Biomechanics of the Armbar
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The armbar, arguably the most iconic technique in grappling, is a submission that involves hyperextension of the elbow joint (humeroulnar joint). While there are two other joints around the elbow region:
1️⃣humeroradial
2️⃣radioulnar joint
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the humeroulnar joint is the primary joint involved in elbow extension. After years of grappling many martial artists may lose a bit of range, but the average person should have about 5 degrees of hyperextension.
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There are a variety of structures that limit hyperextension:
1️⃣muscle
2️⃣ligaments
3️⃣the bony articulation of the humerus and the ulna.
4️⃣The joint capsule is connective tissue that wraps around all three joints and is supported by ligamentous structures.
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The primary ligament to limit excessive extension is the ulnar or medial collateral ligament (not to be confused with the MCL in the knee), particularly the anterior fibers which are the strongest and thickest fibers of the ligament. In addition to ligamentous support muscles of the elbow and forearm help to support and resist excessive motion, particularly the wrist flexors and pronators which act as dynamic medial stabilizers.
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As the UCL is a fairly thick ligament and it is a highly innervated structure, the mechanoreceptors within the structure help detect passive tension. This is why there is a relatively moderate amount of time from when the submission begins and when damage occurs compared to a leg attack.
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Humeroulnar hyperextension often leads to ligament strain of rupture, however if force is continued then dislocation of the ulna from the humerus can occur. While the muscles may become strained they are not likely to tear, more likely the muscle’s attachment on the ulna will rip off the main structure (avulsion fracture).
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Here is an example of Keanu Subba demonstrating an armbar in @onechampionship . Initially Subba was unable to isolate the elbow due to his opponent wiggling his shoulder. Despite the arm being extended Subba could not impart enough force to disable the elbow. Once Subba had proper leg position he was able to isolate the elbow and elicit the tap.