Week 7 Muscle II: Contraction- Force and Movement

parameters of muscle performance

strength, speed, power

main element/purpose of muscle

contraction to generate force

motor unit 

functional unit of contraction: Alpha motor neuron and all the muscle fibers it innervates 

innervation ratio 

number of muscle fibers innervated by a given alpha motor neuron, lower ration means less fibers recruited, likely a smaller motor neuron

hand, extraoculuar muscles

examples of muscles with low innervation ratios, involved in fine movements and precision

large innervation ratios

muscles groups with these innervation ratios tend to be better at large force production and not so much fine movements

heart muscle 

• no motor units, one syncitium (single thing) connected by gap junctions 

• 10 fold from rest to aerobic max 

• doesn’t rest 

• produces only twitches 

skeletal muscle

• range of motor units 

• can be 1000 fold from rest to aerobic max 

• can rest 

• produces twitch, steady tetanus, impulse, any level in between 

function of motor units

allow us to recruit a greater or lesser portion of the muscle, determining force range

S (Slow)

motor unit classification that characterizes a motor unit that generates generally less force but sustains it over time

FFR (fast fatigueable resistant)

motor unit type that is characterized by medium to larger generation of force that descreases incrementally, is resistant to fatigue

FF (fast fatiguable)

motor unit type that is characterized by large force generation that rapidly decreases over time

CT (contraction time)

time it takes for peak tension to be achieved

½ RT (half relaxation time)

time it takes for force to drop to half of its peak force

2-3 times stronger

how much stronger FF are than FFR 

50% as strong

how strong S motor units are compared to FFR

2x as long

twitch contraction time of S compared to FF

example showing the vast differences across motor unit types in diffeerent muscles

• type FF in diaphragm has a TCT of 34 ms

• type S in peroneus longus has a mean TCT of 30.9 ms

generally proportional

relation of size of neuron to the number of fibers innervated by an alpha motor unit

FF > FFR > S (generally)

general schema of number of muscle fibers innervated by the different motor unit types

recruitment (number of units firing) and rate coding (discharge frequency of the motor unit)

two factors that modulate force in skeletal muscle, can be done expirementally by applying electrical stimulation

size principle 

smaller motor units tend to be activated first and subsequent units were recruited in order of increasing size (Henneman’s, holds true for S motor units)

Fast motor units

motor unit type that doesn’t necessarily follow the size principle

recruitment of fast motor units 

tend to be recruited in order of increasing force, not necessarily size 

conduction velocity

technique that Henneman used in order to extrapolate the size of the alpha motor neuron tested in his size principle experiments

rate coding

aka discharge frequency, temporal summation in order to increase force of a muscle contraction (increase of frequency of firing)

MU are recruited and derecruited in order- the first ones on are the last ones off

interesting fact about recruitment and derecruitment 

force frequency relationship 

as the frequency of stimulus increases, so does the force generated- seen at the level of the muscle fiber, motor unit, whole muscle 

Ca2+

interaction of the force frequency relationship and excitation contraction coupling interaction can be seen through behavior of contraction in the reduction of this ion at low frequencies

greater effect 

reduction of Ca2+ release has what kind of effect at low frequencies because our bodies operate at a lower frequency of about 30Hz and according to graph a small change in Ca2+ will result in a greater drop in force

less effect

reduction of Ca2+ release has what kind of effect on force generation at high frequencies 

greater Ca release (less at lower intensities)

calcium release behavior when there is greater intensity stimulus

decreased maximum and mean discharge rates

things that may worsen force reduction caused by decreased calcium release

histochemical technique in identifying fiber types 

based on the pH lability of myosin ATPases, resulted in 2 divisions Type I and Type II, correlated to slow and fast fibers, newer refinement has ID up to 7 recognized human fiber (some may be hydrid)

basic pH

pH which resulted in stronger staining of fast muscle fibers (Type II)

acidic pH

pH which resulted in stronger staining of slow muscle fibers (Type I)

biochemical technique in identifying fiber type

activity of specific enzymes associated with metabolic process can be used to classify the fibers- can ID when combined with contractile properties as SO, FOG, or FG using citrate synthase, lactose dehydrogenase, SDH, GPDH, or glycogen content

current method of classifying fiber types 

done by targeting spefic antibodies to different myosis heavy chain isoforms, groups fibers into 3 main groups- Type I, Type IIa, Type IIx 

hybrid fibers

multiple (usually 2) MHC isoforms expressed in a fiber, often seen in transitions like detraining, training, or bedrest

Type IIb

NOT found in humans

characteristics of Slow (I) muscle fibers

low myosin ATPase activity and glycogen content, anaerobic enzymes; high fatigue resistance, oxidative capacity, myoglobin content; small fiber diameter; many capillaries, mitochondria; slow speed of contraction

characteristics of fast (IIa) muscle fibers 

high mysoin ATPase activity, oxidative capacity, myoglobin content; fast speed of contraction; many mitochondria and capillaries; intermediate fatigue resistance, anaerobic enzymes, glycogen content 

characteristics of fast (IIx) muscle fibers

high myosin ATPase acitvity, anaerobic enzymes, glycogen content; fast speed of contraction, low fatigue resistance, oxidative capacity, myoglobin content; few mitochondria, capillaries; fast speed of contraction

muscle spindle

essentially a sensory mechanoreceptor within the muscle, housed within the extrafusal fibers (typical myofibers of the muscle), contains intrafusal fibers that is innervated by the Ia/II afferent nerve fibers which send signals back to the spinal cord and responds to the stretch via the gamma motor neuron to contract the muscle

gamma motor neuron

aka fusimotor neurons, cell bodies in motor pools interspersed with alpha MNs, controlled from above by CNS, not impacted by spinal nerves, project to intrafusal fibers of the muscle spindle, cause contraction of the polar ends of spindles

alpha motor neuron 

lower motor neurons in ventral horn of spinal cord, innervates extrafusal skeletal muscles, voluntary movement 

alpha-gamma co-activation

normal movement and posture involves co-activation of these, as Ia afferent activity is required to enable max motor unit discharge rates during voluntary contraction, the tension on muscle spindle maintained as muscle shortens through activation of gamma afferent nerves as the alpha motor nerve fires, sensitivity of muscle spindle to stretch is maintained

golgi tendon organs 

infor on tension is transmitted through IIb afferents, reflex inhibition of antagonist alpha motor neurons at spinal level, no motor innervation

GTOs and spindles

muscles sensors that also provide signals to the brain to factor into motor control

the number of strongly bound crossbridges and physiological cross sectional area

what force is a function of

in parallel 

stronger muscles will have more sarcomeres in this pattern 

in series

higher velocity muscles will have more sarcomeres in this pattern

physiological cross sectional area 

accounts for pennation angle of muscle, CSA x cos(pennation angle), typically lose some force generating ability per fiber but we are able to fit more fibers in when they are pennated so altogether we are able to produce more force 

lowest/unloaded

velocity of shortening is the fastest when load is what

force-velocity curve

• muscle maximally activated then allowed to shorten, because longer muscles have more sarcomeres in series they shorten at a greater rate

• when extends into the eccentric range- negative shortening velocity

• note higher forces vs shortening or even isometric

lower cross sectional area 

green muscle line indicates this amount of cross sectional areal compared to the red muscle line (since its generates less force and they move through the same ROM)

more sarcomeres in series

orange line muscle has how many sarcomeres in series as compared to the black line muscle (but equal PCSA bc generate the same force)

both force and velocity

what power is a function of, factors affectign one will impact power, changes in both will have an additive effect

power 

perhaps the most functional parameter of muscle, ability to do generate force rapidly, often more important than absolute maximum force/torque production 

whole body mechanics

factors that contribute to this are at the fiber level (length-tension and force-velocity), multiple motor units (force frequency, recruitment and rate coding, contractile properties), connective tissue components (intramuscular, fascia, tendon which all add passive tension), moving in space

mechanisms affecting muscle force can act at any of these sites 

• neural sites: cortical excitability, spinal excitability, maximal motor unit discharge, nerve conduction 

• intramuscular sites: muscle architecture, muscle mass, increased myocellular lipid content, excitation contraction coupling 

so many areas where things can go wrong, not just at the muscular mass level! 

isometric contraction 

contraction that results in constant length, more metabolically demanding that eccentric but less so than concentric 

dynamic isotonic contractions

• constant tension- against a weight

• will result in change in muscle length or joint angle

• eccentric and concentric are types

concentric contraction

most metabolically demanding contraction, constant tension that results in muscle or joint angle shortening 

eccentric contraction

least metabolically demanding contraction, constant tension that results in muscle or joint angle lengthening, may be able to generate greater amounts of force due to gravity assistance, stored elastic energy, and passive tension, deceleration, less metabolically demanding (physically breaking crossbridges instead of using ATP to do so) 

dynamic isokinetic contraction

constant velocity, requires special equipment, few functional occurrences, can be performed concentrically or eccentrically (usually concentric)

joint angle (affectign angle of force application and lever arm and muscle length), visco-elestic properties, angle of force application

factors that effect deviation from the classical length tension curves, or force and power predicitons, as muscles don’t work in isolation as is normally tested experimentally (and was confirmed in the intact gracilis muscle date from Lieber and Binder Markey)