Inclusion of image-based in vivo experimental data into the Hill-type muscle model affects the estimation of individual force-sharing strategies during walking.

The study of muscle coordination requires knowledge of the force produced by individual muscles, which can be estimated using Hill-type models. Predicted forces from Hill-type models are sensitive to the muscle's maximal force-generating capacity (Fmax), however, to our knowledge, no study has investigated the effect of different Fmax personalization methods on predicted muscle forces. The aim of this study was to determine the influence of two personalization methods on predicted force-sharing strategies between the human gastrocnemii during walking. Twelve participants performed a walking protocol where we estimated muscle activation using surface electromyography and fascicle length, velocity, and pennation angle using B-mode ultrasound to inform the Hill-type model. Fmax was determined using either a scaling method or experimental method. The scaling method used anthropometric scaling to determine both muscle volume and fiber length, which were used to estimate the Fmax of the gastrocnemius medialis and lateralis. The experimental method used muscle volume and fascicle length obtained from magnetic resonance imaging and diffusion tensor imaging, respectively. We found that the scaling and the experimental method predicted similar gastrocnemii force-sharing strategies at the group level (mean over the participants). However, substantial differences between methods in predicted force-sharing strategies was apparent for some participants revealing the limited ability of the scaling method to predict force-sharing strategies at the level of individual participants. Further personalization of muscle models using in vivo experimental data from imaging techniques is therefore likely important when using force predictions to inform the diagnosis and management of neurological and orthopedic conditions.

Does different activation between the medial and the lateral gastrocnemius during walking translate into different fascicle behavior?

The functional difference between the medial gastrocnemius (MG) and lateral gastrocnemius (LG) during walking in humans has not yet been fully established. Although evidence highlights that the MG is activated more than the LG, the link with potential differences in mechanical behavior between these muscles remains unknown. In this study, we aimed to determine whether differences in activation between the MG and LG translate into different fascicle behavior during walking. Fifteen participants walked at their preferred speed under two conditions: 0% and 10% incline treadmill grade. We used surface electromyography and B-mode ultrasound to estimate muscle activation and fascicle dynamics in the MG and LG. We observed a higher normalized activation in the MG than in the LG during stance, which did not translate into greater MG normalized fascicle shortening. However, we observed significantly less normalized fascicle lengthening in the MG than in the LG during early stance, which matched with the timing of differences in activation between muscles. This resulted in more isometric behavior of the MG, which likely influences the muscle-tendon interaction and enhances the catapult-like mechanism in the MG compared with the LG. Nevertheless, this interplay between muscle activation and fascicle behavior, evident at the group level, was not observed at the individual level, as revealed by the lack of correlation between the MG-LG differences in activation and MG-LG differences in fascicle behavior. The MG and LG are often considered as equivalent muscles but the neuromechanical differences between them suggest that they may have distinct functional roles during locomotion.