The physical body can be a window of sorts to your entire being. Everything is interconnected, all One,
and when you apply things such as herbs, yoga, diet, etc., on the physical plane, you affect your mental,
emotional, and spiritual aspects as well.
When you apply mental thoughts that involve spiritual energies to things like yoga, and what you eat, it
aids the spiritual effects of the physical applications, making these things even more powerful.

Peraps we should be talking about posture more than fitness, sitting up straight can be very tiring and slumping stops the diaphragm from working efficiently. If you have a problem sitting up straight unaided or if you feel your back muscles 'locking' then get up and stretch them, in fact I firmly believe that stretching is far more important than any other fitness routine.
If you are really unfit, use a straight backed chair or sit on a couch with cushions supporting you, if you feel you really must sit a lotus on the floor then do it against a wall with one cushion behind your back.

Slight digression:  In a Spiritualist service the medium for the evening arrived on two canes and struggled up to the front to her position.  She sat quietly through the service until it was time for her demonstration.

She rose steadily, without the canes, came forward on the rostrum and immediately proceeded to pace and gesture with both hands in a strong attitude.  As soon as the spirit left her, she hobbled back to her seat and required the canes to leave the platform.  

Spirit sustains us.  Our attunement can be complete and effective.