## 2 Characteristics, Benefits, and Risks of Eccentric Muscle Action  
Eccentric exercise is considered a unique muscle action modality based on the active lengthening of muscle tissue. Muscles exposed to eccentric actions show a unique response compared to other types of muscle contraction and are characterized by the following (Figure 2):  
1. A high force/tension ratio [3; 13; 14]. However, one study suggests that isometric actions could produce higher force values than eccentric actions exert force at an optimal joint angle [15]. 2. A lower metabolic cost for muscular work produced at the same intensities as concentric muscular work (Figure 2B) [13; 16; 17]. 3. Increased muscle strength in all forms of muscle contraction [18; 19; 20]. 4. Selective regional hypertrophy and the architectural remodeling of the muscles, mainly in muscle length change [20; 21; 22]. 5. Respond differently to the force/velocity curve, generating a mechanical paradox; the higher the execution velocity, the greater the force generated (Figure 2A) [13; 19]. 6. Rely on different morphological structures that support the eccentric action (e.g., titin, fascia, aponeurosis, and tendons (i.e., mainly connective tissue)) [23; 24; 25]. Considering all these factors, the muscle acts eccentrically as a shock absorber or elastic spring (Figure 1), where strength needs vary depending on the sport, motor activity, age, training level, health-related parameters, muscle strength capacity, training period, and rehabilitation phase [26; 27; 28]. Accordingly, their development and implementation require organization, structuring, and methodological control to guide training goals while considering their benefits and risks. Eccentric training is broadly used in sports training to improve performance during eccentric tasks. The use of eccentric strength training enhances maximal muscle strength, explosive force (i.e., rate of force development), muscle hypertrophy, and muscular power, improving athletic performance [3; 29]. Also, evidence supports its broad prescription in sports rehabilitation, notably in treating tendonopathies [30; 31; 32; 33] and preventing/rehabilitating  
Figure 1: Underlying factors that influence eccentric muscle activity (adapted from [4; 10]). sports-related injuries such as muscle strain [34; 35]. Additionally, since eccentric training provides high mechanical stress with low metabolic cost, it could be appropriate for training special populations with health-related problems such as muscle wasting and reduction in muscle strength, mobility, aerobic capacity, cardiorespiratory problems, sarcopenia, cachexia, type 2 diabetes, and neurological and musculoskeletal diseases [35]. Eccentric strength training implementation directly relates to training methods and desired adaptations (e.g., sporting performance and rehabilitation, muscle development, injury prevention, and special populations). For young athletes in the early stages of muscle development, the stimulation of muscle strength through resistance training methods is essential [36; 37]. The use of overloading in young athletes is paramount in developing strength and power qualities that support athletic performance, where the physiological adaptations that support such benefits are evident [36]. Similarly, a large body of evidence highlights the importance and efficacy of using eccentric overload methods to improve young male and female athletes' physical capacities [36; 38; 39]. Accordingly, the athlete's maturity stages, development, and technical learning of the exercises must be considered. Additionally, as with any other training modality, their training history, technical proficiency, and long-term physical development guidelines must be carefully considered [36]. Due to its specific physiological and mechanical properties, eccentric exercise is of great interest in rehabilitation processes of the muscle-tendon complex in special populations such as older adults or patients with chronic diseases (e.g., neuromuscular pathologies) [13; 21; 35; 40].