Check to make sure there is good alignment with the left and right side of the quads. Balance with the foot in alignment and the weight in alignment. Remember, the body follows the eyes. Be careful to do the postures correctly, because you are creating an imprint that is hard to undo. Breath is the only way to affect the involuntary systems.
Oxygen deprivation is a major cause of spasms, other muscle pains, and sciatica.
Deep diaphragmatic breathing is best when dealing with lots of resistance. And finally, honor your boundaries, but expect them to expand. The blood supply in arteries and veins is being cut off, creating pressure. When released, a lock gate effect is created, causing blood to rush through veins and arteries, flushing them out. Also, pressure is applied to the heart by its relative position to the rest of the body.