Architectural design and fiber‐type distribution of the major elbow flexors and extensors of the monkey (cynomolgus)

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Abstract
Because the architectural and biochemical properties of a skeletal muscle dictate its force, velocity, and displacement properties, the major extensors (triceps brachii) and flexors (biceps brachii, brachialis, and brachioradialis) of the elbow in a primate (cynomolgus, monkey) were studied. Functional cross‐sectional areas (CSA) were calculated from muscle mass, mean fiber length (normalized to a 2.20 μm sarcomere length), and angle of fiber pinnation measurements from each muscle. Fiber‐type distributions were determined and used as a gross index of the biochemical capacities of the muscle. The extensor group had a shorter mean fiber length (31 vs. 47 mm), a larger CSA (13 vs. 8 cm2), and a higher overall percentage of slow‐twitch fibers (47 vs. 26%). Consequently, the elbow extensors had a relatively greater potential for force production and force maintenance than the flexors. In contrast, the flexors were designed to optimize their length‐velocity potentials; i.e., they had relatively long fibers and a higher fast‐twitch fiber composition than the extensors. These morphologic differences between antagonistic muscle groups should be considered when evaluating the motor control mechanisms regulating reciprocal movements about the elbow.