Effect of Eccentric Exercise on Musculus Triceps Surae Muscle Architecture, Muscle Strength, and Performance

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.

Although muscle strength is primarily determined by genetic structure, it is also influenced by factors such as the level of physical activity, age, gender, motivation, and nutrition. The magnitude of the cross-sectional area of a muscle - which includes the number of engaged fibrils and the size of these fibrils - accounts for the muscle's strength. Developing these aspects is achievable through exercise.

Exercise is defined as a planned, structured, intentional, and continuous activity aimed at enhancing physical fitness. Strength training exercises are designed to increase muscle strength and endurance by applying resistance.

One type of exercise, eccentric exercises, involves movements against gravity using body weight or additional loads. Eccentric exercises are utilized to increase muscle strength and mass. More muscle strength can be generated through these exercises compared to concentric or isometric exercises. Due to their features in injury prevention, rehabilitation, and improving physical fitness in healthy individuals, eccentric training has become quite popular. However, findings regarding the effects of eccentric training on the m. triceps surae are contradictory. Some studies show that eccentric training promotes an increase in muscle fiber length, pennation angle, and muscle thickness, while others find no changes in these architectural outcomes. This might be due to the non-uniform distribution of loads among synergistic muscles and the observation of different mechanical loads for different components of the m. triceps surae. Additionally, short muscle fibers are more sensitive to muscle damage caused by eccentric training compared to long muscle fibers. Considering the differences in architectural features of m. triceps surae segments (GM, GL, SO, and PL), eccentric exercises may produce different results.

The aim of the study is to investigate the effects of 8 weeks of eccentric exercises, performed three times a week for a total of 24 sessions, on the muscle architecture of the m. triceps surae in healthy female participants. Ultrasonography will be utilized to measure fascicle length, pennation angle, and muscle thickness in the gastrocnemius medialis (GM), gastrocnemius lateralis (GL), soleus (SO), and plantaris (PL) muscles before and after the exercise program. It is hypothesized that an 8-week eccentric exercise program will increase fascicle length, pennation angle, and muscle thickness in the m. triceps surae muscles. The architectural adaptations to eccentric exercises must be understood to develop effective exercise programs for improving muscle strength and function.