Skeletal positions of strength
Understanding how you skeleton and muscles create movement is important in determining the most efficient way of moving. For example, hold your hand up, there are 4 ways to move it, one is forwards and backwards from the wrist, second bend at the elbow moving the foreearm with the hand back and forth. Third, moving the whole arm up and down moves the hand along with the arm, and lastly tak a step to the side and the hand moves with the whole body.
The question to think about is which way of moving the hand gives you the most efficient movement in terms of energy required and impact of the hand on whatever you want it to do.
Now take the arm as a whole, it is strong when it moves up and down however lateral movement tends to be weak. This is because chest and back muscles are more involved in up and down movement than lateral movement. For lateral movements use the shoulder muscles which are smaller and weaker. However, if you hold the arm in place with elbow against the body and turn the waist or turn the whole body the arm will move laterally but the whole body is the driver of the movement and not the relatively smaller shoulder muscle.
Through the practice of Tai Chi we seek to fully understand all these complex relationships and ways of creating movement using the most effective positions of strength when one is in skeletal alignment.