As the site says, for health, biokinesis, etc. the Spiritual Square is best as this deals with the soul and the body. The Material Square, although it does work, will have less of an effect on your body, more [this is simplifying it considerably] flinging the things of this world in your direction to accomplish whatever you desire.
Using the Spiritual Square of the Sun is optimal. You can use Mars too for this purpose but Mars is more about functional masculine appearance and contour than muscles specifically, since Mars is more suited to being ripped and lean.