Explain the answer of the following problems

Problem 1- An exercise for skiers involves sitting against a wall as if you were sitting in a chair, but without a chair. Thighs should be parallel to the floor and the knees should make a 90 degree angle.

Part 1- What kind of exercise is this?

Part 2- Which muscles are doing most of the work (which ones begin to hurt)?

Part 3- Which do you think would be easier: 3 minutes of this exercise or 3 minutes of jogging? Explain.

Problem 2- Find some old tennis balls or any type of similar object and try juggling two balls with one hand, or three balls with two hands.

Part 1- Explain how muscle sense is involved in juggling.

I need assistance to explain how muscle sense is involved in juggling.