If you want to use your back to force when the pull-up is up, we need to let the shoulder blades sink when hanging on the bar. We can practice this action repeatedly to feel the force of the back muscles. Then tighten the big arm inward, and finally bend the arm to pull the whole body up.
Pull up is an exercise that many people use now, but not everyone can do it. However, sometimes a person can't pull up not because of its insufficient strength, but because of the wrong way of force.
Many people know that pulling up depends not only on the strength of the arm, but also on the strength of the back muscles. When doing pull-up, we should first hold the horizontal bar with both hands to suspend our body, and then sink our shoulder blades.
This action can make me better master the feeling of back force. After we sink the shoulder blades and lift the body a little, we can tighten the big arm in, and finally bend the arm to pull the whole body up.