To explain further look at this screenshot.

1. The starting point.
2. Keyframe to enable the outward swing of the leg to be adjusted
3. Full extent of the forward swing. Spline break key frame.
4. Keyframe to enable the backward swing to be adjusted.
5. The rear swing keyframe.
6. Final frame to bring the motion back to the start.

Note that because keyframe 3 is a spline break then moving keyframe 2 will not affect the back swing. Similarly moving keyframe 4 will not affect the front swing. Moving keyframe 4 will however affect what happens after keyframe 5, notice the over swing.

Where ever possible I use Spline Section, breaking the separate motions up with spline breaks. If you would like the PZ3 of this scene please email me at pcooke@philc.net and I'll be glad to send it by return.