Muscle forces and the demands of human walking

Abstract

Reconstructing the locomotor behavior of extinct animals depends on elucidating the principles that link behavior, function, and morphology, which can only be done using extant animals. Within the human lineage, the evolution of bipedalism represents a critical transition, and evaluating fossil hominins depends on understanding the relationship between lower limb forces and skeletal morphology in living humans. As a step toward that goal, here we use a musculoskeletal model to estimate forces in the lower limb muscles of ten individuals during walking. The purpose is to quantify the consistency, timing, and magnitude of these muscle forces during the stance phase of walking. We find that muscles which act to support or propel the body during walking demonstrate the greatest force magnitudes as well as the highest consistency in the shape of force curves among individuals. Muscles that generate moments in the same direction as, or orthogonal to, the ground reaction force show lower forces of greater variability. These data can be used to define the envelope of load cases that need to be examined in order to understand human lower limb skeletal load bearing.

Sensitivity of musculoskeletal models to variation in muscle architecture parameters

“…The University of Washington's Institutional Review Board approved all aspects of this study (IRB#: STUDY00001125). The experimental protocol for the kinematic data collection process has been described in detail elsewhere (Sylvester, Lautzenheiser, & Kramer, 2021a;Sylvester et al, 2021b). We refer to the motion and force data of these participants as dynamic cases to distinguish them from subjects whose muscle parameters were used in the simulations described below.…”

“…Each participant completed five trials, but the data from two trials for two participants were unusable, resulting in 46 total dynamic cases with two consecutive stance periods (92 total steps). As detailed in our previous reports using these dynamic cases (Sylvester et al, 2021a(Sylvester et al, , 2021b, after initial scaling to the participant's body mass and stature, segment lengths are further adjusted to improve the match of the model with the motions described by the markers. This results in an individualized model for each participant based on their anthropometrics and motions.…”

“…Both N and p are model assumptions. We used the strength algorithm with an exponent of 3 as the baseline, consistent with our previous work (Sylvester et al, 2021a(Sylvester et al, , 2021b and others (e.g. Simonsen et al 2021).…”

“…The MoCap model is available from: https://anyscript.org/ammr-doc/index.html. The kinematics and kinetics are available from the Supplementary Information of Sylvester et al (2021b). The muscle parameter changes are detailed in Tables 2 and 3.…”

“…Assessing the influence of these assumptions on outcome variables is critically important. Using kinematic and kinetic information from 10 individuals, we have evaluated the muscular forces among individuals (Sylvester et al, 2021b) and found that the functional muscle groups that are critically important to propel (i.e., the ankle plantarflexors) and stabilize (e.g., the hip abductors) the body are consistent among individuals and steps.…”