Skeletal Muscle Biomechanics

lecture 5

what are the behavioral properties of musculotendinous units?

1. Extensibility

2. Elasticity

3. Irritability

4. Ability to develop tension
   

   • these properties are common to all muscle, including the cardiac, smooth and skeletal muscle of human beings, as well as the muscles of other mammals, reptiles, amphibians, birds, and insects

what extensibility?

ability to stretch or to increase in length

what is elasticity?

ability to return to normal length after stretch

• Muscle’s elasticity returns it to normal resting length following s stretch and provides for the smooth transmission of tension from muscle to bone

what is PEC?

parallel elastic component

what is the purpose of PEC?

• provided by the muscle membranes

• supplies resistance when a muscle is passively stretched

what is SEC?

Series elastics component

what is the purpose of SEC?

• residing in the tendons

• acts as a spring to store elastic energy when a tensed muscle is stretched

what is CC?

Contractile component

what is the purpose of CC?

muscle property enabling tension by stimulated muscle fibers

PEC, SEC, CC, these components of muscle elasticity are so named because?

because the membranes and tendons are respectively parallel to and in series (or in line) with the muscle fibers (muscle cells) which provide the contractile component

The parallel elastic component is suggested to consist of?

consist of the membranes surrounding the contractile components which includes the sarcolemma, sarcoplasmic reticulum, the perimysium and the epimysium. Act most effectively against high loads

SEC is most effective as?

most effective against low elastic loads

what is irritability?

• ability to respond to a stimulus

• electrochemical (actin potential)

• mechanical (external blow)

Stimuli affecting muscles are either electrochemical, such as an action potential from the attaching nerve, or mechanical, such as external blow to a portion of a muscle. When activated by a stimulus, muscle responds by developing tension

what is one behavioral characteristics unique to muscle tissue

the ability to develop tension

• historically, the development of tension by muscle has been referred to as contraction, or the contractile component of muscle function

a whole skeletal muscle is considered an organ of the muscular system. Each or muscle consist of?

• skeletal muscle tissue

• connective tissue

• nerve tissue

• and blood or vascular tissue

what is epimysium?

the outermost sheath of connective tissue covering each muscle

in the epimysium, each muscle is made up of groups of muscle fibers called??

fascicles surrounded by a connective tissue layer called perimysium

multiple units of individual muscle fibers within each fascicle are surrounded by_______, a connective tissue sheath

endomysium

what is a muscle fiber?

• muscle cell

• threadlike shape

• single muscle cell surrounded by a membrane called the sarcolemma and containing specialized cytoplasm called sarcoplasm

what are myofibrils?

special organelle, made of myofilaments (actin and myosin)

the two most essential myofilaments that make up the contractile elements of the muscle fiber are?

actin and myosin

• they are arranged distinctively in a striated pattern to form the dark A band, the light I band, and the fundamental unit of contraction, also referred to as a sarcomere

what forms the dark A band?

the sarcomere consists of a central M line, and attached to it on either side are the thick myofilaments of myosin. This forms the dark A band

the sarcomere is bordered by the ?

by the Z-line which serves as the site of origin of the thin myofilaments of actin that project towards each other as they partially overlap the myosin filaments

the regulatory proteins, namely troponin____ and tropomyosin, play a key role in the myofilaments sliding mechanism leading to contraction

troponin C, I, T

what are the other major proteins that contribute to the mechanical properties of the muscle?

Titin and nebulin

what is a unique T-tubule?

There is a unique T-tubule system in place for the conduction of neuronal action potential to the interior of the muscle cell via invaginations of the sarcolemma to enhance coordination and uniform muscle contraction

what are the summary of events in muscle contraction and relaxation

1. Acetylcholine released from the axon terminal binds to receptors on the sarcolemma

2. An action potential is generated and travels down the T tubule

3. Ca²⁺ is released from the sarcoplasmic reticulum in response to change in voltage

4. Ca²⁺ binds troponin; cross-bridges form between actin and myosin

5. Acetylcholinesterase removes acetylcholine from the synaptic cleft

6. Ca²⁺ is transported back into the sarcoplasmic reticulum

7. Tropomyosin binds active sites on actin causing the cross-bridge to detach

what are some structural organization of skeletal muscle

• some fibers run the entire length of a muscle; other are shorter

• skeletal muscle fibers grow in both length and diameter from birth through adulthood

• fiber diameter can be increased through resistance training

what is a motor unit?

• single motor neuron and all fibers it innervates

• considered the functional unit of neuromuscular system

the activation of one motor neuron will result in?

in a weak muscle contraction

the activation of more motor neurons will result in more muscle fibers being activated, and therefore?

and therefore a stronger muscle contraction

the higher the motor unit recruitment

the stronger the muscle contraction

what are the two types of motor unit?

• Fast twitch (FT)

• Slow twitch (ST)

what is fast-twitch fiber?

a fiber that reaches peak tension relatively quickly

what is slow-twitch fiber?

a fiber that reaches peak tension relatively slowly

what is recruited first?

• ST fibers are always recruited first

• slow twitch (ST) fibers are easier to activate than fast twitch (FT) fibers

• increasing speed, force, or duration of movement involves progressive recruitment of motor units with higher and higher activation thresholds

what are the 2 types of fiber architecture?

• parallel fiber arrangement

• pennate fiber arrangement

what is parallel fiber arrangement?

• fibers are roughly parallel to the longitudinal axis of the muscle

• facilitates muscle shortening (muscle become shorten due to fiber shortening)

• ex: sartorius, rectus abdominis, biceps brachii

what is pennate fiber arrangement?

• shorth fibers attach to one or more tendons within the muscle

• promotes muscle force production

• ex: tibialis posterior, rectus femoris, deltoid

in pennate fiber arrangement, when muscle shorten they rotate about their?

• their tendon attachment

• this will increase the angle of pennation

• greater angle of pennation will induce smaller amount of effective force

what is concentric contraction?

• causes muscles to shorten, thereby generating force

• Work against gravity to raise the body or objects

• muscle contracts

what is eccentric contraction

• cause muscles to elongate in response to a greater opposing force

• work with gravity to lower the body or objects

what is isometric contractions?

generate force without changing the length of the muscle