GURPS MA: Leg Grapple technique

[Kromm]

Even untrained people can apply arm locks without meaning to if they catch an incoming punch. It's almost biomechanically intuitive – for humans, at least. And nearly every grappling style teaches arm locks because they're quick, straightforward, and effective.

By contrast, true leg locks (rather than just grabbing a leg) are tricky. A lot more time and contact is needed to lock up the leg. Most styles don't teach leg locks, and those that do generally teach them as deliberate, offensive strategies (work the guy's leg, eventually get a lock) rather than as defensive responses.

I can think of several styles that transition from stopping a punch to locking up the arm with the aid of the punch's momentum. I can't think of any styles that transition directly from a low block to a full leg lock in one move, though. The intuitive response to stopping a kick is to grab it, not to lock it up; locking it up requires significantly more work than locking up an arm.