📝 STANDARD DESCRIPTION:
Type: Intrinsic, Scapulohumeral
Origin: Infraspinous fossa of scapula
Insertion: Lateral aspect of greater tubercle of humerus
Function: Flexes/abducts shoulder; acts as lateral collateral ligament (prevents abduction).
📚 DETAILED DESCRIPTION:
THE LATERAL COLLATERAL LIGAMENT
Filling the infraspinous fossa, this muscle provides the lateral stabilization of the shoulder joint, functioning as the lateral collateral ligament.
TENDON PROTECTION: The tendon of insertion passes over the caudal part of the greater tubercle, protected by a synovial bursa (bursa subtendinea m. infraspinati). This bursa prevents friction but is prone to inflammation.
FUNCTIONAL DUALITY: Acts to flex or abduct the shoulder joint (non-weight bearing), but its critical role is preventing abduction (lateral deviation) of the humerus during weight bearing. When the supraspinatus and infraspinatus contract together, they create a force couple compressing the humeral head into the glenoid cavity.
SWEENEY SIGNIFICANCE: In suprascapular nerve injury (Sweeny), atrophy of infraspinatus removes lateral support, causing the shoulder to 'pop' laterally (abduct) when weight is placed on the limb—a pathognomonic sign visible when watching the animal walk toward the examiner.
// COMPARATIVE ANALYSIS
| HORSE |
Similar position and function; same vulnerability to suprascapular nerve injury. |
| DOG |
Rotator cuff equivalent; similar vulnerability. |
CLINICAL RELEVANCE
Infraspinatus Bursitis: Inflammation of the subtendinous bursa causes lameness. Sweeny Diagnosis: Atrophy causes lateral shoulder subluxation ('popping') during weight bearing—pathognomonic for suprascapular nerve injury.
