“Extension” is the use of the muscles that unfold joints. But the theme of lessons emphasizing extension is more than unfolding joints.
We’re always working within the system that stands upright on its two feet in gravity. That is, the human being. And the arrangement of the skeleton as we stand upright in gravity is somewhere nearer to the extended position of the joints than the flexed (notice in standing how much farther you can bend your hip joint, lifting your knee up in the air, than you can extend it, lifting your leg back out behind you).
Does that mean we need to strengthen those muscles in order to improve our upright posture? Feldenkrais approached the question in a very different way.
For many of us, those extensors (especially the small of the back, the neck, in the legs) are working so hard all day long against the unnecessary activity of the flexors that it’s fatiguing just to be upright in gravity. We don’t need to give them extra strength to work even harder against the flexors that don’t let go (extra strength to compress the discs between the vertebrae and cause other sorts of injuries). Instead, we need to find our balance on the skeleton itself, so all our muscles (flexors and extensors) are to the greatest degree possible freed of unnecessary postural work and freed for real action when we call on them.
In lessons emphasizing extension, as in all lessons, you want to do less instead of more, turn your interest first to ease and comfort and finding a small and pleasant increment of some new sensation that moves in the right direction, rather than focusing on effort and achievement.
A small change somewhere along the path from your feet to the top of your head can make a world of difference to your ease, balance, availability for action. We aren’t moving backwards in order to strengthen those extensors, but in order to find new options and freedom for the skeleton in a direction we usually spend less time exploring. With that greater freedom and new options, you can at last make use of the strength of those extensor muscles for action, instead of using them so much just to keep your head up.