In cross-country skiing the muscles are primarily voluntary movements. Skeletal muscles surround the bones of the body. They are connected together by tendons. Cross-country skiing engages all of the major muscle groups in the body.
Agonists produce most of the force needed for a movement.
Synergists assist the action and fine tune the movement. The antagonist protect. An example of this would be the quadriceps and hamstrings. This prevents overstretching. The prime movers flex a limb and do the majority of the work.