Lecture 1 Anatomical descriptions Muscle and Tendon (1).pdf

Welcome to Functional Anatomy This week: (a) Anatomical descriptions (Canvas video) (b) Structure and function of muscle and tendon 1 Lecture author: Anthony J Blazevich Prof. Anthony Blazevich is a Professor of Biom...

Welcome to Functional Anatomy This week: (a) Anatomical descriptions (Canvas video) (b) Structure and function of muscle and tendon 1 Lecture author: Anthony J Blazevich Prof. Anthony Blazevich is a Professor of Biomechanics in the School of Medical and Health Sciences, and Director of the Centre for Exercise and Sports Science Research (CESSR). His role includes teaching on biomechanics, neurophysiology and other units, as well as leading research in areas such as sports biomechanics, neurophysiology, strength & conditioning. 2 Semester Overview Important dates on-campus… Week 7: Mid-semester exam (lectures 1-5) Week 9: Assignment 1 due (Research process) Week 12: Assignment 2 due (Testing performance) ECU Exam Week: End-semester exam (lectures 1- 11, with focus on 6 – 11) Important dates off-campus… Week 7: Mid-semester exam, in lab class Week 13: Assignments 1 and 2 due ECU Exam Week: End-semester exam (lectures 1 – 11, with focus on 6 – 11) 3 1  Functional Anatomy Human functional anatomy: area of science examining the functional role of musculoskeletal and neurological structures, both in isolation and as part of a complex, moving human Main purpose: to better understand factors that influence both injury risk and human physical performance Closely with anatomy, biomechanics, physiology, and motor control (motor learning), so it’s an important ‘linking’ area of study 4 In functional anatomy, we study movement… …so how do we move? 5 Muscles To move about joints, through planes of motion, about axes, we need a motor Muscles are the motors that drive us (and other animals) They’re incredible: huge forces, minimum mass How? 6 2  Muscle Structure - Myosin Myosin has a heavy chain (head, neck and tail) and light chains (that influence function) 8 DNA strands The head/neck (motor) domain bends to pull on actin Motor domain is very short (20 nm) so myosin works in low gear (need many strokes to pull actin), like bike, car or truck going up hill. 7 Muscle Structure - Myosin Myosin molecules arrange themselves in a unique way, tails together and heads at the ends And then the myosin ‘walks’ along actin to cause muscle contraction (we won’t deal with energetics); the attachment is called a cross-bridge, and this is the “cross-bridge model” of muscle contraction. 8 Muscle Structure - Sarcomere Actin and myosin are the major constituents of the fundamental unit of muscle: the sarcomere Lattice structure 9 3  Muscle Properties Myosin heads have to rotate, then detach, recover, and reattach to actin – called cross-bridge cycling. The recover-reattach time is constant, so the faster the cycle (stroke) rate, the less time myosins are in contact with actin… …and the less force can be produced. 10 Muscle Properties - Speed So as muscles shorten faster, maximum force decreases; this is called the force-velocity relation Power = F x v, so also a power-velocity relation So we can move slowly and lift heavy, Or we can move quickly and lift light, But we can’t do both (heavy and fast) Force Velocity 11 Muscle Properties - Length Shortening and lengthening of sarcomeres changes the overlap of myosin and actin. This changes the number of bound cross-bridges… …and this affects sarcomere, and muscle, force. Short length Optimum length Long length Fewer bound cross-bridges, Most bound cross-bridges Fewer bound cross-bridges and actin-myosin collision Length = ~2.7 µm (~1/10 width of fine hair) 12 4  Muscle Properties - Length Changes in actin-myosin overlap largely (but not completely) underpin the force-length relation Ascending Descending 100 limb limb Force (%) 50 0 1.0 2.0 3.0 4.0 Length (µm) 13 Muscle Properties - Length Within a muscle, sarcomeres can all be at different lengths Sarcomere lengths and length Isometric contraction (toad muscle) changes Talbot & Morgan (1996; J Muscle Res Cell Motil) Rassier et al. (2003; Proc Royal Soc Lond B) 14 Muscle Properties - Length In vivo operating length relative to optimum differs between muscles 100 100 Medialis Force (%) Gastrocnemius Biceps Brachii Force (%) 60 80 100 120 140 160 -20 -10 0 +10 +20 +30 Flexion Elbow Angle Extension Dorsiflexion Ankle Angle Plantar flexion (o) (o) Leedham & Dowling (1995; EJAP) Maganaris (2003; Clin Anat) 15 5  Muscle Properties - Length Force-length relation shifts as activation changes… Electrically-stimulated forces – at low Here’s all the data put together – notice frequencies (forces) lengthening the muscle the shift in optimum length with is good, but at high frequencies lengthening frequency (force) is not Balnave & Allen (1996; J Physiol) 16 Muscle Properties - Length Largely because actin and myosin closer together at long lengths, so more likely to interact for a given activation level. So when you walk, run or hold something, the ‘best’ length is not the ‘optimum’ length determined during a maximal contraction! Optimum elbow Optimum elbow flexion force flexion hold 17 Sidenote… BUT…if a muscle connects to a long tendon, then higher force stretches tendon, then muscle works at shorter length Or, from maximum force, reducing force will allow tendon to shorten, which stretches the muscle anyway! So, the optimum ‘muscle-tendon unit’ length, and therefore the optimum joint angle, probably changes little with changes in force… 18 6  Muscle Properties - Length Passive forces also contribute when muscle stretched: Parallel elastic component (PEC) Includes membranes surrounding fibres, fascicles and whole muscle Keeps muscle from overstretching Often depicted like this, where total force = active + passive Knudson (2006; J Exerc Sci Physiother) 19 Muscle Properties - Length …but it’s not correct, because the fibres shorten during contraction as they stretch the series elastic component (SEC; tendon, myofilaments, cross-bridges)… …so the PEC passive force decreases, often to zero So in most contractions, the passive (PEC) force contribution is small or zero. 20 Muscle Properties - Eccentric Eccentric portion of curve: Force and power increase (negatively), but have different shape Force Power Force Velocity 21 7  Muscle Properties - Eccentric Greater force in eccentric contractions largely due to: (i) Extra force per cross-bridge as they’re lengthened (if shortened during concentric contraction, force falls since force changes with angle of myosin head); (ii) More cross-bridges attached because of higher energy required for detachment (‘forcibly detached’). But cross-bridge model can’t predict eccentric force accurately; it over-predicts both force and energy cost… …so must be some other mechanism too! (Lecture 6) 22 Muscle Properties In Lecture 6, we’ll talk more about muscle the properties… …and see that our current understanding of muscle contraction is wrong [that is, we can’t yet explain some cool things about muscle contraction, but we think we might know something important] 23 Muscle Architecture At the whole-muscle level, muscles have different designs Strap Unipennate Multipennate Fusiform Bipennate Muscle injury is more likely to occur at the muscle-tendon junctions Flores et al. (2018; Radiographics) 24 8  Muscle Architecture Pennate Can have varying vastus lateralis ‘architecture’. Long vs. short fascicles ultrasound Highly pennate vs. parallel fibred 25 Muscle Architecture Summary Long fibres/fascicles allow large ROM, so more work (F x d). Long fibres/fascicles have high shortening speeds. Short fibres have lower metabolic cost (less ATP use). 26 Fibre Length and O2/ATP Longer fibres have more sarcomeres in series. In-series sarcomeres don’t increase isometric (or slow speed) force. But they use ATP to generate tension. Shorter-fibred muscles are optimum for low- metabolic cost. Examples: gastrocnemius and soleus for running…and short muscles in running animals! Cost = 3 packets of ATP & O2 Cost = 6 packets of ATP & O2 27 9  Fibre Length and Speed Longer fibres have more sarcomeres in series. So less length change per sarcomere and slower shortening velocity. Can thus generate good force at higher shortening speeds. Examples: hamstrings (biceps femoris, semimembranosus, semitendinosus), vastus lateralis. Shortening speed = 3 sl/s Shortening speed = 6 sl/s 28 Fibre Length and Work Many movements require work to be performed, W = F x d. Longer fibres have a greater range of shortening (d), so perform more work. For a given d, long-fibred muscles require less sarcomere shortening…so also slower velocity. Therefore stronger in dynamic movements because of F-v and L-T relationships. Examples: gluteus maximus, hamstrings, quadriceps, pectorals, biceps brachii. 29 Pennation Some muscles have high Fascicle angle/ angles of pennation pennation (fascicle angle), but others low. Allows more contractile tissue to attach to the tendon/aponeurosis…more contractile tissue in parallel. Greater physiological cross-sectional area (PCSA; perpendicular to fibres) for a given anatomical cross-section (ACSA). Force = 5 x fibres Force = 12 x fibres PCSA ACSA PCSA 30 10  Pennation Fibres also rotate as they shorten, so the fibres don’t have to shorten as far…or as fast. Caused by intra-muscular pressure. So fibres can produce more force than if they had to do all the shortening themselves! This helps force production in dynamic contractions 31 Pennation and Rotation That is… Sarcomeres will shorten slower, and therefore generate more force (force-velocity relationship). If the fibres (or sarcomeres), shorten less, they might be able to remain closer to optimum length. Examples: gastrocnemius, vastus lateralis and medialis, triceps brachii. 32 The Tendon But muscles transfer their forces through tendons to the bones So, as you know from your biomechanics class, understanding the tendon is a key to understanding functional movement… 33 11  The Tendon Like muscle, tendon has a hierarchical structure - molecule, fibril, fascicle, tendon The fundamental component is collagen Lee & Elliott (2018; J Anat) It is elastic because covalent bonds between amino acids pull it to shape when stretched Buehler (2006; Proc Nat Acad Sci) 34 The Tendon Cross-linking of the collagen fibrils can increase stiffness and reduce breaking of fibrils, and tendon Eekhoff et al. (2017; Conn Tiss Res) Load sharing occurs between fibrils and between fascicles Shear (sliding) between these constituents also allows for tendon elongation Shear between fibrils Szczesny & Elliott (2015; Acta Biomater) 35 Tendon Force-length Relation Tendons stretch when loaded, but properties vary… Force Toe region Linear region Failure region Length Collagen stretch, fibrillary/fascicular shear Loss of collagen crimp, Collagen damage, some collagen stretch fibrillary/fascicular shear 36 12  Tendon Stress-strain Relation To compare tendons of different sizes and types, we ‘normalise’ force and length… Stress Force Length Strain Stress = force change / cross-sectional area (N/mm2) Strain = length change / initial length (%) Stiffness = force change / length change Young’s modulus = stress change / strain 37 Tendon Stress-strain Relation Can dissipate energy partly through shear between fibrils and fascicles during relaxation (shortening) The energy lost or dissipated is the hysteresis… Area between loading Stress and unloading curves Strain This dissipates energy: good for injury prevention and braking, but bad for propulsion/locomotion 38 Viscoelasticity Biological tissues, including tendon, are viscoelastic. They exhibit both viscous (flow- or reorganisation- related) and elastic (self-reorganising) behaviours Creep: increased strain under constant stress Increased stiffness and failure Stress relaxation: decreased stress with strain velocity stress at constant strain Fast Strain elongation Stress Slow elongation Time Stress Strain Time 39 13  Viscoelasticity Viscoelasticity is great! Largely caused by fibril sliding when under stress Greater protection from injury and over- elongation (strain) when loaded rapidly Stress falls slowly if tendon held at a constant length – decreased injury risk Strain continues if tendon pulled with constant force to new length Remember the influence of tendon stiffness, hysteresis, etc., on movement capacity from your biomechanics class! 40 The Tendon Believe it or not, some other factors influence tendon function during movement… …and give tendon a type of ‘intelligent function’. We’ll cover this in Lecture 6, alongside some other amazing stuff about muscles. 41 Conclusions Your task now is to make your own summary of each section, in your own words We should remember that movement in sports or activities of daily living require muscular forces, which are transferred through elastic tendons. Muscles have remarkable properties, although we still don’t know exactly how they produce the forces that they do. Tendons also have incredible properties that help us to optimise movement capacity. The interaction between muscles and tendons is therefore vital, and will be covered in more detail in Lecture 6. 42 14  QUESTIONS? 43 15