Muscular System

Muscle fibers: elongated, multinucleated cells that make up skeletal muscle tissue, responsible for generating force and facilitating movement through contraction

1. Fascicles: bundles of bound muscle fibers that combine with connective tissue to form a muscle

• Connective tissue surrounding fascicles blend into a muscle's tendon

2. Myofibrils: smaller, individual bundles that make up muscle fibers

   • Sarcomeres: functional, longitudinal divisions of myofibrils capable of changing length or remaining constant when contracting

     • Z-lines: longitudinal divisions separating sarcomeres

     • Actin: thin, contractile protein filaments surrounding myosin filaments and attaches to z-lines

     • Myosin: thick contractile protein filaments surrounded by actin

       • Myosin heads project that contact and bind temporarily to actin filaments when a muscle is stimulated

       • When at rest, actin filaments do not meet at the center of the sarcomere

       • Sarcomere is shortened when muscle contracts, by actin filaments coming together

Sliding Filament Theory: describes the interaction between actin & myosin and the production of force during muscle contraction

2. • During a concentric, shortening contraction, myosin exerts a pulling force that slides the actin filaments closer together to reduce the gap between them at the sarcomere's center

     • z-lines draw closer together as actin filaments come closer --> muscle shortening

   • During an eccentric, lengthening contraction, a pulling force exerted by myosin on actin is overcome by imposition of an external force

     • Forces actin-myosin bonds to pull apart, causing the sarcomeres, and muscle to lengthen

     • Actin-myosin bonds are still generating force but the external force is greater and thus overcomes the internal sarcomere force

   • The # of cross bridges formed determines the amount of force generated

   • The initial length of a muscle determines the # of cross-bridges formed

   • Resting length (optimal length): the length of a muscle when in a condition without stimulation

     • The max # of cross-bridges can be formed in response to stimulation

     • When lengthened beyond normal range, the myosin heads can't reach the binding site on actin = decrease # of cross bridges = decreased force-generating capability

     • When shortened from resting length, actin overlaps and myosin heads can't find new actin to bind with = decreased force-generating capability

1. Strap: long & thin, consistent width entirely

   • Sartorius, rectus abdominus, sternocleidomastoid

2. Fusiform: wider in the middle of the muscle and taper to tendons at each end

   • Biceps brachii, brachialis, brachioradialis

3. Quadrilateral: four-sided, usually flat, with broad attachments at each end

4. Triangular: flat & fan-shaped with fibers radiating from narrow attachment at one end to broad at the other

   • Pectoralis major, trapezius

5. Unipennate: one side of a common feather with a series of short fibers attached to an oblique angle to its tendon

   • Tibialis posterior, semimembranosus, flexor pollicis longus

6. Bipennate: common feather with short fibers attached to an oblique angle along both sides of a central tendon

   • Rectus femoris, Doral interossei

7. Multipennate: common feather with short fibers attached to an oblique angle along both sides of multiple tendons

   • Deltoids, subscapularis

• Provide the forces necessary to stabilize or move joints

1. Agonist: muscle role where contractions produce the required movement for performance of a desired activity

   • Size, line of pull, length of lever arms, and contractile potential determinants

2. Antagonist: muscle role that performs the opposite movement to the agonist muscles

   • Typically, one role is inactive while the other is active

3. Co-contraction: when agonist muscles & antagonist counterparts contract simultaneously to prevent joint movement, and providing joint stability

4. Synergist: term to describe two or more muscles that work in combination to produce a desired movement that cannot be performed by a single muscle

   • Ex. Wrist ulnar deviation requires the flexor capri ulnaris and extensor carpi ulnaris to work in synergy

   • Helps stabilize a body segment while allowing a desired movement ot occur

     • Ex. Push up - triceps brachii is the agonist to produce elbow extension while the shoulder girdle musculature stabilizes the scapula, to permit the brachii muscle to produce elbow extension without scapular movement

1. Isotonic - concentric & eccentric contraction types

2. Isokinetic: contractions occurring at a constant velocity

3. Isometric: occurs when a muscle contracts, producing force, but without a change in joint position

   • Small but insignificant change in muscle length

4. Concentric: (shortening contraction) muscle attachments move closer together

   • Movement occurs against gravity (overcome gravity)

5. Eccentric: (lengthening contraction) muscle attachments move farther apart

   • Movement occurs with gravity (controlling the effect of gravity)

1. Tension: refers to increasing force within a muscle as a result of muscle contraction or application of external load

   1. Passive tension: stretching a muscle involving non-contractile tissues

   2. Active tension: created by contractile muscle

2. Tone: tension present in muscle at all times, even when the muscle is resting; state of readiness that permits muscle to respond quickly

3. Muscle is capable of shortening to 1/2 and lengthened 1.5 times its normal length

4. Excursion: the distance from maximum lengthening to maximum shortening

Insufficiency

1. • Active insufficiency: when the muscle is contracted too much and is not able to generate enough force to move through its full ROM; muscle is too short (in shortened position) to produce maximum tension

     • Occurs when the muscle is contracted too far (unable to generate full force)

     • Hamstring - hip extension & knee flexion simultaneously but not to the full ROM available at each joint individually

       • If hip is limited, knee flexion can increase ROM; if knee is limited, hip can increase ROM

   • Passive insufficiency:  when a muscle is stretched too far and its opposing muscle becomes too tight to allow full ROM at the joint (eg. hip flexion and knee extension = hamstring cannot stretch any further

     • Occurs when the muscle is stretched too far (unable to elongate fully)

Adaptive Changes

1. Adaptive lengthening: result of prolonged stretch in which actin filaments and myosin heads are too far apart to form an optimal number of bonds

1. • • Tissue responds to stretch by adding sarcomere in muscle fibers

     • Additional sarcomeres does no increase strength

2. Adaptive shortening: result of prolonged shortening that accompanies prolonged lengthening of tissues on the opposite side of the affected joints

1. • • Muscles decrease the # of sarcomeres

     • Increased overlap of actin filament and myosin heads to decrease the cross-bridges generated & decreases the ability to generate force during contraction

     • Contracture: permanent adaptive shortening

Tenodesis: closing of fingers through tendon action rather than muscle contraction

1. • Degrees of opening/closing of the hand can be achieved using passive insufficiency

   • Fingers flex passively as a result of passive insufficiency of finger flexor muscles

   • Permits functional grasp and release of light objects

Stretching: movement/exercises designed to lengthen muscle(s) & tissues spanning a joint beyond current extensibility

Trigger points: hyperirritable points that are painful, existing within a tight band of muscle tissue and refer pain to other areas

• Often found in overworked muscles