One of the most common problems coaches face when coaching beginning and intermediate players is getting them to relax their arm when stroking. This writer has not only faced this problem hundreds of times as a coach but has also faced it as a lifetime weakness in his own playing game.
If the muscles in the playing arm (or any other muscle) are tight, then they will not stroke properly. The tight muscles (both the ones you are using and the opposing muscles for the opposite movement) will fight you as you stroke, costing both power and control. Instead, try to keep your arm loose--like a rubber band.
Some players can relax their muscles at will. But many think their arm is relaxed, but it's not as relaxed as it should be. If your arm isn't relaxed, then you are at a disadvantage when you play. How can you cure this problem?
To get the arm warm and loose, take a long warm-up, or perhaps shadow-stroke. Then, as you set up to receive at the start of a point, relax both arms. Let them drop by your side loosely. Take a deep breath, and make sure your jaws and shoulders are relaxed. (If you are tense, these are the most likely spots to tighten up. If they tighten up, the rest of you probably will.) Then, as the point is about to start, bring your arms up as lightly as possible. You can do the same thing on your serve - relax your arms at your side, and then bring them up when you are ready to serve.
If you absolutely cannot relax the arm on your own, it's time to take drastic action. Tense the arm muscles tightly for about five seconds. Then relax. This should help relax the muscles.
A good test as to whether your arm is loose or tight is to imagine someone grabbing your arm as you stroke. They should have no problem in pulling your arm up or down. If you resist, then your arm muscles are too tight.