As the great Arnold Schwarzenegger has put it in his Encyclopaedia,
“Bodybuilding is the process of developing and shaping the muscles of the body by means of progressive-resistance weight training. It can be used to make the body stronger, to enhance sports performance, and to rehabilitate injured body part. […]
Since you can only shape and develop your body in this manner by means of extremely difficult physical effort and precise exercise techniques, bodybuilding must be defined as a sport; but the aesthetic goal of achieving just the right blend of muscularity, symmetry, proportion, and muscle shape, and the need to show it off by a mastery of stage presentation, also makes bodybuilding a highly demanding art form“.
(Schwarzenegger 1985: 68, italics added).
As the Greeks (and Plato, above all) had correctly understood, beauty leads to knowledge, and knowledge leads to the divine. However, bodybuilding is not merely an ‘art form’: it is a hard discipline that demands constant practice, extreme self-control and isolation from ‘mundane issues’. Bodybuilding is also a lifestyle, and bodybuilders live in the asceticism of their clean diet and self-improvement. In light of both reasons — namely, if we agree with the Platonic idea of beauty leading to knowledge and knowledge leading to the divine, and assume that bodybuilding can be regarded as a form of ascetic practice — one can surely argue that bodybuilding represents a kind of (secularised) spirituality.
References
Schwarzenegger, Arnold. 1985. The New Encyclopaedia of Modern Bodybuilding. New York: Simon and Schuster, 1998 (2nd edition).