Aims. The conventionally described mechanism of distal biceps tendon rupture (DBTR) is of a ‘considerable extension force suddenly applied to a resisting, actively flexed forearm’. This has been commonly paraphrased as an ‘eccentric contracture to a flexed elbow’. Both definitions have been frequently used in the literature with little objective analysis or citation. The aim of the present study was to use video footage of real time distal biceps ruptures to revisit and objectively define the mechanism of injury. Methods. An online search identified 61 videos reporting a DBTR. Videos were independently reviewed by three surgeons to assess forearm rotation, elbow flexion, shoulder position, and type of muscle contraction being exerted at the time of rupture. Prospective data on mechanism of injury and arm position was also collected concurrently for 22 consecutive patients diagnosed with an acute DBTR in order to corroborate the video analysis. Results. Four videos were excluded, leaving 57 for final analysis. Mechanisms of injury included deadlift, bicep curls, calisthenics, arm wrestling, heavy lifting, and boxing. In all, 98% of ruptures occurred with the arm in supination and 89% occurred at 0° to 10° of elbow flexion. Regarding muscle activity, 88% occurred during isometric contraction, 7% during eccentric contraction, and 5% during concentric contraction. Interobserver correlation scores were calculated as 0.66 to 0.89 using the free-marginal Fleiss Kappa tool. The prospectively collected patient data was consistent with the video analysis, with 82% of injuries occurring in supination and 95% in relative elbow extension. Conclusion. Contrary to the classically described injury mechanism, in this study the usual arm position during DBTR was forearm supination and elbow extension, and the muscle contraction was typically isometric. This was demonstrated for both video analysis and ‘real’ patients across a range of activities leading to rupture. Cite this article: Bone Jt Open 2022;3(10):826–831

The objectives of this study were to elucidate the function of Brachioradialis during forearm rotation to determine whether it is a neutralizing muscle and a protector of hyper-rotation by eccentric contraction. The distance from the brachioradialis (BRAR) origin to insertion was measured on 10 left fresh frozen cadaveric arms using an electromagnetic tracking system. This was done in 10¢. a. increments over the full range of forearm rotation. In addition, fine-wire electrodes were placed in the BRAR of twelve living subjects. EMG data was collected as the subject rotated the forearm in both a pronating and a supinating direction. The muscle length data shows that length is shortest at neutral and greatest closer to full rotation in either direction. When rotating from full pronation to neutral the EMG data show a steady increase while the muscle length decreases indicating a concentric contraction. When rotating from neutral to full pronation the muscle length increased and with load the EMG level increased indicating an eccentric contraction. During rotation from full supination to neutral, the EMG activity increased slightly with the muscle length, indicating a concentric contraction. When rotating from neutral to full supination, the EMG level remained variable while the muscle length increased indicating an eccentric contraction or a passive stretch. EMG activity can occur during isometric, eccentric, or concentric contractions, the accompanying muscle length data is useful for establishing the direction of the activity. We conclude BRAR is a neutralizing muscle as it has a linear relationship with EMG activity when returning the forearm to neutral. It also acts eccentrically slowing extreme pronation and thus it has a dynamic effect on DRUJ stability. This knowledge will assist surgeons in Tendon Transfer surgery and injury to the Brachioradialis muscle