2. Functional Anatomy of Muscle

This chapter describes the main anatomical structures giving skeletal muscle its striated appearance, the location of the “motor” and of other passive elastic elements within its functional unit, the half-sarcomere, which contains all the ingredients characterizing muscular contraction. It is shown that the length change of each myofibril equals the sum of the length change of all the sarcomeres set in series within the myofibril, whereas the force at the extremities of each myofibril equals the force exerted by each sarcomere. The contrary is true for the myofibrils set in parallel within the muscle fiber: the total force exerted by the fiber is the sum of the forces exerted by all myofibrils, whereas the length change of the fiber equals that of each myofibril. Series elastic elements transmitting the force exerted by the contractile component are located within the tendons whereas other elastic elements are located both in series and in parallel within the sarcomeres. Since these structures can shorten quickly without appreciable losses they are called undamped elastic elements and are essential for the storage and recovery of mechanical energy in the stretch-shortening cycle of muscle-tendon units. Other damped structures within the sarcolemma and the sarcomeres are not suitable for storage-recovery of mechanical energy, but have the function to contain and stabilize the actin-myosin filaments during sarcomere lengthening and contraction.