In vivo measurement of fascicle length and pennation of the human anconeus muscle at several elbow joint angles

Ultrasound imaging has facilitated the reliable measure of the architectural variables fascicle length (L-F) and pennation angle (PA), at rest and during static and dynamic contractions in many human skeletal muscles in vivo. Despite its small size and very modest contribution to elbow extension torque, the anconeus muscle has proven a useful model for the study of neuromuscular function in health and disease. Recent single motor unit (MU) studies in the anconeus have reported discrete and identifiable individual trains of MU potentials from intramuscular electromyography (EMG) recordings during dynamic elbow extensions. It is unknown whether the anconeus has unique architectural features related to alterations in L-F and PA throughout the elbow joint range of motion that may help explain these high-quality recordings. Previous anatomical studies have investigated this muscle in cadavers and at mainly one elbow joint angle. The purpose of this study was to measure in vivo PA and L-F of the anconeus muscle in a relaxed state at different degrees of elbow flexion using ultrasonography. Ultrasound images were collected from 10 healthy males (25 +/- 3years) at 135 degrees, 120 degrees, 90 degrees, 45 degrees, and 0 degrees of elbow flexion. Average values of L-F decreased by 6mm (10%), 6mm (12%), and 4mm (9%) from 135-120 degrees, 120-90 degrees, and 90-45 degrees of elbow flexion, respectively, whereas average PA values increased by 1 degrees (9%), 1 degrees (8%), and 2 degrees (14%) from 135-120 degrees, 120-90 degrees, and 45-0 degrees, respectively. The results indicate that anconeus muscle architecture is dynamic, undergoing moderate changes with elbow joint excursion that are similar to other limb muscles reported elsewhere. The data obtained here are more comprehensive and representative of architectural changes at various elbow joint positions than those data reported in cadaveric studies. Furthermore, the results of this study indicate that despite experiencing similar relative changes in muscle architecture to other skeletal muscles about the elbow joint, the minimal absolute changes in L-F of the anconeus likely contribute to the clarity of intramuscular EMG previously reported in this muscle.