Finite element model of intramuscular pressure during isometric contraction of skeletal muscle

Abstract

The measurement of in vivo intramuscular pressure (IMP) has recently become practical and IMP appears well correlated with muscle tension. A numerical model of skeletal muscle was developed to examine the mechanisms producing IMP. Unipennate muscle is modelled as a two-dimensional material continuum that is incompressible and nonlinearly anisotropic. The finite element technique is used to calculate IMP and muscle stress during passive stretch and during isometric contraction. A novel element models the contractile portion of muscle, incorporating sarcomere length-force and velocity-force relations. A range of unipennate muscle geometries can be modelled. The model was configured to simulate the rabbit tibialis anterior muscle over a range of lengths. Simulated IMP and stress results were validated against animal experimentation data. The simulation agreed well with the experimental data over the range of 0.8-1.1 of the optimal length. Severe pressure gradients were produced near the musculo-tendinous junctions while IMP was more uniform in the central muscle belly. IMP and muscle stress in relaxed (unstimulated) muscle increased nonlinearly with muscle length. IMP and stress in isometrically contracting muscle showed a local maximum at optimal length and were reduced at shorter lengths. At muscle lengths longer than optimal, stress and IMP increased predominately due to tension in the passive elastic structures.

Three-Dimensional Representation of Complex Muscle Architectures and Geometries

“…We found that fiber moment arms have a considerable variation within each muscle, indicating that the common assumption that all fiber lengths and excursions within muscle are the same may not be valid, at least for the muscles examined here. Previous studies have described continuum representations of muscle 23,30,31,43 and used them, for example, to investigate intramuscular pressures 28 and understand myofascial force transmission. 48 These models represent the complex nonlinear behavior of muscle tissue; however, they have not incorporated realistic geometric arrangements of muscle fibers or muscle-bone and muscle-muscle surface contact.…”

Mechanotransduction in skeletal muscle

“…The deformations within the whole muscle have been extremely difficult to quantify directly, although experimental observations are beginning to appear (23,24). Several authors have presented continuum or finite element models of muscle during force generation (26)(27)(28)(29). Each of these models confirms that the distribution of deformation within a muscle is not uniform and that shear strains, both parallel and perpendicular to the fiber axis, contribute to the overall deformation ( Figure 1).…”

“…Along the axis of fibers, a materials science analysis suggests that the tension borne by each group will be in proportion to its stiffness and volume fraction (48), which means that very little tension will be transmitted to the passive fibers within a partly activated muscle. It is even possible, due to the intramuscular pressure generated by the active fibers (28), that the principal load borne by the passive fibers is a transverse compression, rather than an axial tension.…”

Intermuscular pressure between synergistic muscles correlates with muscle force

“…Furthermore, IaMP was reported to correlate linearly with contraction force in skeletal muscles of humans, cats and rabbits (Baskin and Paolini, 1967;Sadamoto et al, 1983;Parker et al, 1984;Petrofsky and Hendershot, 1984;Sejersted et al, 1984;Aratow et al, 1993;Nakhostine et al, 1993;Davis et al, 2003;Ward et al, 2007;Winters et al, 2009;Macias et al, 2012). According to present knowledge, muscle contraction causes a reversible distortion of muscle architecture accompanied by an increase of IaMP, owing to muscle incompressibility (Swammerdam, 1737;Hill, 1948;Jenkyn et al, 2002).…”