Muscle short-range stiffness can be used to

Abstract 30 The mechanical properties of the human arm are regulated to maintain stability across many 31 tasks. The static mechanics of the arm can be characterized by estimates of endpoint stiffness, 32 considered especially relevant for the maintenance of posture. At a fixed posture, endpoint 33 stiffness can be regulated by changes in muscle activation, but which activation-dependent 34 muscle properties contribute to this global measure of limb mechanics remains unclear. We 35 evaluated the role of muscle properties in the regulation of endpoint stiffness by 36 incorporating scalable models of muscle stiffness into a three-dimensional musculoskeletal 37 model of the human arm. Two classes of muscle models were tested: one characterizing 38 short-range stiffness, and two estimating stiffness from the slope of the force-length curve. 39 All models were compared to previously collected experimental data describing how 40 endpoint stiffness varies with changes in voluntary force. Importantly, muscle properties 41 were not fit to the experimental data, but scaled only by the geometry of individual muscles 42 in the model. We found that force-dependent vari ations in endpoint s tiffness were accurately 43 described by the short-range stiffness of active arm muscles. Over the wide range of 44 evaluated arm postures and voluntary forces, the musculoskeletal model incorporating 45 short-range stiffness accounted for 98±2%, 91±4% and 82±12% of the variance in stiffness 46 orientation, shape and area, respectively, across all simulated subjects. In contrast, estimates 47 based on muscle force-length curves were less accurate in all measures, especially stiffness 48 area. These results suggest that muscle short-range stiffness is a major contributor to 49 endpoint stiffness of the human arm. Furthermore, the developed model provides an 50 important tool for assessing how the nervous system may regulate endpoint stiffness via 51 changes in muscle activation. 52 53