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The groin muscles are a group of muscles situated high on the leg in the inner thigh. This group includes the adductor magnus, adductor longus, and adductor brevis muscles, as well as the pectineus and gracilis. Collectively referred to as the hip adductors, the groin muscles are responsible for adduction of the hip, or drawing the leg in sideways toward the midline of the body.
Found to either side of the groin area along the inside of the thigh, the groin muscles all find their origins on the pubic bone. The pubis, as it is known, is a curved bone on the bottom of the pelvis. It sits medial to, or to the inside of, the ischium bone in the lower pelvis and below the ilium bone, which is the large butterfly-wing-shaped bone of the pelvis.
Arising from the top of the pubis on a surface known as the superior ramus is the pectineus, a flat, band-shaped muscle and one of the most superficial of the groin muscles. It extends downward and outward from the pelvis, obliquely traversing the upper inner aspect of the thigh. The pectineus inserts along a nearly vertical ridge, known as the pectineal line, on the medial posterior or inner rear aspect of the shaft of the femur bone in the thigh. This muscle runs parallel to the other adductors but only assists in hip adduction. It is more accurately a hip flexor, acting to bend the leg forward at the hip.
Deep to the pectineus and traveling in the same direction is the adductor brevis, a muscle that is narrower but roughly equal in length. The adductor brevis originates below the pectineus on the inferior ramus of the pubis and inserts just posterior to the pectineus along a line on the back of the femur known as the linea aspera. Its only action is hip adduction.
Situated deep to the pectineus but slightly overlapping the inferior or lower border of the adductor brevis is the adductor longus. With fibers fanning out as it approaches the femur, it is larger than the brevis. This muscle also finds its origins on the superior ramus of the pubis, and it attaches to the femur farther down the bone than the pectineus and adductor brevis, inserting along the linea aspera about midway down the shaft. The adductor longus is responsible for hip adduction but also plays a minor role in hip flexion.
Another of the groin muscles is the adductor magnus, which is the largest of the adductors and which is situated deep to the brevis and longus. Triangular in shape, it is narrow where it originates on the inferior ramus of the pubis as well as on the inferior ramus of the adjacent ischium bone, and it fans out to insert on the femur along most of its length. The upper portion of the muscle attaches to the greater trochanter on the top of the femur and to the linea aspera, the middle portion inserts along the linea aspera, and the lower portion attaches to the medial condyle of the femur, which is the rounded bony protuberance on the bottom of the femur at the inside of the knee. This muscle is involved not only in hip adduction but in hip extension, as its upper fibers help draw the leg inward while the lower fibers assist the hamstrings in pulling the leg backward relative to the hip joint.
The last of the groin muscles is the gracilis, a long, narrow muscle running down the inner thigh like the inseam of a pant leg. A superficial muscle, meaning that it lies close to the skin, the gracilis stretches from the pubic symphysis, the joint between the two pubic bones, to the top of the tibia bone in the shin along its medial or inside surface. As it is the only adductor muscle that crosses the knee joint, it is involved not only in hip adduction but also in flexion of the knee, and it assists in flexion and internal rotation of the hip as well.