2.3.4. Muscle Mechanics I/II Flashcards
2 Functions of skeletal muscle
- Produce Skeletal Muscle movement. Performs physical work and develops force, tension or stress
- Maintain posture and body position
What are the two functional elements of skeletal muscle and what are they in charge of?
- Contractile Element - Consists of a cross bridge system that generates muscle force
- Viscous Element/Elastic Element - Element that resists motion. Turns that energy into heat.
Formula for calculating the elastic or viscous functional element force value?
F = -K*V
What occurs with an isometric contraction?
The ends of the muscle are fixed (holding weight without moving it)
Two platforms: one fixed and one adjustable (but immovable during contraction)
What occurs with an isotonic contraction?
Use passive stretch to set initial muscle length and apply a supermaximal electrical stimulus
Muscle shortens until there are just enough cross bridges to barely support the load, then motion stops
What is a muscle twitch?
A muscle twitch, or fasciculation, is the tension (force) produced in response to a single muscle AP
Isometric Contraction
How are twitch and tetanus related?
Twitch may also be used to describe the force and speed of contraction of a single muscle fiber. Total tension developed increases: can result in fused or unfused tetani
Describe the difference between Fused vs. unfused Tetanus.
Unfused: Not “continuous” activation, but a summative result (bumpy graph)
Fused: Fused is a mechanical summation overall (smooth graph)
What is muscle efficiency based on?
The ratio of mechanical work output to total metabolic cost (aka O2 consumption)
Work equation
Work = F x D (joules)
Power equation
Power = work/time (watts)
Stress equation
Stress = Force / Area of muscle
Is movement required to generate stress?
NO
Voluntary activation of skeletal muscle
AP in α-motor neuron propagates to NMJ; AP releases transmitters into synaptic cleft; depolarization of sarcolemma; muscle AP; release of Ca; cross bridges cycle and muscle contracts
How does muscle develop force?
Depends on: Type Length Stimulus Number of cells firing
What is the greatest determinant for the development of force?
Muscle length
Contractile Element of muscles
Cross-bridge systems that generates muscle force
Viscous element
Resists motion; acts like a shock absorber
Only produces force while in motion
Dissipates energy (turns it into heat)
Series elastic element
Made up of tendon and connective tissue (small part made up of titin)
Parallel elastic element
Largely made up of titin and connective tissue
What does a bigger stimulus mean?
More fibers are activated (each individual fiber goes “all out” with each stimulus, large stimuli are just able to activate more than small ones)
Place these muscles in increasing order of contraction time:
Gastrocnemius, Soleus, Occular
Occular < Gastrocnemius < Soleus
eyeballs don’t need a lot of force, but need to move quick < running and jumping < posture
Difference b/w muscle twitches and APs
Twitches can be summed
Mechanism behind fused tetanus
The viscous elements in the muscle don’t allow it to fully relax before the next stimulus arrives (intracellular Ca isn’t completely removed from the cytoplasm either)
Is increasing the stimulus our primary means of controlling muscle force?
No (generally a fused tetanus has a magnitude of only 3-5 times that of an isolated twitch)
Passive muscle strength
The muscle as a rubber band (no stimulation, no contraction of the muscle in questions)
Stretch the muscle and measure the force produced
How is force generated by passive muscle strength
By the muscle trying to get back to its resting length
Total Tension
= Passive + Active Tension
Sliding filament theory
Force is generated by making and breaking cross-bridges b/w the thick and thin filaments
Sarcomere shortens by thick and thin filaments sliding past each other
Active Tension Force Magnitude
Related to the number of cross bridges formed
More cross bridges = greater force (related to thick and thin filament overlap and overlap is related to MUSCLE LENGTH)
What is the thick filament made up of?
Myosin molecules pointed in opposite directions bound together in center of H-band by myomesin that forms the M-line
What is the thin filament up of?
Actin, tropomyosin & troponin
Attached to the z-disks
Cross Bridge Cycling
Thick filaments globular heads form cross bridges with thin filaments; heads rotate inward toward the M-line pulling the thin filaments together; heads release from thin filament (need ATP); heads reset
As long as Ca level is high, the muscle can keep contracting/crawling
Too much thick/thin interaction
Thin filaments shield one another from thick filaments (decrease in cross bridges and less force)
Thick filaments contact the Z line and resist contraction
Tension falls to zero when initial length is set slightly before the thin filament contacts the Z line
Skeletal muscle force is regulated by two things
Motor unti recruitment (primary) Firing pattern (secondary)
Skeletal muscle relaxation
Isotonic relaxation followed by isometric relaxation
Heart muscle relaxation
Isovolumetric
Voluntary control of skeletal-muscle force
Length changes in skeletal muscle are not significant in force regulation
Restricted by bones and ligaments (the muscle works near the flat peak of the active tension curve)
Control of cardiac muscle force
Length changes are significant
Motor Unit
All of the muscle fibers innervated by the motor unit
Muscle fibers are not innervated by more than one motor neuron
Small motor unit
Quick movement but little force
Large motor unit
Slower movement but lots of force/strength
Types of muscle fibers
Slow-oxidative
Fast-oxidative
Fast-glycolytic
Type I
Slow Oxidative; slow myosin ATPase activity; high oxidative capacity
Fatigue resistance; used continuously
RED
Type IIx
Fast glycolytic; fast myosin ATPase activity; high glycolytic
Fast fatigue; used intermittently
WHITE
Type IIa
Fast oxidative; fast myosin ATPase activity; moderate oxidative capacity
Moderately fatigue resistant
Muscle organization
We have many small motor units and very few large motor units
Largest motor units are recruited last, only during the highest forces
