"Say, if I want to fix the model's upper body (e.g. all parts above abs), and make the hips / legs move accordingly to make some kicking / running poses, is there any ways to do it?"

Hi if  I am under standing your stated goal correctly I submit that there is a way to put a hard Fix  or a "hold" on any body part during an animation while moving the others.

step one:
Decide in advance how many frames you animation will be and ADD them to the time line /frame counter.

Set two: put or even animate the desired body parts (e.g. all parts above abs) in the "fixed" position you need/desire.

Step  three: save the pose to the pose library and choose the 'select subset" option and the hierarchy editor will open

Step four:
 Deselect the entire "universe" and check box only those parts whom's fixed pose you wish to save.(e.g. all parts above abs)   and save

Step five:
when you ok your selections in the hierachy editor and the option to save one frame or multiple frames will be presented .. choose MULTIPLE frames using the number of frames you decided in step one.

Step six: do whatever animation you had planed for the non fixed bodypart (legs,hips etc.)
DONT worry that you are "messing up" your upper body pose.

Step seven: when you are satisfied with your lower body animation load that multi-frame "fixed "pose you saved  to your library in step four and it will apply that fixed pose (e.g. all parts above abs)  across the entire animation
while leaving your lower body movement untouched as you made them.

  I use this method all the time to Fix natural looking  poses across entire animated sequences to hands that get all weird from BVH/mocap files

P.S. BTW I am sure the process I just described above can be done with poser animation layers introduced in P7 but Alas I am still on poser 6.

Cheers

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