chapter 03 week 2 lecture 2

Week 2: Lecture 2 Elaine Wilson, PT

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Describe concentric, eccentric, and isometric activation of muscle

Identify the anatomic components that comprise a whole muscle

Describe the sliding filament theoryDescribe 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

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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

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Structure and Function of Skeletal Muscle

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

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Mosby items and derived items © 2009 by Mosby, Inc., an affiliate of Elsevier Inc.

Concentric (shortening or contracting)o Muscle produces active force and

simultaneously shortens Eccentric (attempting to resist elongation)

o Muscle attempts to contract but is pulled to a longer length by a dominant external force

Isometric (remaining at a constant length)o Muscle generates active force while

remaining at a constant length

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Relative points of muscle to bone attachment

Proximal attachment (origin) o Point of attachment closest to the

midline or “core” of the body in the anatomic position

Distal attachment (insertion)o Point of attachment farthest from the

midline or body “core”

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Agonist o Muscle or muscle group most directly related

to performing a specific movement• e.g., quadriceps are agonists for knee

extension Antagonist

o 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

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Co-contraction o Occurs when agonist and antagonist

muscles are simultaneously activated in an isometric fashion

Stabilizer o Muscle that “fixes” or holds a body

segment relatively stationary so that another muscle can more effectively perform

Synergists o Muscles that work together to perform a

particular action10

Force-coupleo Synergistic action occurring when

muscles produce force in different linear directions but produce torque in the same rotary direction

Excursion o Shortening and lengthening of a muscleo Typically a muscle can only shorten or

elongate about half of its resting length

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Muscle bellyo Muscle body, composed of numerous fasciculio Epimysium • Surrounds belly of the muscle; helps to hold

muscle shape Fasciculus

o Bundle of muscle fiberso Perimysium • Surrounds and supports individual fasciculi;

serves as a vehicle to support nerves and blood vessels

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Muscle fibero An individual cell with multiple nuclei;

contains all the contractile elements within muscle

Endomysium o Dense collagen fibril meshwork surrounding

each muscle fiber; helps transfer contractile force to the tendon

Myofibrilo Composes muscle fiber; contains contractile

proteins, packaged within each sarcomere 13

Basic contractile unit of muscle fiber Composed of actin and myosin

protein filaments Sliding filament hypothesisoActin filaments slide past the myosin filaments, resulting in contraction of an individual sarcomere

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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

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Three factors determine functional potential of a muscle:oCross-sectional areaoShapeoLine of pull

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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 oA person with larger muscles can usually generate larger muscular forces

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Shape is one important indicator of a muscle’s specific action

Most muscles are one of four shapes:oFusiformoTriangularoRhomboidaloPennate

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Fusiform muscleso Fibers run parallel to one anothero Built to provide large ranges of motiono e.g., biceps brachii

Triangular muscleso Expansive proximal attachments

converging to a small distal attachmento Provide a stabilized base for generating

forceo e.g., gluteus medius

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Rhomboidal muscleso Expansive proximal and distal

attachments o Shaped like large rhomboids or off-

set squares o Suited to stabilize a joint or provide

large forces, depending on cross-sectional area

o e.g., gluteus maximus 20

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

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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

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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 activitieso e.g., testing and strengthening of

muscles23

Active length-tension relationshipo Force generated by such a process is

highly dependent on sarcomere length o This relationship in a single sarcomere

helps explain how the relative length of a whole muscle affects its force production

oA muscle’s active force is generally greatest at its midlength and least at both extremes

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Passive length-tension relationshipoBecause of its elasticity, a muscle

also produces force passivelyo Like a rubber band, a muscle

generates greater internal elastic force when stretched

o Elastic behavior is demonstrated by a muscle’s passive length-tension curve

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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

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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

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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

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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

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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

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Therapists often increase patients’ muscular strength, employing overload and specificity

Overload principleoMuscle must receive sufficient level of

resistance to stimulate hypertrophy Training specificity

oMuscle adapts to the way in which it is challenged

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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

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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

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Please read Chapter 4 in textbook prior to lecture on Tuesday 01/31/12

Quiz #2: Chapters 3 & 4 – Tuesday 01/31/12

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chapter 03 week 2 lecture 2

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