The second most difficult exercise is latissimus dorsi.
For those who are proficient in training, the most difficult thing to practice is the leg.
Why?
Because ordinary people have a problem, scapula stability is poor.
The training action of the deltoid posterior bundle is basically the extension and rotation of the big arm at the shoulder joint. Intuitively, it is the action of pulling the arm back. But the position of the posterior bundle of deltoid muscle is from scapula to arm. If the scapula is unstable during exercise, then pulling the arm backwards means that the scapula pulls the arm, and there is no activity between the scapula and the arm. Therefore, the posterior deltoid muscle between the scapula and the arm is difficult to train.
Then the second one is latissimus dorsi. The reason is the same as the last one. The stability of scapula is poor, and the action of practicing back is either pulling down or pulling back. However, if you pull your shoulder so hard, it will run, that is, shrug. When you run up, your pull-down and pull-back actions become "scapular arm around the arc" I've done a lot less pulling and I haven't practiced all my latissimus dorsi, but at least I can practice a little. So it ranks second.
Then why are legs the most difficult for people who are proficient in training? This is actually very simple. Leg muscles are big and powerful, which requires a lot of training intensity and quantity. But you should practice fully and don't overtraining yourself. This is not easy to grasp.