MOVEMENT ANALYSIS

Study Guide: Motor Units, Muscle Mechanics, and Biomechanics

II. Short Answer

1. What is a motor unit, and why is it considered the final output of motor commands?

2. Explain the role of acetylcholine in muscle contraction, and describe how cholinesterase affects its function.

3. Outline the sliding filament theory in three steps, highlighting the roles of actin and myosin.

4. What are the key differences between slow-twitch (Type 1) and fast-twitch (Type 2) muscle fibers?

5. Give an example of a synovial joint and explain how its structure allows for a wide range of motion.

6. Distinguish between concentric and eccentric muscle contractions, providing a real-world example of each.

7. Explain the concept of reciprocal inhibition and how it contributes to coordinated movement.

8. Differentiate between scalar and vector quantities, providing an example of each.

9. How does the position of the center of mass affect stability?

10. Describe the three classes of levers, indicating which one is most prevalent in the human body.

Quiz Answer Key

1. A motor unit is the smallest functional unit of the nervous system, comprising a single motor neuron and all the muscle fibers it innervates. It’s considered the final output because the activation of a motor unit directly leads to muscle contraction.

2. Acetylcholine is a neurotransmitter released at the neuromuscular junction, triggering muscle fiber depolarization and initiating contraction. Cholinesterase breaks down acetylcholine, allowing the neuron to repolarize, and muscle to relax.

3. Calcium binds to troponin, causing tropomyosin to shift and exposing actin-binding sites. 2) Myosin heads bind to actin, forming cross-bridges and pulling the actin filaments towards the M-line. 3) ATP binds to myosin, breaking the cross-bridge, and the myosin head recocks, ready to repeat the cycle.

4. Slow-twitch fibers are fatigue-resistant, aerobic, and suited for endurance activities, while fast-twitch fibers generate more power but fatigue quickly and are anaerobic. Slow-twitch fibers have more mitochondria and myoglobin.

5. A ball-and-socket joint, like the hip, allows movement in multiple planes due to the rounded head of one bone fitting into the cup-like socket of another. This structure enables flexion, extension, abduction, adduction, and rotation.

6. A concentric contraction is when a muscle shortens while generating force (e.g., lifting a weight during a bicep curl). An eccentric contraction is when a muscle lengthens while generating force (e.g., lowering a weight slowly during a bicep curl).

7. Reciprocal inhibition is the relaxation of antagonist muscles to allow the agonist muscles to contract efficiently. This process is coordinated by the central nervous system to ensure smooth and controlled movement.

8. A scalar quantity has magnitude only (e.g., mass, 60 kg), while a vector quantity has both magnitude and direction (e.g., force, 100 Newtons downward).

9. A lower center of mass and a center of mass over the base of support increase stability. When the line of gravity, which extends from the center of mass, falls outside the base of support, stability is compromised.

10. In a first-class lever, the fulcrum is between the effort and the load; in a second-class lever, the load is between the fulcrum and the effort; and in a third-class lever, the effort is between the fulcrum and the load. Third-class levers are the most common in the human body.

11. Acetylcholine: A neurotransmitter responsible for transmitting signals at the neuromuscular junction, initiating muscle contraction.
    

12. Actin: A thin protein filament that forms part of the contractile machinery in muscle cells; it interacts with myosin to cause muscle contraction.
    

13. Agonist Muscle: The muscle primarily responsible for producing a specific movement; also known as the prime mover.
    

14. Antagonist Muscle: The muscle that opposes the action of the agonist muscle; it relaxes to allow the agonist to contract.
    

15. Axon: A long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma, to the axon terminals in order to transmit those impulses to other neurons.
    

16. Center of Mass: The point in an object or body at which the mass is evenly 

17. distributed; it is critical for determining stability.

18. Cholinesterase: An enzyme that breaks down acetylcholine, allowing a neuron to return to its resting state.

19. Concentric Contraction: A type of muscle contraction in which the muscle shortens while generating force.

20. Eccentric Contraction: A type of muscle contraction in which the muscle lengthens while generating force.
    

21. Fulcrum: The pivot point of a lever.
    

22. Isotonic Contraction: A muscle contraction in which the length of the muscle changes. Includes both concentric and eccentric contractions.
    

23. Isometric Contraction: A type of muscle contraction in which the muscle length remains constant while generating force.
    

24. Motor Neuron: A nerve cell that transmits signals from the central nervous system to muscles, causing them to contract.
    

25. Motor Unit: A single motor neuron and all the muscle fibers it innervates.
    

26. Myosin: A thick protein filament that forms part of the contractile machinery in muscle cells; its heads bind to actin and pull the filaments together, causing muscle contraction.

27. Neuromuscular Junction: The synapse between a motor neuron and a muscle fiber, where neurotransmitters are released to initiate muscle contraction.
    

28. Reciprocal Inhibition: The relaxation of antagonist muscles to accommodate the contraction of agonist muscles, ensuring smooth and coordinated movement.

29. Sarcomere: The basic contractile unit of a muscle fiber, composed of actin and myosin filaments.
    

30. Scalar: A quantity that has magnitude only (e.g., mass, temperature).
    

31. Sliding Filament Theory: The widely accepted explanation of how muscle contraction occurs, involving the sliding of actin filaments past myosin filaments.
    

32. Synovial Joint: A freely moving joint that contains synovial fluid for lubrication and shock absorption.

33. Torque: A twisting force that tends to cause rotation.

34. Vector: A quantity that has both magnitude and direction (e.g., force, velocity).