BIOMECHANICS of HUMAN SKELETAL MUSCLE ENT 214 Biomechanics BIOMECHANICS of HUMAN SKELETAL MUSCLE Mohd Yusof Baharuddin BBiomedEng(UM) MBiomedEng(Melbourne) myusof@unimap.edu.my Picture from MisterBisson Flickr pages

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

Elastic Behaviour Parallel elastic component (PEC) Muscle membrane 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

Muscle Tissue Behavioral Properties Irritability ability to respond to a stimulus Ability to develop tension the contractile component of muscle function

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

Skeletal Muscle Silverthorn, Human Physiology, Benjamin-Cummings, ISBN 0- 321-22687-9, Fig. 12-3, p. 392

Skeletal muscle Silverthorn, Human Physiology, Benjamin-Cummings, ISBN 0- 321-22687-9, Fig. 12-1a, p. 390

Image taken from http://courses. cm. utexas

Muscle fibers some fibers run the entire length of a muscle; others are shorter skeletal muscle fibers grow in both length and diameter from birth through adulthood fiber diameter can be increased through resistance training

Muscle Fiber (cont’d)

Muscle Fiber (cont’d)

Muscle Fiber (cont’d)

Myosin & Actin

Motor unit single motor neuron and all fibers it innervates considered the functional unit of the neuromuscular system There are 2 types Fast twitch (FT) Slow twitch (ST)

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

Skeletal Muscle Fiber Characteristics TYPE IIA Type I Fast-Twitch Type IIB Slow-Twitch Oxidative Fast-Twitch Oxidative Glycolytic Glycolytic CHARACTERISTIC (SO) (FOG) (FG) Contraction Speed slow fast Fatigue rate intermediate Diameter small large ATPase concentration low high Mitochondrial concentration Glycolytic enzyme

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

Sample Problem 1 How much force is exerted by tendon of pennate muscle when tension in fiber is 100 N, given that the angle of pennation is 60 degree. Ft α Ff

Solution Ft = 100 cos 60 = 100 (0.5) = 50 Ft α Ff

Recruitment of Motor Units (MU) Slow twitch (ST) fibers are easier to activate than fast twitch (FT) fibers ST fibers are always recruited first Increasing speed, force, or duration of movement involves progressive recruitment of MUs with higher and higher activation thresholds

Terms used to describe muscle contractions concentric: involving shortening eccentric: involving lengthening isometric: involving no change

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

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

Length – Tension Relationship 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. 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. Tension 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

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

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