Musicians often have the misconception of being emotionally intense with being physically tense. Having intense expressiveness and power does not come from over-tightening of your muscles that produces the sound but from freeing it to work smoothly, effectively and efficiently.
If you begin with a posture that is relaxed, it will give you maximum strenghth and you will be able to use your playing to the best advantage. Since your body is relaxed, the energy can flow directly to the keys, producing a rich, and sonorous sound. The music will not feel repetitive, and you begin to notice slight variations in sound each time your body is free and relaxed.
Let yourself discover the ease and joy of moving naturally and not being tensed when you play. You will notice a world of difference in the sound you produce. You will also begin to enjoy your playing as you hear each nuances being produced and created.
Enjoy your Music!