Vladimir Janda identified the following as tonic muscles (from this site):
- tibialis posterior
- hip adductors
- rectus femoris
- tensor fascia lata
- thoraco-lumbar extensors
- quadratus lumborum
- pec major
- upper trapezius
- levator scapulae
- upper limb flexors
All of these muscles are focused on in the Rolfing 10-series.
The key to lengthening postural muscles is to identify a vector for that muscle--most often, just get a sense of up-down. When you put your awareness on a postural muscle--say the bicep--project the length of the muscle out into space. Straighten your own arm, and push against the back of your forearm to try to bend your own arm. First put all your focus into only the muscle. Feel how the tricep struggles. Then think of the vector of your bicep and extend it out into space, projecting as if your bicep is very long. And notice how the muscle becomes very solid and almost relaxed even as you push against yourself. You have become about five times stronger simply by changing your focus.
All the postural muscles deal with your presence in space, the relationship between inside and outside. They can be considered space muscles, not body muscles--and your way of looking at space can maximize their efficiency.
So as you're walking around during your day with a soft, peripheral gaze, think of extending vectors from each of the long muscles that support you upright. See how it feels.