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Structural Specialization • Architecture-function matching – Sarcomere arrangement can “tune” function – Define “Optimal” • Force transmission – Myotendinous junction – Costamere Structural optimization • Long muscles are optimized for speed – Length change is proportional to length – Sarcomeres in series • Short muscles are optimized for force – Isometric force is proportional to area – Sarcomeres in parallel • Tuning conditions for power production – ~200 W/kg Force, work, and power • Force (N) • Work = force * distance (Nm = J) • Power = Work/time (J/s = W = N m/s) Force (PCSA) Velocity (Lf) Isotonic force and power in muscle PCSA * Lf = Volume Long vs short fascicles • For fixed mass, doubling Lf halves PCSA • Double Lf double operating range • Nonlinear force-velocity gives extra power boost Math V-Vmax • Force-velocity: P= V/a+Vmax/Po V(V-Vmax) • Power-velocity: Pwr = V/a+Vmax/Po dPwr • = dV aPo(a Vmax(Vmax-2V) –Po v2) (Po V+a Vmax)2 • dPwr/dV==0v=0.3Vmax Architectural diversity • Similar within compartment (anterior thigh, posterior calf) • Functional • Physical constraints 10x range of PCSA 5-10x range Lf Scup musculature • Red myotome – Midline fibers – Parallel to body – Superficial • White myotome – Helical – ~45° to body – Deep Rome, 1991 Scup swimming modes • Normal – Slow – Low power – Small body curvature • Escape – Fast – High power – Large body curvature Rome, 1991 Functional optimization • Muscle types are optimized for their task – Red muscle near Lo during normal swimming – White muscle near Lo during escape Relative force Slow swim Sarcomere Length Escape Quads/Hams • Quads – Pennate – Short fibers – Knee flexor • Hams – Parallel – Long fibers – Hip Extensor – Knee Flexor Functional Capacity Is muscle arch functionally optimized? • VI: weight support, propulsion, heavy lifting – M=172g, Lf=9.9cm, PCSA=17 cm2 • ST: inertial loading, leg swing, fast – M=100g, Lf=19 cm, PCSA=4.8 cm2 Walking/running Running joint angles • Stance – Hip flexion – Knee Fx/Ex – Quads • Swing – Hip Ex – Knee Fx/Ex – Hams Cappellini et al., 2006 Riley et al 2008 Muscle length changes • Muscles have similar (relative) length changes • Muscle activation is not coordinated with shortening • Power • Length change Allen (unpublished) Tendon transfer • Restore lost function – Nerve palsy, spasticity – Force – Range of motion Wrist flexors & extensors are in different limb compartments, with different capabilities and requirements. Choosing a donor muscle with appropriate structure may improve outcome. Extensors Flexors Architectural adaptation • Runners vs Cyclists – Upright posture, hip extended – Aero posture, hip flexed • Rectus femoris: biarticular hip-knee – Active shorter in cyclists – Cyclists (seem to) have shorter fibers runners Herzog & al., 1991 cyclists Force transmission • Interfaces – Myofibrilcytoskeleton – CytoskeletonECM • Myotendinous junction • Costamere • Stress – Longitudinal – Shear Myotendinous junction • Ruffled/invaginated • Increase surface area • Force transmission in shear Ridge, et al., 1994 Z A Z MTJ invaginations Collagen of periand epi-mysium Some deep Ciena et al 2010 Epimysium • “Lateral” force transmission • Fibrous network – Collagen – Laminin – Elastin – Fibronectin Purslow & Trotter, 1994 “Chinese finger trap” Endomysial collagen network short length Endomysial collagen network long length At short lengths, collagen is arranged mostly as rings around muscle. With stretch, the fibers become longitudnal. Large shape change without stretching collagen fibrils Purslow & Trotter, 1994 Desmin intermediate filaments and non-muscle g-actin form a cohesive intracellular network anchoring cellular components to the ECM Redundant adhesion complexes: integrin, dystroglycan Capetanaki & al., 2007 Adhesion structure is different at NMJ. •Agrin in ECM •MuSK (muscle specific kinase) •NCAM •utrophin replaces dystrophin •AchR Berthier & Blaineau, 1997 Summary • PCSA and Lf describe functional capacity – Assuming fixed volume of muscle • Optimal function – Isometric force: Lo – Isotonic power: Vmax/3 • Molecular networks transmit force – Shear – Desmin/integrin/dystroglycan
