Effect of Eccentric Exercise on Musculus Triceps Surae Muscle Architecture, Muscle Strength, and Performance in Young Volleyball Players

Skeletal muscles are organized masses of muscle fascicles covered with connective tissue, attached to bones at both ends through tendons. The volume of a muscle is largely determined by the total number of sarcomeres within that muscle. Sarcomeres are approximately 1 µm in diameter and 2-3 µm in length. These functional units are aligned end-to-end to form myofibrils, which are packed parallel to create muscle fibers (approximately 50 µm in diameter in humans). Bundling these muscle fibers in parallel forms fascicles (approximately 1 to 3 mm in diameter in humans), which, in turn, combine in parallel to create muscles.

Muscle architecture is defined as the geometric arrangement of muscle fiber bundles concerning the axis that generates force. Muscles with fibers extending longitudinally along the muscle length possess longitudinal muscle architecture, while muscles with fibers running at a certain angle along the muscle length and containing shorter fibers possess pennate or multipennate muscle architecture.

This arrangement affects a muscle's contraction speed, force generation capacity, and range of motion known as 'excursion'. In pennate muscles, bundles of fibers, referred to as fascicles, are positioned obliquely and attach to the muscle's aponeuroses. The angle at which a fascicle attaches to the aponeurosis defines the pennation angle. The distance between epimysiums (superficial and deep aponeuroses in ultrasonography) defines the anatomical muscle thickness. These parameters of skeletal muscle architecture are measured through muscle physiology and biomechanical studies to determine the anatomical and contractile characteristics of the muscles. Typical parameters included in architectural analysis are fiber length, pennation angle, and physiological cross-sectional area.

Ultrasonography enables the examination of muscle morphology without radiation exposure. Fascicle length, pennation angle, and muscle thickness can be measured in vivo using two-dimensional (2D) B-mode ultrasonography. In ultrasonographic imaging, normal muscle tissue appears as a structure with low echo intensity. As the epimysium surrounding the muscle is quite reflective, the muscle's boundaries are clearly visible.

While skeletal muscles show significant structural similarities at a microscopic level, muscle architecture is the fundamental factor creating differences in strength and functional capabilities. Understanding the architectural features of muscles allows for the effective assessment and improvement of muscle function. Loading placed on muscles results in an adaptive process leading to muscle development. Muscle architecture allows the macroscopic understanding and interpretation of this adaptation process.