The cue to "lift and lengthen your spine" can be interpreted from an anatomical perspective as referring to the use of muscles to counter the tendency of gravity to slightly "collapse" the normal sagittal curves of the spine (A). With wellcoordinated contraction of the spinal muscles, the curves can be slightly decreased and the length of the spine very slightly increased. However, the challenge of this cue is that because the curves of the spine go in different directions, a complex co-contraction of muscles in different regions of the spine is necessary to achieve the desired effects. For example, if too much contraction of the abdominal muscles occurs, it will decrease the lumbar curve but increase the thoracic curve, pulling the front of the rib cage too far down and creating a "collapsed" versus "lifted" look. In contrast, if excessive thoracic extension is used, it will tend to bring the shoulders and upper back behind the gravity line and again fail to create the desired "lifted" look.
Sometimes it can be helpful to use supplemental cues, for example the cue of imagining a line of energy starting with the front of the bottom of the pelvis, going under the rib cage (along the front of the spine), and out the top of the head (just behind the middle of the top of the head) as seen in the figure (B). For dancers that still appear "collapsed" in the chest, thinking of lifting the top of the sternum up toward the ceiling can facilitate desired positioning. For dancers who still tend to shorten the low back (lumbar hyperlordosis), thinking about letting the low back "lengthen" and reach down toward the floor, or imagining having a small weight hanging from the coccyx or having fingertips on the back of the sacrum that pull the sacrum slightly down toward the floor, while still maintaining the "lift" on the front of the body, can be helpful.
This is an excerpt from Dance Anatomy and Kinesiology.