BIOMECHANICS of HUMAN SKELETAL MUSCLE ENT 214 Biomechanics BIOMECHANICS of HUMAN SKELETAL MUSCLE Mohd Yusof Baharuddin & Assoc. Prof. Dr. Mohammad Iqbal Omar (Biomedical Electronic Engineering Program)

Introduction Muscular system consists of three muscle types: cardiac, smooth, and skeletal Muscle types: cardiac muscle: composes the heart smooth muscle: lines hollow internal organs skeletal (striated or voluntary) muscle: attached to skeleton via tendon & movement

Skeletal muscle most abundant tissue in the human body (40-45% of total body weight) Human body has more than 430 pairs of skeletal muscle; Most vigorous movement produced by 80 pairs Skeletal muscles provide strength and protection for the skeleton, enable bones to move, provide the maintenance of body posture against gravity Skeletal muscles perform both dynamic and static work

Lecture outlines: Composition & structure of skeletal muscle Mechanics of Muscle Contraction Force production in muscle Muscle remodeling Summary

I. Structural Organization of Skeletal Muscle What is a muscle fiber? Muscle fiber = muscle cell Because of threadlike shape single muscle cell surrounded by a membrane called the sarcolemma and containing specialized cytoplasm called sarcoplasm

Muscle Structure- contd Structural unit of skeletal muscle is the multinucleated muscle cell or fiber (thickness: 10-100 m, length: 1-30 cm Muscle fibers consist of myofibrils (sarcomeres in series: basic contractile unit of muscle) Myofibrils consist of myofilaments (actin and myosin)

Connective Tissue Wrappings of Skeletal Muscle Figure 6.1

Microscopic & Macroscopic Structure of Skeletal Muscle

Microscopic Anatomy of Skeletal Muscle Figure 6.3a

Microscopic Anatomy of Skeletal Muscle Myofibrils are aligned to give distinct bands I band = light band Contains only thin filaments A band = dark band Contains the entire length of the thick filaments

Microscopic Anatomy of Skeletal Muscle Figure 6.3b

Bands of myofibrils A bands: thick filaments in central of sarcomere I band A bands: thick filaments in central of sarcomere Z line: short elements that links thin filaments I bands: thin filaments not overlap with thick filaments H zone: gap between ends of thin filaments in center of A band M line: transverse & longitudinally oriented linking proteins for adjacent thick filaments M Z Z sarcomere

Microscopic Anatomy of Skeletal Muscle Sarcomere—contractile unit of a muscle fiber Organization of the sarcomere Myofilaments Thick filaments = myosin filaments Thin filaments = actin filaments

Muscle Structure (continued) Composition of sarcomere Z line to Z line ( 1.27-3.6 m in length) Thin filaments (actin: 5 nm in diameter) Thick filaments (myosin: 15 nm in diameter) Myofilaments in parallel with sarcomere Sarcomeres in series within myofibrils

Molecular basis of muscle contraction Sliding filament theory: relative movement of actin & myosin filaments yields active sarcomere shortening Myosin heads or cross-bridges generate contraction force Sliding of actin filaments toward center of sarcomere: decrease in I band and decrease in H zone as Z lines move closer

ATP ATP

Muscle Structure (continued) Motor unit Functional unit of muscle contraction Composed of motor neuron and all muscle cells (fibers) innervated by motor neuron Follows “all-or-none” principle – impulse from motor neuron will cause contraction in all muscle fibers it innervates or none

Muscle Tissue Behavioral Properties Extensibility Elasticity Irritability Ability to develop tension

Extensibility & Elasticity What is extensibility? Ability to stretch or to increase in length How about elasticity? Ability to return to normal length after stretch Figure from http://www.flashplayer.com/forum/showthread.php?t=20485

Muscle Tissue Behavioral Properties Irritability or Excitability (also called responsiveness) ability to receive and respond to a stimulus Not necessarily a contraction Ability to develop tension – Contractility ability to shorten when an adequate stimulus is received

Elastic Behaviour Parallel elastic component (PEC) Muscle membrane (epimysium, perimysium, endomysium, sarcolemma) Supply resistance when muscle is passively stretched Series elastic component (SEC) Tendons Act as spring to store elastic energy Contractile component (CC) Muscle property enabling tension by stimulated muscle fibers

Muscle Elastic Behavior Model

Structural Organization of Skeletal Muscle Fast twitch (FT) fibers both reach peak tension and relax more quickly than slow twitch (ST) fibers. Peak tension is typically greater for FT than for ST fibers. Twitch Tension Time FT ST

Size Principle Smallest motor units recruited first Smallest motor units recruited with lower stimulation frequencies Smallest motor units with relatively low levels of tension provide for finer control of movement Larger motor units recruited later with increased frequency of stimulation and increased need for greater tension

Motor unit considered the functional unit of the neuromuscular system

Smallest MU recruited at lowest stimulation frequency As frequency of stimulation of smallest MU increases, force of its contraction increases As frequency of stimulation continues to increase, but not before maximum contraction of smallest MU, another MU will be recruited

Size Principle- contd. Tension is reduced by the reverse process Successive reduction of firing rates Dropping out of larger units first

Muscle Structure (continued) Motor unit Vary in ratio of muscle fibers/motor neuron Fine control – few fibers (e.g., muscles of eye and fingers, as few as 3-6/motor neuron), tetanize at higher frequencies Gross control – many fibers (e.g., gastrocnemius,  2000/motor neuron), tetanize at lower frequencies Fibers of motor unit dispersed throughout muscle

Motor Unit Tonic units – smaller, slow twitch, rich in mitochondria, highly capillarized, high aerobic metabolism, low peak tension, long time to peak (60-120ms) Phasic units – larger, fast twitch, poorly capillarized, rely on anaerobic metabolism, high peak tension, short time to peak (10-50ms)

Muscle Structure (continued) Motor unit (continued) Weakest voluntary contraction is a twitch (single contraction of a motor unit) Twitch times for tension to reach maximum varies by muscle and person Twitch times for maximum tension are shorter in the upper extremity muscles (≈40-50ms) than in the lower extremity muscles (≈70-80ms)

Muscle Differentiation (types of fibers) I (slow-twitch oxidative) IIA (fast-twitch oxidative glycolytic) IIB fast-twitch glycolytic Contraction speed Slow fast Myosin-ATPase activity Low High Primary source of ATP production Oxidative phosphorylation Anaerobic glycolysis Glycolytic enzyme activity Intermediate No. of mitochondria Many Few Capillaries Myoglobin contents Muscle Color Red White Glycogen content Fiber diameter small Large Rate of fatigue slow Fast

Contraction of Skeletal Muscle Graded responses can be produced by changing The frequency of muscle stimulation The number of muscle cells being stimulated at one time

Types of Graded Responses Twitch Single, brief contraction Not a normal muscle function

Types of Graded Responses Figure 6.9a

Types of Graded Responses Tetanus (summing of contractions) One contraction is immediately followed by another The muscle does not completely return to a resting state The effects are added

Types of Graded Responses Figure 6.9b

Types of Graded Responses Unfused (incomplete) tetanus Some relaxation occurs between contractions The results are summed

Types of Graded Responses Figure 6.9c

Types of Graded Responses Fused (complete) tetanus No evidence of relaxation before the following contractions The result is a sustained muscle contraction

Types of Graded Responses Figure 6.9d

Muscle Response to Strong Stimuli Muscle force depends upon the number of fibers stimulated More fibers contracting results in greater muscle tension Muscles can continue to contract unless they run out of energy

II. Mechanics of Muscle Contraction Neural stimulation – impulse Mechanical response of a motor unit - twitch T: twitch or contraction time, time for tension to reach maximum F0: constant of a given motor unit Averaged T values Tricep brachii 44.5 ms Soleus 74.0 ms Biceps brachii 52.0 ms Medial Gastrocnemius 79.0 ms Tibialis anterior 58.0 ms

Generation of muscle tetanus 100Hz 10 Hz Note: muscle is controlled by frequency modulation from neural input very important in functional electrical stimulation

Motor unit recruitment All-or-nothing event 2 ways to increase tension: - Stimulation rate Recruitment of more motor unit Size principle Smallest m.u. recruited first Largest m.u. last

Muscle fatigue Drop in tension followed prolonged stimulation. Fatigue occurs when the stimulation frequency outstrips rate of replacement of ATP, the twitch force decreases with time

III. Force production in muscle Force production depends on length velocity muscle composition & morphology

Terms used to describe muscle contractions concentric: involving shortening eccentric: involving lengthening isometric: involving no change isokinetic: no change in the resulting muscle length occurs

Types of contraction

Types of Muscle Contraction Type of Contraction Definition Work Concentric Force of muscle contraction  resistance Positive work; muscle moment and angular velocity of joint in same direction Eccentric Force of muscle contraction  resistance Negative work; muscle moment and angular velocity of joint in opposite direction Isokinetic Force of muscle contraction = resistance; constant angular velocity; special case is isometric contraction Isometric Force of muscle contraction  resistance; series elastic component stretch = shortening of contractile element (few to 7% of resting length of muscle) No mechanical work; physiological work

Force Production - Load - Velocity Relationship Concentric contraction (muscle shortening) occurs when the force of contraction is greater than the resistance (positive work) Velocity of concentric contraction inversely related to difference between force of contraction and external load Zero velocity occurs (no change in muscle length) when force of contraction equals resistance (no mechanical work)

Fiber architecture There are 2 types of fiber architecture parallel fiber arrangement fibers are roughly parallel to the longitudinal axis of the muscle example: sartorius, rectus abdominis, biseps brachii pennate fiber arrangement short fibers attach to one or more tendons within the muscle example: tibialis posterior, rectus femoris, deltoid

Fiber architecture (cont’d) parallel fiber arrangement Muscle become shorten due to fiber shortening pennate fiber arrangement When muscle shorten, they rotate about their tendon attachment This will increase the angle of pennation Greater angle of pennation will induced smaller amount of effective force

Effect of Muscle Architecture on Contraction Fusiform muscle Fibers parallel to long axis of muscle Many sarcomeres make up long myofibrils Advantage for length of contraction Example: sartorius muscle Force of contraction along long axis of muscle   of force of contraction of all muscle fibers Tends to have smaller physiological cross sectional area

Fusiform Fiber Arrangement Fa = force of contraction of muscle fiber parallel to longitudinal axis of muscle Fa = sum of all muscle fiber contractions parallel to long axis of muscle Fa

Effect of Muscle Architecture on Contraction (continued) Pennate muscle Fibers arranged obliquely to long axis of muscle (pennation angle) Uni-, bi-, and multi-pennate Advantage for force of contraction Example: rectus femoris (bi-pennate) Tends to have larger physiological cross sectional area

Pennate Fiber Arrangement Fa = force of contraction of muscle fiber parallel to longitudinal axis of muscle Fm = force of contraction of muscle fiber  = pennation angle Fa = (cos )(Fm) Fa = sum of all muscle fiber contractions parallel to long axis of muscle Fa Fm 

Effect of Muscle Architecture on Contraction (continued) Force of muscle contraction proportional to physiological cross sectional area (PCSA); sum of the cross sectional area of myofibrils Velocity and excursion (working range or amplitude) of muscle is proportional to length of myofiblril

Muscle Fiber Types (continued) Smaller slow twitch motor units are characterized as tonic units, red in appearance, smaller muscle fibers, fibers rich in mitochondria, highly capillarized, high capacity for aerobic metabolism, and produce low peak tension in a long time to peak (60-120ms).  Larger fast twitch motor units are characterized as phasic units, white in appearance, larger muscle fibers, less mitochondria, poorly capillarized, rely on anaerobic metabolism, and produce large peak tensions in shorter periods of time (10-50ms).

Muscle Fiber Types (continued) Nerve innervating muscle fiber determines its type; possible to change fiber type by changing innervations of fiber All fibers of motor unit are of same type Fiber type distribution in muscle genetically determined Average population distribution: 50-55% type I 30-35% type IIA 15% type IIB

Histology of muscle Type IIA Type IIB Type I Eye muscle (Rectus lateralis); Myofibrillar ATPase stain, PH 4.3

Muscle Fiber Types (continued) Fiber composition of muscle relates to function (e.g., soleus – posture muscle, high percentage type I) Muscles mixed in fiber type composition Natural selection of athletes at top levels of competition

Electrical Signals of Muscle Fibers At rest, action potential of muscle fiber  -90 mV; caused by concentrations of ions outside and inside fiber (resting state) With sufficient stimulation, potential inside cell raised to  30-40 mV (depolarization); associated with transverse tubular system and sarcoplasmic reticulum; causes contraction of fiber Return to resting state (repolarization) Electrical signals from the motor units (motor unit action potential, muap) can be recorded (EMG) via electrodes

Muscles Roles Agonist: acts to cause a movement Antagonist: acts to slow or stop a movement Stabilizer: acts to stabilize a body part against some other force Neutralizer: acts to eliminate an unwanted action produced by an agonist

Agonist Agonist – a muscle or group of muscles that causes the motion Without this muscle (or group) the motion is o longer considered functional The muscle contracts isotonically to produce a motion or isometrically to maintain a position The isotonic contraction would be concentric since motion results from the shortening of the muscle Example: The quadriceps, through concentric isotonic contraction, is the primary muscle-----responsible for knee extension

Antagonist performs the opposite motion of the prime mover It contracts eccentrically or "relaxes" and lengthens to prevent, slow or control a motion Examples: the quadriceps contracts eccentrically to slow down knee flexion, the motion opposite to its prime mover motion of knee extension. This is seen in slow squats. Since motion is occurring, this example would be of an isotonic contraction. Antagonistic muscle contractions may, however, be isometric With elbow extension, the triceps would be the agonist and the biceps would be the antagonist. With elbow flexion, the biceps would be the agonist and the triceps would be the antagonist

Stabilizer the muscle may contract to hold a body part immobile while another body part is moving. The sustained stabilizing contraction is frequently isometric. In most normal activities, proximal joints are stabilized by muscle contractions during movement of more distal joints - proximal stabilization. For an isolated movement at one joint to occur, the muscles that control the joints proximal to that joint must stabilize the proximal joints so that no motion occurs there. The antagonists for each motion at the proximal joint co-contract or contract against each other to prevent motion. Example: the quadriceps may stabilize the knee in an extended position of permit plantar flexion of the ankle as when the individual rises to tip-toe in erect position To perform isolated elbow flexion the proximal shoulder joint must be stabilized by flexors/extensors, abductors/adductors and internal/external rotators.

Neutralizer prevents unwanted motion so muscle can perform a specific motion can occur. Mostly dependant on the angle of pull. Examples: the biceps can flex the elbow and supinate the forearm. If only elbow flexion is wanted, the supination component must be ruled out. Therefore, the pronator teres, which pronates the forearm, would contract to counteract the supination component of the biceps, and only elbow flexion would occur. Neutralizers act to cancel out an unwanted movement

Neutralizer (2) With wrist ulnar deviation the flexor carpi ulnaris will cause flexion and lunar deviation of the wrist. The extensor carpi ulnaris will cause extension and ulnar deviation. If ulnar deviation is desired, these muscles would contract doing two things: they would neutralize each other's flexion/extension component, and they would act as agonists in wrist ulnar deviation Stabilizers and Neutralizers both use co-contraction to prevent motion and have an relationship. Stabilizers are associated with joints; Neutralizers are associated with muscle.

Two Joint and Multijoint Muscles active insufficiency: failure to produce force when slack passive insufficiency: restriction of joint range of motion when fully stretched

Active Insufficiency Failure to produce force when muscles are slack (decreased ability to form a fist with the wrist in flexion)

Passive insufficiency Restriction of joint range of motion when muscles are fully stretched (decreased ROM for wrist extension with the fingers extended)

Factors Affecting Muscular Force Generation Velocity Force (Low resistance, high contraction velocity) The force-velocity relationship for muscle tissue: When resistance (force) is negligible, muscle contracts with maximal velocity.

Force – Velocity Relationship isometric maximum The force-velocity relationship for muscle tissue: As the load increases, concentric contraction velocity slows to zero at isometric maximum.

Length – Tension Relationship Length (% of resting length) 50 100 150 Active Tension Passive Tension Total Tension Tension Length (% of resting length) 50 100 150 Active Tension Passive Tension Total Tension The length-tension relationship: Tension present in a stretched muscle is the sum of the active tension provided by the muscle fibers and the passive tension provided by the tendons and membranes.

Force – Time Relationship What is electromechanical delay (EMD)? Myoelectric activity Force Stimulus Electromechanical delay EMD is the time between arrival of a neural stimulus and tension development by the muscle Time needed for muscles reached maximum isometric is 0.2 - 1 second after EMD

Muscular Strength the amount of torque a muscle group can generate at a joint

How do we measure muscular strength? Ft The component of muscle force that produces torque (Ft) at the joint is directed perpendicular to the attached bone.

Sample Problem 2 How much tork is produced at the elbow by biceps brachii inserting at an angle 60 degree on the radius when tension in the muscle is 400 N? α

Solution Find Fp which is perpendicular to bone Calculate tork Tm = Fp x d

Factors affect muscular strength tension-generating capability of the muscle tissue, which is in turn affected by: muscle cross-sectional area training state of muscle

Factors affect muscular strength moment arms of the muscles crossing the joint (mechanical advantage), in turn affected by: distance between muscle attachment to bone and joint center angle of the muscle’s attachment to bone

Effects for differences angle B C The mechanical advantage of the biceps bracchi is maximum when the elbow is at approximately 90 degrees (A), because 100% of muscle force is acting to rotate the radius. As the joint angle increases (B) or decreases (C) from 90 degrees, the mechanical advantage of the muscle is lessened because more and more of the force is pulling the radius toward or away from the elbow rather than contributing to forearm rotation.

Muscle Power the product of muscular force and the velocity of muscle shortening the rate of torque production at a joint the product of net torque and angular velocity at a joint

Muscular Power Force Velocity Power Power-Velocity Force-Velocity The general shapes of the force-velocity and power-velocity curves for skeletal muscle.

Muscular Endurance the ability of muscle to exert tension over a period of time the opposite of muscle fatigability

Effect of muscle temperature (Warm up) the speeds of nerve and muscle functions increase

Effect of muscle temperature (Warm up) cont’d With warm-up, there is a shift to the right in the force-velocity curve, with higher maximum isometric tension and higher maximum velocity of shortening possible at a given load. Normal body temperature Elevated body temperature Force Velocity

IV. Muscle Remodeling Effects of Disuse and Immobilization Immediate or early motion may prevent muscle atrophy after injury or surgery Muscle fibers regenerate in more parallel orientation, capillariaztion occurred rapidly, tensile strength returned more quickly Atrophy of quadriceps developed due to immobilization can not be reversed by isometric exercises. Type I fibers atrophy with immobilization; cross-sectional area decreases & oxidative enzyme activity reduced Tension in muscle afferent impulses from intrafusal muscle spindle increases & leading to increase stimulation of type I fiber

Effects of Physical Training Increase cross-sectional area of muscle fibers, muscle bulk & strength Relative percentage of fiber types also changes In endurance athletes % type I, IIA increase Stretch out of muscle-tendon complex increases elasticity & length of musculo-tendon unit; store more energy in viscoelastic & contractile components Roles of muscle spindle & Golgi tendon organs: inhibition of spindle effect & enhance Golgi effect to relax muscle and promote further lengthening.

Summary Basic behavioral properties of the muscle unit Relationships of fiber types and architecture to muscle function Skeletal muscles function to produce coordinated movement of the human body Effects of the force-velocity and length-tension relationships and EMD on muscle function Muscular strength, power, and endurance

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