As a rule, Orthodox don't kneel on Sundays. Usually, weeknights during Lent you will see kneeling and prostrations. On a weekday liturgy (mass, always done in the morning) people will usually kneel at least for the Lord's Prayer (this is in the US)
Could you share your magnesium preparation and dose? If you are deficient in magnesium, the mechanism I would guess is that neck muscles are unable to relax and constrict the action of weaker muscles used for breathing.