Biomechanics & Muscles

Organisms in physical world deal with

external and internal loads

Organisms in water have what external loads?

fluid flow

What external loads to organisms on land have?

fluid flow, gravity, muscle force, predation

Force

interaction that alters object’s motion and/or shape

Force Equation

mass x acceleration

Force units

Newton (N) = kgm/s²

What way does structure respond to applied force?

depends on shape and material

Stress

how much force is applied over an area; amount of energy used to deform or move a volume

Stress Equation

Force/ Area

Stress Units

pascals (Pa) = Newtons/m²

How does a material respond to an applied force?

measured by stress

Structures fail (break) when ____ gets too high

stress

Higher the stress, _____ likely to break

more

Increase area, ______ stress

decrease

Types of Load

tension (pull), compression (squish), bending

Tension (Pull) Load

pulling apart molecules; failure likely; ex: Achilles Tendon

Compress (Squish) Load

only happens at very small scale; pushing molecules together; failure unlikely

Bending Load

tension & compression; on opposite sides with neutral axis in middle

What does resistance to bending depend on?

structure & material

How to make a structure better at resisting bending?

put material where stress is highest

Neutral Axis

region of a bending structure where there is no applied load; magnitude of load decreases

Material Properties

stress & strain

Strain

unit-less measure of deformation; %

Stress- Strain Curve

show relationship between applied stress and deformation for a material; strain on x-axis and stress on y-axis; has stiffness, strength, and work

Stiffness

Pa; how deformable a material is; yellow dashes

High stiffness, _____ to deform

hard

Low stiffness, ____ to deform

easy

Compliance

inverse of stiffness

High compliance, _____ stiffness

low

Low compliance, ______ stiffness

high

Deformity

how much bending

Strength

Pa; maximum stress; where failure occurs; red dashed line

Work

Pa; area under the curve; work put into the system to deform the material/object; dark blue

External Forces/Loads

gravity, flow (water currents and wind), competitors and predators

Internal Forces

animals use muscles pulling on stiff structures to moveM

Muscle contractions generate

force

Biochemical precursors to muscles

used at cellular level in cell division, amoeboid movements, cytoskeleton

Myocytes

derived from mesoderm; 3 types (cardiomyocytes, gut myocytes, somatic myocytes)

Cardiomyocytes

muscles that power the heart

gut myocytes

smooth muscle that line the gut

somatic myocytes

skeletal muscles used to move the body

Contractile cells

organized into tissues where the cells tend to contract to various degrees together to magnify their individual effects

What groups have muscles?

cnidaria and bilateria

Ductile

deform before failure; not a lot of stress to break

Elastic Deformaiton

stores energy and release to return to original shape; beginning of graph

Plastic Deformation

uses energy to permanently change shape; does not return to original form (bent); between yellow and red

Vertebrate Skeletal Muscle

SkM; produces voluntary movments; controlled exogenously; roughly 36-42%of total body mass; modified force output; contains mechanoreceptors

Exogenously

external signals; sometimes nervous system

What does vertebrate skeletal muscle do when fully activated?

dominate metabolism

Vertebrate Skeletal Muscle Force Output

undergo elastic deformation; stores and releases energy

Vertebrate Skeletal Muscle mechanoreceptors

report movement and position

Do vertebrate skeletal muscles have intrinsic activity?

nope; only contracts when commanded by neurons

What is the only tissue in mammals that engages in significant anaerobic metabolism?

skeletal muscle

Origin

anchoring muscle attachment; typically proximal; on other side of a joint from the insertion; little motion

Insertion

mobile muscle attachment; typically distal; on other side of a join from the origin

proximal

closer to the center of the body

distal

further from the center of the body

Tendons

connect muscle to bone; connective tissue dominated by collage and elastin; undergoes elastic deformation

Belly of a muscle

generates force; muscle fibers (SkM cells), stem cells, blood vessels, sensory cells, neurons

Joint

where two bonds meet; some allow motion (hinge & ball-and-socket) and some do not (sutures)

Fascia

the sheet of connective tissue that covers a muscle

Fascicles

bundles of muscle fibers that make up the belly of a muscle

Muscle fibers

multiple myocytes fuse into a large syncytium; inside fascicles; a cell

Myocytes

embryonic precursor cells that fused together to form one muscle fiber (single cell); used to be separate cells but fused during development

Muscle Fiber Cytoplasm

lots of mitochondria, extensive membrane system with t-tubules, sarcoplasmic reticulum, and myofibrils

T-Tubules

deep in-folding of plasma membrane; action potential goes down this (gets AP to whole muscle fiber)

Sarcoplasmic reticulum

modified endoplasmic reticulum; uses ATP to pump Ca²⁺ in → stores Ca²⁺; Ca²⁺ gradient

Myofibrils

strands of sarcomeres

Largest to Smallest Skeletal Muscle Anatomy

muscle organ and tissue, fascicle, muscle fiber (cell), myofibrils, sarcomeres, actin + myosin filaments

Sarcomeres

repeating sections; actin filament and myosin filament

Actin Filaments

pulled by myosin; anchor on Z-disks; proteins that regulate the interactions between myosin and actin; thin; lighter color

Myosin Filaments

pull actin; anchor on M-line; each has a “head” containing ATPase and actin binding site; bundle together → thick; darker

Actin monomers

myosin binding sites; blocked by tropomyosin, held in place by troponin; repeating spheres

Myosin heads

actin binding site; ATP binding site

How do skeletal muscles generate force?

Excitation Contraction Coupling

Excitation Contraction Coupling

receive signal (AP); AP excites cell → release of Ca²⁺; allows Actin-Myosin bond; cross-bridge cycle & contraction

Neuromuscular junctions

where skeletal muscle action potentials are generated

Receive Signal in Excitation Contraction Coupling

AP on pre-synaptic motor neuron → releases acetylcholine (Ach) across synaptic cleft

AP Excites cell → release of Ca²⁺ in Excitation Contraction Coupling

Ach binds to receptors on myofibril → Excitatory Post-Synaptic Potentials (EPSPs) → AP → AP travels down T-Tubule, close to sarcoplasmic reticulum → sarcoplasmic reticulum releases Ca²⁺ → floods cell with Ca²⁺ ions

Allows Actin-Myosin bond in Excitation Contraction Coupling

released Ca²⁺ binds to Troponin → changes its shape → moves Tropomyosin off myosin binding sites → clears myosin binding sites

Cross-Bridge Cycle & Contraction in Excitation Contraction Coupling

myosin binds to Actin → release of phosphate → myosin changes shape (bends back toward M-line) → release of ADP from myosin → bind to ATP → release Actin → hydrolysis of ATP → myosin changes shape, ready to bind

Proteins change in shape, depending on

binding partner

Change in affinity (likelihood of binding)

myosin head & ATP/ADP-P_i; Myosin head & Actin

What happens when there is no ATP?

myosin never lets go of actin

What happens when there is no Ca²⁺?

myosin can’t bind to actin; no contractions

What generates force?

muscle contractions

What happens when myosin heads form cross-bridges with actin filaments and pull?

shortens sarcomere

Small Force

myofibrils composed of thousands of sarcomeres in series → sarcomeres shorten and myofibrils shorten

Large Force

muscle fibers composed of thousands of myofibrils in parallel → myofibrils shorten and pull together

Maximum muscle force scales with 

cross-sectional area of the muscle

How do muscles generate different amounts of force?

tetany, recruitment, muscle fiber type, resting fiber length, muscle usage

Tetany

summation of twitches generated by a series of closely spaced action potentials; all or nothing; lasts as long as AP signal

Twitch

single short contraction; single AP

Why do muscles experience fatigue?

muscle is used for too long or too often, using up ATP in the cell; cannot produce as much force

Recruitment

activation of additional motor units yields mroe force

Muscle force depends on the 

number of actively contracting muscle fibers

Motor Unit

all the muscle fibers controlled by one neuron

Activation of any motor unit yields

certain amount of force

increase activation of motor units, _____ force

larger

maximum force

all motor units activated; proportional to muscle x-section area