Muscle Mechanics and Relative Age Effect in Sports

Motor Learning - Week 4

• Physical Abilities: Highly modifiable through training (e.g., muscle strength, muscle mass, flexibility, maximal oxygen uptake)
• Static Abilities: Limited potential to change (e.g., muscle fiber type, height, lung size). Largely genetic. Not to be confused with static vs. dynamic exercises.

Movement Continuity

• Discrete Movements: Throwing a punch (marked in presentation).
• Serial Movements: Steering a car (marked in presentation).
• Continuous Movements: Playing piano composition, triple jump, running (marked in presentation).

Triple Jump

• Phases: Approach run, hop, step, jump.
• World Record: (Men) 18.29 m (Edwards, 1995)
• Long Jump Record: 8.95m

Reaction Time

• Simple Reaction Time: One stimulus, one response (e.g., sprinting after a starting pistol shot).
• Complex Reaction Time (choice): Two or more stimuli, specific responses (e.g., choosing the correct response from multiple options).
• Complex Reaction Time (discrimination): Two or more stimuli, one response paired with specific stimuli (e.g., pressing a button only when a red light is lit).

Relative Age Effect

• Preference: Preference for selecting athletes born earlier in the year.
• Example: January 15, 2024 selection for 9-year-olds. April 1, 2014 selection for 9-year-olds.

Unit 3: Peripheral Neuromuscular Mechanisms

• Learning Objectives: Understand the basic mechanical properties of muscles, describe the organization of the central and peripheral nervous system, explain functions of neurons and their communication, and describe the physiology and organization of the motor units.

Muscle Properties

• Skeletal Muscles: Central area of muscle tissue with tendons on both ends.

• Mechanical Properties: Extensibility (stretch and recoil), contractility (shortening to produce force), and elasticity.

• Simplified Model: Contractile element (CE), Parallel Elastic Elements (PE), Serial Elastic Elements (SE).

• Stretch and Recoil Properties: Vary greatly, depend on shortening/lengthening velocity, and tissue thickness/health. Nervous system regulates mechanical properties (stiffness, force absorption, recoil) by timing/amount of muscle contractions.

• External Forces: Body weight and stretching of muscle-tendon complex. Forces are stored in EE (tendon) and released through recoil. Muscle work is reduced.

• Tasks: Running, Hopping, Walking.

• Muscle Contraction:

• Concentric: Muscle shortens, force generated (e.g., jumping, acceleration).

• Eccentric: Muscle lengthens (stretched), force absorbed (e.g., declin running, deceleration).

Force-Length Relationship

• Passive Force: Resistance of relaxed muscle to stretch (EE).

• Resting Length: Length at which passive force begins to develop.

• Active Force: Produced by active cross-bridges during contraction (CE). Sarcomere is basic force generating unit.

• Total Force: Active + passive force.

• Optimal Length (Lo): Length at which the greatest active force occurs.

Stretch-Shorten Cycle (SSC)

• Concentric vs. Eccentric Contraction: Contractions occurring simultaneously.
• Concentric: Length shortens, force generated.
• Eccentric: Lengthens, force absorbed.
• SSC Mechanism: Preload effect (build up of stored elastic energy within tissues during eccentric phase, released during concentric phase), Optimal length (muscle stretched during eccentric phase), Excitation or reflex mechanism (e.g., muscle spindle feedback).

Exercise Training and Neuromechanics

• Flexibility Training (static stretching): May increase range of motion (ROM) and decrease stiffness, tolerance to stretching (how this impacts SSC?).
• Strength Training: May increase muscle-tendon stiffness and tissue density/thickness (hypertrophy).
• Plyometric Training: Increases muscle stiffness and maximizes elastic recoil (the energy return). Targets the SSC mechanism; forceful eccentric followed by rapid concentric phase (e.g., box jumps). Ballistic training, light-load, maximal speed of concentric phase. How would that change the force-length relationship?
• Considerations: How would this training impact the force-length relationship of specific muscles?

Organization of the Nervous System

• Central Nervous System: Brain and spinal cord; integration and command centre for the entire nervous system.
• Peripheral Nervous System: Motor/efferent (somatic, autonomic, sympathetic, parasympathetic) pathways; skeletal muscle connections.
• Direction of Information: Central nervous system to peripherals.

Neuron Structure

• Soma, dendrites: Signal incoming
• Axon: Signal out
• Classifications: Sensory (afferent), Motor (efferent), Interneurons
  • Sensory: Receptors to central nervous system
  • Motor: Central nervous system to muscle contractions
  • Interneurons: Connect other neurons.

Action Potentials

• Bioelectric Signal: Neuron to neurons/muscles
• Spread through Synapses: Neurons communicate through changes in charges across the membrane (sodium or potassium ions).
• Depolarization: Membrane potential becomes less negative, Na+ channels open (in to out).
• Repolarization: Membrane potential returns to resting state, K+ channels open (in to out).
• Hyperpolarization: membrane potential becomes more negative.
• Overshoot: Period after depolarization where the membrane potential is more positive than the resting potential.

Neuron Function

• Neurotransmitters: released from presynaptic neuron to induce depolarization in postsynaptic neuron(s)

• EPSP: excites postsynaptic neuron; more likely to fire an action potential

• IPSP: inhibits postsynaptic neuron; less likely to fire an action potential

• Summation: Multiple EPSPs may summate to a threshold level to generate an action potential; summation can be temporal or spatial

• Temporal: Repeated stimuli in short time frame

• Spatial: Multiple stimuli at same time

• Ratio of EPSPs to IPSPs: Balance determines if a neurons fires an action potential or not.

Motor Neurons

• Axons of Motor Neurons Branch Off: Axons branch, affecting multiple muscles.
• Motor Unit (MU): Lower motor neuron (found in spinal cord) and all muscle fibres it innervates; Muscle Fibers are found in MU innervation.

Motor Unit (continued)

• Innervation Ratio: Number of fibers controlled by one neuron; lower ratio= finer movement.
• All-or-none Principle: All muscle fibers within one motor unit contracts or none contract in response to the neuron's action potential.
• Fine Movement: Lower innervation ratio (e.g., hand).
• Gross Movement: Higher innervation ratio (e.g., gastro-cnemius)