Blender / animation: how to keep the feet on the ground?

[Kyle Woohoo]

I just started to create my first animation. To keep it simple I started with a single man who stands on the floor and jerks. I managed to get the animation into the game thanks to turbodrivers webpage and this forum. To make the animation more realistic and less stiff I wanted the man to move his ass/hip a little bit and to move a little bit up and down with his body... you know this tiny little movement up and down when someone slightly bends the knees a little bit. In my eyes the simpliest solution in Blender would be that I could move the top sim body and then the legs simply follow keeping the feet on the same position. By now I am struggling with that little detail because as soon as I move the knees in Blender a little bit to do the up/down movement the feet loose their position. So everytime I do this I need to adjust the z-axis position of the whole sim object (to bring it back on the gound) and I need to find the prior position of the feet on x- and y-axis so that when playing the animation the feet does not start to slide on the ground in a unnatural manner. 

 

A further example for my problem would be another animation I have in mind: a man exercising with a barbell for leg muscles. They usually put the barbell above the head and then go down with the whole body and then go up again. Here I have the same problem: when bending the knees in Blender the feet move up, I need to move down the whole sim object and I need to find the old x- and y-axis positions of the feed so that the feet does not start to slide on the ground when I play the animation (due to the changed x- and/or y-axis position). 

 

I am a total noob to Blender and animation, I went through many tutorials and I already used  "Inverse Kinematics" for the knees (and for the hands) for example so when I move the knee bones the whole leg moves. I think I read somewhere something that this might be used for such a problem as well but I did not get it. I hope someone here might be able to help me.

[Kyle Woohoo]

Just for the record: the rig from https://www.loverslab.com/topic/74481-small-guide-for-creating-animations-for-ts4-tips-and-tricks/page/2/?tab=comments#comment-2176691 does pretty good what I needed