When i do sit ups i can feel a strain in my legs what might this be?

Iliopsoas muscle: If you are doing sit ups, especially if your feet are fixed, you are also contracting some of the muscles that cross the hip joint and help to stabilize your pelvis. The iliopsoas muscle and indirect head of rectus femoris are 2 most common.

Your hip flexors.: With poor form sit ups work the hip flexors the psaoas muscles try crunches or pilates youll get a better ab work out and leave your hip flexors out of it.