In the years since he stopped running, his joint had deteriorated badly.

Slow movements ensure you are performing the exercise properly, not using the joints and momentum to complete your repetition.

Stretching improves flexibility of the joints, which is always a good thing.

We need water to lubricate our joints and maintain our muscles.

Even in the latter case the line is only elliptical, a form of movement natural to the human body because of the joints, so this hardly demands any great skill from the operator.