A Short Progression for doing Plank Posture
A common mistake made doing plank pose is using the hip flexors to support the weight of pelvis. This is shown in the photo below.
If the hip flexors are active, they tend to tilt the pelvis forward (anteriorly), relative to the rib cage. This results in a mis-alignment between the pelvis and the rib cage and inhibits the deep abdominal and pelvic floor muscles, severely compromising pelvic and trunk stability and placing much more of the load on the shoulder girdle and lower back.
When the hip flexors are relaxed, it allows the pelvis to align with the rib cage in a way that allows the deep abdominal and pelvic floor muscles to respond to the weight of the pelvis and lower trunk. With the help of the quadriceps and toes, the total weight of the body is then distributed evenly between the feet, legs, pelvis, trunk and shoulder girdle.