Regionalizing muscle activity causes changes to the magnitude and direction of the force from whole muscles—a modeling study

Abstract

Skeletal muscle can contain neuromuscular compartments that are spatially distinct regions that can receive relatively independent levels of activation. This study tested how the magnitude and direction of the force developed by a whole muscle would change when the muscle activity was regionalized within the muscle. A 3D finite element model of a muscle with its bounding aponeurosis was developed for the lateral gastrocnemius, and isometric contractions were simulated for a series of conditions with either a uniform activation pattern, or regionally distinct activation patterns: in all cases the mean activation from all fibers within the muscle reached 10%. The models showed emergent features of the fiber geometry that matched physiological characteristics: with fibers shortening, rotating to greater pennation, adopting curved trajectories in 3D and changes in the thickness and width of the muscle belly. Simulations were repeated for muscle with compliant, normal and stiff aponeurosis and the aponeurosis stiffness affected the changes to the fiber geometry and the resultant muscle force. Changing the regionalization of the activity resulted to changes in the magnitude, direction and center of the force vector from the whole muscle. Regionalizing the muscle activity resulted in greater muscle force than the simulation with uniform activity across the muscle belly. The study shows how the force from a muscle depends on the complex interactions between the muscle fibers and connective tissues and the region of muscle that is active.

The effect of intramuscular fat on skeletal muscle mechanics: implications for the elderly and obese

“…The simulations in this study (M1-M6) were for muscle with uniform distributions of activity and intramuscular fat. It should be remembered that muscle force additionally depends on regional variations in muscle activity [22,42], fat distribution (figure 4) and fibre-type composition [43,44], and that muscle contribution to joint torque also depends on its moment arm that can vary with ageing and obesity [45,46]. Nonetheless, the results from this study show that the inclusion of intramuscular fat and the basematerial properties of the muscle tissue have an important effect on muscle force.…”

“…The neo-Hookean strain energy for the fat (W fat ) was adopted based on modelling work on human breast tissue [31] and is defined as where I 1 is the first invariant of the Cauchy-Green deformation tensor. The adipose tissue (fat) had a larger stiffness than the isotropic muscle base material (figure 2) for the lean muscle (eqn 8 in [22]). However, fat stiffness was less than the passive response of the whole muscle tissue that includes both the base-material and fibre properties (figure 2).…”

“…Also, whenever assuming an X% loss of contractile elements of the fibres, the W muscle component of isochoric strain energy was reduced by a factor of 1 2 X/100. The bulk modulus, k, for the volumetric part of the strain energy W vol (eqn 5 in [22]) was chosen to be k fat ¼ 0.25 Â 10 6 for fat. This was based on the fat compressibility properties used in modelling the human heel pad [32].…”

The influence of patellar tendon and muscle–tendon unit stiffness on quadriceps explosive strength in man

“…Indeed, modelling studies demonstrate that greater aponeurosis stiffness results in a reduction in its stretch that decreases muscle fibre strain (Rehorn & Blemker, 2010;Rahemi et al 2014). Conceptually, our method reflects the elongation of distal (to the ultrasound measurement site) tendinous tissues (aponeurosis-tendon).…”

Multidimensional models for predicting muscle structure and fascicle pennation

“…It is possible that the inability of the models to predict and operate at large aponeurosis strains may be a factor influencing such errors. Recently, we have shown that the aponeurosis stiffness and stretch greatly influence the extent to which the muscle pennation changes during contraction and the magnitude of the whole muscle force (Rahemi et al, 2014). Understanding the role and contribution of the aponeurosis compliance is a necessary aspect of understanding the structure and dynamics of muscle contraction (Kawakami et al, 1998;Rahemi et al, 2014).…”