Transcript Structural Specialization - Georgia Institute of Technology

Structural Specialization
• Architecture-function matching
– Sarcomere arrangement can “tune” function
– Define “Optimal”
• Force transmission
– Myotendinous junction
– Costamere
Structural optimization
• Long muscles are optimized for speed
– Length change is proportional to length
– Sarcomeres in series
• Short muscles are optimized for force
– Isometric force is proportional to area
– Sarcomeres in parallel
• Tuning conditions for power production
– ~200 W/kg
Force, work, and power
• Force (N)
• Work = force * distance (Nm = J)
• Power = Work/time (J/s = W = N m/s)
Force (PCSA)
Velocity (Lf)
Isotonic force and power in muscle
PCSA * Lf = Volume
Long vs short fascicles
• For fixed mass, doubling Lf halves PCSA
• Double Lf  double operating range
• Nonlinear force-velocity gives extra power
boost
Math
V-Vmax
• Force-velocity: P=
V/a+Vmax/Po
V(V-Vmax)
• Power-velocity: Pwr =
V/a+Vmax/Po
dPwr
•
=
dV
aPo(a Vmax(Vmax-2V) –Po v2)
(Po V+a Vmax)2
• dPwr/dV==0v=0.3Vmax
Architectural diversity
• Similar within compartment (anterior thigh, posterior calf)
• Functional
• Physical constraints
10x range of PCSA
5-10x range Lf
Scup musculature
• Red myotome
– Midline fibers
– Parallel to body
– Superficial
• White myotome
– Helical
– ~45° to body
– Deep
Rome, 1991
Scup swimming modes
• Normal
– Slow
– Low power
– Small body curvature
• Escape
– Fast
– High power
– Large body curvature
Rome, 1991
Functional optimization
• Muscle types are optimized for their task
– Red muscle near Lo during normal swimming
– White muscle near Lo during escape
Relative force
Slow swim
Sarcomere Length
Escape
Quads/Hams
• Quads
– Pennate
– Short fibers
– Knee flexor
• Hams
– Parallel
– Long fibers
– Hip Extensor
– Knee Flexor
Functional Capacity
Is muscle arch functionally optimized?
• VI: weight support, propulsion, heavy lifting
– M=172g, Lf=9.9cm, PCSA=17 cm2
• ST: inertial loading, leg swing, fast
– M=100g, Lf=19 cm, PCSA=4.8 cm2
Walking/running
Running joint angles
• Stance
– Hip flexion
– Knee Fx/Ex
– Quads
• Swing
– Hip Ex
– Knee Fx/Ex
– Hams
Cappellini et al., 2006
Riley et al 2008
Muscle length changes
• Muscles have similar (relative) length changes
• Muscle activation is not coordinated with
shortening
• Power
• Length change
Allen (unpublished)
Tendon transfer
• Restore lost function
– Nerve palsy, spasticity
– Force
– Range of motion
Wrist flexors & extensors are in different limb
compartments, with different capabilities and
requirements. Choosing a donor muscle with
appropriate structure may improve outcome.
Extensors
Flexors
Architectural adaptation
• Runners vs Cyclists
– Upright posture, hip extended
– Aero posture, hip flexed
• Rectus femoris: biarticular hip-knee
– Active shorter in cyclists
– Cyclists (seem to) have
shorter fibers
runners
Herzog & al., 1991
cyclists
Force transmission
• Interfaces
– Myofibrilcytoskeleton
– CytoskeletonECM
• Myotendinous junction
• Costamere
• Stress
– Longitudinal
– Shear
Myotendinous junction
• Ruffled/invaginated
• Increase surface area
• Force transmission in shear
Ridge, et al., 1994
Z
A
Z
MTJ invaginations
Collagen of periand epi-mysium
Some deep
Ciena et al 2010
Epimysium
• “Lateral” force transmission
• Fibrous network
– Collagen
– Laminin
– Elastin
– Fibronectin
Purslow & Trotter, 1994
“Chinese finger trap”
Endomysial collagen network short length
Endomysial collagen network long length
At short lengths, collagen is
arranged mostly as rings
around muscle. With stretch,
the fibers become
longitudnal.
Large shape change without
stretching collagen fibrils
Purslow & Trotter, 1994
Desmin intermediate
filaments and non-muscle
g-actin form a cohesive
intracellular network
anchoring cellular
components to the ECM
Redundant adhesion
complexes: integrin,
dystroglycan
Capetanaki & al., 2007
Adhesion structure is different at
NMJ.
•Agrin in ECM
•MuSK (muscle specific kinase)
•NCAM
•utrophin replaces dystrophin
•AchR
Berthier & Blaineau, 1997
Summary
• PCSA and Lf describe functional capacity
– Assuming fixed volume of muscle
• Optimal function
– Isometric force: Lo
– Isotonic power: Vmax/3
• Molecular networks transmit force
– Shear
– Desmin/integrin/dystroglycan