This lecture discusses the structure and function of skeletal muscle. It describes the different types of muscle contractions - concentric, eccentric, and isometric. It explains the sliding filament theory of muscle contraction and how cross-sectional area, shape, and line of pull determine a muscle's functional potential. It also describes how the length of a muscle affects its force production and discusses principles of stretching and strengthening muscle.

1. Week 2: Lecture 2 Elaine Wilson, PT

3. Describe concentric, eccentric, and isometric activation of muscle Identify the anatomic components that comprise a whole muscle Describe the sliding filament theory Describe how cross-sectional area, line of pull, and shape help determine the functional potential of a muscle Describe the active length-tension relationship of muscle

4. Describe the passive length-tension relationship of muscle Explain why the force production of a multi-articular muscle is particularly affected by its operational length Describe the principles of stretching muscular tissue Describe the basic principles of strengthening muscular tissue

5. Structure and Function of Skeletal Muscle

6. Sole producer of active force in the body Stimulated by the nervous system, muscle contracts and pulls on bone to create movement When a muscle contracts, the freest (or less constrained) segment moves Mosby items and derived items © 2009 by Mosby, Inc., an affiliate of Elsevier Inc.

7. Concentric (shortening or contracting) Muscle produces active force and simultaneously shortens Eccentric (attempting to resist elongation) Muscle attempts to contract but is pulled to a longer length by a dominant external force Isometric (remaining at a constant length) Muscle generates active force while remaining at a constant length Mosby items and derived items © 2009 by Mosby, Inc., an affiliate of Elsevier Inc.

8. Relative points of muscle to bone attachment Proximal attachment (origin) Point of attachment closest to the midline or “core” of the body in the anatomic position Distal attachment (insertion) Point of attachment farthest from the midline or body “core”

9. Agonist Muscle or muscle group most directly related to performing a specific movement e.g., quadriceps are agonists for knee extension Antagonist Muscle or muscle group that can oppose the action of the agonist e.g., during elbow flexion, biceps are agonists and triceps are antagonists, passively elongating as the elbow is flexed

10. Co - contraction Occurs when agonist and antagonist muscles are simultaneously activated in an isometric fashion Stabilizer Muscle that “fixes” or holds a body segment relatively stationary so that another muscle can more effectively perform Synergists Muscles that work together to perform a particular action

11. Force-couple Synergistic action occurring when muscles produce force in different linear directions but produce torque in the same rotary direction Excursion Shortening and lengthening of a muscle Typically a muscle can only shorten or elongate about half of its resting length

12. Muscle belly Muscle body, composed of numerous fasciculi Epimysium Surrounds belly of the muscle; helps to hold muscle shape Fasciculus Bundle of muscle fibers Perimysium Surrounds and supports individual fasciculi; serves as a vehicle to support nerves and blood vessels

13. Muscle fiber An individual cell with multiple nuclei; contains all the contractile elements within muscle Endomysium Dense collagen fibril meshwork surrounding each muscle fiber; helps transfer contractile force to the tendon Myofibril Composes muscle fiber; contains contractile proteins, packaged within each sarcomere

14. Basic contractile unit of muscle fiber Composed of actin and myosin protein filaments Sliding filament hypothesis Actin filaments slide past the myosin filaments, resulting in contraction of an individual sarcomere

15. Created when myosin filaments containing numerous “heads”attach to thinner actin filaments Myosin head binds an actin filament, flexes, and produces a power stroke between the actin and myosin Actin filament slides past the myosin, generating force and shortening a sarcomere Simultaneous contraction of sarcomeres shortens entire muscle

16. Three factors determine functional potential of a muscle: Cross-sectional area Shape Line of pull Mosby items and derived items © 2009 by Mosby, Inc., an affiliate of Elsevier Inc.

17. Thickness of a muscle, an indirect measure of contractile elements available to generate force The larger a muscle’s cross-sectional area, the greater its force potential A person with larger muscles can usually generate larger muscular forces

18. Shape is one important indicator of a muscle’s specific action Most muscles are one of four shapes: Fusiform Triangular Rhomboidal Pennate

19. Fusiform muscles Fibers run parallel to one another Built to provide large ranges of motion e.g., biceps brachii Triangular muscles Expansive proximal attachments converging to a small distal attachment Provide a stabilized base for generating force e.g., gluteus medius

20. Rhomboidal muscles Expansive proximal and distal attachments Shaped like large rhomboids or off-set squares Suited to stabilize a joint or provide large forces, depending on cross-sectional area e.g., gluteus maximus

21. Resemble shape of a feather Muscle fibers approach a central tendon at an oblique angle Large force potential; limited excursion Further classified as uni-pennate, bi-pennate, or multi-pennate on the basis of number of fiber sets attached to central tendon

22. Muscle forces can be described as vectors because they possess both a direction and a magnitude Direction of a muscle’s force is referred to as line of pull (or line of force) e.g., a muscle’s line of pull that courses anterior to the medial-lateral axis of rotation of the shoulder performs flexion; coursing posterior performs extension

23. Degree to which muscle is either stretched or shortened at the time of its activation Significantly impacts force output of muscle Concept that muscle length strongly influences muscle force influences many clinical activities e.g., testing and strengthening of muscles

24. Active length-tension relationship Force generated by such a process is highly dependent on sarcomere length This relationship in a single sarcomere helps explain how the relative length of a whole muscle affects its force production A muscle’s active force is generally greatest at its midlength and least at both extremes

25. Passive length-tension relationship Because of its elasticity, a muscle also produces force passively Like a rubber band, a muscle generates greater internal elastic force when stretched Elastic behavior is demonstrated by a muscle’s passive length-tension curve

26. Mono-articular muscles cross one joint; multi-articular muscles cross multiple joints A multi-articular muscle can be elongated to a much greater extent than a mono-articular muscle The range in force output of a multi-articular muscle can be very large, much greater than a mono-articular muscle

27. Velocity of a muscular contraction can significantly affect force production During a concentric contraction, a muscle produces less force as the speed of contraction increases At higher speeds of contraction, actin-myosin cross bridges lack sufficient time to form—pull—and re-form; therefore force is decreased

28. Isometric activation creates greater force than any speed concentric contraction During an eccentric activation, force production increases slightly as the speed of the elongation increases Mosby items and derived items © 2009 by Mosby, Inc., an affiliate of Elsevier Inc.

29. Muscle held in a shortened position will shorten; muscle held in an elongated position will lengthen Disease, immobility, or simply poor posture often results in some degree of “adaptive” shortening Contracture is a muscle so tight that it severely restricts joint movement

30. Overly tight muscle causes associated joints to assume a posture mimicking the muscle’s primary actions—e.g, a tightened hamstring causes hip extension and knee flexion Generally, optimal stretching of a muscle requires the therapist to hold a limb in a position opposite to all its actions

31. Therapists often increase patients’ muscular strength, employing overload and specificity Overload principle Muscle must receive sufficient level of resistance to stimulate hypertrophy Training specificity Muscle adapts to the way in which it is challenged Mosby items and derived items © 2009 by Mosby, Inc., an affiliate of Elsevier Inc.

32. Although ligaments and capsules can stabilize joints, only muscle can adapt to the immediate and long-term external forces that can destabilize the body Many types of injuries such as ligamentous rupture can significantly destabilize a joint Physical therapists and physical therapist assistants often improve stability of a joint by strengthening the surrounding muscles

33. Force generated by muscle is the primary means of balancing stable posture and active movement Injuries or disease can impair muscular function, causing tightness, weakness, or postural instability Fundamental understanding of the nature of muscle can be extremely helpful in determining and advancing a particular course of treatment

34. Please read Chapter 4 in textbook prior to lecture on Tuesday 01/31/12 Quiz #2: Chapters 3 & 4 – Tuesday 01/31/12 