Upper chest "crunch" to pull Shoulder joint down (and perhaps a
little backward)
Waist and hip flexor muscles to pull chest and shoulder down
plus
(possibly) Knee Flexors (hamstrings) to make butt-drop aspect of the
double-pole-push quicker and more forceful.
Knee Extensors (quadriceps) to recover butt and hips up forward and
high. Which also raises the weight of the upper body, which is
real work in building "potential energy", which can be released in a
later phase to help add force to the pole-push.
(possibly some) Torso Rotation sideways -- slight reactive
side-force to help the skate-push.
but in V2 Skate the side-to-side upper body forces are
usually minimized in order to focus on up-and-down motion and on
optimally pushing the poles straight back in the line of overall motion.
