Skeletal Muscles and Functions Huei-Ming Chai, PT, Ph.D. School of Physical Therapy National Taiwan University Classification of Muscles striated muscles skeletal muscles: voluntary contraction cardiac muscles non-striated (smooth) muscles 1
Skeletal Muscle Statistics the most abundant tissue in the body, accounting for 40-45% BW > 430 skeletal muscles Most movements are completed by < 80 pairs of skeletal muscles Skeletal Muscle and Function Structure of skeletal muscle Muscle contraction Muscle coordination 2
Properties of Skeletal Muscle Muscle fiber extensibility elasticity contractility tendon or aponeurosis viscoelasticity non-contractility Organization of Skeletal Muscle epimysium muscle muscle fasciculus perimysium endomysium muscle fiber muscle fibril 3
Muscle Fiber a long cylindrical cell with hundreds of nuclei 10-100 m in diameter 1-30 cm in length Contractile component: myofabril Non-contractile component: endomyosium Organization of Muscle Fiber endomysium muscle fiber muscle fibril sarcomere actin filament cross section myosin filament 4
Contractile Component -- Sarcomere actin myosin crossbridge I band A band H band Z line Types of Muscle Fibers slow twitch fiber (Type I) red in color slow to peak when contracted fatigue resistant fast twitch fiber (Type IIA) white in color fast to peak when contracted easy fatigue intermediate (Type IIB) Smith, p.88 5
Fiber Architecture parallel fiber arrangement: parallel to the longitudinal axis of the muscle longitudinal: sartorius quadrate or quadralateral: rhomboid triangular or fan-shaped: pectoralis major fusiform or spindle-shaped: biceps brachii pennate fiber arrangement: at an angle to the longitudinal axis of the muscle, unipenniform: extnesor digitorum longous bipenniform: flexor hallucis longus multipenniform: middle deltoid Effect of Pennation pennation angle effective force transmitted to the tendon tension in the muscle fibers pennation angle pennate arrangement: to allow packing of more fibers given the same space. 6
Skeletal Muscle and Function Structure of skeletal muscle Muscle contraction Muscle coordination Functions of Skeletal Muscle To move the body limb by creating motion To provide strength by generating active force To protect joints by absorbing shock specific functions of connective tissues within muscle To provide gross structure to muscle To generate passive tension against stretch To transmit force to the bone and across the joint 7
Sliding Filament Mechanism AF Huxley & HE Huxley, 1964 active shortening of sacromere, resulting from the relative movement of actin and myosin filaments with retaining its original length force of contraction is developed by the crossbridges of myosin Movement of Cross Bridges Lengths of myosin and actin keep the same shortening 8
Types Based on Changes in Length resting concentric (shortening) isometric (static) eccentric (lengthening) Abdominal Muscle Contraction concentric contraction to create trunk flexion (resisting gravity) eccentric contraction to control trunk extension (checking gravity) 9
Triceps Brachii Action downward motion elbow flexion elbow extensors (antagonist) eccentric contraction upward motion elbow extension elbow extensors (agonist) concentric contraction Abdominal Muscle Actions concentric eccentric direction of motion muscle length concentric gravity-resisted shortening eccentric gravity-assisted lengthening 10
Elbow Flexion at 90 of Shoulder ABD elbow flexor, eccentric elbow extensor, concentric elbow flexor, concentric elbow extensor, eccentric Quadriceps Actions open kinematic chain closed kinematic chain motion? gravity? muscle contraction? muscle length? 11
What if. shoulder extensor concentric contraction to create force Shoulder Extension 12
Types Based on Tension Development isotonic contraction isometric contraction isokinetic contraction Isotonic Contraction iso = equal; tonus = tension defined by muscle physiologists as a kind of muscle contraction that develops constant tension throughout the whole muscle excursion as isotonic contraction seldom seen in the living body clinically refer to a muscle contraction that causes a joint to move through some range of motion Even though the resistance remains the same, the tension generated by the muscle is not equal tension because moment arm to the joint axis changing throughout the motion resistance with respect to the gravity changing throughout the motion 13
quadriceps action calf action shoulder flexor elbow flexor Slight Squatting Isokinetic Contraction iso = equal; kinetos = move first introduced by Hislop and Perrine in 1967 definition: one kind of muscle contraction that occurs when the rate of movement is constant not occur in the living body without using special machine (isokinetic dynamometer) equal motion speed with accommodating resistance 14
amplitude Isokinetic Testing Isokinetic Testing System Cybex: torque Kin-Com: force ext. angle flex. Cybex: dynamometer Kin-Com: load cell 15
Isokinetic Contraction dynamometer M joint moment Isokinetic Isotonic F mg joint angle Comparison of Muscle Contraction isotonic contraction varying tension varying length varying speed Isometric contraction varying tension equal length zero speed isokinetic contraction accommodating resistance (various tension) varying length equal speed 16
tension Tension Developed by A Single Muscle Fiber Contraction active tension resting length length Mechanical Model of Musculotendinous Unit Keele, Neil, Joels, 1982 parallel elastic component contractile component series elastic component 17
force tension Length-Tension Curve -- maximum isometric contraction total tension active tension passive tension resting length length Force-Velocity Curve Hill s model eccentric concentric isometric 0 contraction velocity 18
Skeletal Muscle and Function Structure of skeletal muscle Muscle contraction Muscle coordination Muscle Activities During Motion focal muscle agonist (prime mover) antagonist synergist stabilizer neutralizer postural muscle anticipatory postural adjustment (APA) 19
Agonist the principal muscle that produces a joint motion or maintains a static posture can be concentric, isometric, or eccentric 拮抗肌 Antagonist the muscle that contracts in the opposite direction of the agonist passively elongates or shortens to allow motion acted by agonist 20
Synergist Syn = together; ergon = work the muscle that contracts together with the agonist stabilizer: to stabilize the proximal component of the joint involved neutralizer: to rule out unwanted motions Stabilizer teres minor scapular muscles stabilize the scapula deltoid can elevate the arm teres minor can rotate the arm externally 21
Neutralizer upper trapezius scapula adduction neutralize lower trapezius Cocontraction co-contraction: agonist and antagonist contract simultaneously co-contraction joint approximation 22
Single- vs. Multi-Joint Muscle single-joint muscle: a muscle that cross one joint only, e.g. the brachialis, the short head of the biceps brachii two-joint muscle: a muscle that cross two joints, e.g. the long-head of the biceps brachii, the grastrocnemius, etc. multi-joint muscle: a muscle that cross more than one joint e.g. the long finger flexors, the long finger extensors, etc. Action of Two-Joint Muscle active insufficiency unable to reach the contraction force because of the limit of muscle length e.g. make a fist as wrist extended vs. that as wrist flexed passive insufficiency unable to reach full range of motion because of the limit of muscle length e.g. open the hand as wrist extended vs. that wrist flexed 23
Active Insufficiency unable to reach the contraction force because of the limit of muscle length examples: Making a fist with the wrist extended is stronger than that with the wrist flexed the strength of the elbow flexor decreases as the shoulder joint is more flexed Mechanism of Active Insufficiency the contractile tension of the agonist is markedly weak when a multi-joint muscle is attempt to contract at a shortened position 24
Passive Insufficiency unable to reach full range of motion because of the limit of muscle length examples: automatically open the hand as wrist flexed difficult to reach the toes with the knee extended as compared to that with the knee flexed Even though the agonist may contract strongly, motion may be limited because of the lack of excursion of the antagonist Practice of Two Joint Muscles biceps brachii hamstring gastrocnemius 25