TENDON BIOMECHANICAL PROPERTIES ENHANCE HUMAN WRIST MUSCLE SPECIALIZATION

Biomechanical properties of human wrist tendons were measured under lo ads predicted to be experienced by those tendons under physiological c onditions. This was accomplished by measuring the architectural proper ties of the five prime wrist movers-extensors carpi radialis brevis (E CRB), extensor carpi radialis longus (ECRL), extensor carpi ulnaris (E CU), flexor carpi radials (FCR), flexor carpi ulnaris (FCU)-and predic ting their maximum tension (P-0) using a specific tension value of 22. 5 N cm(-2). Loading the corresponding tendons to P-0 resulted in signi ficantly different strain among tendons (p < 0.01) with the largest st rain observed in the FCU (3.68 +/- 0.31%) and the smallest strain obse rved in the ECRL (1.78 +/- 0.14%). Further, strain magnitude was signi ficantly positively correlated with the tendon length-to-fiber length ratio of the muscle-tendon unit, a measure of the intrinsic compliance of the muscle-tendon unit. Theoretical modeling of the magnitude of m uscle sarcomere shortening expected based on the measured biomechanica l properties revealed a maximum sarcomere length decrease of about 0.6 mu m for the FCU to a minimum of about 0.2 mu m for the ECRB at P-0. Thus, tendon compliance may, but does not necessarily, result in signi ficant modification of muscle force generation. The significant variat ion in tendon biomechanical properties was not observed using traditio nal elongation-to-failure methods on the same specimens. Thus, the use of elongation-to-failure experiments for determination of tendon prop erties may not be reasonable when the purpose of such studies is to in fer physiological function. These data indicate that muscle-tendon uni ts show remarkable specialization and that tendon intrinsic properties accentuate the muscle architectural specialization already present.