How do muscles work?

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

4. Macroscopic to microscopic

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

6. Membrane systems - - - - + + + + + + + + - - - - + + - - - - - - + + + + + + - - - - - - + + + + + + - - - - - - + + + + + + Ca2+ Ca2+ Ca2+

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

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

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

15. 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)

16. Increased force

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

18. Connective tissuesMechano-transduction - signaling Barton, 2006

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

20. 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

21. Connective tissuesForce transmission – through MTJs Huijing, 2003

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

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

24. 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

25. Sorry about the rain!