Skeletal muscle transverse strain during isometric contraction at different lengths

Area of Science:

• Biomechanics
• Skeletal Muscle Physiology
• Computational Modeling

Background:

• Two-dimensional (2D) numerical models of skeletal muscle contraction often assume plane strain conditions.
• These models may overlook crucial three-dimensional (3D) deformations occurring in the muscle section.

Purpose of the Study:

• To investigate the validity of the plane strain assumption in 2D skeletal muscle models.
• To quantify 3D muscle surface deformation during isometric contractions at varying muscle lengths.

Main Methods:

• 3D muscle surface deformation was measured using marker displacements.
• Measurements were taken during isometric contractions across a range of initial muscle lengths.
• Longitudinal and transverse strains of superficial muscle fibers and aponeurosis were analyzed.

Main Results:

• Longitudinal and transverse strains in muscle fibers and aponeurosis varied with initial muscle length.
• Transverse strains were comparable at intermediate lengths but diverged at short and long lengths.
• Muscle fiber surface area change was approximately -11% and independent of muscle length, suggesting a 3D strain of 11% perpendicular to the surface.

Conclusions:

• The plane strain assumption is insufficient for studying the effect of muscle length on skeletal muscle deformation.
• 2D models may be suitable for contractions with ~ -9% longitudinal strain where transverse strains are minimal.
• Superficial muscle fibers likely flatten during isometric contractions, a phenomenon not fully captured by 2D models.