56 Chapter Three Figure 3-2. Organization of muscle from macro- scopic to microscopic levels. Reprinted with permis- sion from Oatis CA. Kinesiology: The Mechanics and Pathomechanics of Human Movement. Philadelphia, Pa: Lippincott Williams & Wilkins; 2004:46. which contains only actin (thin) filaments. The darker area is the A-band, which contains alternating actin and myosin (thick) filaments. The Z-line consists of a connective tissue network that bisects the I-band, anchors the thin filaments, and provides structural integrity to the sarcomere. The H- Figure 3-1. Successive connective tissue sheaths with- zone, located in the middle of the A-band, is the region of in muscle. Reprinted with permission from Oatis thick filaments not overlapped by thin filaments. The M- CA. Kinesiology: The Mechanics and Pathomechanics band bisects the H-zone and represents the middle of the of Human Movement. Philadelphia, Pa: Lippincott sarcomere. The M-band consists of protein structures that Williams & Wilkins; 2004:47. support the arrangement of the myosin filaments. During muscle contraction, the sarcomere I-band and H-zone decrease in length while the length of the A-band remains constant.2,3 THE MUSCLE SPINDLE The muscle spindle is a long, thin structure located adjacent and parallel to muscle fibers and is composed of multiple components that have both afferent and efferent innervation (Figures 3-4a and 3-4b). The muscle spindle functions as a stretch receptor and responds to static and dynamic length changes of skeletal muscle.4-6 This complex receptor is found in all muscles, primarily in extremity, inter- costal, and cervical muscles. Figure 3-3. Actin-myosin organization of two succes- sive sarcomeres showing A, H, and I bands as well Anatomical Structures of the Spindle as Z-lines. Reprinted with permission from Oatis Apparatus CA. Kinesiology: The Mechanics and Pathomechanics Intrafusal muscle fibers are generally only 2 to 4 mm in of Human Movement. Philadelphia, Pa: Lippincott length. They are fusiform in shape, widest in the center Williams & Wilkins; 2004:46. and tapered at ends. These fibers are composed of two types of fibers: © SLACK Incorporated 2006. Moffat M, Rosen E, Rusnak-Smith S. Musculoskeletal Essentials: Applying the Preferred Physical Therapist Practice Patterns.SM.