Triceps Surae Muscle Forces

Musculoskeletal modeling can be used to estimate forces during locomotion. These models,
however, are dependent on underlying assumptions about the model inputs, such as muscle
volumes and fiber lengths, to calculate muscle forces. Triceps surae (gastrocnemius medialis,
gastrocnemius lateralis, soleus) muscle volume distributions vary among humans. Here
we quantify how this muscle volume variation impacts maximum estimated lower limb muscle
forces during the braking and propulsive phases of the stance phase of walking. Three
triceps surae muscle volume distributions (AnyBody Modeling System standard cadaver
[MS], average of 21 cadavers [C], average of 21 young, healthy adults [YHA]) were evaluated
in a standard musculoskeletal model using the kinetic and kinematic data of 10 healthy
individuals at three walking velocities. Maximum muscle forces were calculated using
inverse dynamics and an algorithm to solve the muscle redundancy problem in the AnyBody
Modeling System. Repeated measure ANOVAs were used to test for significant differences
among the three muscle distribution configurations for each muscle/muscle group at each
velocity. Triceps surae muscle volume distribution significantly affects gastrocnemius lateralis
and soleus maximum muscle forces for both braking and propulsion at all three velocities
(p < 0.001), with relatively larger muscle volumes typically producing relatively larger muscle
forces. There was no significant difference in gastrocnemius medialis maximum force among
configurations (p > 0.124) except at the self-selected spontaneous velocity during braking.
Significant differences exist at some velocities for the hamstrings and gluteus maximus
during braking (p < 0.046) and the other plantarflexors, dorsiflexors, evertors, hamstrings,
quadriceps, sartorius, and gluteus maximus during propulsion (p < 0.042). Muscle volumes
used in musculoskeletal models impact estimated muscle forces of both the muscles of
interest and other muscles in the biomechanical chain. This is consistent with recent analyses
demonstrating that input values can substantially impact results and suggests individualized
muscle parameters may be needed depending on the research question.