My mother was a college lecturer for many years and latterly suffered from sciatica. She was advised by her doctor that her problem was exacerbated by "resting her leg, like an old horse", when asked to show him how she stood in front of a class. He explained that such a posture tips your pelvis and stresses the lower back nerves and muscles. He gave her a simple piece of advice to ameliorate it I stand like the Queen: keep both feet firmly on the floor, the same width apart as your pelvis.
My daughter (a postgraduate certificate in education student), who has already had treatment for a "dodgy" back and knows how debilitating back pain can be, has taken her grandmother's advice ... as have I (a teacher for more than 20 years). I remember this "instruction" when my back starts to ache, and, as yet, I have not had to consult an osteopath.
Of course, the basic problem can be overcome by never standing still in front of a class, or going back to a "four-footed" stance as nature intended for vertebrates - but both might prove difficult for class management.
LIZ WRIGHTON 7 Park Lane The Vine Sevenoaks Kent