Receiving the ball, passing underhand, executing a dig, reaching for the ball or setting underhand entail some action of the arms, mainly moving the arms upward or working the arms in front of the body. The arm actions involved in these skills do not require great force but you do need good control and accuracy of the movements. By doing exercises to strengthen the muscles involved in the specific movements that duplicate what occurs during game play, you can get better control and feel for the movements involved. One of the best exercises to do this is the front arm raise.
The usual recommendation when doing this exercise is to raise the arms upward from alongside the body until they are level to the ground. But for volleyball players, stopping at shoulder level is not recommended as it may cause shoulder tightness. You need full ranges of motion in the shoulders for effective play. For example, when you jump upward to spike or block, you swing the arms in front of the body completely overhead. Because of this, you should do front arm raises through the full range of motion–from alongside the body to completely over the head.
More specifically, assume a comfortable standing position holding dumbbells in the hands alongside the body. Inhale slightly more than usual and hold your breath as you keep the arms straight (but not locked in the elbows) and raise them directly forward and upward. Raise the arms until they are overhead and you experience slight arching in the lower back which indicates your body is completely extended. Then exhale and relax as you return the arms under control, back down to the sides of the body. Repeat for 10-15 repetitions. Start with light weights so that you can go through the full range of motion for all the repetitions.
As you do this exercise, difficulty increases after the arms pass the level position. This is the range of motion in which the deltoid muscle is most strongly involved in this exercise. When you stop at the level position, you do not get full strengthening of this muscle since it is most active from slightly below level to the overhead position. The supraspinatus does most of the early work in raising the arm from alongside the body.
Keeping the arms straight throughout execution creates a long lever so that a light weight will feel quite heavy as you go through the full range of motion. If you bend the elbows, which will happen if you use too much weight, you change the positioning of the arms and do not get the development that is more specific to execution of the volleyball skills. For variety, rather than using a dumbbell in each hand, hold one dumbbell in both hands to execute the exercise. Raise the arms as high as possible so that the hands are together as typically occurs in the pass or dig. Also, raise the arms up slightly to each side rather than directly in front from some repetitions. As a result, you will have better muscle control and overall development that can help prevent injury and enhance your arm actions.