THE RELATIONSHIP BETWEEN ELBOW FLEXOR VOLUME AND ANGULAR STIFFNESS ATTHE ELBOW

Objective. The purpose of this experiment was to determine if a correl ation exists between the volume of the elbow flexors and angular stiff ness at the elbow, and to determine the contribution of the biceps bra chii and the brachialis muscles to angular stiffness. Design. This stu dy is a descriptive, correlational study and presents a graphical mode l of the passive properties of muscle. Background. The correlation bet ween arm volume and angular stiffness has been shown, but the measurem ent of arm volume was not specific to the structures being strained. P re-positioning a bi-articular muscle by stretching over one joint decr eases the range of motion at the other joint and may affect the stiffn ess. Methods. Angular stiffness at the elbow of 14 female and 15 male volunteers was measured, and the volume of the elbow flexors was calcu lated from compounded ultrasound imaging. Initial biceps length was se t by pre-positioning the shoulder in two different positions. Results. A significant linear relationship was observed between the slope of p hase 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 s houlder. Phase 2 of the stiffness curve showed no linear relationship to muscle volume in either shoulder position (flexion, r=0.22; extensi on r=0.33). The slopes of phases 1 and 2 were significantly greater wi th the shoulder in horizontal extension than in horizontal flexion. Co nclusion. The volume of the elbow flexor muscles is a good predictor o f angular stiffness in phase 1 of the curve. A model of the additive c ontribution of the biceps and brachialis muscles is presented to accou nt for the increased stiffness in the shoulder extended position.