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

Membrane systems Ca 2+

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

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

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

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

Length-tension relationships Isometric – constant length 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,

Length-tension relationships Isotonic – constant load Muscle force (% max tension) Contractile velocity (%V max ) Adapted from Lieber, Skel Musc Struct Funct Plasticity, Isometric length Maximum isometric tension eccentric concentric

Levers First Class Second Class Third Class RRREEE 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 force 2 Increase length of moment arm 3 Direct force perpendicular to radius 1 2 3

Increased force

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

Connective tissues Mechano- transduction - signaling Barton, 2006

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

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

Connective tissues Force transmission – through MTJs Huijing, 2003

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

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

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

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