Lecture 4-5: Muscles Flashcards
What are muscles?
biological actuators that drive the stiff levers of the musculoskeletal system
Why do muscles attach so close to the fulcrum?
because they are good at generating force, but not very good at getting shorter – it also keeps them out of the way
What is the pattern of contraction of a muscle at the end of a limb? What does this require?
contract a short distance, but produce long movement – requires small dE and large dL
What is the structure of a muscle? (4)
- myofibril – basic unit of the muscle that contracts to shorten the muscle and generate force
- muscle fibre – formed by many myofibrils
- muscle fascicle – formed by many muscle fibres
- muscle – formed by many muscle fascicles
What type of muscle is skeletal muscle?
striated muscle
What is a sarcomere?
functional unit of the muscle
What causes muscle contractions?
myosin thick filament heads form cross bridges with actin filaments, then pull on them to cause contractions
In what direction do striated muscle fibres shorten?
in the direction of the contracting muscle
By how much do striated muscles shorten?
by only 20-25% of their relaxed length
What determines the (absolute) contraction distance of a muscle?
all muscle sarcomeres shorten by approximately 20% when they contract, therefore absolute distance a muscle can contract is due to its length – proportional to the number of sarcomeres in series
ie. 1 m muscle can shorten by 20 cm (20% of 1 m)
ie. 10 cm muscle can only shorten by 2 cm (20% of 10 cm)
Do shorter or longer muscles have a higher contraction speed?
longer muscles are faster – because the sarcomeres in series are shortening simultaneously
What determines contraction force?
force produced by a muscle is proportional to the number of sarcomeres in parallel
more sarcomeres in parallel = more force generated
What is the cross-sectional area of muscle proportional to?
number of fibres
therefore, also proportional to the force it can exert
What is displacement (contraction distance) of a muscle proportional to?
muscle length
therefore, the work a muscle can do is proportional to its volume (W = F x d)
What is muscle volume?
cross-sectional area (force) x length (muscle shortening or displacement)
Do shorter or longer muscles have longer maximum displacement?
longer
How much can sarcomeres contract?
by around 20% of its relaxed length
What is contraction speed determined by?
number of sarcomeres in series
important metric: length
What is contraction force determined by
number of sarcomeres in parallel
important metric: cross-sectional area
How do vertebrate fibres types differ?
- different mechanical properties
- different composition and activity of myosin heavy chain (myosin head has many isoforms)
- different myofibrillar ATPase activity
How are vertebrate fibre types similar?
sarcomere lengths are invariant
Type I Muscle
- Motor Unit Type
- Contraction Force (High/Low)
- Contraction Speed (High/Low)
- Time to Fatigue (Long/Short)
- ATPase Activity (High/Low)
- motor unit type: slow twitch oxidative (SO)
- contraction force: low
- contraction speed: low
- time to fatigue: long
- ATPase activity: low
Type II Muscle
- Motor Unit Type
- Contraction Force (High/Low)
- Contraction Speed (High/Low)
- Time to Fatigue (Long/Short)
- ATPase Activity (High/Low)
- motor unit type: fast twitch oxidative (IIA), glycolytic (IIB)
- contraction force: medium
- contraction speed: high
- time to fatigue: short
- ATPase activity: high
How do invertebrate muscles differ?
- different ATPase activity
- different sarcomere lengths
What is the force (high/low) and speed (high/low) of invertebrate long sarcomeres?
high force, low speed
- more myosin/actin cross-bridges pulling directly on the load
- can only pull the load as fast as each myosin head can move
- each myosin head generates 1 unit of force
What is the force (high/low) and speed (high/low) of invertebrate short sarcomeres?
low force, high speed
- myosin/actin cross-bridges pull on each other, as well as on the load
- each sarcomere will pull on adjacent sarcomeres and their speed will add up
What is specific force production?
force / cross-sectional area of muscle
What is the unit for tension?
Pa
When is tension measured?
during isometric (non-moving) contraction
How is force related to shortening velocity (speed)?
force decreases as shortening velocity increases
- cross-bridges are being made and broken more quickly as speed increases – at any given moment, there are fewer actin/myosin cross-bridges and therefore less force
What is isometric contraction?
all force, no speed
What is unloaded contraction?
all speed, no force
Describe the relationship between muscle force and power?
maximum power is reached at 15-40% of the maximum force (depending on the muscle), then it begins to decline
When is there no work being done?
no displacement of weight = no work done and no energy expended
Does a rope need to expend any energy generating a tension opposing the weight?
no
Does a muscle need to expend energy to maintain a tension to support the weight?
no
- myosin heads continually make and break contact with actin filaments, consuming ATP
- can think of it like a motor running against a slipping rope – if the motor turns, it keeps tension on the rope (which consumes energy), and if the motor stops, the rope will slide through and the weight will drop
What has a big effect on a muscle’s functional properties?
arrangement of fascicles (and therefore fibres)
What is the shape of pennate muscle fibres?
parallelogram-shaped (area = width x length)
this does not change as fibres contract, therefore volume is approximately constant
Do pennate muscle fibres bulge when contracting?
no, due to their shape
Parallel vs. Pennate Muscle Fibres
Length
parallel: long fibres – can contract further
pennate: short fibres – short contraction distance
Parallel vs. Pennate Muscle Fibres
Number of Fibres in a Given Muscle Volume
parallel: few fibres – produce lower forces
pennate: more fibres – produce higher forces
Parallel vs. Pennate Muscle Fibres
Force Orientation
parallel: oriented along muscles’ line of action – ie. both muscle and fibres contract along the same direction
pennate: oblique to the muscles’ line of action (pennation angle θ)
Parallel vs. Pennate Muscle Fibres
Bulge
parallel: outward
pennate: (mostly) do not bulge, therefore occur where space is an issue and/or there is a requirement for generating large forces
What are the 3 types of pennate muscle?
- unipennate
- bipennate
- multipennate
What are unipennate muscles?
all fascicles of the pennate muscle are on the same side of the tendon
What are bipennate muscles?
fascicles lie to either side of the tendon
What are multipennate muscles?
central tendon branches within a pennate muscle
What are the 3 components of the force generated by the fibre?
- F fibre – force in line with the fibre
- F muscle – force in line with the muscle
- F perpendicular to muscle – force perpendicular to the muscle
F muscle = cos(θ) x F fibre
Pennate Pump Muscles – Calculations
see slides
What is gearing?
trading force for distance (same as how levers conserve work)
The skeletal system can alter what part of a force?
how the force generated by a muscle translates into high force/short distance (MA) OR low force/long distance (DA)
What can muscle fibre arrangement within a muscle alter?
- velocity of contraction
- force generated by the muscle
What is the architectural gear ratio (AGR)?
ratio of whole muscle contraction velocity to fibre contraction velocity
What is AGR in parallel muscle?
AGR = 1
individual muscle fibres are oriented in the same direction as the whole muscle, therefore muscle contraction velocity is equal to fibre contraction velocity, and therefore AGR = 1
What is AGR in pennate muscle?
AGR ≠ 1
rate at which pennate muscle contracts depends on pennation angle θ of the fibres, therefore muscle contraction velocity is NOT equal to fibre coontraction velocity
What happens to muscle contraction speed when pennation angle θ increases?
speed increases
What part of a right-angle triangle is a muscle fibre in a pennate muscle equivalent to?
hypotenuse
What part of a right-angle triangle is muscle length equivalent to?
adjacent side
How is muscle fibre (hypotenuse) and muscle length (adjacent side) related?
if the muscle fibre (hypotenuse) contracts at a constant rate, muscle length (adjacent side) grows shorter faster
Does muscle contraction velocity exceed fibre contraction velocity, or vice versa?
muscle contraction velocity exceeds fibre contraction velocity – it increases as pennation angle increases
Which muscle fibre type generates more force?
type II
What length of invertebrate sarcomeres generate more force?
longer sarcomeres
Does more or less muscle cross-sectional area generate more force?
more cross-sectional area (more sarcomeres in parallel)
Do parallel or pennate muscle fibres generate more force?
pennate muscle fibres – force is highest at low pennation angle θ < 30º
In a lever system, does MA or DA generate more force?
MA as large as possible
What type of muscle fibre is faster?
type II
What length of sarcomeres are faster?
short sarcomeres
- speed increases as more sarcomeres in series contract simultaneously
- short sarcomeres = more units contracting per fibre length = faster
- long sarcomeres = fewer units contracting per fibre length = slower
Are parallel or pennate muscle fibres faster?
parallel – speed increases as more sarcomeres in series add up
In a lever system, does MA or DA generate faster speed?
DA as large as possible
Muscle fibres of identical resting length, but different sarcomere length.
- fibre with longer sarcomeres generates more force – sarcomere length determines force
- fibre with shorter sarcomeres contracts faster – contraction velocities of more (shorter) sarcomeres in series add up
Muscle fibres of different resting length, but identical sarcomere length.
- both generate the same amount of force – only sarcomere length determines force, therefore same sarcomere lengths results in same force generated
- fibre of longer resting length contracts faster – more sarcomeres in series, therefore faster contraction
Muscle Speed and Force Variation
- muscles with short sarcomeres: fast, but less forceful
- pennate muscles: slow, but more forceful
- type II fibres: more forceful than type I fibres (but still limited)
Can a lever system increase a muscle’s power?
NO
levers conserve work (W = F x d)
- F and d are inversely related and occur during the same time, for the same duration
- can never increase power in a lever system because work and time are the same
What can the work a muscle does be stored as?
can be stored by an elastic mechanism as potential energy, which can then be released very rapidly to move a lever system, without the force/velocity contrasting that afflict molecular motors
Catapult Example – Calculations
see slides
Describe elastic energy storage.
rather than using gravitational potential energy to store the work done by slow, forceful muscles, animals can use elastic potential energy – invertebrates store this in specially modified parts of their exoskeleton
How does a power amplifier work?
take the slow, low power contraction and turn it into a rapid, high power release
