Muscle Biomechanics PDF

Muscle biomechanics 1. Composition and structure of skeletal muscle TYPES OF MUSCLES Muscle: muscle tissue organ of mesodermal origin whose fundamental property is contractility. - Striped muscle tissue: organized structure of myofilaments: - Skeletal muscle: voluntary contracti...

Muscle biomechanics 1. Composition and structure of skeletal muscle TYPES OF MUSCLES Muscle: muscle tissue organ of mesodermal origin whose fundamental property is contractility. - Striped muscle tissue: organized structure of myofilaments: - Skeletal muscle: voluntary contraction to allow motion - Cardiac muscle: involuntary contraction to allow cardiac rhythm - Smooth muscle tissue: without striations: - Location: walls of viscera and vessels, and dermis - Involuntary contraction to allow the motion of the walls of the internal organs CELLS OF SKELETAL MUSCLE - Muscle cell = muscle fiber - Colour: red → myoglobin: oxygen-binding - Cylindrical shape protein - Multinucleated - Each cell receives an axonal ending from a - Cytoplasm with transverse striations motor neuron forming the motor plate CELL ELEMENTS - Sarcoplasm: muscle cell cytoplasm, which has transverse bands, organelles and myofibrils - Sarcolemma: plasmatic membrane - T-tubules: invaginations associated to the sarcoplasmic reticulum that enter into the sarcoplasm surrounding each myofibril at the level of the bands A-I. - Dystrophin: protein that links the actin cytoskeleton to a protein complex in the sarcolemma, which binds to the extracellular matrix. - Nuclei: flattened shape and there are several hundred for each cell MYOFIBRILS - Cylindrical and elongated structures formed by actin and myosin myofilaments - 200 - 3000 / muscle cell. - Transverse striation --> alternative bands: dark and light. - Ultrastructure: - Thick myofilaments: one protein --> MYOSIN II. - Thin myofilaments: three proteins --> ACTINE, TROPOMYOSIN and TROPONIN. - Sarcoplasmic reticulum: surrounds each myofibril and is formed by a network of smooth endoplasmic reticulum tubules, which converge in 2 terminal cisterns, in the center of which is a T-tubule --> TRIAD. TYPES OF BANDS - Bands: - A: dark band - I: light band - H: light band in the center of the A band - Lines: - M: dark line in the center of the H band - Z: dark line in the center of the I band - Sarcomere: morphofunctional unit of striated muscle. It is the zone between two Z lines ORGANIZATION OF SKELETAL MUSCLE - Muscle - Fascicles or bundles - Muscle cells - Myofibrils - Myofilaments 2. Molecular basis of muscle contraction MYOFIBRILS - Contractile proteins → transform chemical energy into mechanical energy - Myosin: - Thick myofilament - It is formed by: 2 heavy chains and 4 light chains - In the globular head there are two hinge regions to interact with actin - Actin: - Thin myofilament - G-actin monomers assembled to form long helical F-actin polymers - F-actin binds troponin and tropomyosin IONS AND MOLECULES FOR MUSCLE CONTRACTION - Ca2+: calcium ion released from sarcoplasmic reticulum → conformational change in tropomyosin → union tropomyosin - actin - ATP: binds to the globular head of the myosin → hydrolysis → ADP + Pi - Na+: sodium ion input → depolarization - K+: potassium ion output → depolarization - Acetylcholine: neurotransmitter that acts on the motor neuron MOLECULAR BASIS OF CONTRACTION (5 stages) 1. ATP is attached to the head of the myosin → hydrolysis → energization and reorientation of myosin (90º angle between head and tail) 2. Calcium binds to troponin in the TnC subunit → change of structure os the troponin that pulls on tropomyosin → actin-myosin junction area remains free 3. The actin-myosin bridge is formed 4. A sliding occurs due to the dissociation of the phosphorus → 50º angle between heal and tail → another phosphorus is dissociated → 45º angle 5. Another molecule of ATP gets in → if there are calcium ions, the cycle is repeated 3. Mechanics of muscle contraction - A band will remain constant while I band shortens - Sliding of thin filaments over thick ones - Z lines get closer - Sarcomeres shorten EXCITATION / CONTRACTION COUPLING 1. The nerve impulse reaches the axon terminal and acetylcholine is released thanks to the input of calcium ions 2. Acetylcholine diffuse to nicotinic receptors on the surface of the sarcolemma, opening sodium channels that get in the cell → threshold action potential 3. Action potential propagates across the sarcolemma and down the T-tubules 4. Voltage-gated receptors are stimulated → calcium is released from the L-tubules 5. Calcium binds to troponin, pulls on tropomyosin → junction actin-myosin is free → bridge actin-myosin → muscle contraction MUSCLE RELAXATION - Acetylcholinesterase breaks down acetylcholine in the synaptic cleft - Calcium pumps (Active transport) bring calcium back into the sarcoplasmic reticulum - A calcium-binding protein (calsequestrin), helps get calcium into the sarcoplasmic reticulum 4. Force production in muscle KEY CONCEPTS - Contraction: type of muscle activity in response to a stimulus, which may or may not lead to muscle shortening, generating changes in length or tension - Force: physical capacity to perform a work or a movement - Tension: force developed by a muscle per unit area - Load: force-weight of an object on the muscle - Power: amount of work per time unit TYPE OF CONTRACTIONS - Isometric: - The developed tension is equal to the load - No length change in the muscle  - Isotonic: - The developed tension is not equal to the load - Length change in the muscle - There is a delay from the muscle activation to the length change (time to reach the necessary tension) - Two subtypes - Concentric: tension is greater than the load and the muscle shortens - Eccentric: tension is minor than load and the muscle stretches CONTRACTION FORCE GRADUATION - It depends on the motor unit’s recruitment - Small motor units to allow precise movements - Large motor units to produce great tensions - Motor units firing asynchronously to delay muscle fatigue and be able to maintain contraction - Wave summation: increase of the contraction force as a result of the application of a second stimulus before the muscle has relaxed from the previous stimulus - Tetanus: at a certain critical value of frequency, the muscle reaches maximum tension, and it stops responding 5. Muscle fiber differentiation TYPES OF FIBERS - Type I - Slow or red fibers - To allow repeated contractions during long periods - More resistant to fatigue - Aerobic metabolic processes - Energy substrate: glucose and glycogen - Type II - Fast or white fibers - Two subtypes: IIa and IIb (faster) - To allow strong and fast gestures - Fatigue quickly - Anaerobic metabolic processes - Energy substrate: phosphocreatine and ATP INDIVIDUAL DIFFERENCES - Each person has a different proportion of fiber types → this factor plus others factors generate the individual athletic qualities - At the same time, the individual athletic qualities and the training influence the formation of one type or another of fibers - Strength training → sonverts some IIa fibers to IIb fibers, that are larger, stronger and faster - Endurance training → converts some IIa fibers to I fibers, that work better in long-term efforts - It is more difficult for type I fibers to differentiate into other types, since they are not as versatile as IIa fibers. MUSCLE EFFECTS IN MUSCLE FUNCTION - Improvement in capillarization → more resistance - Increase of the diameter of muscle fibers → strength and muscle mass (hypertrophy) - Increase of the enzyme activity (aerobic or anaerobic), depending on the specificity of the training Adaptations are not only morphological, but also physiological  6. Muscle remodeling PLASTICITY OF SKELETAL MUSCLE - Primary myofibers are patterned and specified according to developmental cues - Adult myofibers exhibit a high degree of plasticity and can phenotypically “remodel” in response to environmental and physiological cues - This ability occurs through activation of signals transduction pathways which remodel the fibers through alterations in gene expression. GENES INVOLVED IN MUSCLE REMODELING - Calcineurin: - Heterodimeric protein phosphatase - It is activated by sustained, low amplitude calcium waves - Calcineurin dephosphorylate nuclear factor of activated T cells, translocation of NFAT from the cytoplasm to the nucleus - NFAT targets and activates muscle remodeling genes. - Myocyte enhancer factor 2 (MEF2): - Transcription factor regulator of skeletal muscle development - It is activated by class II histone deacetylases (IIHDACs) - It drives slow fibers formation and prompte myocyte survival MITOCHONDRIA: KEY FACTOR IN MUSCLE REMODELING - Mitochondria can adapt to numerous stimuli - The metabolic stress of exercise provides ample signal for the adaptation → to meet the energy demands needed to sustain work - Changes at molecular level can modulate muscle fiber types shifts and promote antioxidant enzyme expression - Exercise → mitochondrial biogenesis → increase of the mitochondrial content and function → tissue remodeling MUSCLE INJURY Any alteration of muscle tissue. There are 4 large groups: - Myopathies and nervous system abnormalities: spasticity, dystonia, Duchenne muscular dystrophy… - Anatomical or traumatic injuries: contusion, strain, breakage… - Minor muscle injury: diffuse muscle pain (soreness), myofascial pain syndrome… - Muscular distress syndromes: core sensitization (fibromyalgia, chronic fatigue)... RECOVERY STAGES IN ANATOMICAL INJURY - Inflammation phase: - 1-6 days - Muscle cell disruption → metabolic alteration and release of coagulation factors - Vasoconstriction (10-15 minutes) → vasodilatation and hyperaemia (24-36 hours) - Proliferation phase: - 3-20 days - Bridges between the edges of the injury → dense network of capillaries and connective tissue - “Race” between repair and regeneration - Remodelling phase: - From day 20th - Collagen fibers are oriented according to the tensile lines → “functional scar” TRAINING IN MUSCLE INJURY PREVENTION - Biarticular muscles → maximum strength exercises cannot be performed on one end of the muscle if the other end is in maximum stretch - Fatigue → we have to know how to “play with the fatigue”, we cannot prescribe the most demanding exercise in a state of maximum muscle fatigue. - Do not make explosive gestures without a warm-up TREATMENT - Recovery is long and training without the supervision of a healthcare professional should not be included until the last phase (maximum safe) - Be careful with the eccentric contractions