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How do muscles work?
Microscopic to macroscopic structure Myofilaments, membrane systems Muscle architecture
Force production, excursion Length-tension, mechanics Joint moments and torque Eccentric, concentric, isotonic, isometric
Connective tissues Tendon flexibility, energy storage
Myofilament organization
Z disc Z disctitin M line myosin
nebulin actin
Adapted from Alberts, Molec Biol Cell, 1994
sarcomere
A = Anisotropic I = Isotropic
Membrane systems
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- - - -
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Ca2+
Ca2+Ca2+
Muscle architectureForce or excursion?
Physiological cross-sectional area
Fiber length
Relation to force generating axis Parallel or longitudinal Unipennate – 0o to 30o angle Multipennate – multiple angles
Pennation reduces force along axis, but allows for increased packing of shorter fibers
PCSA (cm2) = Mass (g) x Cos pennation angle
Density (g/cm2) x Fiber length (cm)
Netter, Icon Learning
45
6.5
Fiber length + PCSA dictate function Hamstrings
Excursion 11.2 cm fiber length 35.4 cm2 PCSA Low pennation angle
Quadriceps Force production 6.8 cm fiber length 87 cm2 PCSA High pennation angle Lieber, 2002
Synergists with distinct architecture
Gastrocnemius 3.5 – 5.1 cm fiber length 23 – 11 cm2 PCSA Great for excursion
Soleus 2.0 cm fiber length 58.0 cm2 PCSA Great for force
Fiber length is proportional to
excursion
PCSA is proportional to maximal force
Length-tension relationshipsIsometric – constant length
0
20
40
60
80
100
120
1.0 1.5 2.0 2.5 3.0 3.5 4.0
Pe
rce
nt m
axim
um
ten
sio
n
Sarcomere length (m)
Myosin filament 1.6 m longActin filament 1.0 m long
Adapted from Lieber, Skel Musc Struct Funct Plasticity, 2002
0.0
Length-tension relationshipsIsotonic – constant load
Mu
scle
forc
e (%
max
ten
sio
n)
Contractile velocity (%Vmax)
Adapted from Lieber, Skel Musc Struct Funct Plasticity, 2002
-75 -50 -25 0 25 50-100 75 1000
20
40
60
80
100
120
140
160 Isometric length
Maximum isometric tensioneccentric
concentric
Ways to increase torque
Force (N)
Moment arm (m)
Torque (N.m)
1 Increase force2 Increase length of moment arm3 Direct force perpendicular to radius
1
2
3
Connective tissuesForce transmission – through sarcolemma
Grounds et al., 2005
50% of force transmission is lateral!
Connective tissuesForce transmission – through perimysium
Accommodate shear strains during contraction and extension
Shear is greater at fascicle border than within fascicle
Large fascicles and thick perimysium in muscles with high force
Small fascicles and thin perimysium in muscles with large excursion
Connective tissuesForce transmission – through tendon to bone
Collagenous tendon
Fibrocartilage Mineralized
fibrocartilage Mineralized
bone
Doschak and Zernicke, 2005
Connective tissuesForce transduction – in fascial compartments
Increases efficiency of muscle contraction
Increases the effective muscle stiffness in active contraction, leading to increased force production
Tendon flexibility
Tendons strain approx 3% at maximal muscle contraction
Increasing tendon length:fiber length ratio increases operating length for muscle/tendon unit
Sarcomere shortening occurs with tendon lengthening – stored energy
Recoil of shortened tendon provides movement from the stored energy