LAB 9.2 Hindquarter muscles


Loin muscles

At the end of  LAB 9  we  reached as far back as the loin muscles.  We will repeat these again because of their
importance, and then continue into the hindquarter.  Remember LD is the longissimus dorsi, PM is the psoas major and GM is the gluteus medius.

   The loin muscles give rise to tender meat with a desireable taste, and they command a high price when presented for sale as steaks or chops. The longissimus dorsi extends posteriorly from the rib region, it runs through the loin, and most of the muscle terminates on the anterior face of the ilium. Thus, the longissimus dorsi is seen in cuts of meat taken through the ribs and loin, but not in cuts of meat such as sirloin steaks that are posterior to the anterior face of the ilium. The longissimus dorsi is dorsal to the transverse processes of the lumbar vertebrae, and it is dorsal to the ribs in the thoracic region. For most of the length of the ribcage, there are no major muscles immediately ventral to the heads of the ribs. Thus, in a rib steak, there is only one main eye of meat, and that is the longissimus dorsi dorsal to the ribs. However, in the loin, there are muscles both above and below the level of the transverse processes of the lumbar vertebrae. The dorsal muscle above the transverse processes is the longissimus dorsi. The ventral muscle below the transverse processes is the psoas major or filet muscle. The psoas major originates ventrally to the last couple of ribs in the ribcage. The cross sectional area of the psoas major increases towards the sirloin. Medial to the psoas major, almost under the centra of the vertebrae, is a smaller psoas muscle called the psoas minor. The letter P in the word psoas is silent when the word is spoken.

   Immediately lateral to the dorsal spines of the vertebrae (medial to the longissimus dorsi) are some small multifidus dorsi muscles. The longissimus costarum is a relatively small rope‑like muscle, dorsal to the ribs. It appears as a small eye of meat at the separation between the forequarter and the hindquarter. The multifidus dorsi and the longissimus costarum have little commercial significance, since they are such small muscles, but they may create a problem in the measurement of rib‑eye areas since they might be accidently included with the longissimus dorsi.

Hindlimb muscles

   The muscles of the hindlimb are relatively large and produce a large volume of moderately tender meat. The gracilis is a thin sheet of muscle spread over the medial face of the hindlimb. The gracilis, together with the small sartorius muscle anterior to the gracilis, may be used to orient hindlimb cuts of meat such as a beef round steak or a slice of ham. Orientation is necessary for the identification of the remaining muscles of the hindlimb which are grouped around the femur. The quadriceps femoris muscles form a group of four large muscles that pull on the patella when the leg is extended. The vastus medialis is medial, the vastus lateralis is lateral, the vastus intermedius covers the anterior face of the femur, and the rectus femoris covers the vastus intermedius.

   The biceps femoris is a single large muscle on the lateral face of the hindlimb. In cross section, it often appears to be divided into two parts because it has a very deep cleft along part of its length. The biceps femoris appears as a single muscle in cuts of meat which miss the cleft, but sections through the cleft make the muscle appear double. To add to the possibility of confusion, the small segment of muscle cut off by the cleft is often paler than the main part of the muscle.

   The semitendinosus and semimembranosus are two large muscles located on the posterior face of the hindlimb. The semimembranosus is medial to the semitendinosus. The adductor and pectineus are located in the medial part of the hindlimb, near to the femur. The pectineus is anterior to the adductor. In lean carcasses, it may be difficult to separate the adductor from the semimembranosus, and these two muscles may appear as a single muscle. Given the possibility that the biceps femoris muscle on the other side of the limb may look like two muscles,  caution is needed in the identification of these muscles for the first time. The gastrocnemius is a large muscle located deep in the hindlimb and is  covered by distal extensions of some of the proximal muscles of the limb. The gastrocnemius pulls on the Achilles tendon at the hock, and is the equivalent of the calf muscle in the human leg. A major anatomical difference between the human leg and the hind limbs of meat animals is that the human gastrocnemius is not covered by the posterior thigh muscles.

Sirloin muscles

   Between the hindlimb and the loin, and located laterally to the pelvis, are several large muscles that form the rump and sirloin of the carcass. The positioning and naming of the first of these muscles is rather misleading. Despite its name, the gluteus medius is located laterally in meat animals. It covers the lateral face of the ilium and appears as the large muscle area in sirloin steaks and chops. From lateral to medial, there are three layers of muscles, gluteus medius, gluteus accessorius and gluteus profundus. The gluteus medius is the largest of the three.

   The psoas major continues posteriorly from the loin into the sirloin. It is joined by another muscle, the iliacus, and the two together may be given a compound name, the iliopsoas. Little, if any, of the longissimus dorsi appears in the sirloin since most of the muscle terminates on the anterior face of the ilium.


Flank muscles


   The flank and belly of the animal are formed by sheets of muscle and connective tissue. The muscles are relatively tough and need not be identified individually. The layers of abdominal muscles appear as the parallel layers of lean in a slice of side bacon. The tensor fascia lata is a triangular muscle located in the angle between the animal's flank and its hindlimb. The fascia lata is a sheet of connective tissue that covers the anterior surface of the hindlimb, and it is stretched by the tensor fascia lata muscle during locomotion. It is important to remember that, although an animal walks into the abattoir on four legs, it is suspended by its hindlimbs when it leaves the abattoir. As a carcass is hoisted onto the overhead rail, there is an extreme rotation of the hindlimb relative to the vertebral column. Consequently, the tensor fascia lata is spread through the stretched muscle mass of the sirloin.