The Rectus Femoris muscle is the largest hip flexor by volume, and is also one of the quadriceps. Rectus Femoris connects all the way from the pelvis to the kneecap. This length allows it to do two functions: 1) flex the hip and 2) extend the knee.
Rectus Femoris Function
Rectus Femoris is unique in its dual function - 1) flexion of the hip and 2) extension of the knee. Rectus Femoris is considered one of quadriceps muscles, but it is the only of the four quads with hip flexion function. Rectus Femoris is able to do this because unlike the other 3 quad muscles, Rectus has an upper connection to the pelvis instead of the femur .
To locate your Rectus, stand up and lift one leg in the air. Feel a muscle contract right below your waistline around the top of the thigh - this muscle is Rectus Femoris.
Rectus Femoris Pain
Rectus Femoris pain typically manifests deep in the kneecap, as shown in the left portion of the image above . This is a great example of referred pain. Referred pain is the concept that the hurt and the cause of the hurt are in two different locations. Pain caused by Rectus Femoris is likely to manifest as deep knee pain, even though the source is in the middle of the thigh.
Rectus Femoris trigger points can be rolled out with a lacrosse ball, or with a versatile massage tool like QL Claw. QL Claw is great for massaging the Rectus Femoris as well as other pain-inducing hip flexors like TFL, Psoas, and Iliacus.
 Donnelly, Joseph M. Travell, Simons & Simons Myofascial Pain and Dysfunction: the Trigger Point Manual. 3rd ed., Wolters Kluwer Health, 2019.
 Davies, Clair, and Amber Davies. The Trigger Point Therapy Workbook: Your Self-Treatment Guide for Pain Relief. 3rd ed., New Harbinger Publications, Inc., 2013.