## 2 Characteristics, Benefits, and Risks of Eccentric Muscle Action
Aging, obesity, chronic illness, physical inactivity, muscular unloading, and prolonged states of injury or rehabilitation affect the skeletal muscle and the tendon, causing quantitative and qualitative tissue alterations that affect muscle function and mobility [33; 35; 41; 42], potentially exposing them to long periods of adaptation and recovery of their functions [33; 35; 43; 44]. Consequently, eccentric training can be an alternative to counteract these effects in older adults or special populations; however, it is essential to consider multiple factors, such as modality, intensity, frequency/volume, and safety. Eccentric exercise as a therapeutic model varies during the different phases of the muscle-tendon complex rehabilitation processes, and in terms of application, it is usually used as an injury prevention strategy in professional and amateur athletes [13; 35; 40; 45; 46]. The  
Figure 2: Distinctive responses of eccentric actions vs. other muscle contraction types (adapted from [3; 13; 14]). (**A**) The force-velocity curve that applies to the muscle action in eccentric and concentric phases. (**B**) metabolic cost (VO\({}_{2}\)) according to the magnitude of work (mean power). VO\({}_{2}\): oxygen consumption. W: watts. muscle-tendon complex interactions and adaptations related to mechanical load seem to be directed toward regional or specific responses by muscle or muscle groups [21; 33; 35; 47]. Accordingly, muscle remodeling and the reordering of tendon collagen fibers are the basis of the therapeutic action of eccentric exercise [35; 43]. Furthermore, eccentric mechanical work can treat tendonopathies since the tendon responds to mechanical forces by adapting its metabolism and structural properties, altering its genetic pattern, protein synthesis, and phenotype, which could improve the healing process [33; 35; 43; 44]. Finally, training regimes should consider delayed-onset muscle soreness (DOMS). This muscular phenomenon generates a physiological alteration of the muscle's functional unit (the sarcomere). It presents structural and functional damage generated by applying excessive eccentric actions in sedentary people, young athletes, and athletes with little experience. It is a complex of symptoms, pain experienced upon movement, weakness, stiffness, and swelling of the muscles and areas of the muscle-tendon junction [48]. In addition, it can cause muscles to lose their functional characteristics, such as force generation or decreased proprioceptive function [35]. Multiple factors, such as muscle architecture, muscle phenotype, individual fitness, age, sex, and genetic variability, may contribute to the wide inter-subject variability in response to eccentric exercise [32; 35; 49; 50]. Symptoms usually appear 8 to 10 h after work, reaching their highest peak after 24-48 h and progressively decreasing until total cessation after 3 or 4 days [40; 51]. While an initial unaccustomed high-intensity eccentric exercise bout can induce a remarkable amount of muscle damage, the magnitude of this muscle damage is usually attenuated in the subsequent bouts of the same exercises (a phenomenon referred to as the repeated bout effect) [52]. Additionally, it is essential to note that low-intensity eccentric actions attenuate muscle damage induced by maximal eccentric actions [53], highlighting the relevance of managing different eccentric training methods and considering the different factors that constitute the training load (e.g., volume, intensity, rest, among others). Strength and conditioning practitioners should consider several factors in order to avoid these potentially undesirable associated effects, such as training experience, relative strength, the adaptations aimed during specific training phases, the progression of the complexity and technical requirements of the exercise, and the ability to integrate eccentric training into a holistic resistance training program to benefit a subject's overall performance.