KEVINM. CROSS,MEd, ATC, PT Spectrum Therapy

Injuries to the musculotendinous unit (MTU) Anatomy of the are common amongphysically active people. bBusculotsndinous Unit The incidence of overuse injuries alone in the (omposition athletic population varies from 20 % to 50 % , with being the primary structure in- MTUs are composed of contractile and non- volved (Jarvinen, contractile tissues. Muscle tissue forms the 1992).Injury severity contractile units, which are composed of can vary from a com- muscle fibers containing hundreds to thou- plete disruption be- sands of myofibrils. tissue and sar- tndmuscle tween fibers coplasm form the ground substance in which ,connectin the myofibrils are secured. Myofibrils are com- .I . I to an irritation of the novial sheath that posed of thousands of actin and myosin pro- weaknes(,QSand are injury. otects the tendon as tein filaments that are arranged into smaller it courses over a bony compartments called sarcomeres. The myo- prominence. Regard- sin filaments are attached to actin filaments :at studips less of injury severity by chemical bonds. Thesebonds require en- Ionsdiagnf or structure, the ath- ergy to successively release and reconnect the 5'' areactu :iatedwith letic trainer or thera- actin filaments over the adjacent myosin fila- Inon. pist must understand ments in a cyclic fashion, which shortens the the anatomy and sarcomeres' length and produces a contrac- pathophysiology of tion (Ciullo & Zarins, 1983). MTU injuries in order Tendon tissue forms the noncontractile to implement a suc- component of the MTU. The primary cellular cessful intervention componentsof tendons are , which plan. The purpose of produce collagen and . Collagen, espe- this article is to dis- cially Type I collagen, is the main fiber com- cuss MTU anatomy ponent of tendon tissue; it accounts for d the pathophysiol- 70-95 % of the dry weight of a tendon (Amiel, KeyWords: mustle, ter y-that"-results-from* -- -- F,rank,--& Harwood,_1984; .Gross, J 992; ' pathology Ute and overuse O'Brien, 1992). Elastin composes approxi- trauma. mately 5 % of the dry weight of tendon and is

" ZOO0Human Kinetics. ATT 5(4), pp. 6-13 6 1 JULY 2000 ATHLETICTHERAPY TODAY very important because of its resis- tance to tensile forces. The collagen and elastin fibers are embedded in a viscous ground substance, which provides lubrication and spacing among the collagen fibers to prevent tissue immobility. It also provides the tendon with additional "elastic" properties (Zachazewski, 1989).

Muscles are arranged in a hierarchi- cal order of progressively larger bundles of muscle fibers. The fibers are arranged into bundles that are termed fasciculi. Fasciculi vary in size and pattern. Endomysium, which is loose aereolar , lies within each fasciculus to fillthe "dead space" between the muscle fibers. The perimysium is a stronger connective tissue sheath that encap- sulates many fasciculi. Finally, the epimysium, a denser connective tis- sue, surrounds all the fasciculi to pro- Figure 1 Muscle structure. From S. Power and E.Howley, Exercise physiology: vide a muscle with its shape (Gray, Theory and application tofitness and performance (3rd ed.). 0 1997 McGraw-Hill Publishing Companies. Reproduced with the permission of the McGraw-Hill 1973; see Figure 1). The point of attachment between muscle and tendon tissue is termed the musculotendinous junction (MTJ).At this junction, tensile stress (O'Brien, 1992). Fibrils are then parti- the endomysium, perimysium, and epimysium con- tioned into small groups, called fascicles, by verge to form the primary collagen components of endotenon sheaths. Numerous fascicles are enclosed the tendon. Muscle fibers integrate into the connec- by the epitenon sheath, which creates the general tive tissue convergence either by tapering, ending in structure of the tendon. The next and most superfi- a flat alignment, or showing terminal expansion. The cial sheath is the paratenon, which encloses the en- muscle fibers transmit force to the connective tissues tire tendon. Small amounts of fluid separate the via viscous adhesions between the muscle fiber and paratenon from the immediate underlying epitenon ground substance (Gray, 1973). This juncture is the to prevent friction during movement (O'Brien; Gross, weakest aspect of the MTU and is most prone to acute 1992). In addition, a synovial sheath might encapsu- injury. late the tendon to provide further protection from fric- Tendons are also organized in a hierarchical ar- tion generated by the surrounding anatomy, such as rangement of progressively larger units. Likewise, the friction between the wrist and thumb extensor these units are separated by low-density connective- tendons and the osseous tunnels of the wrist (Figure tissue sheaths. The basic unit of the tendon is tropo- 2). collagen, which consists primarily of the mature and Tendons insert into by different mecha- strong Type I collagen. Five tropocollagen units join nisms, depending on the depth of the fibers. Superfi- to form a fibril that lies parallel to the direction of cial fibers attach to bone by simply maintaining their

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