© SLACK Incorporated 2006. Moffat M, Rosen E, Rusnak-Smith S. Musculoskeletal Essentials: Applying the Preferred Physical Therapist Practice Patterns.SM

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-zone, located in the middle of the A-band, is the region of thick filaments not overlapped by thin filaments. The M-band bisects the H-zone and represents the middle of the sarcomere. The M-band consists of protein structures that 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.

Anatomical Structures of the Spindle Apparatus Intrafusal muscle fibers are generally only 2 to 4 mm in

length. They are fusiform in shape, widest in the center and tapered at ends. These fibers are composed of two types of fibers:

56 Chapter Three

Figure 3-1. Successive connective tissue sheaths with-in muscle. Reprinted with permission from Oatis CA. Kinesiology: The Mechanics and Pathomechanics of Human Movement. Philadelphia, Pa: Lippincott Williams & Wilkins; 2004:47.

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.

Figure 3-3. Actin-myosin organization of two succes-sive sarcomeres showing A, H, and I bands as well as Z-lines. Reprinted with permission from Oatis CA. Kinesiology: The Mechanics and Pathomechanics of Human Movement. Philadelphia, Pa: Lippincott Williams & Wilkins; 2004:46.