Biomechanics of Skeletal Muscle Flashcards
Muscle Action
Muscles have active contractile component that develops force-
Active force dependent on: Neural factors Mechanical factors Fiber type Muscle architecture
Muscle force transmitted through tendon to bone-
Muscle force creates joint torque or moment → motion
Joint torque/moment dependent on:
Muscle force
Moment arm / Lever arm
Joint position (angle of pull)
Breakdown of Skeletal Muscle
Epimysium
Perimysium
Endomysium
Sarcolemma
Sarcomere
Basic contractile unit of muscle that develops force
Actin and Myosin (myofilaments) cycling
Sliding Filament Theory
↑ cross-bridge formation = ↑ force
Muscles “do work”
Work refers to the product of force and displacement (Work = Force x distance)
Displacement = the parallel displacement component relative to the force applied
Motor Unit
Single motor neuron and all the muscle fibers it innervates
3 to 2,000 fibers innervated (innervation ratio)
Functional unit of muscle
Smallest unit of muscle contraction
All muscle fibers respond as one
“All or None”
Synergist
Two or more muscles working together to produce a movement
Agonist
Cause or assist movement
Stabilizer
Active in one segment so that a movement in an adjacent segment can occur
Antagonist
Perform movement opposite of agonist
Neutralizer
Active to eliminate an undesired joint action of another muscle
Concentric action
Shortening of fibers to cause joint movement
Eccentric action
Lengthening of fibers to control or resist joint movement
Isometric action
Minimal change in fiber length
No joint movement
Factors Influencing Active Muscle Force Production
Neural Factors
Fiber type
Mechanical Factors
Muscle Architecture
Neural Factors Affecting Active Muscle Force
Activation & Discharge Rate
Motor unit recruitment
Muscle Fiber Activation & Discharge Rate
Twitch
Summation
Tetanus
Twitch
response of muscle to single stimulus
Summation
the overall effect of added stimuli
Tetanus
sustained maximal tension due to high frequency stimulation
Motor Unit (m.u.) Recruitment
Muscle force is proportional to number of m.u.’s recruited # crossbridge formations
Muscle force is proportional to rate of stimulation (or firing)
Rate of crossbridge cycling
Synchronization of firing impulses may increase muscle force
Important in fatiguing exertions
Fiber Type Comparison
Fiber type affects muscle force, rate of force production,
& recruitment order
*look at chart
Fast Twitch vs Slow Twitch
FT peak force > ST peak force
FT rate of force production > ST rate of force production
Fiber Type (cont.)
All fibers within a m.u. are the same type
Within a muscle, however, there are a mixture of fiber types
Ordered recruitment (Henneman’s size principal)
Type I recruited 1st (lowest threshold)
Type IIa recruited second
Type IIb recruited last (highest threshold)
Reduction in tension accomplished in reverse order
Allows for controlled, smooth gradation of force
Largely genetic, but may change with training
Mechanical Factors Affecting Muscle Force
Length
Velocity
Force-Length Relationship
Optimal (~resting length) (max crossbridges → ↑ force) Shortened = Crossbridge overlap (↓ crossbridges → ↓ force) Lengthened = Crossbridges are pulled apart (↓ crossbridges → ↓ force)
Force-Length Relationship: Concentric vs. Eccentric
Same muscle length:
ForceECC > ForceCON
More crossbridges in ECC
ECC uses less energy
No ATP used to break crossbridge like in CON
Stretched – therefore more elastic energy stored More force
Less Friction
F-L relationship occurs during concentric & eccentric muscle action
Force-Length Relationship: Active & Passive
Stretch beyond resting length causes:
↓ Active tension
↑ Passive tension
Total tension may be greater at extremes of motion
Due to passive tension
Force-Velocity Relationship
Why Concentric F-V?
More crossbridges in release stage at a given time
Concentric - some CBs stay bound to too long (braking effect)
In eccentric – pulling apart makes it easier for crossbridges to cycle
Concentric
Shortening
CB Resists = “Negative Braking effect”
Eccentric
Resisting Lengthening
CB Assists: “Positive braking effect”
F-V Impact
At a given level of activation, muscles contracting more slowly produce greater force than muscles contracting more quickly
To achieve a given force, muscles contracting slowly need less activation as muscles contracting quickly
Muscle Power
Power refers to the rate at which work is performed
Power = Work / Time
OR
Power = Force x Velocity
Muscle power is especially important to functional and athletic tasks in comparison to strength – examples?
Activation History
Stretch-Shorten Cycle
↑ muscle work when shortening immediately follows stretching
Squat jump vs. Counter-movement jump
If stretch is held too long before shortening the effects are lost
Basis is not well understood
Stored elastic energy
Reflex activation
Muscle Architecture
Arrangement of contractile components (sarcomere) affects force production, excursion, & velocity
Muscle fiber arrangement:
Parallel: side to side arrangement
Series: end to end arrangement
Longitudinal
Esophagus
Unipennate
Lumbricals
Bipennate
Gastrocnemius m.
Fusiform
Wider in middle than ends (biceps brachii m.)
Muscle Architecture & Force
Arrangement of contractile components (sarcomere) affects force production
Muscle force is proportional to number of fibers / crossbridges active in parallel
Parallel = greater force production
Series = greater shortening velocity
Parallel Muscle Fiber Arrangement
side to side arrangement
Series Muscle Fiber Arrangement
end to end arrangement
Muscle Displacement and Velocity
Muscle displacement & velocity are proportional to number of fibers / crossbridges in series
Each fiber undergoes a change in length
Δ LengthTOTAL= ∑ ΔLengthSERIES FIBERS
↑ Δ LengthTOTAL in same time → ↑ Velocity
Muscle Force
Muscle force is proportional to number of fibers / crossbridges active in parallel
Muscle Force = ∑ForcePARALLEL FIBERS
Muscle Force = Avg ForceSERIES FIBERS
Angle of Pennation
Alignment of muscle fibers relative to line of pull
Pennation θ = 0° → Fibers aligned with line of pull
↑ resultant force directed along line of pull in less pennated fiber arrangements
↑ overall force in less pennated (longitudinal) arrangements???
Muscle Architecture
For the same muscle volume, longitudinal arrangements produce
less force than pennate arrangements
Energy
The capacity to do work Scalar quantity We are most interested in mechanical energy (associated with motion and position) Kinetic: energy of motion Potential: energy of position
Types of energy
Mechanical, chemical, heat, sound, light, etc
Mechanical Energy
Strain or Elastic Energy
Special form of potential energy
Energy due to deformation
This type of energy arises in compressed springs, squashed balls ready to rebound, stretched tendons inside the body, and other deformable structures (like muscles!!)
Tennis Ball Bounce
Stiffness
Force response to a mechanical stretch
Muscle fibers possess stiffness
Stiffness (K) = ΔF/ΔL
Stiffness Recruitment
Stiffness can be controlled
Preparatory muscle activation (intrinsic stiffness)
Reflex activation (reflex mediated stiffness)
Co-activation (joint stiffness)
How do reflexes maintain muscle stiffness?
Muscle spindle excitation facilitates increased motor unit recruitment
Change in muscle length
Change in rate of muscle lengthening
Increased number of fibers aligned in parallel
Increased muscle stiffness
Co-Activation and Joint Stiffness
↑ number of fibers aligned in parallel
Agonist & antagonist muscles
Antagonist muscle activity → ↑ agonist muscle activity
Offset antagonist muscles → ↑ number of fibers aligned in parallel
↑ joint compression → ↑ friction
Role of Stiffness
Stiffness creates stability
Musculoskeletal injury (joint stability)
Balance (postural stability)
Preventing Joint Injury
Biomechanical stability is required to prevent joint injury
Ability of a loaded structure to maintain static equilibrium after perturbation around the equilibrium position (Bergmark, 1989)
Joint Biomechanical Stability
Perturbation to Joint
(e.g. anterior tibial shear force) > Limited Athrokinematic Motion > Return to Resting Position > Maintenance of Joint Biomechanical Stability
Biomechanical Stability Factors
Sufficient potential energy (PE) is needed to maintain biomechanical stability after perturbation (Bergmark, 1989)
System able to return to equilibrium position
Quantifying Biomechanical Stability
Work (J) performed during perturbation ≤ PE (J) inherent to the system → Stable system
Forms of Potential Energy (PE)
PE due to object height above reference
PE = mgh
(m = mass, g = gravitational acceleration, h = height)
PE due to elastic deformation
PE = ½kx2
(k=stiffness, x = distance stretched)
Elastic Energy, Stiffness & Stability
PE in form of elastic energy is most important for musculoskeletal applications
PE = ½kx2
↑ stiffness → ↑ PE → ↑ stability
Thus, stiffness creates biomechanical stability
Ankle Stiffness
Stiffness of the evertor muscles can limit excessive inversion Peroneus longus Peroneus brevis ↑ stability ↓ injuries
Summary
Stiffness creates joint stability
Active muscle stiffness most important
Insufficient stiffness → inability to maintain stability
Joint injury
Too much of a good thing?
could increase injury risk of bone and muscle injury
