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Terms in this set (89)
Contractile (CE) Mechanics: include (4)
1) Macro level: Muscle Architecture
2) Micro level: Structural unit (fiber)
3) Mechanical model muscle-tendon unit
4) Muscle Mechanics
Macro level: Muscle Architecture
~ ACSA vs PCSA
~ The fascicles attach to the aponeuroses and form an angle (the pennation angle) to the load axis of the muscle.
~ The greater the angle of pennation, the smaller the amount of force transmitted to the tendon & bcoz the pennation angle increases w/ contraction, the force producing capabilities will reduce.
simply cross-section area
physiological cross-section area
Micro level: Structural unit (fiber)
~ Connective Tissue
~ Fascicle, muscle fiber, myofibril
~ Functional unit (sarcomere)
• Cross-bridge theory, actin-myosin
Mechanical model muscle-tendon unit
~ Contractile element (CE)
~ Parallel elastic element (PE)
~ Series elastic element (SE)
~ Direction of contraction
1) Excitability, or irritability
Excitability, or irritability
ability to receive and respond to stimuli
ability to recoil (elastic energy storage, connective tissue)
~ ability to be stretched or extended beyond resting length
~ protective mechanism -shock or energy absorption/transfer (lengthening contraction)
ability to shorten forcibly (50-70% of rest length)
3 Functions of Skeletal Muscle
1) Produce movement
2) Maintain postures and positions
3) Stabilize joints
Other Functions of Skeletal Muscle
~ Support and protect visceral organs
~ Alter and control cavity pressure
~ Maintain body temperature
~ Control entrances/exits to the body
Anatomy of Skeletal muscle
Gross Structure of Muscle
~ Muscles are typically arranged in functional groups and contained within compartments by sheets of fibrous tissue (fascia).
~ Muscle in a group have a similar, mutual role (synergy).
~ Muscle on the contralateral side of the segment usually have the opposite role at a joint.
Muscle organization, macro level
1) Parallel (fusiform) fiber types
Parallel (fusiform) fiber types
~ Fascicles & therefore force and length changes are parallel to long axis of muscle-tendon complex.
~ Generally, greater range of shortening and movement velocity.
~ Fibers diagonal to central tendon of muscle; therefore, change in fiber length is not equal to change in muscle-tendon length.
~ Generally, shorter range of motion but stronger due to greater PCSA (more fascicles).
Skeletal Muscle Architecture
fascicles run parallel to the long axis of the muscle (e.g., sartorius)
originates from aponeurosis , (rectus abdominis)
spindle-shaped muscles (e.g., biceps brachii)
uniform diameter, (sartorius)
fascicles converge from a broad origin to a single tendon insertion (e.g., pectoralis major)
~ Fibers attach obliquely to a central tendon running the length of the muscle (e.g., rectus femoris).
~ Fibers are shorter than muscle origin-to-insertion distance.
Angle of pull
causes less tendon insertion force generated by a single fiber but there are more fibers in the muscle so larger force overall.
~ Muscle origin-to-insertion length change is different than muscle fiber length change.
Non-pennate vs pennate contraction
Muscle Architecture ---> Force
Anatomical Cross-Section Area
~ Cross-sectional sample of muscle group
seen w/ Ultrasound, CAT scan, MRI
~ ⊥ to the long axis of the muscle
Cross-sectional area (CSA) =V/L
L = fiber length (cm)
V = volume (cm3)
V = m/ρ
m = muscle mass (g)
ρ = density
ρ = 1.056 g/cm2
Physiological cross-sectional area (PCSA)
Considers cross-sectional area of tissue pulling through the tendon (effective force).
PCSA = volume (cm3) / fiber length (cm).
PCSA = volume*cos (Θ) / fiber length
=(m/ρ)*cos (Θ) /L
= (m*cos (Θ)) / (ρL)
If # muscle fibers were equal between two muscle types
F(t) = F (m)
F(t) = F(m ║)
= F(m) * cos(θ)
If θ = 45degrees
F(t) = F(m) * 0.707
If # of muscle fibers are the same,
F(t-penniform) = 0.707 F(t-fusiform)
If # muscle fibers (penniform)= 2 * # muscle fibers (fusiform)
Fusiform & Penniform
F(t) = F (m)
Penation allows more fibers to be packed into a confined volume
If there are 2x's muscle fibers:
F(t-penniform) = 2 * 0.707 F(t-fusiform) = 1.414 F(t-fusiform)
Excitable vs. Non-Excitable
~ Muscle tissue IS excitable
~ Connective is NOT excitable
Passive Elastic Components
~ Dense fibrous connective tissues
Dense fibrous (irregular) connective
Anatomy of Muscle
Covering on outside of muscle that helps transmit muscular tension to the tendon.
Surrounds fascicle and provides a pathway for nerves and blood vessels to reach the muscle fibers.
Protective sheath for each muscle fiber that carries the capillaries and nerves for each muscle fiber.
A bundle of muscle fibers
A cylindrical, multinucleate cell composed of numerous myofibrils that contract when stimulated.
~ Densely packed, thread like contractile elements
~ They make up most of the muscle volume
~ The arrangement of myofibrils within a fiber is such that a perfectly aligned repeating series of dark A bands and light I bands is evident
Arrangement of Filaments in Sarcomere
Structural Integrity and Titin
~ Titin -links thick filament to Z line & is part of thick filament (provides a scaffold for sarcomere assembly).
~ An elastic filament thought to contribute to passive force with stretch.
~ Appears to have important signaling properties through its interaction with other proteins.
Muscle Fiber Arrangement
~ Not all muscles have fibers arranged in parallel from origin to insertion.
~ Some began at their proximal end (0) others within a fascicle.
~ Fiber-to-connective tissue arrangement can be somewhat serial.
Force Transfer Among Muscle Fibers
~ Z-line to proximal point of actin.
~ may have a role for thin filament assembly.
~ cytoskeletal support.
~ an efficient mechanical coupling for force generation from one myofilament to another.
Mechanical Analog of Muscle-Tendon Unit Hill-type model
1) Contractile (CC)
2) Parallel Elastic (PEC)
3) Series Elastic (SEC)
activation (excitation) > sarcomere cross-bridging > length, velocity, type of contraction, PCSA, fiber length.
Parallel Elastic (PEC)
Titan, epimysium, perimysium, endomysium, sarcolemma
Series Elastic (SEC)
, fiber, actin-myosin cross-bridges.
Force-Length (CE) Relationship - the magnitude of force (CE) produced depends on what
~ its length
~ taking the cross-bridge theory into play - - > the interaction of actin and myosin filaments generates force and length change
Force-Length (CE) Relationship - force is dependent on what (actin wise)
dependent on the number of actin-myosin binding sites
Force-Length & Passive Elements
~ Force from the CE is transferred to the bone through connective tissue in series and in parallel with the CE.
~ Passive element compliance will influence the muscle-tendon F-L relationship (bottom figure) and the time-course of tension transferred to the bone.
Force-Length & Passive (SE) Element - under static conditions
if unaccounted for, SE compliance will influence the
F-L relationship for muscle tissue
Force-Length & Passive (SE) Element - under dynamic conditions
SE tissue will influence
the time course of muscle tension transferred to the bone
Force-Length & Passive (SE) Element -
Different tissues in series will have different load-strain (force-length)
Force-Length & Passive (SE) Element - Under isometric conditions
the CE of muscle will shorten at the expense of SE elements lengthening. Hence, an isometric contraction at a fixed joint angle can result in muscle (CE) shortening, and therefore, the CE force as a function of muscle length will have changed without a change in joint angle.
Force-Velocity (CE) Relationship for shortening
Force-Velocity (CE) Relationship
~ The tension the CE of muscle can generate decreases dramatically as the velocity of muscle shortening (concentric) increases under load.
~ Cross-bridge theory suggests force decreases at faster shortening velocities because there is less time for actin-myosin bonding to take place.
2 theories doesn't hold for eccentric condition are
1) with lengthening, actin-myosin bonds are broken rather than formed so this requires more force
2) there is rotation of the actin-myosin bonding heads and stretching of elastic elements that contribute, thereby increasing force.
CE force is dependent on
muscle length, velocity of contraction, and type of contraction (concentric or eccentric)
stress strain curve - PE vs SE
~ PE = parallel elastic comp (fascia)
~SE = tendon
CE force vs length and force-velocity curves
How Muscle Fiber Length affects V_max - sarcomere length and speed of contraction
~ single sarcomere undergoes a 0.5 μm shortening in 1 sec
the more sarcomeres you have (ie the longer the mm) the faster it contracts
How Muscle Fiber Length affects V_max - For a given velocity of shortening 2.5microm/sec how many micrometers for 20 sarcomeres
How Muscle Fiber Length affects V_max - For a given velocity of shortening 2.5microm/sec how many micrometers for 10 sarcomeres
Force & Velocity of Contraction: 3 points
1) The longer the fibers of muscle (sarcomeres in series) the greater the ability of muscle to shorten (ΔL=n(Δl)) per unit time (velocity).
2) Greater muscle cross-sectional area (sarcomeres, fibers, fasicles in parallel) the greater the force (F=nF).
~ Generally, more fibers packed in pennate muscle architecture. Advantage force, even though fiber force (Fm) through tendon (Fm,t) is less (Cosβ•Fm)
3) Muscle contraction time and maximum velocity of shortening is related to the biochemical properties of the muscle (fiber type). A subject for a different course.
Joint moments of force (torque)
~ Muscle-tendon force
~ Muscle-tendon moment arm
~ Muscle Architecture
~ Passive elastic elements
~ Contractile muscle dynamics
Contractile muscle dynamic
~ Force-velocity (and direction of contraction)
~ Level of excitation & motor unit dynamics (EMG section)
• Firing rate
• Fiber type
Combined Muscle Length & Moment Arm - as joint angles change - what happens
Schematic examples of moment arm and length change with changing joint angles
muscle moment arm decreases but muscle lengthens as insertion angle becomes more acute (θ=20 degree). Note opposite effect for more obtuse insertion angle (θ =50 degree).
Optimal muscle length (top) and greatest moment arm (bottom) may not occur at same joint angle. Peak Torque-Angle relation would be a combination of the two.
Combined Muscle Length & Moment Arm - optimal joint moment-angle relationship does not coincide with
optimal length or moment arm but is somewhere between.
Two muscles of equal PCSA but different fiber lengths generate equal peak muscle force but have different working range, optimal length and maximum contraction (shortening) velocity. Which generates the most force at 10 cm/s? Review
Two muscles of equal fiber lengths but different PCSA have the same working range, optimal length and maximum contraction (shortening) velocity but generate different peak force. Which generates the most force at 5 cm/s?
Schematic comparison of two different muscles, one with short fibers and large PCSA relative to another. Differences in force-length and force velocity relationship (real units) for the different types of muscle are shown.
The combined effect a muscle's ability to generate force (PCSA) & velocity (length)
torque - velocity - power chart
~ A muscle contracts faster when warm (dashed line), shifting the F*V curve to the right (> Vmax).
~ For increasing velocities, force is greater, therefore muscle power increases with its peak reached at a muscle shortening
force - velocity - power chart -
Different synergist muscles have different F-V characteristics;
therefore, they will have different muscle power profiles
as velocity increases what happens to force
~ it decreases
~ peak power occurs at 30% (F*V)
muscle tendon mechanics - putting it all together slide
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