Muscles contain many different systems on which the evolutionary selection of preflex stabilization can operate. The deltoid muscle, for example, consists of at least seven segments with different bone attachments and neural control. Within each muscle segment, there exists a complex internal structure that goes down to one in which each muscle unit consists of a tendon, aponeurosis, and a fascicle of active contractile and passive elements. Another source of variation is in the internal architecture of the fiber orientation relative to a muscle’s line of action, for example, as found in pennate muscles. The complexities of the different visco-elastic length- and velocity-force relationships of these subparts provides the opportunity for the adaptive selection of structurally complex muscle biocomposites with highly task-tuned nonlinear visco-elastic length- velocity- force relationships. This nature of muscles to be composite structures thus provides the adaptive opportunity for evolution to modify the visco-elastic reactions of the musculoskeletal system so they counteract perturbations without the need for spinal or higher levels of control.
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