Kin 235 Quiz 4

Describe how muscle activation determines strength?

# of active fibers + motor unit firing rate Some people can't recruit all their motor units (less force than predicted by muscle size) -increases with training -dynamic movements (vs isometric) increases potential for neural adaptations Frequency motor units are recruited at ⇒ determines force production -fast = more force

How does acetylcholine release into the NMJ lead to muscle contraction?

-ACh diffuses across neuromuscular junction leading to depolarization of muscle fiber -Depolarization travels down T tubules reaching SR -Causes release of calcium into sarcomeres -Ca binds to troponin -Troponin has binding site with tropomyosin -At rest, tropomyosin covers myosin binding site on actin -Troponin binds calcium ⇒ pulls tropomyosin off binding sites on actin -Cross bridge cycling can occur ⇒ power stroke requires 1 ATP- used for releasing myosin head from actin) -Reuptake of Ca into SR -Muscle relaxes

2 reasons for muscle fiber type differences:

-Muscle dependent (slow vs fast) -Training (endurance vs power)

Describe how muscle size determines strength?

-Physiological CSA -# of sarcomeres in parallel -Linear relationship between muscle size and strength (not perfect though)

Training impacts on muscle length

-Runners and cyclists have optimal force production at different muscle lengths based on training -Some evidence that eccentric contractions will be able to add sarcomeres in series (add muscle length)

4 components that impact voluntary muscle action:

1. CNS (signal) 2. PNS (ability to release ACh) 3. NMJ (muscle depolarization ability) 4. Muscle fiber (reaction to calcium)

Sequence of Events in Muscle Action

1. Generation of AP in motor neuron causes terminal axon to release ACh, which diffuses across synaptic cleft and attaches to specialized ACh receptors on sarcolemma 2. Muscle action potential depolarizes the transverse tubules at the sarcomere's A-I junction 3. Depolarization of T-tubule system causes Ca2+ release from lateral sacs of sarcoplasmic reticulum 4. Ca2+ binds to troponin-tropomyosin in actin filaments, releasing the inhibition that prevented actin from combining with myosin 5. Actin combines with myosin-ATP; this activates myosin ATPase, which splits ATP. The reaction's energy produces myosin crossbridge movement and creates tension 6. ATP binds to myosin crossbridge which breaks actin-myosin bond, allowing actin disassociation from the crossbridge. Thick and thin filaments then slide past each other and muscle shortens 7. Crossbridge activation continues when Ca2+ concentration remains high enough in muscle to inhibit troponin-tropomyosin system (and sufficient ATP for cycling) 8. When muscle stimulation ceases, intracellular Ca2+ concentration rapidly decreases as Ca2+ moves back into lateral sacs of sarcoplasmic reticulum through active transport; requires ATP hydrolysis 9. Ca2+ removal restores inhibitory action of troponin-tropomyosin. In presence of ATP, actin and myosin remain in dissociated, relaxed state

4 characteristics of fast twitch fibers and what do they result in?

1. High capability for electrochemical transmission of action potentials (depolarization from NMJ to muscle = fast) 2. High myosin ATPase activity (enzyme that allows ATP to be used is efficient) 3. Rapid Ca2+ release and uptake by efficient sarcoplasmic reticulum (quick contraction and relaxation) 4. High rate of cross bridge turnover -rapid energy generation for quick, powerful muscle actions -Intrinsic speed of shortening and tension development is 3 to 5 times faster -Well-developed, short-term glycolytic system for energy transfer (generate ATP) -Predominate anaerobic sprints, power actions, and stop/go sports

4 characteristics of slow twitch fibers and what do they result in?

1. Low myosin ATPase activity (cycles slower) 2. Slow calcium handling ability and shortening speed (slow contraction/relaxation) 3. Less well-developed glycolytic capacity than fast-twitch fibers 4. Large and numerous mitochondria (stay below LT longer) -Highly fatigue resistant -Predominate prolonged aerobic exercise -Oxidative metabolism

2 divisions of the human nervous system

1. Peripheral (nerves leaving brain/spinal cord going out to body to sense & provide stimulus) 2. Central (brain & spinal cord)

You hold an isometric bicep curl with 30% of your 1RM. Explain which motor units would likely be recruited. Would this change if you held the contraction until you were forced to drop the weight due to fatigue?

30% 1RM isometric bicep curl would recruit type I fibers and many some type IIa, you will not recruit type IIx fibers because it is not heavy Holding a contraction becomes harder over time/fatigable -As type I fatigue (& peripheral fatigue- Ca causes less force production), we send more electrical activity (brain effort) to recruit type IIa and then type IIx -Increasing fiber rate and recruitment to do same task (easy at first ⇒ hard over time)

About ____% of crossbridges make contact with actin to form protein complex actomyosin

50

How many actin/thin filaments encircle a thick myosin filament?

6

All or none principle:

A stimulus strong enough to trigger a motor neuron action potential; activates all muscle fibers in the motor unit to contract synchronously Does not exert a force gradation After the motor neuron fires and the impulse reaches the NMJ, all fibers of the motor unit fire simultaneously

How does NMJ work?

ACh is released every time nerve fires ACh diffues across NMJ and attaches to recepotors on muscle Depolarization of muscle Ion channels of muscle open so K+ leaves and Na+ enters

What powers cross bridge cycling?

ATP hydrolysis

4 reasons for fatigue

Acidosis Lactate (correlative) Pi ROS

What accounts for 85% of myofibrillar complex

Actin and myosin Contractility!

What is motor unit recruitment?

Adding motor units to increase muscle force Size principle: orderly recruitment (small to large) of specific motor units to produce a smooth muscle action

Explain how adding sarcomeres in series and parallel would alter muscle force production characteristics

Adding sarcomeres in parallel (resistance training) -Muscle hypertrophy -Force production Adding sarcomeres in series -Increased velocity (faster contraction) -Increased ROM -More efficient in producing force for a specific sport

What are the advantages and disadvantages of increasing the angle of pennation in a muscle?

Advantages of Increasing Angle of Pennation in a Muscle -For force production -Increased packing of muscle fibers -Hypertrophy Disadvantages of Increasing Angle of Pennation in a Muscle -Slow velocity -Less sarcomeres in series ⇒ each change in length of the whole muscle changes length of each sarcomere more ⇒ smaller sweet spot for force production (long and short, drops off) -Long muscles have bigger range (fusiform)

Why is length-tension relationship in a muscle a smooth parabola?

All cross bridges and muscle fibers add up together (not all sarcomeres are same length at same time)

Describe force production at a optimal sarcomere length:

All cross bridges are overlapped with actin ⇒ produce maximal force Sarcomere is strongest Flat bit: area in middle of sarcomere with no cross bridges (small changes in length that cross this area ⇒ doesn't change # of cross bridges formed)

What is the purpose the size princple?

Allows the CNS to fine tune skeletal muscle activity to meet motor task demand

Describe events of a single twitch

Alpha motor neuron depolarizes ⇒ ACh releases ⇒ muscle depolarizes ⇒ releases Ca ⇒ Ca binds to troponin ⇒ cross bridge cycling ⇒ force production ⇒ muscle repolarization ⇒ Ca sucked into SR ⇒ resting level

What is striation

Alternating thick and thin filaments

Explain how crossbridge cycling leads to force production/ movement

Analogy: climbing rope hand over hand -each cross bridge reaches up from myosin molecule and binds to actin, moving it slightly, next CB binds again -little bit of force, little bit of movement -thousands binding at different rates ⇒ adding up to moving quickly or adding up to create force

What is muscle architecture? Types?

Arrangement of muscle fibers 2 classes: fusiform and pennate

Reactive oxygen species and fatigue

Associated with peripheral fatigue Redox reactions for generation of ATP (ETC in mitochondria) These reactions in high amounts = errors occur and ROS formation occurs ROS: oxygen with extra e- (O3) -super reaction and cause tissue damage, DNA damage, mutation, lipid oxidation and protein dysfunction -charged and unable to permeate membrane -reduces to H2O2 to diffuse and create OH- (unstable) -affects excitability of sarcolemma and its ability to release Ca from SR and cross bridge binding -oxidative stress and ROS increases with intense exercise (a little bit is not bad, but high levels are bad)

Describe the contributions of muscle fiber types in near-max aerobic and anaerobic exercise:

At top anaerobic capacity ⇒ recruit everything Low intensity ⇒ fast twitch aren't recruited

Athlete A has 60% type I fibers Athlete B has 60% type II fibers explain the mechanism which would affect their performance in the 100 M dash and 10,000 M race

Athlete B -More force production, but quicker fatigue -Higher capacity for anaerobic glycolysis -More glycogen stores -More glycolytic enzymes ⇒ produce more lactate -Large muscle fibers ⇒ more sarcomeres in parallel ⇒ more force production -Quick force generation/relaxation due to rapid Calcium movement -Efficient myosin heavy chain for ATP breakdown -Faster cross bridge cycling (but fatigue quicker) Athlete A -More mitochondria for aerobic metabolism -More fat in fiver to provide energy -Weaker force generation -Slower myosin heavy chain isoform -Slower calcium release and uptake -Highly fatigue resistant so can maintain force generation for long time

What does axon size mean?

Axon size changes its resistance/urge to depolarize for a given electrical input Because of axon size, they will depolarize at different rates based on a given signal from the brain

Why don't we eat high amounts of antioxidant to reduce fatigue?

Because antioxidants would completely block inorganic phosphate and ROS formation which would reduce the effect of training We need these signals for muscle adaptations and mitochondrial growth

Why do we cross bridge cycle and not synchronously activate?

Because we wouldn't be able to maintain force Keeps constant force vs jerky movement

What are the main factors which explain sex differences in strength?

Bigger is better in terms of absolute strength Strength/Body Mass ⇒ men still appear stronger Healthy women have higher body fat than healthy men so because of this healthy women will always have less force relative to body mass

Describe the binding sites on actin at rest:

Blocked by troponin

Issue with comparing men and women strength when looking at force / body mass

Body composition differences, a healthy woman will always have a higher body fat % than a healthy man Shows men have advantage over women in all movements

How do skeletal muscles generate force?

By shortening (draw bones closer together)

Acidosis and fatigue:

Ca is required for positioning filaments for cross bridge formation High concentrations of H+ and Pi directly impact the power stroke in contractile mechanism of muscle fibers (limiting factor in the speed of muscular contraction- shortening velocity) Decreases sarcolemma sensitivity for Ca2+ Ca2+ maintains membrane depolarization for AP

Describe force production at a long sarcomere length:

Can't produce any more active force Stretch in muscle = actin and myosin don't overlap ⇒ no cross bridge ⇒ no force Linear decrease in overlap and force production

What surrounds muscle fibers?

Capillaries for gas and nutrient exchange

What is the H zone?

Center of A band where there is only thick filament (myosin)

How would central fatigue affect [Ca+] within working muscle?

Central fatigue would not decrease Ca sensitivity (this happens only with peripheral fatigue), but with central fatigue we will always have a lower Ca concentration in the muscle because the only way Ca is released is from neural stimulation (depolarization of muscle/motor unit = Ca release) Lower neural stimulation from CF ⇒ less electrical signals ⇒ less motor units depolarize or they depolarize at a slower rate ⇒ less Ca

Pennate fiber length-muscle length ratio

Closer to 0 (0.2)

Fusiform fiber length-muscle length ratio

Closer to 1 (0.6)

Name two locations where ATP is used during muscle contraction

Cross bridge cycling Reuptake of Ca into SR Na-K ATPase (rebalances ions to repolarize muscle after contraction)

Describe a power stroke:

Cross bridge undergoes a power stroke (shortens) to create force Requires ATP hydrolysis (breakdown ATP to ADP + P +G) Pulls z lines at end of sarcomere together

Anatomical cross sectional area:

Cut muscle in half between origin and insertion Area = proportional to force

Physiological cross sectional area:

Cut muscle perpendicular to fibers Cut in 2 spots for quads due to fibers going in 2 directions Add up areas

Fatigue

Decline in force/power generation capacity with repeated stimulation or during a given time period

What does any type of fatigue result in ?

Decrease in force generating capacity

Reasons for peripheral fatigue

Decreased O2 Increase [H+] Increase Pi Decreased substrate (glu,gly,pcr) Decreased Ca release and/or Ca sensitivity for actin-myosin cross bridge formation

What happens to I band when Z bands are pulled together (concentric)? What happens to A band?

Decreases to nothing at max Doesn't change

Muscle spindles

Detect mechano-sensory information about changes in muscle fiber length and tension Respond to stretch of muscle by initiating a stronger muscle action to counteract stretch Two sensory afferent fibers and one motor efferent fiber innervate spindles

Describe force production at a short sarcomere length:

Disrupts actin & myosin binding Thin filaments start to overlap each other Changes distance between cross bridge heads and actin (can't get binding and force) More leverage here so it is less noticeable than muscle stretched too long

Where does peripheral fatigue start?

Distal from motor unit Processes associated with mechanical and cellular changes in muscular system

How do you know peripheral fatigue has occurred?

Don't see an increase in force production with extra stimulation Chain from brain to muscle is fine (additional stimulation doesn't help) Muscles is just peripherally fatigued

Central fatigue & EMG

EMG decreases with force decrease This occurs because body is worse at sending neural signals to muscle causing decreased force generation capacity

Peripheral fatigue & EMG

EMG increases with force decrease This occurs because body is trying to overcome this decrease in Ca sensitivity (for a given amount of Ca, less force is produced) by having the body send greater neural signals to cause more depolarization of muscle

How to determine site of fatigue (peripheral vs central)

EMG- measures signals from muscle; harder contraction = bigger signal Twitch interpolation

Describe staining muscle fiber types:

Each muscle fiber type has a specific myosin heavy chain isoform Each myosin heavy chain isoform has a specific antibody that binds to it Secondary antibodies with fluorescent tags bind to these primary antibodies Microscope shines fluorescent light at muscle and the antibodies bound to myosin heavy chain isoform will show up in different colors

How do we reduce ROS in body?

Eat antioxidants (may help slightly with fatigue)

How does box help one jump higher?

Effects of stretch shortening cycle are increased: -faster, more forceful eccentric contraction -greater stretch -high initial force -store more elastic energy in tendon

How is a max stimulus given and level determined?

Electrical nerve stimulation or magnetic brain stimulation By turning up current to point that no more force is produced

What type of exercise is central fatigue more present in?

Endurance exercise

What two general roles does myosin have in muscle action?

Enzymatic and structural

True or false: There are times when we only recruit type II fibers

False

True or False: A single measurement of force is adequate to detect fatigue.

False -Measure force generation decline via 2 measurements

What are muscle fibers arranged into?

Fascicles

Type IIx Fibers: Contraction Time Size of motor neuron Resistance to fatigue Activity used for Max duration of use Force production Mitochondrial density Capillary density Oxidative capacity Glycolytic capacity Major storage fuel Myosin-heavy chains, human genes

Fast Large Intermediate Short-term anaerobic <5 min High Medium Low Intermediate High Creatine phosphate, glycogen MYH1

Architectural properties of pennate muscle:

Few sarcomeres in series Many sarcomeres in parallel Higher force production/peak force (parallel) Less velocity (series) Time they can produce this force is smaller because each sarcomere has to change more in length when there is fewer sarcomeres in series e.x., quad

Describe how force is affected by fiber angles:

Fibers at more extreme angles ⇒ lower % of force going end to end and higher % force pulling side to side Angle increases, you lose more force end to end At 30 degree angle, 87% force going end to end, rest is to lateral movement

Myosin

Filaments are bundles of molecules with polypeptide tails and globular heads

Actin

Filaments have two twisted monomer chains bound by tropomyosin polypeptide chains

Describe the anatomical vs physiological cross sectional area for fusiform and pennate muscles:

For fusiform muscle, anatomical and physiological CA are the exact same For pennate muscle, physiological CA is always bigger than anatomical CA because anatomical CA underestimates force production ability because fibers aren't running end to end

Describe force as a vector:

Force divided into straight (origin to insertion) and lateral (to side) components

Gradation of Force

Force of muscle action varies from slight to maximal via two mechanisms: 1. Increased number of motor units recruited - muscle generates considerable force when activated by all of its motor units 2. Increased frequency of motor unit discharge - repetitive stimuli that reach a muscle before it relaxes and increases the total tension

Why do successful Olympic rock climbers tend to be small and lean rather than larger and lean?

Force production relative to body mass favors small people. Weight is proportional to strength Bigger muscles (volume) = heavier, so when a muscles job is to lift body weight, they become less efficient with size because weight is disproportionate to strength Body weight to the 2/3 exponent corrects for differences in body mass when looking at strength

Both resistance and endurance training get transition away from what muscle fiber type to what instead?

From type 2x to type 2a

Compare force production and velocity when muscle is fusiform (series) vs pennate (parallel)

Fusiform: -Force doesn't add up, only get the one greatest force out of the sarcomeres -Velocity adds up (units of velocity x # of sarcomeres in series) Pennate -Force adds up, unit of force x # of sarcomeres in parallel -Velocity (units of velocity x # of sarcomeres in series)

Describe how a cross bridge is formed?

Globular myosin heads extend perpendicularly to latch onto actin strands to create a structural and functional link between myofilaments ATP hydrolysis activates myosin's two heads, placing them in an optimal orientation to bind actin's active sites

What activity causes more central fatigue. How is this shown on graph:

Heavy sets You see minimal peripheral fatigue due to minimal difference in twitch amplitudes You see central fatigue due to spike in force during max contraction from extra electrical input

Pi and fatigue

High Pi and ADP correlate with fatigue Pi and ADP are produced with any use of ATP => muscle contraction, Ca pumping, K/Na movement Inability to regenerate ATP from ADP correlates with muscle fatigue

Describe events of fused tetanus:

High calcium concentration in muscle is wanted for high force Stimuli down motor neuron at fast rate Second depolarization arrive at muscle BEFORE calcium reuptake to SR occurs, so just additional calcium release (no stair case pattern- no slight decrease in force) Just increase in force until peak is hit Generate as much force as you can ⇒ fused tetanus

Type of contraction on fatigue

High intensity contractions are more fatiguing than low intensity contractions

What type of fatigue is peripheral fatigue more present in?

High intensity exercise

What are the advantages of a longer fascicle length?

High velocity contraction Big range of motion

What is central fatigue? 4 sites:

Higher centers -neurotransmitter availability (ACh) -signal sending ability from motor cortex to spinal cord -affected by long duration exercises -motivation plays role Spinal motor neurons -excitability of nerve that leaves brain and travels down spine decreases -requires a bigger electrical input to depolarize Motoneuron excitability -excitability of alpha motor neuron decreases -requires a bigger electrical input to depolarize NM transmission -run out of choline for ACh

What band represents the light area vs dark area?

I band = light A band = dark

What happens to muscle fibers in hypertrophy?

Increased angle of inclination

If time to exhaustion while running on a treadmill at a constant velocity is improved by a carbohydrate sports drink what is the likely site of fatigue during this task?

Ingestion of CHO sports drinks during 90+ min duration exercise can improve time to exhaustion -Fatigue due to decreased CHO availability RINSING of CHO sports drinks during shorter duration exercise can improve time to -CHO rising increases time to exhaustion, but not through increasing availability of CHO to muscle -Neural component: body senses taste of CHO ⇒ thinks more availability ⇒ decreases central fatigue

Lactate and fatigue

Lactate is correlative to fatigue, but not causative of fatigue Lactic acid dissipates to H+ which increases acidity of muscle H+ ions drop the pH which causes fatigue

What are myofibrils

Lie parallel to fiber's long axis

Name some common athletic situations where you can be sure fatigue has occurred

Lifting Weights at Gym Task failure Running Stop due to cramping Muscle can't produce force/less force Sprinting (200m) Reach a peak velocity and slowly lose velocity Trying as hard as you can whole time, but performance is dropping off Running 5km Pacing ⇒ constant velocity and maybe even increase speed at end Might feel tired, but can't prove fatigue occurred No maximal performance and then lower maximal performance Or no submaximal performance that can no longer be maintained Wingate Test VO2 Max Test

Stretch Shortening Cycle (SSC)

Loading of a muscle eccentrically to prepare it for concentric contraction Stretching spindles increases neural stimulation (stretch reflex) High initial force => develop force in eccentric contraction so less time to develop force than if starting with concentric Series elastic component taken up force and stores elastic energy

Muscular fatigue

Loss in the capability to generate skeletal muscle force and/or velocity that is accompanied by recovery during rest Is reversible with rest

Multipennate

Lots of strips of connective tissues Many short fibers inserting a different angle ex., Deltoid

Oxygen availability & fatigue

Lowering the oxygen will increase development of fatigue (the degree of fatigue seems to be similar, but time exercising/intervening is shorter) Increasing the FiO2 (breathing extra O2 in air) will decrease fatigability

Describe the arrangement of sarcomeres in a large muscle

Many sarcomeres in parallel (beside each other)

Architectural properties of fusiform muscle:

Many sarcomeres in series Few sarcomeres in parallel Lower force production/peak force (parallel) Time they can produce this force is longer because each sarcomere can change length less when there is more sarcomeres in series Quick velocity of contraction (series) e.x., hamstring

Describe arrangement of sarcomeres in a long muscle:

Many sarcomeres in series (end to end)

What is the best way to correct muscle size when comparing strength between people?

Measure Body Comp -Use lean mass/fat free mass -Strength / FFM -Eliminates strength differences between lower body Measure CSA -Relative distribution of muscle differs between men and women in upper body -Men have more upper body strength -Women have relatively more lower body than upper Measure physiological CSA -Corrects for inaccuracy of anatomical CSA in pennate muscles -Best way to compare strength and shows female and males muscle strength are the same, they just differ in distribution/amount

How can we measure fatigue:

Measure firing rate of motor neurons -firing rate decreases indicating central fatigue Measure excitability of motor neurons -stimulate motor neuron with consistent electrical stimulus -if motor neuron becomes less excitable/less likely to fire, this indicates a decrease in excitability Measure amount of ACh in NMJ -less due to availability of Choline

What is EMG

Measures the signals that come from muscle Harder contraction = bigger signal Increases with exercise With fatigue EMG increases, but no further increases in force production occur

Why does using strength / FFM not always work when looking a male and female strength?

Men and women place muscle mass differently: men have more upper body muscle mass and women have more lower body muscle mass, relatively Results in scores of females being 50% lower than men for upper body strength and 30% lower for leg strength

What do comparisons of muscular strength between men and women on an absolute score basis indicate?

Men possess considerably greater strength than women for all muscle groups (incorrect)

Type IIa Fibers: Contraction Time Size of motor neuron Resistance to fatigue Activity used for Max duration of use Force production Mitochondrial density Capillary density Oxidative capacity Glycolytic capacity Major storage fuel Myosin-heavy chains, human genes

Moderately fast Medium Fairly high Long-term anaerobic <30 min Medium High Intermediate High High Creatine phosphate; glycogen MYH2

Describe how biomechanics determines strength?

Moment arm length -Where our muscles insert affects force generating capacity -Bicep insertion for example: Insert lower on arm ⇒ produce more force

Describe a Wingate result for someone with type I vs type II fibers:

More type II fibers ⇒ high peak power, but declined quick, higher FI More type I fibers ⇒ lower peak power, but declined less over 30s, lower FI

Do motor neurons or muscle fibers modulate fiber properties and adaptive responses:

Motor neurons

Motor unit functional characteristics:

Motor unit contains only one specific muscle fiber type (type I or type II) or a subdivision of the type II fiber with the same metabolic profile Classified based on 3 physiologic and mechanical properties 1. Twitch characteristics (fast or slow) 2. Tension/force characteristics (high or low) 3. Fatigability (high or low)

How can we measure muscle fiber type:

Muscle biopsy of vastus lateralis

3 reasons of how muscles fatigue

Muscle fiber composition (I vs II) Type and intensity of contraction Oxygen availability

Describe how specific force determines strength?

Muscle fiber type -type I, IIa, IIx -II = increased force/CSA due to isoform of myosin heavy chain, CBC, and ATP hydrolysis rate Quality of muscle -fat stores are droplets of fuel for healthy individual -fat stores are streaks within and between muscles so larger proportion of muscle is connective tissue and fat than sarcomere

Pennate

Muscle fibers come in on an angle Run to central tendon at an angle

Bipennate

Muscle fibers run at angle to one another with central tendon in middle of muscle Pulling off to side ex Rectus femoris

Fusiform

Muscle fibers run in a straight line from origin to insertion Not all complete length of muscle necessarily, if one fiber ends there is connective tissue and next fiber just goes in straight line from first fiber All sarcomeres are parallel ⇒ force is added up ⇒ equals muscle force Not losing any force to lateral pull on muscle

Describe how weight lifting impacts O2 availability:

Muscle force compresses arteries (particularly isometric) blood flow is blocked making O2 availability low

Describe the sliding filament model:

Muscle shortens or lengthens because thick and thin filaments slide past each other without changing length Individual filaments DON'T SHORTEN Globular head of myosin crossbridge provides mechanical power stroke for actin and myosin to slide past each other Produces change in relative size within sarcomere's zones and bands; and produces a force at Z bands

What determines muscle strength?

Muscle size Muscle activation Specific force (force / cross sectional area) Biomechanics

The stretch reflex

Muscle spindles detect, respond to, and modulate changes in extrafusal muscle fiber length to provide important regulatory function for movement and posture maintenance Stretch reflex consists of three components: 1. Muscle spindle: responds to stretch 2. Afferent nerve fiber: carries sensory impulse from spindle to spinal cord 3. Efferent spinal cord motor neuron: activates stretched muscle fibers

What is a nerve

Myelinated group of axons

What are muscle fibers made of?

Myofibrils

What do myofibrils contain?

Myofilaments (actin & myosin) that lie parallel to long axis of myofibril

What actions does ATP hydrolysis enable in muscle:

Myosin crossbridges cyclically attach, rotate, and detach from actin filaments Powerstroke

3 ways to confirm fatigue:

Need serial measurements 1. Decline in performance of max effort -Wingate test: pedal as hard as you can -decreases as PCr depletes & H+/Pi concentrations rise 2. Sub max effort with brief maximal test -Lift 50% of max load and measure how this effects maximal contraction -Hand grip: maximal hand grip for 2 seconds and then 10 contractions at 50% max, do another max test -Submaximal task causes fatigue in second max test -Used because we do a lot of sub max exercise and know intuitively it causes fatigue, but can't be sure without measurement 3. Sub max effort until task failure -applicable to everyday life -submaximal weight for 10 reps of bench press -feel tired by 8th rep but no measure of fatigue -try and do 11th rep and can't => task failure => indicates fatigue

What is the dividing line between central and peripheral fatigue?

Neuromuscular junction

What is the central fatigue hypothesis?

Neurotransmitters get out of balance => feeling of fatigue At motor cortex level: balance between dopamine and serotonin and norepi changes => causes fatigue Increased serotonergic => lethargy => loss of drive => reduced motor unit recruitment Supraspinal fatigue => excitability of spinal motoneurons Decreases excitability of alpha motor neurons that go to muscle Feedback from afferents in muscle (sense movement & metabolites like H+ ions)

Describe variability of strength of men and women related to muscle's total cross-sectional area

No difference when comparing CSA and strength relatively (strength/unit muscle CSA)

Sex differences in fibers of a muscle:

Not huge effect Men and women have about the same number of muscle fibers of each type Women may have smaller type IIa fibers

Why does the body use this inefficient way of organizing muscle (we lose force)?

Packing strategy (fit more fibers) More fibers beside each other if packed on angle then on straight line We sacrifice efficiency of force production, but it allows us to fit more fibers in to make muscles stronger We make up for this loss

What locations within a muscle cell could sarcomeres be added to increase muscle force?

Parallel

How would peripheral fatigue affect [Ca+] within working muscle?

Peripheral fatigue is primarily a decrease in Ca sensitivity, but could include decreased Ca release due to low sensitivity to ACh During submaximal contractions we can overcome peripheral fatigue, by increasing central drive to muscle

4 activities that use stretch shortening cycle:

Plyometrics Locomotion Counter movement jump Drop jump

Twitch interpolation at rest:

Pre: Stimulated maximally for 1 twitch Complete maximal activity (using heavy or light weight) Post: Stimulated maximally for 1 twitch Interpretation: Peripheral Fatigue ⇒ twitch amplitude is smaller post than pre

What 3 characteristics differentiate between fast and slow muscle fibers?

Primary mechanism used to produce ATP (oxidative vs anaerobic) Type of motor neuron that innervates Type of myosin heavy chain expressed (isoforms work at different rates)

What does the frequency of stimuli determine?

Rate of recruitment

Fiber Length-Muscle Length Ratio

Ratio of individual fiber length to muscle's total length usually varies between 0.2 and 0.6

Sarcomere Length-Isometric Tension Curve

Relationship between length of sarcomere and amount of tension developed (how much force produced at given length) Muscle produces various force throughout ROM The degree of overlap between actin and myosin determines the force of an individual sarcomere

What is the signal that starts muscle contraction?

Release of calcium from sarcoplasmic reticulum (activates actin filaments)

Motor neuron pool

Represents all the alpha-motor neurons that innervate one muscle

Motor unit

Represents an alpha-motor neuron and the fibers it innervates Alpha motor neuron leaves spinal cord and branches off to innervate individual fiber Contraction/firing= all or none (in a motor unit all fibers fire or none)

What is peripheral fatigue? 4 sites:

Sarcolemma excitability -membrane around fiber requires more ACh binding to depolarize -Na/K balance fluctuating from normal, net K+ efflux -Takes a bigger electrical input to depolarize Excitation-contraction coupling -when sarcolemma depolarizes, Ca is released for binding, but with additional Pi and H+ sarcolemma sensitivity to Ca decreases -less cross bridges form -Ca release decreases Contractile mechanism -impairment of contractile force or velocity distal to the NMJ -Pi competes with Ca for troponin binding -Decreases cross bridge formation Metabolic energy supply -run out of fuel => produce less force -low glycogen is associated with a faster decrease of availability in the sarcoplasmic reticulum Ca2+ during dynamic contraction -change substrate => rate of energy production reduced (fat)

What is the smallest functional unit of a muscle?

Sarcomere

How do you know central fatigue has occurred?

See an increase in force production with max electrical stimulation

Sarcomeres lie in _______ and their filaments have a ________configuration within a given fiber.

Series Parallel

Unipennate

Simplest Fibers come on angle to central tendon Pull off to side

Type of muscle fiber that dominates each muscle: Soleus: Tricep: Vastus lateralis Endurance athlete Power athlete

Slow Fast 50/50 Slow Fast

Type I Fibers: Contraction Time Size of motor neuron Resistance to fatigue Activity used for Max duration of use Force production Mitochondrial density Capillary density Oxidative capacity Glycolytic capacity Major storage fuel Myosin-heavy chains, human genes

Slow Small High Aerobic Hours Low High High High Low Triacylglycerol MYH7

How does stretch shortening cycle work when you jump down from box and jump up again?

Starting Position: on box, stepping off box Lengthen: eccentric phase when you land on ground; stretching spindles and storing elastic energy for jump up Load: bottom of landing (amortization phase); brief isometric contraction Fire: concentric phase where athlete explodes up

Describe events of unfused tetanus

Stimulate nerve ⇒ allows some Ca to re uptake Stimulation keeps happening ⇒ increasing Ca concentration ⇒ high force (staircase)

Allometric scaling

Strength proportional to CSA (force) and body mass is proportional to muscle volume (weight). With this, small people are better at lifting a given % of their body mass. The best way to correct this relationship for varying body masses is to go strength / body mass to the power of 2/3

What is the best way to compare strength of men and women:

Strength score (weight lifted) /FFM Creates ratio scores that considerably reduce/eliminate the large absolute strength differences usually observed between genders

Why might stepping off a box before jumping increase the height of that jump?

Stretch Shortening Cycle -Higher box = rapid more forceful eccentric contraction ⇒ leads to stronger concentric contraction during jump (all components of stretch shortening cycle are enhanced) -Increased storage elastic energy -Increased initial force -More forceful stimulation of muscle spindles -Box height has upper limit

You fall asleep in class, your head begins to drop then jerks back to an upright position, why?

Stretch Shortening Cycle -Muscle spindles sense stretch of head dropping (rapid stretch) -Afferent signal sent to spinal cord which synapses with alpha motor neuron of same muscle -Muscle contracts and head jerks back upright -More rapid/forceful eccentric contraction = stronger concentric contraction as a result

Series elastic component

Stretches with force and allows for force transfer to muscle (takes time) Stores energy

Describe motor unit recruitment for moderate intensity efforts:

Stronger signal/greater stimuli) More motor units recruited (type IIa) PLUS ones we already turned on (type I)

Describe motor unit recruitment for high intensity efforts:

Strongest signal Even more motor units recruited, type IIx, in addition to type IIa and I

Twitch interpolation during MVC:

Superimpose an involuntary stimulus while the subject is performing a maximal voluntary contraction (MVC) of the muscle Central fatigue -Spike in force during MVC from extra electrical input -Difference MVC and electrical stim No central fatigue -No spike in force during MVC from additional electrical input -Indicates brain was already sending a max signal down to muscle -No difference in MVC vs electrical stim

Describe the purpose of gels and endurance exercise;

Supposedly to help transport nutrients across blood brain barrier Doesn't really work in this way, it is just sugar

Transverse tubules

T-tubule system that runs perpendicular to myofibril and terminates in saclike vesicles that store calcium. Functions as a micro-transportation network that spreads action potential from outer membrane The waves of depolarization that travel down t-tubule system stimulates release of Ca from SR

Patella tendon stretch reflex

Tap stimulates muscle spindle causing contraction of same muscle from alpha motor neuron

Describe the A band during an eccentric contraction:

The A band (thick filament) widens in eccentric action as the fiber lengthens during force generation

When lifting 30% of 1RM to failure Ca+ sensitivity within type I muscle fibers may decrease how would the body compensate for this?

The body overcomes decrease Ca sensitivity within type I muscle fibers (peripheral fatigue) by sending more central drive/electrical signals from motor cortex, down spinal cord to alpha motor neuron pool which would recruit more motor units or recruit motor units at higher firing weight

Describe I and A band spacing during an isometric contraction:

The fiber's length remains unchanged so relative spacing of the I band and A band remains constant Cross bridge binding, but no sliding ⇒ no change in length

What happens if box is too high?

The load phase period is too long and we lose elastic energy in tendons causing lower force Eccentric contraction is too hard that the body can't resist it => force overwhelms tendon capacity so box is no longer beneficial

Why do type II fibers fatigue more quickly?

They have the ability contract and relax at a faster rate, but the consequence of this is that both actions use ATP Higher force from each stimulation, faster cross bridge cycling ⇒ fatiguing Fewer mitochondria ⇒ less optimized for aerobic ATP production Use of PCr and anaerobic glycolysis ⇒ limited time span Central fatigue ⇒ less electrical activity ⇒ type II are first to drop out

Skeletal muscle

Tissue that connects bone to bone via tendon on each end

What is the most important role of ATP in muscle action

To release cross bridge

What regulates make and breaks in contact between myofilaments during muscle action?

Tropomyosin and troponin

True or false: For intense exercise we recruit type I, type IIa and then type IIx

True

Describe turkey makes you tired statement:

Tryptophan amino acid is very high in turkey It shares receptors with leucine to get across blood brain border It binds to these and alters the release of other neurotransmitters This is probably not fully true

Describe events of wave summation

Twitch ⇒ electrical impulse down nerve ⇒ Ca release ⇒ force increases ⇒ Ca back into SR Second twitch comes before all Ca back in SR ⇒ more Ca release ⇒ higher than after 1 twitch Result: Two twitches close together = more force than first twitch alone via mechanism = Ca concentrations in muscle start to rise

How would calcium concentrations in the muscle be changed during twitch, unfused tetanus and fused tetanus contraction?

Twitch: lowest Ca concentration (released and completely reuptaken- relaxation) Unfused: moderate Ca concentration; higher than during single twitch because Ca doesn't have time to be completely reuptaken into sarcoplasmic reticulum before second depolarization (some accumulation- staircase pattern) Fused: highest Ca concentration; there is no reuptake of Ca into SR so accumulation of Ca occurs with each depolarization

3 types of muscle fibers

Type I Type IIa Type IIx

Describe the H+ and Pi concentrations in type I and type II fibers

Type I: low H+/Pi concentration -aerobic system reduces production of H+ ions from lactic acid -slow hydrolysis of ATP to ADP/Pi and good resynthesis of Pi to ATP keeps levels lower Type II: high H+/Pi concentration -Good at using ATP so make lots of Pi -Not good at resynthesizing Pi to ATP -Not good at using O2 so lots of lactic acid and H+ ions made

What muscle fiber type is preferential to hypertrophy?

Type IIx

What mechanism differentiate fused and unfused tetanus?

Uptake of calcium into sarcoplasmic reticulum

Two examples of skeletal muscle:

Ventilatory muscles Peripheral muscles

Why does lifting a light load become hard overtime?

We need more neural activity to get same force as peripheral fatigue occurs

Describe motor unit recruitment for light intensity efforts:

We only recruit type I motor units Type II a/x are inactive Don't need to produce a lot of force We want to maintain force this activity over time

Describe the stretch reflex and dozing off:

Weight of hanging head leads to rapid stretch in neck ⇒ detected by muscle spindles ⇒ alpha motor neuron in spinal cord stimulates head to pop up

What activity causes more peripheral fatigue. How is this shown on graph:

You see peripheral fatigue due to large difference in twitch amplitudes You see minimal central fatigue due to minimal spike in force during max contraction from extra electrical input ⇒ means brain was already sending max signal

What bisects the I band? Providing what?

Z line Structural stability

What is the z line

end of each sarcomere

3 layers of connective tissue

epimysium: surrounds individual fibers perimysium: surrounds fascicles endomysium: surrounds entire muscle

What is the m band? Consists of?

middle of a sarcomere bisects H zone consists of protein structures that support arrangement of myosin filaments

What is a cross bridge?

myosin bound to actin

Individual fibers in the longest muscles (e.g., such as those of the upper and lower limbs) remain significantly _______than the muscle's overall length

shorter

What does sarcomere length determine?

the muscles functional properties

When does electrical excitation cease?

when Ca2+ concentration in cytoplasm decreases, and the muscle relaxes

When does muscle action begin?

when myosin filament cross bridges attach to active sites on actin