It is the most superficial muscle on the radial side of the forearm. It forms the lateral side of the cubital fossa. It's often fused proximally with the brachialis. It has a thin belly that descends in the mid forearm, where its long flat tendon begins, then the tendon continues to the radius.
Humerus (lateral supracondylar rigid) and lateral intermuscular septum.
Radius (just proximal to styloid process) 
The brachioradialis is innervated by the C5-C6 segments. Due to its location in the posterior compartment of the forearm it is innervated by the radial nerve. (C7 innervation sometimes cited)
Radial recurrent artery from the radial artery.
The brachioradialis flexes the forearm at the elbow. Depending on the position of the hand during the flexion, the brachioradialis can tend to move the hand to, neutral. A recent study EMG study found that while neutral forearm is thought the be the strongest, the greatest EMG activity from the brachioradialis occurs during elbow flexion tasks regardless of forearm position. The study also found that during rotational tasks, more EMG activity was recorded during pronation as compared to supination tasks.
If this muscle is too tight and/or contains trigger points, it can send pain to your elbow and forearm.
Additionally, you might feel pain on the back of your hand or between your thumb and index finger. Thus it may contribute to the tennis elbow pain. 
Palpation will be on the proximal volar surface of the forearm
The MMT is usually done for the three main elbow flexors (biceps, brachialis and brachioradialis) by asking the patient to bend his/her elbow against resistance and start grading, but in try to focus on the brachioradialis, we will ask the patient to bend the the forearm with some degree of pronation (midposition)
Note: the wrist flexor muscles should remain relaxed throughout the test, because strongly contracting wrist flexors may assist in elbow flexion. 
In supine, sitting or standing position and by having the forearm in midposition, slowly start bending with any suitable type and degree of resistance given by the therapist, such as dumbbells, sand packs or theraband.
- Gray, Henry. Anatomy of the Human Body. Philadelphia: Lea & Febiger, 1918; Bartleby.com, 2000. www.bartleby.com/107/125.html
- Helen J.Hislop Jacqueline Montgomery,Muscle Testing,2007,8th edition.
- Brachioradialis Anatomy: Origin, Insertion, Action [Internet]. The Wellness Digest. 2014 [cited 2018 Jul 4]. Available from: https://thewellnessdigest.com/brachioradialis-anatomy/
- Marieb EN, Hoehn K. Human anatomy & physiology. 10th ed. Boston, Ma: Pearson; 2016.
- Boland MR, Spigelman T, Uhl TL. The function of brachioradialis. J Hand Surg Am. 2008 Dec;33(10):1853–9.
- Brachioradialis muscle pain & trigger points [Internet]. Muskel und Gelenkschmerzen. [cited 2018 Jul 4]. Available from: https://www.muscle-joint-pain.com/trigger-points/trigger-point-self-treatment/brachioradialis/