Muscle L1: Biomechanics of musculotendinous unit Flashcards
What are the 4 types of tissue?
- Epithelial tissue
- Muscle tissue
- Nervous tissue
- Connective tissue
What are tissues?
Tissues are groups of similar cells and their extracellular products, organised to perform a common function.
What is epithelial tissue?
covers body surfaces, lines cavities & forms glands
What is muscle tissue?
responsible for movement, interaction with the environment
What is nervous tissue?
receives, transmits & integrates information to control the activities of the body
What is connective tissue?
supports the other 3 tissues
A muscle (organ) = _____ + ______
muscle tissue + connective tissue
What is extracellular matrix (ECM)?
- A substance produced by the cells of a specific tissue
- Can contain protein, salts, H2O, and dissolved macromolecules
- Located outside of cells
- Respond to physical stresses
- Muscle tissue has very little (if any) ECM – rather, it is surrounded by connective tissue **referred to as ECM in Oatis and some other sources. Connective tissue has a significant ECM
Microstructure of muscle belly: Muscle + Connective Tissue influence function
I
What is endomysium?
An array of “tubes” into which muscle fibres insert Image
What is perimysium?
Thickened area surrounding groups of the muscle fibres Image
Muscle is made up of a large number of bundles (_____) of muscle fibres (muscle cells), surrounded by _______ tissue
fascicles connective tissue
A single muscle fibre (cell) is very long. It runs ____ to other fibres.
parallel
The fibres is made from a large number of ________, therefore a single muscle fibre has _____ nuclei.
fused embryonic cells many
Inside each cell is a bundle of _____ (muscle- slender fibre), that lie in parallel
myofibrils
These are the contractile filaments that convert the electrical signal (action potential) initiated in the nervous system to ______ are within the myofibril
muscle force
A series of _______make up each myofibrils. The _______is the basic contractile unit of skeletal muscle
sacromeres sacromere
Thin filaments (actin) attach at the ______.
Z line
When at rest, actin and myosin overlap _____.
partially
Whole muscle shortening results as the filaments overlap more, pulling the _____ closer
Z line
Whole muscle lengthening results as the filaments overlap _____
less
Contraction can occur during_____ or _____.
lengthening or shortening
Thick filaments (myosin) anchor at the _____ in the _____ of the sarcomere
M line; centre
Individual muscle force (N) is influenced by…
- Muscle architecture, muscle length
- Muscle fibre length, pennation angle - PCSA
- Specific tension – fibre types
- Number/discharge rate of active motor units i.e. neural drive
- Type of contraction i.e. isometric, concentric, eccentric speed of contraction, force relative to length
- Passive force
- Muscle fatigue and damage
Characteristics of muscle architecture
- Different muscles have different force generating capacity.
- Greatest predictor of force is the muscles physiological cross sectional area (PCSA)
- Most common calculation used in the literature:
- PCSA = Muscle volume/ Fiber length
- When also considering pennation angle: PCSA = (Muscle volume x CosΦ)/ Fiber length
- Fiber length and pennation angle [CosΦ] changes with contraction and joint angle… PCSA will change depending on muscle condition when measured
What are the characteristics of Muscle volume ?
- Does not change with contraction and joint angle
- Eg. vastus lateralis is larger (in volume) than vastus medialis
What are 2 components to make note of when measuring fascicle length?
- length of a muscle fascicle between the superficial and deep aponeuroses (dashed lines) along the line of collagenous tissue
- pennation angle (α), defined as the angle between the line of collagenous tissue and the deep aponeurosis.
Why does fibre length changes with contraction and joint angle?
- Shorter length during contraction compared to rest
- Due to the sarcomeres (actin and myosin overlap)
What is the Muscle-Length-Tension relationship?
- Increases to an optimal point and decreases again
- Vastus medialis has a 30o pennation angle
- Vastus laterialis has a 18o pennation angle
- More force produced
- Larger muscle (volume)
- Smaller pennation angle
How does pennation angle changes with contraction and joint angle?
- Muscle force transfer to tendon is reduced when muscle fascicles insert with a greater pennation angle
- Changes in pennation angle during contraction make a small difference to overall force within a muscle.
How does force generate capacity of individual muscles?
PCSA = Muscle volume/ Fiber length
- Greater force generating capacity from VL than VM
PCSA = (Muscle volume x CosΦ)/Fiber length
- Even greater force generating capacity from VL than VM if pennation angle was considered
What is specific tension?
Specific tension (Tspe)= the maximum force exerted by the fibers per unit of PCSA
Total muscle force = PCSA x Specific tension
- Type depends on muscle typology, with a higher specific tension associated with Type II muscle fibres.
- Skeletal muscles contain different proportions of Type I, IIA and IIB motor units depending on their function:
- POSTURAL MUSCLES: high proportion Type I (e.g. soleus 86% slow twitch)
- Don’t fatigue, keep using, decreased force
- DYNAMIC MUSCLES: high proportion Type II (e.g. gastrocnemius 56% fast twitch)
- Easily fatigue-able, force
- Most human muscles are mixed (40% - 60% fast twitch)
- POSTURAL MUSCLES: high proportion Type I (e.g. soleus 86% slow twitch)
What the 2 fibre types?
Type I Type II
What are type I fibres?
Slow twitch, low force, fatigue resistant Lower specific tension
What are type II fibres?
Fast twitch, more force, fatigue quickly Higher specific tension
Less slow twitch fibres = _____ specific tension
Higher
What are characteristics are the number/discharge rate of active motor units (ie. neural drive)
- Each muscle fibre is innervated by a motor neuron (from the spinal cord) at a neuromuscular junction (motor point).
- Motor unit = 1 motoneuron, its motor axon and all of the muscle fibres it innervates
- Motoneuron receives excitatory and inhibitory input from descending pathways, spinal interneurons & afferent fibres
- When sufficient excitatory input to reach firing threshold, an action potential is generated
- Every action potential generated in the motor neuron generates an action potential in the motor units muscle fibres
- actin/myosin cross-bridging
- active force production
- Force is altered by number and discharge rate of single motor units (SMU)
What is Henneman’s size principle?
- Small motor units (within a motor unit pool) are recruited first
- Same input = increased change in membrane potential in the smaller units >> reaches threshold sooner
- Increased drive = increased motor units and bigger motor units
- With increased excitatory input = increased number of motor units and increased size of the recruited motor units
- Assumption: Motor units within the same pool receive the same drive.
- Increased drive = more motor units and bigger motor units
What is eupnea?
small motor neurons
What is hypercapnea?
recruit larger- more slow twitch
What is airway occulution?
60% diaphragm recruited = 50% max. force
What is expulsive behaviour?
explosive diaphragm all fast fatigue (fast twitch)
What are the 3 types of contractions?
- Concentric
- Isometric
- Eccentric
What is a concentric contraction?
During concentric muscle contractions the muscle shortens while contracting
What is an isometric contraction?
Neither lengthening or shortening
What is an eccentric contraction?
- During eccentric muscle contractions the muscle lengthens while contracting.
Eccentric= higher torque for same motor drive
- Same torque in each condition below
- Motor unit discharge rate lower during eccentric torque matched contractions For each motor unit discharge more torque is produced during eccentric contractions
Force opposing the contraction is _____ than the contraction force. Decelerates the movement (absorbing energy)
larger
The maximum force that can be produced is dependent on the direction such that: _____ > _______ > ______; and rate of change of muscle length such that the faster you contract concentrically the less force is produced
ECCENTRIC > ISOMETRIC > CONCENTRIC
An eccentrically contracting muscle can produce _____ force at the same muscle length than a concentrically contracting muscle
more
Can hold _____ load than you can lift, and lower _____ load than you can hold steady.
more; more
What are some mechanisms for higher muscle force for same neural drive during eccentric contractions?
- Active force is generated through the actin and myosin cross-bridge.
- In eccentric movements actin and myosin ‘hold on’, more stretch of the elastic components (eg titin) > more force for same energy (# action potentials)
- Decreased rate of cross-bridge detachments >> greater force production on the eccentric bout.
What is force-velocity relationship?
- The force generated by a muscle is also dependent on its (lengthening/shortening) velocity.
- Mechanisms for higher muscle force at lower shortening speed:
- The force generated by a muscle depends on the total number of actin-myosin cross bridges. • It takes time for cross-bridges to attach.
- As filaments slide past each other faster (i.e. as the muscle shortens with increased velocity), fewer cross bridges are able to attach and generate force.
- As the relative filament velocity decreases, more cross bridges have time to attach and generate force.
- The potential to produce force….. Considering fascicle length, contraction direction and velocity
- Greatest force is produced when the muscle fibres are at optimal length, and the muscle is lengthening
Explain Hill’s Mechanical Model and the components. What are the 3 structures that contribute to the 3 components? What makes up the muscle-tendon unit and why are these components important when considering muscle force?
POSSIBLE EXAM QUESTION
- Contractile component (CC) – muscle fibers, actin and myosin cross bridges
- Series elastic component (SEC): intracellular titin, tendon
- Parallel elastic component (PEC): connective tissue - epimysium and perimysium,endomysium & passive cross bridge connections
What is muscle fatigue?
exercise-induced reduction in ability to produce force peripheral (muscle): failure to produce force appropriate to drive central (brain, spinal cord): failure to drive motoneurons adequately
Peripheral (muscle): failure to produce force appropriate to drive Peripheral sites may include?
Peripheral mechanisms will be task dependent, peripheral sites may include:
- neuromuscular junction [reduced release of Ach]
- changes in the muscle cell membrane potential [Na+/K+ pump]
- excitation-contraction coupling due to a change in the calcium release
- accumulation of metabolites [build up of lactate and H+ will change the pH and alter cell membrane]
- depletion of fuels [ATP]
Central (brain, spinal cord): failure to drive motoneurons adequately. Central factors include?
Central factors include:
- motivation
- discomfort
- attention
- drugs (e.g. caffeine)
How does muscle damage reduce ability to generate force?
Force-velocity relationship
TOTAL (purple) muscle force = ACTIVE (blue) + PASSIVE (red) tension
Active Tension = Force produced by active sarcomeres, driven by CNS
Passive Tension = Force produced by stretched connective tissue, cross bridges
