Mitch is demonstrating Standing Single Leg Stretch.  His back is aligned, which means his abdominals are working hard to keep the balance.  This is a good one to practice for balance.  

Below we find Mitch doing a chest lift.   All of the back muscles are active in this move.   A great one to correct postural deficiencies and lower back weakness.

 Kneeling Arm Exercises offer increased strength and flexibility for all range of movements.  Here we work the rotator cuffs in one direction while balancing on the moving Reformer.  This is an Intermediate Level exercise.

Liz is demonstrating Single Leg Stretch on the Mat.  The abdominals are working hard while she's getting a great stretch from hips to hamstrings to toes.

 
This move shows lots of core strength to stay aligned while one leg reaches back at a time.  Great move for obliques, buttocks, and hamstrings, but actually the whole body is "on" because of the need to balance during movement.

An Advanced Level exercise that requires the entire body to reach and feel long while controlling the Reformer.  An especially effective one for the core.