Ludvig, D. | bioRxiv Science

bioRxiv2021

Multidirectional measures of shear modulus in skeletal muscle

The material properties of muscle play a central role in how muscle resists joint motion, transmits forces internally, and repairs itself. While many studies have evaluated muscles tensile material properties, few have investigated muscles shear properties. None of which have taken into account muscles anisotropic structure or investigated how different muscle architecture affect muscles shear properties. The objective of this study was to quantify the shear moduli of skeletal muscle in three orientations relevant to the function of whole muscle. We collected data from the extensor digitorum longus, tibialis anterior, and soleus harvested from both hindlimbs of 12 rats. These muscles were chosen to further evaluate the consistency of shear moduli across muscles with different architectures. We calculated the shear modulus in three orientations: parallel, perpendicular, and across with respect to muscle fiber alignment; while the muscle was subjected to increasing shear strain. For all muscles and orientations, the shear modulus increased with increasing strain. The shear modulus measured perpendicular to fibers was greater than in any other orientation. Despite architectural differences between muscles, we did not find a significant effect of muscle type on shear modulus. Our results show that in rat, muscles shear moduli vary with respect to fiber orientation and are not influenced by architectural differences in muscles.

physiology↗

bioRxiv2021

Leveraging joint mechanics simplifies the neural control of movement

Behaviors we perform every day, such as manipulating an object or walking, require precise control of interaction forces between our bodies and the environment. These forces are generated by active muscle contractions, specified by the nervous system, and by joint mechanics, determined by the intrinsic properties of the musculoskeletal system. Depending on behavioral goals, joint mechanics might simplify or complicate control of movement by the nervous system. However, whether humans can exploit joint mechanics to simplify neural control remains unclear. Here we evaluated if leveraging joint mechanics can simplify neural control by comparing performance in three tasks that required subjects to generate specified torques about the ankle during imposed sinusoidal movements; only one task required torques that could be generated by leveraging the intrinsic mechanics of the joint. We developed a novel approach that used continuous estimates of impedance, a quantitative description of joint mechanics, and measures of muscle activity to determine the mechanical and neural contributions to the net ankle torque generated in each motor task. We found that the torque resulting from changes in neural control was reduced when ankle impedance was consistent with the task being performed, resulting in a task that required less muscular effort. Subjects perceived this task to be easier than those that were not consistent with the impedance of the ankle and were able to perform it with the highest level of consistency. These results demonstrate that leveraging the mechanical properties of a joint can simplify task completion and improve performance. KEY POINTSO_LIInteracting with our environment requires production of interaction forces, which are generated by muscle contractions, specified by the nervous system, and by joint mechanics, determined by the intrinsic properties of the musculoskeletal system. C_LIO_LIWe assessed whether leveraging joint mechanics can simplify neural control by having subjects complete 3 tasks, only one of which could be accomplished by leveraging joint mechanics. C_LIO_LIWe found that subjects reduced their muscular effort, perceived the task to be easier and completed the task more consistently when the mechanics of the ankle were consistent with the task. C_LIO_LIThese results highlight the importance of considering limb mechanics when interpreting measures of neural control related to movement and may benefit the design of mechanical interfaces that optimize human performance. C_LI

neuroscience↗