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How do muscles work?

How do muscles work?

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How do muscles work?

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  1. How do muscles work? Kimberly S. Topp, PT, PhD Phys Ther & Rehab Sci Anatomy UCSF

  2. 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

  3. Macroscopic to microscopic

  4. Myofilament organization Z disc Z disc myosin M line titin nebulin actin A = Anisotropic I = Isotropic sarcomere Adapted from Alberts, Molec Biol Cell, 1994

  5. Membrane systems - - - - + + + + + + + + - - - - + + - - - - - - + + + + + + - - - - - - + + + + + + - - - - - - + + + + + + Ca2+ Ca2+ Ca2+

  6. 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

  7. Pennation reduces force along axis, but allows for increased packing of shorter fibers 45 6.5 Mass (g) x Cos pennation angle PCSA (cm2) = Density (g/cm2) x Fiber length (cm) Netter, Icon Learning

  8. 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 cm2PCSA • High pennation angle Lieber, 2002

  9. 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

  10. Length-tension relationshipsIsometric – constant length Myosin filament 1.6 mm long Actin filament 1.0 mm long 120 100 80 Percent maximum tension 60 40 20 0 0.0 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Sarcomere length (mm) Adapted from Lieber, Skel Musc Struct Funct Plasticity, 2002

  11. Length-tension relationshipsIsotonic – constant load Isometric length 160 140 eccentric 120 Maximum isometric tension 100 concentric Muscle force (% max tension) 80 60 40 20 0 0 100 -100 -75 -50 -25 25 50 75 Contractile velocity (%Vmax) Adapted from Lieber, Skel Musc Struct Funct Plasticity, 2002

  12. Levers biceps brachii R E R E E R F F F First Class Third Class Second Class

  13. Joint moments and torque Torque (N.m) Moment arm (m) f Force (N)

  14. Ways to increase torque Torque (N.m) 2 Moment arm (m) f 3 1 1 Increase force 2 Increase length of moment arm 3 Direct force perpendicular to radius Force (N)

  15. Increased force

  16. Connective tissuesTwo-way exchange of information - force Kjaer, 2004

  17. Connective tissuesMechano-transduction - signaling Barton, 2006

  18. Connective tissuesForce transmission – through sarcolemma 50% of force transmission is lateral! Grounds et al., 2005

  19. 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

  20. Connective tissuesForce transmission – through MTJs Huijing, 2003

  21. Connective tissuesForce transmission – through tendon to bone • Collagenous tendon • Fibrocartilage • Mineralized fibrocartilage • Mineralized bone Doschak and Zernicke, 2005

  22. Connective tissuesForce transduction – in fascial compartments • Increases efficiency of muscle contraction • Increases the effective muscle stiffness in active contraction, leading to increased force production

  23. 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

  24. Sorry about the rain!