How do muscles work?

Kimberly S. Topp, PT, PhD

Phys Ther & Rehab Sci

Anatomy

UCSF

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

Macroscopic to microscopic

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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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

Levers

First Class Second Class Third Class

R R RE E E

F F F

biceps brachii

Joint moments and torque

Force (N)

Moment arm (m)

Torque (N.m)

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

Increased force

Connective tissuesTwo-way exchange of information - force

Kjaer, 2004

Connective tissues

Mechano-transduction - signaling

Barton, 2006

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 MTJs

Huijing, 2003

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

Sorry about the rain!