In this set of movements, the key points of the exercise are: dumbbells are aimed at the posterior bundle of deltoid muscle in reverse, and the front flat lift is aimed at the anterior bundle of deltoid muscle.

When the dumbbell moves backward to the front level, if the arms are completely straight and the center of gravity of the dumbbell is just at the support point of the shoulder joint, the weight of the dumbbell will be supported by the bone (this kind of weight is supported by the bone instead of the muscle, which is called "locking"), so that the muscle is relaxed and it is difficult to get the feeling of stretching and muscle contraction. Affect the exercise effect. So the arm does not need to be completely straight, and the elbow joint can be slightly flexed.