HUTCHINSON, J.R.; DENNENY, D.; ROBILLIARD, J.; WILSON, A.; Royal Veterinary College, Univ. London; London South Bank Univ.; Royal Veterinary College, Univ. London; Royal Veterinary College, Univ. London: The function of muscle-tendon units during sit-stand transitions in humans and other animals.
Locomotor economy generally relies on keeping muscle fibre length changes low, whereas length changes occur more in passive elastic components. Animals however also have to go from an upright standing to a crouched sitting position. Theoretically, such large-amplitude movements might require extreme length excursions of extensor muscle fibers. This seems at odds with specialization for efficient locomotion, and may represent a design constraint. Here we analyse experimental and anatomical data from a variety of taxa to determine what length changes of muscle-tendon units are required for sit-stand motions.
For example, in humans, in vivo ultrasound data for movement from a full squatting to a standing pose show M. soleus and M. gastrocnemius medialis mean length changes of -23% (shortening from a lengthened state in squatting) and +32% (lengthening from a shortened state in squatting) relative to standing fascicle length. Interindividual variation, however, was high for all triceps surae muscles. A musculoskeletal computer model estimated different length changes for the same kinematics: -42% for M. soleus and +7% for M. gastrocnemius medialis. This difference may be because of independent length changes occurring in series elastic elements. Hence muscle fascicle length changes can be very different from expectations, especially during large joint excursions. Better measurement techniques and more complex models of muscle-tendon unit geometry, both able to tease apart fascicle length changes from changes in other parts of muscle-tendon units, are essential in such cases.
