A muscle is stretched when its overlapping actin and myosin protein fibers slide past each other. Once this occurs, a lengthening of myofilaments past their current resting length occurs. We cannot make stretches happen; we allow them to.  

Pictured here is the Pilates exercise called the Thigh Stretch.  It can be performed on the Tower or Cadillac.  Using the Roll Back bar, you kneel on the mat with your upper body perpendicular to the floor. After ensuring that your body placement is squared off and stabilized and you've engaged your abdominals, you shift backward approximately 45 degrees (or more if you're including a back bend), feel the lengthening of the quadriceps, then gently thrust the hips forward and return to the starting position.  The spring resistance of the bar gently stretches your arms, shoulders and chest.

It's important to maintain muscle length and symmetry in the weight-bearing lower extremities. Optimal functioning of these muscles coupled with properly aligned ligaments and joints limit stress, strain and the gradual shortening of these structures.

The Pilates method employs eccentric contractions whereby muscles oppose a stronger force (the springs) which causes the muscles to lengthen as they contract.  With the controlled movements that are essential when doing Pilates, muscles are strengthened too.