At the level of the single muscle fiber the number of sarcomeres in series and in parallel can vary. • The number of sarcomeres in series determines the length of the myofibril and thus the length of the muscle fiber.• Because one sarcomere can shorten by a certain length with a given maximal velocity, both the range of motion and the maximal shortening velocity of a muscle fiber are proportional to the number of sarcomeres in series. • The force that a myofibril can exert is equal to the average sarcomere force and is not influenced by the number of sarcomeres in series. • The force capacity of a fiber, however, depends on the number of sarcomeres in parallel and hence on the diameter or cross-sectional area of the fibe