An Elastography-Driven Biomechanical Model for Individual Muscle Force Estimation

The estimation of muscle forces is a long-standing problem in the field of muscle biomechanics. This is often referred as a classic indeterminate problem, as the unknowns (individual muscles forces) are greater than the measurable properties of the system. Although there exist approaches based mainly on electromyography (EMG), this approach is currently discussed due to the electrical nature of the measures and the susceptibility of the EMG signal to physiological and non-physiological factors. In this context, in recent years, elastography has become a reference method to characterize the longitudinal shear elastic modulus of skeletal muscle. As shown in previous works, this variable is directly related to muscle strength. In this sense, the mechanical and non-electrical nature of the estimates obtained through this methodology makes it a good alternative for calculating muscle forces. Thus, this work proposes a model that, based on muscle elasticity values, allows the calculation of individual muscle forces under a specific loading condition. Particularly, we analyzed the isometric flexion of the elbow joint, where the biceps brachii, brachioradialis, and brachialis muscles act synergically. This way, depending on the muscle and torque level, the model provided reliable force values, which stabilized the external torque on the elbow joint so that the net torque was 0. Furthermore, the model results showed that the ∼10% MVC appears to be a breakpoint in the load sharing between these muscles, thus retrieving the phenomenology observed in previous studies regarding their synergistic action during the isometric elbow flexion.