I see here people quoting "Patanjali Yoga Sutras" as a definitive text on everything Yoga. Not so; it is definitive only so far as "Raja Yoga" is concerned. In fact, it has got far more in common with Buddhist texts rather than Hatha Yoga (also Tantra) which is where you should look for insights on everything physical. To that end, the three main Hatha Yoga texts are;

1) The Gheranda Samhita

2) The Hatha Yoga Pradipika

3) The Siva Samhita