You can often improve character animation by filtering: using selected data rather than all data.
In the Motion Mixer, you use the trackgroup filter to select the biped parts that will be affected by motion clips and transitions on tracks within the trackgroup. See Filtering Mixer Tracks.
Filtering is also a motion-capture technique. Motion-capture and marker data typically have keys at every frame. Filtering motion-capture data reduces the number of keys, making the job of altering or personalizing the motion data simpler. Other filtering options include footstep extraction, applying the skeletal structure stored in the motion-capture file to the biped, looping the data, importing a portion of the motion capture file, and selecting tracks to load. See Filtering Motion-Capture and Marker Data.