Your hipflexors consist of two muscles that form a group called the Iliopsoas, the primary hip flexor is the Psoas Major, you have two, one on each side of the spine. The Psoas Major is the deepest muscle in your core it attaches to both sides of the spine at T12 – L5 vertebraes and inserts at the lesser trochanter of the femur connecting the spine to your legs. The second muscle of the Iliopsoas is the Iliacus , again you have 2 on each side which originate at the ilium of the pelvis and inset along the Psoas Major into the lessor trochanter connecting your pelvis to your legs (you could have a Psoas Minor here as well however approx only 50% of the population has one) . Both of these muscles (or all 3 if you’re a chosen one ) are responsible for flexing your hip (bringing your knees to your chest). In addition to hip flexion the Psoas stabilizes the spine therefore it is also used when rotating and side bending.
The hip flexors can become tight from overuse or from underuse. Sitting for long periods weakens the muscles they become short and tight which also effects your posture increasing the curve in the lumbar spine (Lordosis) causing pain and dysfunction. Tight Hip flexors can even cause other muscles like the glutes to not properly fire. In Pilates the hip flexors are included and activated in much of the work bringing you awareness and education on work to benefit and lengthen the flexors. A great stretch you can do anywhere that will help stretch these muscles is the lunge stretch, or bridging, this can be done on the floor or in reformer work