Week 2 - Muscle Mechanics and Reflex Control PDF

Muscle mechanics MEDI258: HUMAN NEUROMECHANICS Lecture learning outcomes Muscle mechanics u Understand some of the factors constraining how muscles contribute to joint torque and movement production Mechanics of single muscle fibres The relationship between muscle force and length...

Muscle mechanics MEDI258: HUMAN NEUROMECHANICS Lecture learning outcomes Muscle mechanics u Understand some of the factors constraining how muscles contribute to joint torque and movement production Mechanics of single muscle fibres The relationship between muscle force and length u The amount of cross-bridge overlap is a key determinant of muscle fibre force capacity u Why do you think this is true? (Open rectangle above plot shows the physiological range of sarcomere lengths in a wrist extensor muscle) Mechanics of single muscle fibres The relationship between muscle force and velocity u The speed of contraction is another major factor in the force capacity of muscle fibres u Higher shortening speed = less force u Why would this be true? u What happens during lengthening contractions? Lengthening Shortening Hill’s model of whole muscle mechanics u Archibald Vivian Hill (in 1938) described the muscle-tendon unit mathematically u Contractile element (CE) (sarcomere) u Parallel elastic element (PE) Connective tissue u Series elastic element (SE) u Active elasticity (within sarcomeres) u Passive elasticity (tendon) u Which physical components of muscle and tendon might these elements represent? The arrangement of muscle fibres is important! u In-series fibres u If motor neuron firing causes each muscle fibre to change length by 1 unit, the muscle changes length by 3 units u Force = average of each fibre u Contraction speed high u In-parallel fibres u Muscle changes length by 1 unit u Force = sum of fibre forces u Contraction speed low How does fibre arrangement affect force production? Inc. in-series sarcomeres Inc. in-parallel u Does muscle force capacity vary sarcomeres with cross-sectional area? u It depends. u Physiological cross sectional area (PCSA) is the relevant measure u Estimates number of parallel fibres u The number of parallel fibres varies enormously across different muscles u Reflects functional roles u e.g. ankle plantarflexors tend to have larger CSA and pennation angle than dorsiflexors How well does tendon transmit force? u Tendon structure varies greatly between muscles u Structure determines the contribution of individual muscle fibres to overall force How do active and passive elements contribute to force? u Tendon mechanics probed by measuring the response to applied loads u Stress = Force/tendon CSA (MPa) u Elastic region of stress/strain curve lasts until ~8% strain u Plastic region indicates tendon damage prior to failure at ~12% strain u Is tendon as elastic as you thought? How do passive (elastic) elements contribute to force production? u Large open circles: Maximum voluntary force at various muscle lengths u Filled circles: Force produced at various muscle lengths with no contraction u Small open circles: Contribution of active force (difference between MVC and no contraction data) Wow, elasticity contributes a lot beyond resting muscle length! Joint-level muscle mechanics Lecture learning outcomes Joint-level muscle mechanics u Understand how a muscle’s contribution to joint torque depends upon its mechanical and physiological properties How much joint torque can a muscle produce? It depends. u Muscle moment arms u Perpendicular distance b/t line of action of a muscle and the joint centre u Dictates the torque capacity of a muscle u Muscle torque (Nm) = muscle force (N) x moment arm (m) How much joint torque can a muscle produce? It depends. u Muscle moment arms u Are not constant during movement Remember: the CNS must predict the muscle torque capacity at Wrist Wrist every instant in time! flexion extension Most muscles are multifunctional u Off-axis attachments mean that most muscles produce torque about at least two axes u Biceps brachii: Elbow flexion + Forearm supination u Muscle torques contributions depend on joint angle (due to changing moment arms) One- & Two- joint muscles u Monoarticular muscles: cross one joint u Biarticular muscles: cross two joints a. Co-contraction of GM and RF transforms hip ext into knee ext! b. Vasti produce knee ext but hamstrings must contract with GM to produce force in the optimal direction. How is force shared between muscles? u Each joint is served by multiple synergist muscles u CNS has to decide how to share force production between synergist muscles u This is complex, but involves: u Muscle moment arms u Motor unit properties u Muscle mechanics The stretch-shortening cycle Lecture learning outcomes Stretch-shortening cycle u Describe how the stretch-shortening cycle can benefit force production  The stretch-shortening cycle u The stretch shortening cycle can increase: u Positive work done by a muscle work = force x displacement u Power produced by a muscle How?  The stretch-shortening cycle 1. Storage and release of elastic energy (mechanical model) 2. Increased time for muscle force development 3. Reflex action (neurophysiological model) 4. Force potentiation u Where stretching a muscle increases the amount of force it produces at a given length Sensing muscle actions MEDI258: HUMAN NEUROMECHANICS Lecture learning outcomes Reflex actions u Describe in detail the operation of the spinal stretch reflex u Understand the nature and purpose of Ib and reciprocal Spinal reflexes u All spinal reflexes have five common elements 1. Sensory receptor 2. Afferent neuron 3. CNS processing (through 1 or more synapses) 4. Efferent neuron 5. Muscle The Proprioceptive System u Provides the central nervous system with information about limb position and motion from: u Muscle receptors u Tendon receptors u Skin (cutaneous) receptors u Joint receptors u Nociceptors Muscle receptors u Muscle Spindles within each muscle provide information about u Muscle length u Muscle lengthening velocity Muscle spindle – a complex sensorimotor unit u Intrafusal muscle fibres u Contractile fibres with an non- contractile centre u Gamma motor neurons u Excite intrafusal fibres u Afferent nerves u Stretch-sensitive receptors (spiral around intrafusal fibres) How do muscle spindles signal both length and velocity? u Change in muscle length and lengthening velocity DR = discharge rate Monosynaptic reflexes u Spinal stretch reflex u Muscle stretch activates muscle spindle receptors u Muscle spindle afferents excite alpha motor neurons of same muscle u Muscle contracts to counter stretch u Involved in regulating muscle (and therefore joint) stiffness u Important for dealing with external instability/force Reciprocal Inhibition u Ia afferents also synapse onto Ia inhibitory interneurons u These synapse onto alpha motor neurons of antagonist muscles u Outcome: u Antagonist is inhibited by muscle stretch u Allows agonist activation to have full counteracting effect Monosynaptic reflexes u H-reflex u Electrical analogue of stretch reflex u Afferent neurons are stimulated electrically u Reliable test of reflex function u Produces 2 contractions of same muscle – why? Tendon receptors u Tendons contain sensory receptors called Golgi Tendon Organs u Respond to tendon stretch u Signal related to the force being transmitted through the tendon u Activation of GTOs results in inhibition of extrafusal muscle fibres in the same muscle How? Why? Who cares about reflexes? u Movement relies heavily on u They are the basic building proprioception blocks of movement u They offer protective functions u The brain primarily controls against injury movement through reflex pathways u They offer rapid and subconscious mechanisms for u By activating reflex pathways, regulating posture we can understand how the motor system is controlled and u They have shorter delays than how it adapts voluntary control u They are over-expressed in many movement disorders (CP, stroke, spinal cord injury, Parkinson’s disease)