Anatomy and Physiology for Yoga Instructors
Movement and support
206 bones in the adult skeleton (about 300 at birth); the vertebral column has 33 vertebrae - 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral and 4 coccygeal - with curvatures that yoga cues must respect (keep the natural lumbar lordosis, avoid flattening or over-arching). Major joint types: hinge (knee, elbow), ball-and-socket (hip, shoulder), pivot (atlanto-axial) and plane/gliding (intervertebral facets). Muscles work in pairs: agonist, antagonist, synergist and stabiliser. Isotonic contraction changes length; isometric contraction holds length under tension - many asanas are largely isometric.
Breath and circulation
The diaphragm is the primary muscle of inspiration: on inhalation it contracts and flattens, ribs lift, thoracic volume rises and pressure drops, drawing air in. Accessory inspiratory muscles include external intercostals, scalenes and sternocleidomastoid; forced exhalation uses internal intercostals and abdominal muscles. Typical tidal volume is about 500 mL; vital capacity roughly 4-5 litres. The heart has four chambers; the right side pumps to the lungs, the left to the body.
Nervous and endocrine regulation
Slow, rhythmic breathing with a longer exhalation raises vagal (parasympathetic) tone, lowering heart rate and shifting the body toward the relaxation response. Sympathetic activation (fight-or-flight) raises heart rate, BP and cortisol via the HPA axis. The endocrine system's stress axis and the limbic system (amygdala) explain why pranayama and Savasana calm emotion.
Traps and a rule of thumb
Traps: mixing up the number of cervical vertebrae, saying the diaphragm relaxes on inhalation, or claiming that forward bends always lower BP. Rule of thumb: inhale = diaphragm down and flat; exhale = diaphragm up and dome-shaped. Worked example: in Ustrasana, the hips must stay over the knees and the lumbar curve should be shared along the whole spine - cue thoracic extension, not dumping into L5.