The relationship between elbow flexor volume and angular stiffness at the elbow

Objective. The purpose of this experiment was to determine if a correlation exists between the volume of the elbow flexors and angular stiffness at the elbow, and to determine the contribution of the biceps brachii and the brachialis muscles to angular stiffness. Design. This study is a descriptive, correlational study and presents a graphical model of the passive properties of muscle. Background. The correlation between arm volume and angular stiffness has been shown, but the measurement of arm volume was not specific to the structures being strained. Pre-positioning a bi-articular muscle by stretching over one joint decreases the range of motion at the other joint and may affect the stiffness. Methods. Angular stiffness at the elbow of 14 female and 15 male volunteers was measured, and the volume of the elbow flexors was calculated from compounded ultra-sound imaging. Initial biceps length was set by pre-positioning the shoulder in two different positions. Results. A significant linear relationship was observed between the slope of phase 1 of the stiffness curve and volume of the elbow flexors in both horizontal flexion (r = 0.92) and horizontal extension (r = 0.79) of the shoulder. Phase 2 of the stiffness curve showed no linear relationship to muscle volume in either shoulder position (flexion, r = 0.22; extension r = 0.33). The slopes of phases 1 and 2 were significantly greater with the shoulder in horizontal extension than in horizontal flexion. Conclusion. The volume of the elbow flexor muscles is a good predictor of angular stiffness in phase 1 of the curve. A model of the additive contribution of the biceps and brachialis muscles is presented to account for the increased stiffness in the shoulder extended position.