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Questions and Answers
Which type of muscle crosses only one joint?
What contributes to the changing torque of a muscle?
What factor is NOT involved in how the CNS shares force production between synergistic muscles?
Which mechanism is NOT part of the stretch-shortening cycle?
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Which of the following describes the role of reciprocal inhibition in spinal reflexes?
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What is an outcome of the stretch-shortening cycle regarding muscle performance?
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Which of the following is part of the common elements of all spinal reflexes?
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What describes biarticular muscles?
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What aspect of muscle contraction is affected by the moment arm?
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Which component is crucial for the storage and release of elastic energy during the stretch-shortening cycle?
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What primarily determines the force capacity of a muscle fibre?
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What happens to muscle force production as shortening speed increases?
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In Hill's model, which component of the muscle-tendon unit represents the elastic properties of muscle connective tissue?
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If a muscle has more in-series muscle fibres, how does that affect its contraction?
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What is the effect of adding more in-parallel muscle fibres to a muscle's architecture?
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What occurs during lengthening contractions in a muscle?
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What is the primary role of the series elastic element in Hill's model?
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Which factor is NOT a determinant of muscle fibre force capacity?
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Why is the arrangement of muscle fibres important?
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What is the relationship between contractile element (CE) length change and muscle output in a muscle with all in-series fibres?
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What is the primary factor that influences the variation in muscle force capacity across different muscles?
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How does tendon structure affect its ability to transmit force?
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What parameter is used to calculate stress in tendons?
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At what point does the elastic region of the tendon stress-strain curve typically end?
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What does the small open circles in the representative data indicate?
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What determines the torque capacity of a muscle at a joint?
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Why must the central nervous system (CNS) predict muscle torque capacity constantly?
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What contributes significantly to force production beyond resting muscle length?
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Which of the following statements about muscle moment arms is correct?
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What factor does NOT influence the functional roles among different muscles?
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What is the primary function of muscle spindles in relation to muscle movement?
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What is the role of gamma motor neurons in muscle spindle function?
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What occurs during reciprocal inhibition in the context of muscle stretch?
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Which receptors are involved in signaling changes due to tendon stretch?
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How does the spinal stretch reflex contribute to muscle control?
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What is true about the H-reflex?
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Which statement best describes the relationship between proprioceptors and the central nervous system?
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What happens to extrafusal muscle fibers when Golgi Tendon Organs are activated?
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Which type of receptors collectively contribute to the proprioceptive system?
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What is the effect of muscle stretch on the muscle spindle afferents?
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Study Notes
Mechanics of Single Muscle Fibres
- The amount of cross-bridge overlap within a sarcomere determines muscle fibre force capacity.
- The speed of contraction (shortening or lengthening) is another major factor influencing muscle fibre force.
- Higher shortening speeds lead to less force, while lengthening contractions can generate more force than isometric contractions at a given length.
Hill's Model of Whole Muscle Mechanics
- The muscle-tendon unit can be mathematically described using Hill's model (1938).
- This model decomposes the muscle into three elements:
- Contractile Element (CE): Representing the sarcomere.
- Parallel Elastic Element (PE): Representing connective tissue.
- Series Elastic Element (SE): Containing both active elasticity within the sarcomeres and passive elasticity within the tendon.
Muscle Fibre Arrangement
-
In-series fibres:
- Increase muscle length change per motor neuron firing.
- Average force across fibres.
- High contraction speed.
-
In-parallel fibres:
- Increase muscle force production.
- Sum of individual fibre forces.
- Low contraction speed.
Muscle Force Capacity
- Muscle force capacity is determined by its physiological cross-sectional area (PCSA), which estimates the number of parallel fibres.
- PCSA varies significantly across different muscles, reflecting their functional roles, for example, ankle plantarflexors have a larger CSA and greater pennation angle than dorsiflexors.
Tendon Mechanics
- Tendon structure varies greatly between muscles, impacting the transmission of force from individual muscle fibres.
Passive and Active Elements in Force Production
- The stress-strain curve of tendon reveals its elastic properties.
- The elastic region extends up to approximately 8% strain.
- Further strain leads to tendon damage and eventually failure around 12% strain.
- Passive elements contribute significantly to force production, especially at lengths beyond resting length.
Joint Level Muscle Mechanics
- A muscle's contribution to joint torque depends on its mechanical properties, moment arm length, and physiological properties.
Muscle Moment Arms
- The moment arm is the perpendicular distance between the muscle's line of action and the joint center.
- It dictates the torque capacity of a muscle, with torque calculated as force multiplied by moment arm.
- Moment arms change during movement, requiring CNS prediction of muscle torque capacity at every instant.
Multi-Functional Muscles
- Most muscles produce torque about multiple axes due to their off-axis attachments.
- Muscle torque contributions vary with joint angle due to changing moment arms.
One & Two Joint Muscles
- Monoarticular muscles cross a single joint, while biarticular muscles cross two joints.
- Biarticular muscles can have complex force interactions due to their influence on two joints, requiring joint angle dependent coordination.
Force Sharing Between Muscles
- Multiple muscles act as synergists to produce movement at a given joint.
- The CNS distributes force production between synergists based on factors like moment arms, motor unit properties, and muscle mechanics.
The Stretch-Shortening Cycle (SSC)
- The SSC can increase muscle work and power production by:
- Elastic energy storage and release: Mechanical model.
- Increased time for force development: Mechanical model.
- Reflex action: Neurophysiological model.
- Force potentiation: Neurophysiological model.
Sensing Muscle Actions: The Proprioceptive System
- The proprioceptive system provides the CNS with information about limb position and motion from various receptors including:
- Muscle receptors (Muscle spindles & GTOs).
- Tendon receptors (GTOs).
- Skin receptors.
- Joint receptors.
- Nociceptors.
Muscle Spindles
- These receptors within muscles are sensitive to both muscle length and lengthening velocity.
- They consist of intrafusal fibres (contractile with a non-contractile centre) and gamma motor neurons that excite these fibres.
- Afferent nerves with stretch-sensitive receptors wrap around the intrafusal fibres.
Monosynaptic Reflexes
- The spinal stretch reflex:
- Muscle stretch activates muscle spindle receptors.
- Muscle spindle afferents excite alpha motor neurons of the same muscle, leading to muscle contraction to counter stretch.
- This reflex contributes to regulating muscle stiffness for dealing with external forces.
Reciprocal Inhibition
- Ia afferents also synapse onto Ia inhibitory interneurons that inhibit alpha motor neurons of antagonist muscles.
- This inhibition allows the agonist muscle to fully counteract external forces.
H-reflex
- An electrical analogue of the stretch reflex, stimulating afferent neurons electrically.
- Used to test reflex function, producing two contractions of the same muscle due to activation of both afferent and efferent pathways.
Tendon Receptors: Golgi Tendon Organs (GTOs)
- Located within tendons, GTOs are sensitive to tendon stretch, signaling force transmitted through the tendon.
- Activation leads to inhibition of extrafusal muscle fibres in the same muscle, contributing to protective mechanisms.
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Description
Test your understanding of the mechanics of single muscle fibres, Hill's model, and muscle fibre arrangement. Explore how cross-bridge overlap and contraction speeds influence force generation in muscles. This quiz dives into the intricate components of muscle mechanics and their arrangements.
