A biomechanical model to simulate the effect of a high vertical loading on trunk flexural stiffness

Abstract

A human trunk model was developed to simulate the effect of a high vertical loading on trunk flexural stiffness. A force-length relationship is attributed to each muscle of the multi-body model. Trunk stiffness and muscle forces were evaluated experimentally and numerically for various applied loads. Experimental evaluation of trunk stiffness was carried out by measuring changes in reaction force following a sudden horizontal displacement at the T10 level prior to paraspinal reflexes induction. Results showed that the trunk stiffness increases under small applied loads, peaks when the loads were further increased and decreases when higher loads are applied. A sensitivity analysis to muscle force-length relationship is provided to determine the model's limitations. This model pointed out the importance of taking into account the changes in muscle length to evaluate the effect of spinal loads beyond the safe limit that cannot be evaluated experimentally and to predict the trunk instability under vertical load.

Numerical Identification of Deep Muscle Residual Tensions (Tones) Based on Multi-Directional Trunk Stiffness Data

“…The identification procedure solves the boundaries of the domain of feasible tone values by matching the tone-dependent model stiffness with the experimental stiffness taken as a reference. In accordance with the common practice in biomechanical modeling, the muscle tone has been represented by its ratio to the maximum active force of the muscle, called tone ratio (RMTT) [2]. In the following, the RMTTs will be represented by a vector in a space denoted T.…”

“…The tone is taken proportional to the maximum active muscle force. The calculation of the model trunk stiffness, in the specified displacement direction, proceeds as follows [2,10]. An equilibrium problem (Equation ( 1)) is run for the trunk subject to a specified horizontal force, applied at the T10 level, with the muscle stiffnesses held constant.…”

“…The trunk stiffness [2] measured through the trunk test is dependent on the changes in muscle activation and muscle-tendon stiffness, concurrent with the perturbation. As the trunk regains a static post-perturbation state before the induction of the paraspinal reflex, the effective trunk stiffness should be calculated based on the force and horizontal displacement at this state [10].…”

“…Muscle tone is one of the most common parameters governing human movement and posture [1,2]. It is described as residual muscle tension or contraction [3].…”

“…On the other hand, muscle tone levels influence the behavior of the trunk. Through a sensitivity analysis, [2,10] showed the model trunk flexural stiffness to be significantly influenced by the muscle tones, treated as model parameters. This finding suggests that accurate estimation of muscle tone is helpful for a better assessment of trunk behavior.…”