The Caudal (Hamstring) Muscles
In the horse, the three muscles of this group possess well-developed vertebral heads of origin (in addition to the usual pelvic heads) that account for the characteristic filling and rounding of the croup (Fig.
24.2/5 and 6). The vertebral head of the biceps arises from the sacrum and adjacent part of the sacrosciatic ligament. It descends behind and partly covers the gluteal muscles before it crosses the ischial tuber to be joined by the smaller pelvic head that arises from that process. The muscle inserts by three divisions (Fig. 24.2/5'): the first in the fascia lata and on the patella, the second on the lateral patellar ligament and tibial crest, and the third, the tarsal tendon, on the common calcanean tendon. The vertebral head is supplied by the caudal gluteal nerve, and the pelvic head is supplied by the sciatic nerve.The origin of the vertebral head of the semitendinosus (Fig. 24.2/6) is near that of the biceps. After merging with the pelvic head, the muscle travels to insert on the medial aspect of the tibia and the crural fascia and contributes a tarsal tendon to the common calcanean tendon. The vertebral and pelvic heads are supplied by the caudal gluteal and sciatic nerves, respectively.
The semimembranosus (Fig. 24.2/7) is included in the hamstring group, but topographically it is a muscle of the medial aspect of the thigh. The vertebral head is relatively weak, and the pelvic head is more substantial. The combined muscle is largely covered by the gracilis and follows the caudal margin of the adductor, to which it is closely bound. It inserts by two divisions. The cranial division inserts on the medial epicondyle of the femur and the medial collateral ligament of the stifle joint; the caudal division proceeds distally to the medial condyle of the tibia. The principal nerve supply is from the sciatic nerve (Table 24.2).
» TABLE 24.2
Hamstring Muscles
Muscle Nerve Action
| Biceps femoris | Vertebral head by the caudal gluteal and pelvic head by the sciatic | All can extend the hip. Extend the Stiflewhen the Iimb is bearing weight, and flex the StifIewhen the hoof is raised. Biceps and semitendinosus extend the hock through contributions to the common calcanean tendon. |
| Semitendinosus | Vertebral head by the caudal gluteal and pelvic head by the sciatic | |
| Semimembranosus | Sciatic |
The actions and uses of the three hamstring muscles are complicated and in certain respects enigmatic. It is clear that all three units are well placed to extend the hip. The actions of the hamstring muscles on the stifle can be better understood by dividing them into two functional units: one that inserts proximal to the axis of rotation of the joint and the other distal to it. The "proximal unit" comprises parts of the muscles that are potentially extensor, because they may straighten the stifle by drawing the femur caudally when the limb bears weight. The "distal unit" will flex the stifle when the hoof is raised from the ground but will extend it when the hoof is firmly planted. Through their contributions to the common calcanean tendon, the biceps and semitendinosus extend the hock.
Some of these actions are clearly incompatible because the movements of the stifle and hock joints are linked in their actions by the reciprocal mechanism (see p. 626). It follows that the entire hamstring group, which includes parts that may flex the stifle, cannot always contract en masse.