/will instinctively protect the family jewels, but there is simply NOTHING he can do to protect his knees from being folded backward.
I discovered just how well this works <img src="/images/graemlins/frown.gif" alt="" /> when I got a slight bang on the side of my knee from my exuberant 50# dog and couldn't move at all. I couldn't put any weight on that leg at all. Prepared to plant roots until I leafed out in Spring, I finally saw my neighbor, and called for her to go to the house and fetch my mother's walker. It took a week to get over just this little bump.
You know how you stomp a stick to make it a shorter piece of firewood? That's how you strike. Very, very effective.
Sue