Make sure that the feet are not too far apart. A wide stance may feel more stable but creates a static position that makes it difficult to transfer weight from one foot to the other. With a narrower stance, it is easier to shift the center of weight of the body from one foot to the other as a stroke is done.
Back Extension McKenzie Exercises Effect on Disc Fluid
The authors concluded that little evidence was found supporting the hypothesis that extension McKenzie exercises affect disc fluid content.
Feeling back stiffness may be all in the head, not in the back
Feelings of back stiffness may be a subjective protective construct of the mind, rather than reflecting objective biomechanical properties of the back.