Current knowledge of skeletal muscle contraction mechanics is derived predominantly from cadaver measurements and animal studies. As a consequence, assumptions about muscle architecture and contractile properties are made when modeling the in vivo contraction mechanics of human muscle. A principal goal of this research was to use non-invasive medical imaging techniques to test the common assumption that skeletal muscle contracts uniformly. Ultrasound imaging, magnetic resonance imaging (MRI), and positron emission tomography (PET) were used to characterize the in vivo architecture, contraction mechanics, and metabolic activity of the biceps brachii muscle.

The in vivo architecture and contraction mechanics of the biceps brachii muscle-tendon complex were characterized in 12 volunteers using ultrasound imaging and MRI. The biceps tendon continued as an internal aponeurosis that spanned the distal third of the biceps brachii;​ muscle fascicles inserted along the aponeurosis at mean angles of 12 to 21 degrees. Using cine phase contrast MRI, highly non-uniform shortening was measured within the biceps brachii during elbow flexion against 5% and 15% of subject-specific maximum voluntary contraction strength. Shortening along the centerline, or longitudinal axis, was significantly different from shortening along the anterior boundary of the muscle. Mean shortening was constant along anterior fascicles, averaging 21% for both active elbow flexion conditions. In contrast, mean centerline shortening demonstrated significant heterogeneity during low-force elbow flexion. The internal aponeurosis was likely the principal cause of heterogeneity in centerline shortening;​ however, centerline shortening remained non-uniform proximal to the aponeurosis. Preliminary results from an ¹⁸F-fluorodeoxyglucose (FDG) PET study suggest glucose metabolism is also highly non-uniform within the biceps brachii during elbow flexion.

Common assumptions made about muscles for biomechanical analysis, such as uniform contraction, may be inappropriate for many muscles because of their complex muscletendon architectures. For example, the biceps brachii is often represented as a simple, parallel-flbered muscle that contracts uniformly. However, the complex architecture and non-uniform shortening measured in this study suggests fascicle lengths and sarcomere lengths are heterogeneously distributed within the biceps brachii. Because such heterogeneity has important functional consequences and may alter the muscle's forcegenerating capability, its effects should be included when modeling muscle mechanics. This research emphasizes the important role o f non-invasive imaging data in characterizing in vivo skeletal muscle contraction, improving the accuracy of representations of muscle-tendon mechanics, and furthering the understanding of muscle function.