NEUR30080 L10 SF striated muscle: mechanical properties

learning outcomes

functions of skeletal muscle

• to move the body limb by creating motion

• to provide strength by generating active force

• to protect joints by absorbing shock

specific functions of connective tissues within muscle

• provide gross structure to muscle

• generate passive tension against stretch

• to transmit force to the bone and across the joint

properties of skeletal muscle

tendon or fascia

• viscoelasticity

• non-contractility

• tendon is like a tight-fitting wetsuit

note about tension and muscle length

• increase in tension does not imply decrease in muscle length

mechanism of single fibre contraction

• contraction refers to the turning on of the cross-bridge cycle

• note: whether there is an accompanying change is muscle length depends on the external forces acting on the muscle

• contraction: the thick and thin filaments do not shorten, they increase their overlap

activation of a muscle fibre: 3 types of contraction

1. isometric

2. isotonic concentric

3. isotonic eccentric

isometric contraction

the muscle generates tension but does not change length

e.g. wall sit

tension produces never exceeds load

isotonic concentric contraction

the muscle shortens, moving a load

isotonic eccentric contraction

the external load on the muscle causes the muscle to lengthen during the period of contractile activity

slow movement, ‘negatives’ in gym

types of muscle contraction image

isotonic mnemonic

isotonic - taking gin and tonic (moving)

activation of a muscle fibre - sarcomere level

which involves sarcomeres shortening and muscle remaining the same length?

isometric contraction

which involves sarcomeres shortening even more, shortening the entire muscle?

isotonic contraction

whole muscle contraction

• the tension prod by whole muscle contraction depends on the amount of tension developed by each fibre and the number of active fibres in the muscle

• movement around a joint requires 2 antagonistic groups of muscles

  • one flexes at the limb joint

  • other extends at the limb

• this lever system of muscles and bones req muscle tensions far greater than the load in order to sustain a load in an isometric contraction

• the lever system produces a shortening velocity at the end of the lever arm that is greater than the muscle-shortening velocity

antagonistic muscles example

biceps and triceps

types of skeletal muscle fibres: collection of fibres

• muscle composed of heterogenous collection of different fibres

• range of muscle fibre types facilitate range of muscle capacities

muscle fibre classification criteria

• 3 types of skeletal muscle fibres are classified based on 2 criteria:

  • how fast? (maximal shortening velocities)

  • how do they regenerate ATP? (main pathway)

3 types of muscle fibres

1. slow twitch (Type I) / slow oxidative

2. intermediate twitch (Type IIA) / fast oxidative

3. fast twitch (Type IIB) / fast glycolytic

proportions of fibres in muscle

• most muscles contain varying proportions of each fibre type

• muscles can adapt to changing demands by changing size or fibre type composition

types of muscle fibres image

types of muscle fibres: speed

• maximal shortening velocity determines speed

• different maximal shortening velocity due to different myosin enzymes with high/low ATPase activities - giving rise to fast/slow fibres

• fast-glycolytic have larger avg diameter than oxidative - therefore produce more tension BUT fatigue more rapidly

motor unit and fibre types

all the muscle fibres in a single motor unit belong to same fibre type, and most muscles contain all 3 types

types of muscle fibres depending on requirement

• no. of slow or fast twitch fibres in body varies - genetics

• slow twitch: endurance sports

• fast twitch: sprinting

• can influence with training

muscle twitch of different muscle fibres graph

how are muscle fibres distinguished?

based on the time it takes for them to contract

Type I fibres: overview

• slow oxidative (slow and low intensity contraction)

• red (high myoglobin & rich capillary supply)

• isoform of myosin with low ATPase activity (ATP split slowly)

• thin fibres

• numerous mitochondria

• high aerobic enzymes (ATP prod)

• slow to fatigue, sustained contraction possible

note: red muscles contain predominately, but not exclusively, red muscle cells

Type I fibres: applications

• postural control: maintaining upright posture

• endurance and stability: stabilisation of bones and joints

• limitations: NOT suitable for powerful, fast or high-tension, explosive movements

Type IIA fibres: overview

• fast twitch, oxidative-glycolytic

• red/pink

• numerous mitochondria

• aerobic and anaerobic respiration (ATP prod)

• rapid ATP prod vs TI

• more fatigue resistant than TI

Type IIA fibres: applications

• activities requiring moderate-to-high energy and endurance combined with speed

• e.g. walking, mid-distance running, swimming

Type IIB fibres: overview

• fast-twitch

• white fibres (in white muscles)

• thicker

• less myoglobin

• few mitochondria

• high glycogen

• high ATPase activity (rapid energy use)

• anaerobic glycolysis (ATP prod)

• fast, forceful contraction

• fatigue quickly

Type IIB fibres: applications

• rapid accelerations, short-lasting maximal contractions, quick and powerful movements

• fatigue v quickly

muscle fibre types image

muscle fibres in athletes

• fibre type varies in individuals, heavily dictates athletic predispositions

• quadriceps femoris example:

  • sprinters: <20% slow-twitch TI, many fast-twitch white fibres TIIB

  • marathon runners: >95% slow-twitch TI

summary: types of skeletal muscle fibres

table summary: types of skeltal muscle fibres

muscle twitch

• neuromuscular transmission lasts 1-2 ms

• latent period: time between stimulus and beginning of muscle contraction

• contraction phase: peak tension reached in myocyte after 50 ms

• relaxation phase: Ca2+ actively transported back into SR

graded muscle response

for proper control of skeletal movement, there are variations in strength of muscle contraction in response to demand

graded muscle response: the rate principle

frequency of stimulation - summation

graded muscle response: the size principle

number of motor neurons recruited - multiple fibre summation

rate principle/rate coding

the neural input

summation

the mechanical output

a twitch

smallest unit of muscle contraction, generates little force (is weak)

summation and tetanus

1. single AP - twitch lasts <100 ms

2. summation - 2nd, 3rd etc AP arrives before muscle fibre completely relaxed, force of contraction summates

3. incomplete tetanus - quick cycles of contraction w/ short relaxation

4. complete tetanus - frequency of stimuli so high that fibre cannot relax in between contractions, twitch fuses into oen sustained contraction

summation and tetanus: image

motor unit recruitment

• muscle fibre/myocyte must be innervated by axon terminal of motor neuron to contract

• motor unit = a single alpha1 or 2 motor neuron and all the muscle fibres/myocytes it innervates

• the size of motor unit variable depending on muscle function

small motor units

• smaller and fewer myocytes

• for fine motor control of the muscle

• extraocular eye muscles (eye movement)

• approx. 6 myocytes per unit

larger motor units

• high myocyte amount

• simple, gross movements

• e.g extending limbs

• thigh and back muscles

• single motor neuron innervates 1000s of muscle fibres

fibres per motor unit

• muscles that produce delicate movements have small no. of fibres

• large postural movements have much larger motor units

• fast-glycolytic motor units have large-diameter fibres and tend to have large numbers of fibres per motor unit

large vs small motor unit image

motor unit function

• AP from motor neuron - all fibres in motor unit contract as a group

• strength of contraction depends on how many muscle fibres in the MU

• multiple motor neurons stimulated at once to gen more force

• size principle - smaller MUs recruited first, larger MUs recruited to larger stimuli

asynchronous activation of MUs

• muscles maintaining body upright

• NS alternates between diff motor units of similar size - reduce length of contraction time - fatigue

• prolonged contraction - ATP depletion and dissipation of ion gradients necessary for contraction

MU recruitment: cross-innervation exp

• when a slow motor neuron innervates a fast muscle, the muscle switches its properties to become slow-like (& vice versa)

• muscle fibres are largely dictated by the pattern of neural stimulation they receive for their motor neurons

MU recruitment

• recruited according to the ‘all or none’ principle

• once recruited, the MU will contract to its fullest extent regardless of the strength of impulse causing the contractions

• the order of recruitment - SMALLER MUs FIRST

recruitment definition

the process by which muscle tension is controlled primarily by increasing the number of active motor units in a muscle

• increasing MU recruitment increases velocity of contraction

size principle image summary

rate principle image summary

skeletal muscle fibre APs image

isometric contraction graphs image

isometric contractions prod by multiple stimuli image

muscle fatigue during isometric tetanus image

length-tension curve image

shows myosin-actin overlap

length-tension relationship

• the principle that a muscle’s ability to generate force depends on its resting length, which dictates the optimal amount of overlap between its actin and myosin filaments

tension

• the pulling force gen. by actin and myosin cross-bridge formations

• magnitude det. by stimulation freq. & initial resting length of muscle

• maximal tension readily produced in body as CNS maintains resting muscle length near the optimum

isometric tension

• produced at the optimal sarcomere length (L0)

• stretching fibre beyond its optimal length or decreasing the fibre length below L0 decreases the tension gen

stress=length relationships in isometrically contracting muscle

muscle fatigue

mechanisms of muscle fatigue

causes of muscle fatigue

exercise of muscle to alter susceptibility of fatigue

muscle damage and repair

case-study: decathlete

came 18th place

learning outcomes