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Questions and Answers
What physiological role do muscle spindles primarily serve in the proprioceptive system?
Which receptors are involved in signaling the force being transmitted through a tendon?
In the context of reflexes, what role do Ia inhibitory interneurons play during reciprocal inhibition?
What is the primary outcome of the monosynaptic reflex, particularly the spinal stretch reflex?
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What does the H-reflex electrically simulate in the assessment of reflex function?
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What primarily determines the force capacity of muscle fibers?
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According to the mechanics of single muscle fibers, how does shortening speed affect force production?
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Which of the following best describes the role of parallel elastic elements in muscle-tendon mechanics?
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In an arrangement with in-parallel fibers, how does the change in muscle length compare to the individual fibers?
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What function does the series elastic element serve in the muscle-tendon unit according to Hill's model?
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What effect does an increase in the number of in-series sarcomeres have on muscle contraction speed?
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What is the implication of having motor neuron firing that causes each muscle fiber to change length by 1 unit within in-series fibers?
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What is a major factor in determining how muscles contribute to joint torque?
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What is the physiological cross-sectional area (PCSA) primarily used for in muscle evaluation?
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How does the structure of the tendon affect muscle function?
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At what point does the elastic region of the tendon stress/strain curve end?
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Which of the following factors dictates the torque capacity of a muscle?
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What is indicated by the plastic region on the stress/strain curve of a tendon?
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What primarily varies across different muscles and reflects their functional roles?
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How can joint torque produced by a muscle change during movement?
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What is the relationship between muscle force and tendon cross-sectional area (CSA)?
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What does the contribution of active force in muscles depend on?
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Which characteristic of ankle muscles contributes to their larger cross-sectional area compared to dorsiflexors?
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Which type of muscle crosses only one joint?
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What is a primary benefit of the stretch-shortening cycle concerning muscle performance?
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Which of the following factors does NOT influence the sharing of force between synergist muscles?
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In the context of spinal reflexes, which element is crucial for the initial detection of muscle stretch?
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What role do co-contraction of the gluteus maximus and rectus femoris play in joint movement?
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Which mechanism is NOT involved in the stretch-shortening cycle?
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How does the central nervous system share force production between synergist muscles?
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What physiological effect occurs as a result of muscle stretching during the stretch-shortening cycle?
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Which of the following best describes the action of the biceps brachii?
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What is the relationship between joint angle and muscle torque?
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Study Notes
Muscle Mechanics
- The relationship between muscle force and length: The amount of cross-bridge overlap is a key determinant of muscle fibre force capacity. This is because a greater overlap allows for a higher number of cross-bridges to form, leading to a higher force.
- The relationship between muscle force and velocity: Contraction speed is a major factor in the force capacity of muscle fibres. A higher shortening speed results in less force because there's less time available for the cross-bridges to form and contribute to force production. During lengthening contractions, the muscle's elasticity is engaged, contributing to a passive force component.
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Hill's model of whole muscle mechanics: Hill's model describes the muscle-tendon unit mathematically, with three key components:
- Contractile element (CE): Represents the sarcomere, the fundamental unit of muscle contraction.
- Parallel elastic element (PE): Corresponds to the connective tissue surrounding muscle fibres, which contributes to passive elasticity.
- Series elastic element (SE): Includes both the active elasticity within the sarcomeres and the passive elasticity of the tendon.
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Fibre arrangement and force production:
- In-series fibres: These fibres are arranged end-to-end, resulting in a higher contraction speed and a force equal to the average force of individual fibres.
- In-parallel fibres: These fibres are arranged side-by-side, resulting in a lower contraction speed but a higher force due to the summation of individual fibre forces.
- Physiological cross-sectional area (PCSA): PCSA is a measure of the cross-sectional area perpendicular to the muscle fibres and is a more accurate reflection of the number of parallel fibres than the anatomical cross-sectional area. Muscles with larger PCSAs tend to have greater force production capacity.
- Tendon structure and force transmission: Tendons differ greatly in structure, which influences the contribution of individual muscle fibres to overall force.
- Active and passive elements: Tendon mechanics are probed by measuring its response to applied loads. The stress-strain curve exhibits an elastic region up to approximately 8% strain and a plastic region indicating tendon damage prior to failure at around 12% strain.
- Passive elements: Elastic elements provide a significant contribution to passive force production, especially beyond the resting muscle length.
Joint-level muscle mechanics
- Muscle moment arms: The moment arm is the perpendicular distance between the line of action of a muscle and the joint centre. It determines the torque capacity of a muscle, with larger moment arms generating greater torque.
- Moment arm changes: Moment arms are not constant during movement, changing with the joint angle. This means the CNS must predict the muscle torque capacity at every instant in time.
- Multifunctional muscles: Most muscles have off-axis attachments, allowing them to generate torque around multiple axes. This means their torque contributions depend on joint angle due to changing moment arms.
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One-joint and two-joint muscles:
- Monoarticular muscles: These span a single joint.
- Biarticular muscles: These span two joints, allowing for transfer of force and momentum between joints.
- Force sharing: Multiple synergistic muscles act on each joint. The CNS determines how to share force production between these muscles, taking into account factors like muscle moment arms, motor unit properties, and muscle mechanics.
The stretch-shortening cycle
- The stretch-shortening cycle: This cycle combines a pre-stretch followed by a rapid shortening, resulting in increased work and power output by the muscle.
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Mechanisms:
- Elastic energy storage and release: The pre-stretch allows for elastic energy storage in the muscle-tendon unit, which is released during shortening, contributing to increased power.
- Increased time for muscle force development: The pre-stretch provides more time for muscle force development, leading to a higher force output.
- Reflex action: The stretch activates the muscle spindle receptors, triggering the stretch reflex, which contributes to increased activation and force.
- Force potentiation: Stretching a muscle increases the amount of force it can produce at a given length, enhancing overall force output.
Sensing muscle actions
- Spinal reflexes: These reflexes involve five key elements: sensory receptors, afferent neurons, CNS processing, efferent neurons, and the muscle.
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Proprioceptive system: This system provides the CNS with information about limb position and motion, utilizing various receptors:
- Muscle receptors: Muscle spindles signal muscle length and lengthening velocity.
- Tendon receptors: Golgi Tendon Organs (GTOs) signal tendon stretch and force transmission.
- Skin receptors: Cutaneous receptors provide information about touch, pressure, and pain.
- Joint receptors: Mechanoreceptors signal joint position and movement.
- Nociceptors: These receptors signal pain.
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Muscle spindles: These sensors have:
- Intrafusal muscle fibres: Contractile fibres with a non-contractile centre.
- Gamma motor neurons: Excite intrafusal fibres, allowing them to adjust their sensitivity to muscle stretch.
- Afferent nerves: These nerves contain stretch-sensitive receptors that wrap around intrafusal fibres and transmit information about length and velocity.
- Muscle spindle signalling: They generate signals based on both muscle length and lengthening velocity, with greater discharge rates corresponding to increased length or velocity.
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Monosynaptic reflexes:
- Spinal Stretch Reflex: This reflex activates the muscle spindle receptors, causing simultaneous contraction of the stretched muscle.
- H-reflex: This is an electrical analogue of the stretch reflex, providing a reliable measure of reflex function.
- Reciprocal Inhibition: With stretch, Ia afferents also inhibit antagonist muscle activity via Ia inhibitory interneurons, allowing for more effective agonist contraction.
- Golgi Tendon Organs (GTOs): These receptors signal tendon stretch and force transmission. Activation leads to inhibition of the muscle to protect against excessive force.
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Description
Explore the intricacies of muscle mechanics including the relationships between muscle force, length, and velocity. Understand Hill's model and its key components like contractile and elastic elements. This quiz offers a comprehensive look at how these factors influence muscle performance.
