The latissimus dorsi originates from the spinous processes of T6-T12, thoracolumbar fascia, iliac crests and the inferior 3-4 ribs and inserts onto the intertubercular sulcus of the humerus. It is innervated by the thoracodorsal nerve (C6-C8). It acts to extend, adduct and medially rotate the humerus.
The external oblique originates from ribs 5-12 and inserts onto the iliac crest, pubic tubercle, and linea alba. The external oblique is innervated by the thoracoabdominal nerves (T7-T11) and the subcostal nerve (T12). This muscle acts to rotate the contralateral torso.
The internal oblique lies below the external oblique muscle. It originates from the inguinal ligament, iliac crest and the lumbodorsal fascia and inserts onto the linea alba, pecten pubis and ribs 10-12. The internal oblique is innervated by the thoracoabdominal nerves (T6-T11), subcostal nerve (T12), iliohypogastric nerve (L1) and ilioinguinal nerve (L1). The internal oblique acts to compress the abdomen and aids in unilateral vertebral column rotation.
The gluteus medius originates from the gluteal surface of the ileum, under the gluteus maximus. It inserts onto the greater trochanter of the femur. The gluteus medius is innervated by the superior gluteal nerve (L4,L5,S1). It mainly acts to abduct and medially rotate the thigh. It also helps keep the pelvis level when the opposite leg is raised. A positive Trendelenburg’s sign may indicate a gluteus medius or superior gluteal nerve injury.
The gluteus maximus originates from the posterior ilium, posterior gluteal line, sacrum, sacrotuberous ligament and inserts to the iliotibial tract. It is innervated by the inferior gluteal nerve (L5, S1, S2). It acts to extend the thigh, laterally rotate the thigh and also steadies the thigh to assist in rising from a sitting position.
The erector spinae are a group of muscles that act as the main extensors of the vertebral column. The erector spinae includes the spinalis muscle group, longissimus muscle group and iliocostalis muscle group. The erector spinae muscles attach to the spinous processes of T9-12 and the posterior iliac crest and insert onto the spinous processes of T1 and T2 and the cervical vertebrae. These muscles are innervated by the posterior rami of spinal nerves. Unilaterally, these muscles aid in lateral flexion of the vertebral column. Bilaterally, these muscles extend the vertebral column and head. A functional mnemonic to remember the erector spinae muscles is “I Like Standing” to represent the iliocostalis, longissimus, and spinalis muscle groups, respectively.
The serratus anterior originates from ribs 1 to 8 along the midaxillary line and inserts posteriorly on the medial margin of the scapula. It is innervated by the long thoracic nerve. The serratus anterior acts to protract and stabilize the scapula. A lesion of the long thoracic nerve will cause winging of the scapula.
The serratus posterior is divided into the serratus posterior superior and serratus posterior inferior. The serratus posterior superior attaches to the nuchal ligament and spinous processes of C7- T3 and inserts on the superior borders of ribs 2-5. This muscle is innervated by intercostal nerves 2-5. The serratus posterior superior acts to elevate the ribs. The serratus posterior inferior attaches to the spinous processes of T11- L2 and attaches to ribs 9-12. This muscle is innervated by intercostal nerves 9-11. The serratus posterior inferior acts to depress the ribs.
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