Muscle Structure and Function Part 1 PDF

Muscle Structure and Function: Part I PT 720 APPLIED BIOMECHANICS WEST COAST UNIVERSITY DPT JAMES E. TAYLOR PT, DPT, OCS, COMT SEPTEMBER 25, 2024 Items of Business  Rescheduling 10/23 Lecture  Review of Research Summaries  Rubric for Group Project – coming this week  Review of E...

Muscle Structure and Function: Part I PT 720 APPLIED BIOMECHANICS WEST COAST UNIVERSITY DPT JAMES E. TAYLOR PT, DPT, OCS, COMT SEPTEMBER 25, 2024 Items of Business  Rescheduling 10/23 Lecture  Review of Research Summaries  Rubric for Group Project – coming this week  Review of Exam #1 Exam #1 Review  Questions  Comments  Complaints  Concerns The Sarcomere  The fundamental unit in each muscle fiber  Aligned in series  Comprised primarily of contractile and non-contractile proteins  Contractile proteins generate force  Non-contractile proteins create tension when stretched, provide support and alignment, and transfer active force throughout the length of the muscle Regions Within a Sarcomere Region Components A band Myosin filaments I band Actin filaments H band Region absent of Actin / Myosin filaments M line Thickened region of overlapped Action / Myosin Z disc Connecting junction between sarcomeres Muscle Protein Function Proteins Function Myosin (heavy chain) Acts as motor for muscle contraction – binds with actin Myosin (light chain) Acts contraction velocity of sarcomere; affects crossbridge cycling kinetics Actin Binds with myosin to transfer force and reduce sarcomere length Tropomyosin Regulates interaction b/w actin & myosin, stabilizes actin Troponin Affects position of tropomyosin; binds with Ca²⁺ Non-Contractile Proteins Nebulin Anchors action to Z discs Titin Affects position of myosin in sarcomere; provides elasticity to sarcomere Desmin Stabilizes longitudinal and lateral aspect of adjacent sarcomeres Sliding Filament Hypothesis  According to this theory, force created by stimulation of voltage- gated channels in the muscle  Ca²⁺ release that causes heads from thick myosin to “latch” onto the actin filaments and slide past  leads to shortening of the sarcomere Extracellular Connective Tissue  ECT is a form of non-contractile tissue in muscle that supports and provides elasticity to the muscle  Epimysium – on surface of muscle, separates muscles from other groups  Perimysium – lies within epimysium, divides muscle into fascicles  Endomysium – surrounds individual muscle fibers and represents exchange point between muscle fibers / capillaries Muscle Morphology  Fusiform* - Fibers run parallel towards a central tendon  Pennate* - Fibers approach the tendon at an oblique angle  Can be subdivided as unipennate, bipennate, or multi-pennate  Because of the angulation of the fibers, more fibers contained per unit area  more force development Examples on Muscle Morphology? Fusiform Pennate Biceps Brachii Extensor digitorum Sartorius Semimembranosus Pec major Deltoid Latissimus dorsi Rectus femoris Sternohyoid Gastrocnemius Triceps brachii Muscle Architecture  Physiological Cross-Sectional Area  the # of active proteins (actin/myosin that can generate force)  Fusiform muscles  Calculated by dividing the muscle’s volume by length (cm²) o Maximal force production ~ sum of cross-sectional area of all fibers o Can’t use this same thinking with pennate muscles  different portions may be running at different angles Pennation Angle  The angle of orientation of the muscle fibers relative to the tendon  For example, if fibers are parallel to the tendon, pennation angle = 0  In this scenario, all force is transmitted to the tendon  If pennation angle > 0°, < 100% force will be transmitted to the tendon  If pennation angle = 30°, then the force transmitted to the tendon will only equal 86% of the force being actually produced by the fibers contracting Pennation Angles and Trigonometry  We use the cosine function as we are looking as the force parallel to the tendon  Muscle fiber orientation = hypotenuse  Tendon orientation = adjacent  Remember cos θ =  Determining Max Force Potential in Muscle  Typically, most skeletal muscle fibers produce a specific force of about 30 N/cm² (range: 15 – 60 N/cm²)  Assuming, quad cross-sectional area of 180 cm² * 30 N/cm² = 5400 N * cos 10 deg = 5317 N Quadriceps tendon line of pull Muscle fiber line of pull Pennation angle =10° Why Pennate Then?  In a similar volume of muscle, the pennate muscle structure allows for more fibers fit into a specific muscle length  enables more force production within a specific area  Space-saving advantage  As pennation increases (from uni- to multi-pennate), the overall force production increases – decreased muscle shortening, however more fibers acting on a tendon to generate force Developing the Force Passive Length-Tension Curve  Components in the muscle fibers, either parallel or in series  Passive tension created by stretch  non- dependent on muscle action/activation  Important to have this capacity as muscles lose their force-generating capability with increased length Passive Length-Tension Curve  Critical length = all of the available slack from series and parallel elements has been taken up  Limitations with passive tension  Tissue slow to adapt to external force  Significant amount of lengthening required to generate time Active Length-Tension Curve  As mentioned previously, the ability of a myofibril to generate force is depending on the length of the muscle fiber  degree of overlap between actin and myosin  Ideal resting length  length that allows the greatest number of crossbridges and thus force to develop  Graphically, the peak of the active curve is going to represent the ideal resting length Force Production: The Combined Curve a) Short muscle length  active force is dominant b) Beyond resting length  decreased active force production offset by development of passive tension c) Passive tension accounts for majority of force development Isometric Tests  Isometric testing is typically used to get a “snapshot” of maximal force development in a muscle  By testing against a known external force, we can observe if the muscle is able to meet that demand Influences on Internal Torque Production Independent Clinic Example Internal Torque Clinical Variable Effect Consequence Increased IMA Displacement of Decreased force Decreased muscle greater trochanter required to produce force crossing joint distally to increase a desired level of leads to decreased IMA for hip ABD torque joint compression Decreased IMA Patellectomy Sig increase in knee Increased force  ext force to produce increased joint desired torque compression  wear on TFJ Decreased muscle Damage to portion Decreased force Decreased stability activation of nerve production in desired in muscle crossing a group specific joint Decreased muscle Damage to radial n. Decreased wrist Ineffective grasping length and subsequent extensor strength  patterns paralysis of wrist flexing from finger extensors flexors during gripping ADLs Works Cited 1. Aagaard, P., Andersen, J. L., Dyhre-Poulsen, P., Leffers, A. M., Wagner, A., Magnusson, S. P., Halkjaer-Kristensen, J., & Simonsen, E. B. (2001). A mechanism for increased contractile strength of human pennate muscle in response to strength training: changes in muscle architecture. The Journal of physiology, 534(Pt. 2), 613–623. https://doi.org/10.1111/j.1469- 7793.2001.t01-1-00613.x