I'm trying to do an animation using Poser 4.0.x, and my two keyframes have the head and chest in the exact same position. Only the left leg is moving forward, (the figure has previously been shifted right, so that the weight appears to be over the right leg). Poser moves the chest and head forward between keyframes, and then back again. How do I prevent this unwanted movement?

Use a spline break on the two key frames. You'll find the spline break button on the top right of the animation palette.

To see the difference it makes, select the chest then open up the graph display and select bend. You'll see the graph curve between your two key frames. Creating a break spline will stop the previous motion influencing the chest after the key frame. The curve will change to a straight line indicating constant motion