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 titin M line myosin nebulinactin Adapted from Alberts, Molec Biol Cell, 1994 sarcomere A = Anisotropic I = Isotropic

6. Membrane systems ++++++++ + + + + + + + + + + + + ---- - - - - - - + + + + + + - - - - - - - - - - - - ++ ---- Ca 2+

7. Muscle architecture Force or excursion? Physiological cross-sectional area Fiber length Relation to force generating axis Parallel or longitudinal Unipennate – 0 o to 30 o angle Multipennate – multiple angles

8. Pennation reduces force along axis, but allows for increased packing of shorter fibers PCSA (cm 2 ) = Mass (g) x Cos pennation angle Density (g/cm 2 ) x Fiber length (cm) Netter, Icon Learning 45 6.5

9. Fiber length + PCSA dictate function Hamstrings Excursion 11.2 cm fiber length 35.4 cm 2 PCSA Low pennation angle Quadriceps Force production 6.8 cm fiber length 87 cm 2 PCSA High pennation angle Lieber, 2002

10. Synergists with distinct architecture Gastrocnemius 3.5 – 5.1 cm fiber length 23 – 11 cm 2 PCSA Great for excursion Soleus 2.0 cm fiber length 58.0 cm 2 PCSA Great for force Fiber length is proportional to excursion PCSA is proportional to maximal force

11. Length-tension relationships Isometric – constant length 0 20 40 60 80 100 120 1.01.52.02.53.03.54.0 Percent maximum tension Sarcomere length (  m) Myosin filament 1.6  m long Actin filament 1.0  m long Adapted from Lieber, Skel Musc Struct Funct Plasticity, 2002 0.0

12. Length-tension relationships Isotonic – constant load Muscle force (% max tension) Contractile velocity (%V max ) Adapted from Lieber, Skel Musc Struct Funct Plasticity, 2002 -75-50-25 0 2550-10075 100 0 20 40 60 80 100 120 140 160 Isometric length Maximum isometric tension eccentric concentric

13. Levers First Class Second Class Third Class RRREEE F F F biceps brachii

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

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

16. Increased force

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

18. Connective tissues Mechano- transduction - signaling Barton, 2006

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

20. Connective tissues Force 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 tissues Force transmission – through MTJs Huijing, 2003

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

23. Connective tissues Force 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!