Quote - "Drop figure to floor" script works fine so long as the figure does not leave the floor during the animation, but jumps or parts of a fast run where both feet might leave the ground will be adversely affected. In such cases, you can use another method. Open the graph editor for the hip's Y translation, expand the range so that you can see all frames, click and drag above or below the line of keyframes, high-lighted /selected frames turn black. Hold down Ctrl and drag the keyframes up to the required level.

 

 

Yes, you're right 3Dave, now that I see the animation,  in one part is off the ground and happens what you explain   , I'll try your advice and I tell you the result .

thanks for the tip :)