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Gastrocnemius is the largest muscle in the caudal muscle group of the crus and has 2 heads.
In the crus of the pelvic limb (hindlimb) there are 2 main groups of muscle, the craniolateral and caudal muscles.
Intrinsic = muscle origin + insertion are within the pelvic limb
There are 2 sesamoid bones found in each tendon of origin of Gastrocnemius muscle.
The sesamoid function is to preserve the tendon and also to alter the angle of pull and these particular sesamoid bones used to be called fabellae.
The tendons of origin of the lateral and medial heads of the Gastrocnemius muscle are respectively the lateral and medial supracondylar tuberosities of the femoral condyles of the femur (thigh) bone.
Orientation of Gastrocnemius muscle
Proximally Gastrocnemius is covered laterally by the Biceps Femoris muscle and medially by the Semitendinosus and Gracilis muscles.
Distally the Gastrocnemius lies superficial, deep to the skin and fascia.
The hamstring muscle group of 3 muscles consists of Biceps Femoris the largest and most laterally placed, Semitendinosus medial to Biceps Femoris and Semimembranosus being the most medial and deeply placed.
The two heads of the Gastrocnemius muscle enclose the Superficial Digital Flexor (SDF) muscle between them. SDF is a multipennate muscle and firmly united proximally with the lateral head of Gastrocnemius. These 2 muscles form the main part of the calf muscle bulk in the dog and their tendons are the 2 major contributors of the common calcaneal tendon (achilles tendon).
Deep to the Gastrocnemius muscle and lying directly on the tibia and fibula bones are the muscles: Popliteus, Tibialis Caudalis and 2 heads of Deep Digital Flexor (DDF) which is also known as Flexor Digitorum Profundus.
Clinical Tip: For accurate and relevant information, it's very useful to palpate the dog in different postures. The dog's natural fascial tension changes in the different postures which a dog uses throughout their day. Muscle tension also alters depending on the emotional status of the dog during the palpation experience.
Gastrocnemius muscle injuries
Muscles have the greatest force exerted at their origin and insertion (attachments), and in the case of Gastrocnemius, also at the musculotendinous junction where the common calcaneal tendon (achilles tendon) merges into the muscle bellies.
Muscles that pass over 2 joints will have an action at both joints and are much more prone to injury compared to muscles that pass over just one joint.
Injuries to the common calcaneal tendon (also known as the achilles tendon) have significant impact on the dog's functional activities and motion. These can be partial or full rupture injuries.
Clinical Tip: Full rupture of the common calcaneal tendon for athletic dogs is career ending as it is a key stabilising structure of the tarsus (hock) joint, having implications for hock extension and hip extension through its 5 tendon contributors. Consider your rehabilitation plan carefully for these dogs and manage your owner / trainer expectations.
The key action of the Gastrocnemius muscle is as a hock extensor in stance.
Innervation of Gastrocnemius muscle
Gastrocnemius is innervated by the tibial nerve, which is larger than the common fibula nerve. The tibia nerve supplies all the muscles in the caudal group of the crus.
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