A skeletal muscle’s serial sarcomere number (SSN) is tightly regulated in response to chronic stimuli associated with changes in muscle length. Decreased SSN (~20%) is observed following immobilization in a shortened position, while increased SSN (~15%) is observed following immobilization in a stretched position, with both responses being amplified by simultaneously electrically stimulating the muscle. Furthermore, following cessation of immobilization, SSN rapidly returns to pre-immobilization values. Training interventions biased to concentric and eccentric contractions also decrease and increase SSN, respectively, but to a lesser extent (4–8% changes) than immobilization. Static stretch training may also increase SSN, or at least prevent SSN loss when performed intermittently during immobilization. Decreases in SSN induce a leftward shift and narrowing of the isometric force-length relationship, while increases in SSN induce a rightward shift and widening of the force-length relationship, which often translates to improved force throughout the range of motion and maximum power output. Profound losses of SSN have been noted in certain clinical conditions including aging, cerebral palsy, stroke, and bed rest. There is increasing evidence that interventions aimed to promote SSN addition may mitigate functional impairments in these populations, however, further research is required to fully understand the optimal stimuli in humans.