Triggering sarcomerogenesis: Examining key stimuli and the role attributed to eccentric training —Historical, systematic, and meta-analytic review

Serial sarcomere number (SSN) critically influences muscle function and is hypothesized to protect against injury. While most evidence for SSN addition comes from non-human animal studies, eccentric exercise is often proposed as a key stimulus due to its association with increased fascicle length in humans. However, the most efficient exercise stimuli and the effectiveness of eccentric training in increasing SSN remain unclear. The objective of this study was to provide a detailed historical overview of research exploring the mechanical factors regulating muscle/fiber length and its relationship with function, and to explore more recent evidence that eccentric muscle contractions might be an important stimulus for SSN regulation using a meta-analytic approach. An extensive literature search with snowballing was conducted to build the historical review. A systematic review with random-effect meta analyses was performed to compare proposed types of fiber-lengthening exercises to control conditions. The historical review demonstrated that the application of forces at long fiber lengths, but not specifically the use of greater excursions, plays an important role in increasing SSN. Animal data showed changes in SSN exceeding 20% over several weeks with varied forms of activities. Nonetheless, the meta-analysis revealed a lack of effect of eccentric resistance training in animal models (Δ = 1%; Cohen's d = 0.19 (-0.29, 0.67), p = 0.449). High active or passive muscle forces applied at long fiber lengths appear to be the key stimuli triggering sarcomerogenesis. Eccentric exercise does not seem to be a key promoter of SSN. Too few studies exist to draw conclusions as to the effect of eccentric exercise on SSN in humans. Understanding the mechanical triggers and physiological mechanisms involved in serial sarcomere addition could help in the development of exercise (and other) interventions to optimize muscle function and reduce injury risk.