Tissue Mechanics (Exam 2)

BME 312 - Fall 2025

4 types of tissue in human body

• connective tissues (45%)

• muscle tissue (80%)

• neural tissue (2%)

• epithelial tissue (3%)

3 types of muscle tissues

• skeletal muscle

• cardiac muscle

• smooth muscle

skeletal/striated muscle fibers

• cause the movement of bones/limbs

• occur in muscles attached to the skeleton

• striated in appearance and under voluntary control

smooth muscle fibers

• located in wall of hollow visceral organs, EXCEPT the heart (which is spindle-shaped)

• under involuntary control

• non-striated

cardiac muscle cells

• form the wall of the heart

• under involuntary control

• striated

skeletal muscles

• generate forces for movement

  • converts chemical energy into mechanical work

• composed of muscles fibers and myofibrils

• have viscoelastic behavior

viscoelastic behavior

• you can see the curve and different thickness of muscles

• it will go back to its original (unstretched) size and shape

  • after the muscle is stretched and released

• viscous = internal resistance to motion

types of muscle contractions

• concentric

• static

• eccentric

concentric contractions

occurs simultaneously as the length of muscle decreases

• biceps during the flexion of forearm

static contraction

occurs while muscle length remains constant

• biceps are flexed and held without any movement

eccentric contractions

occurs as the length of the muscle increases

• biceps are extended (forearm)

functions of the muscle 

• agonist

• antagonist

agonist muscle

• causes movement through concentric contractions

• shortens the muscle length to cause joint movement

antagonist muscle

• causes movement through eccentric contractions to control the movement

• lengthens the muscle

  • decelerates the motion of the joint

assumptions and limitations

• locations of muscle attachments are known

• anatomical axes of rotation of joints are known

• line of action of muscle tension is known

• segmental weights and COGs are known

• dynamic aspect of problems are ignored

• friction factors of joints are negligible

• 2D problems will be considered

mechanics of skeletal muscles

• skeletal muscles attach to at least 2 bones controlling the relative motion of one segment with respect to the other

• contraction can occur as a result of muscle shortening/lengthening

  • can occur without any change in the muscle length

How are skeletal muscles attached?

attached via aponeuroses and/or tendons to at least 2 bone

What is a unique ability of muscle tissue?

contraction

What is the result of a muscle contraction?

ALWAYS tension

A muscle can only exert what?

ONLY exert a pull but NOT a push

structure of skeletal muscle

• consist of 100,000 muscle fibers

  • cells acts together to perform the functions of the specific muscle (they are part of)

• voluntary muscle is under conscious control

• causes movement of skeletal system (moving limbs and bones)

• striated muscle

• is also the skin (muscles of facial expressions in the head and neck)

Voluntary muscle is under what?

conscious control

parts of skeletal muscle

• tendon

• tendon sheath

• fascia

• superficial fascia

• deep fascia

• epimysium

• perimysium

• endomysium

• muscle fiber

• fascicle

tendon (skeletal muscle)

• attaches muscle to bone

• tough pale color (whitish)

• cord-like

  • formed many parallel bundles of collagen fibers

• flexible

  • bend around other tissues, changing position as they move

tendon sheath (skeletal muscle)

surround the tendons by tubular double layer sacs

• lined with synovial membrane

• contains synovial fluid

function of tendon sheath (skeletal muscle)

• minimize friction associated with movement at the joint

• facilitate movement of the joint

fascia (skeletal muscle)

• means bandage

• form of sheets of broad bands of fibrous connective tissue 

  • cover muscles or organs

• form an outer-wrapping

superficial fascia (skeletal muscle)

• consist of areola connective tissue and adipose tissue

• referred to as subcutaneous layer of skin

deep fascia (skeletal muscle)

• holds the muscles together

• consists of dense fibrous connective tissue

epimysium (skeletal muscle)

• fibrous elastic tissue that surrounds muscle

• many fascicles that form a single muscle

  • ____________ surrounds the total group of muscle

perimysium (skeletal muscle)

• fibrous sheath that surrounds and protects bundles of muscle fibers

• shown as thing grey line in the cross-section of skeletal muscle

endomysium (skeletal muscle)

connective tissue sheath that surrounds each single muscle fiber

muscle fiber (skeletal muscle)

• aka muscle cells

• special cells that are able to contract, causing movement

fascicle (skeletal muscle)

• expresses as a fascicules

• refers to a bundle of muscle fibers or nerves

structure of skeletal muscle

• Z line

• M band

• I band

structure of skeletal muscles (con’t)

structure of a muscle cell

myofibrils (micro structure of skeletal muscle)

• small contractile filaments located within the cytoplasm of striated muscle cells

• filaments cause the distinctive appearance of skeletal

• consist of bands of alternative high and low refractive index

sacromere

• thick filaments

• thin filaments

• H zone

thick filaments

produce the dark A band 

thin filaments

• extend in each direction from the Z line

• DO NOT overlap the thick filaments

• create the light I band

H zone

portion of the A band where the thick and thin filaments DO NOT overlap

sacromere

• entire array of thick and thin filaments between the Z lines

• shortening of the _________ in a myofibril produces the shortening of the myofibril

electron microscopy model of muscle contractions

• Z lines come closer together

• width of the I bands decreases

• width of the H zones decreases

• there is NO change in the width of the A band

electron microscopy model of muscle stretching

• width of the I bands and H zones increase

• there is NO change in the width of the A band

tendonitis

overuse of the tendon

• weak point is where the muscle and tendon connect (common area for tendonitis)

striated muscle

• striated appearance of the muscle fiber is created by a pattern of alternating dark A bands and light I bands

• A bands are bisected by H zone

• I bands are bisected by Z line

• each myofibril is made up of arrays of parallel filaments

diameters of thick and thin filaments (striated muscle)

• thick filaments = 15 nm

  • composed of the protein myosin

• thin filaments = 5 nm

  • composed mainly of actin with smaller amounts of troponin and tropomyosin

What are actin, troponin, and tropomyosin?

proteins

motor units

• minimum unit of contraction

  • small in muscles over which we have precise control

  • a single motor neuron triggers fewer than 10 fibers in the muscles controlling eye movements

motor units (con’t)

• all motor neurons leading to skeletal muscles have branching axons

  • each terminates in a neuromuscular junction with a single muscle fiber

• nerve impulses passing down a single motor neuron will trigger contraction in all the muscle fibers

  • which the branches of that neuron terminated

motor units (con’t)

• although the response of a motor unit is all-or-none, the strength of the response of the entire muscle is determined by the number of motor units activated

What is the size of the motor units in the muscles controlling the larynx?

small as 2-3 fibers per motor neuron

A single motor unit for a muscle like the gastrocnemius muscle include…

1000 - 2000 fibers (scattered uniformly through the muscle)

tonus

• partial contraction of skeletal muscles

  • even at rest, there is still ________

• maintained by the activation of a few motor units at all times even at rest

fueling muscle contraction

• ATP = immediate source of energy for muscle contraction

  • although a muscle fiber consists of ATP enough for a few twitches, the “ATP pool” is replenished as needed

What are the three sources for high-energy phosphate to keep the ATP pool filled?

• creatine phosphate

• glycogen

• cellular respiration (in mitochondria of the fibers)

creatine phosphate

• phosphate group is attached by a “high-energy” bond like one in ATP

• derives its high-energy phosphate from ATP

  • can donate it back to ADP to form ATP

• pool of creatine phosphate in the fiber is 10 times larger than that of ATP

  • serves as a modest reservoir of ATP

glycogen

• skeletal muscle fibers contains about 1% glycogen

• muscle fiber can degrade glycogen by glycogenolysis

  • produces glucose-1-phosphate

• enters the glycolytic pathway to yield 2 molecules of ATP for each pair of lactic acid molecules produced

  • sufficient to keep muscle functioning if it fails to receive sufficient oxygen to meet its ATP needs by respiration

• limited resource

  • muscle will start to depend on cellular respiration

cellular respiration

• not only required to meet ATP needs of muscle engaged in prolonged activity

  • causes more rapid and deep breathing

  • afterwards, required to enable the body to resynthesize glycogen from the lactic acid produced earlier

    • deep breathing continues for a time after exercise is stopped

  • body must repay its oxygen debt

cellular respiration (con’t)

• oxygen debt

  • demand for oxygen is greater than the supply

    • means that the body is working hard, and breathing in a lot of oxygen (lung cannot absorb enough to cope with the level of activity)

  • if this process happens, the body is mainly utilizing the anaerobic energy system 

    • lactic acid builds up (undesirable waste product)

types of muscle fibers

• convergent

• circular

• multipennate

• parallel

• fusiform

• unipennate

• bipennate

parallel/fusiform muscle fibers

• run parallel to each other

• contract over a great distance

• have good endurance

• NOT very strong

• examples include sartorius, rectus abdominus muscles

convergent muscle fibers

• converge on the insertion to maximize the force of muscle contraction

• examples include deltoid, pectoralis major muscle

pennate muscle fibers

• three types of pennate muscles (depends on location of muscle)

  • unipennate

  • bipennate

  • multipennate

• strong but tie easily

circular muscle fibers

• muscle fibers surrounded opening to act as a sphincter

• examples include orbicularis, orbicularis oculi muscles

types of muscle contractions

• isotonic

• isokinetic

isotonic muscle contraction

force remains constant

isokinetic muscle contraction

• rate of change of muscle is not constant

• angular velocity remains constant

• muscle translations cause joint rotations

• center of joint rotation is not in a fixed position in human joints

What are isokinetic dynamometers?

devices that have been developed for muscle training

process for fueling muscle contraction

isometric contraction

• stimulated muscle is held so that it cannot shorten (“same length”)

• simply exerts tension

isotonic contractions

if the muscle is allowed to shorten (“same tension”)

cardiac muscles

• very strong

• myofibrils of each cell are branched

• myofibrils of cardiac muscle is made of single cells and each with a single nucleus

• branches interlock with those of adjacent fibers by adherent junctions

• strong junctions enable the heart to contract forcefully without ripping the fibers apart

• involuntary, striated muscle that is forced in the walls of the heart

cardiomyopathy

abnormal structure and function of the heart related to heart muscle

• think of a closed system (P1V1 = P2V2)

myocardial infarction (MI) or acute myocardial infarction (AMI)

• heart attack

• occurs when a branch of blood vessels stops supplying blood to a part of the heart causing damage to the heart muscle

symptoms of MI or AMI (heart attack)

• chest pain

• discomfort travels into the shoulder, arm, back, neck, or jaw

What happens in a stroke?

• one-sided

• speech slurring, can’t lift hands, drooping face

diseases of smooth muscle

• multisystemic smooth muscle dysfunction syndromes

• achalasia

multisystemic smooth muscle dysfunction syndromes

• disease relating to activity of smooth muscle throughout the body

  • it is impaired

• blood vessel abnormalities

• a decreased response of the pupils to light

• a weak bladder

• weakened of the muscle used for digestion of food

  • hyperperistalsis

achalasia

• over contracted condition of the smooth muscle portion of the esophagus

• lower esophagus and lower esophageal sphincter

torn quadriceps tendon

torn Achilles tendon