Abstract
Purpose
Based on anatomic studies, it appears that the short head (SH) and long head (LH) of the distal biceps tendons have discreet distal attachments on the radial tuberosity. The SH attaches distally and therefore may function as a stronger flexor, whereas the LH attaches more proximal and ulnar which would make it a greater supinator. The contribution of each of the two heads to flexion and supination has not yet been defined. The rationale of this study was to directly measure the contribution of the SH and LH of the biceps to elbow flexion and forearm supination and provide biomechanical evidence for what is inferred in the anatomical studies.
Method
Twelve fresh-frozen cadaveric arms were secured using in vitro elbow simulator, while controlled loads were applied to the individual biceps tendons short and long heads. Isometric supination torque and flexion force were recorded with the forearm in 45 degrees supination, neutral rotation and 45 degrees pronation.
Results
In all specimens examined the LH and SH of the distal biceps were important contributors to flexion and supination of the forearm. On average, the SH showed 16% more contribution to flexion for all forearm positions. Torque measurements showed that in pronation and neutral positions the SH contributed 11% more than LH. In the supinated forearm, the SH and LH showed no difference in contribution. A repeated measures analysis of variance (ANOVA) and post-hoc Student-Newman-Keuls tests were used for statistical analysis.
Conclusion
In most anatomic studies, it has been suggested that the LH and SH of the distal biceps contribute in different ways to flexion and supination. Based on anatomy only, authors have suggested that the SH is a more important flexor and that the LH is more important in supination. In this biomechanical study we demonstrated that both the LH and SH contribute to flexion and supination. Loss of either the LH or SH will result in weakness in flexion or supination. When assessing contribution to flexion, as anatomic studies suggest, the SH alone is a stronger flexor than the LH. This is most likely due to the fact that the SH has a more distal attachment on the bicipital tuberosity. When assessing contribution to supination, the SH alone was a stronger supinator than the LH consistently in our model. This is in contradiction to previous authors who felt the anatomy of the LH attachment, in a slightly more ulnar position, would make it a stronger supinator.
