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Key Points:

• Brachioradialis flexes the elbow with forearm in neutral (‘thumbs up position’).
• Many patients with loss of biceps and brachialis function (through injury to the upper brachial plexus or to the musculocutaneous nerve) will use brachioradialis to flex the elbow.

Muscle Function:
• Primary: Elbow flexion with forearm in neutral.
• Secondary: Assists with supination and pronation, bringing forearm to neutral position.
Palpation: Lateral to the biceps tendon at the elbow.
Strength Testing:
Gravity-lessened Test: Position – the subject in sitting with 90º of shoulder flexion and internally rotation, 90º elbow flexion, and forearm in neutral with the thumb pointing towards the body. Stabilize – the arm or rest on a table at shoulder height. Support - the arm through the normal range of motion.
Anti-gravity Test: Position – the subject in sitting with shoulder and elbow slightly flexed with forearm in neutral position. Stabilize – the anterior shoulder when giving resistance. Resist – against elbow flexion while keeping the forearm in neutral/’thumbs up’ position (in the direction of elbow extension). Additionally, the subject can place and actively hold elbow flexion at 90º with forearm in neutral position.
Possible Substitutions: Biceps brachii and brachialis – to avoid this, make sure the elbow is kept in forearm neutral position throughout. Muscles for wrist extensors – if this occurs, the forearm would likely turn into pronation. Watching the position of the forearm is important to alleviate these potential substitutions.
Relevant Anatomy:
• Roots: C5, C6.
• Nerve: Radial nerve.
• Innervation Route: C5, C6 → radial nerve → brachioradialis branch.
Origin: Proximal ⅔ of lateral supracondylar ridge of humerus and the lateral intermuscular septum. 
Insertion: Lateral side of base of styloid process of radius.