Muscle Function, Adaptation & Energy Systems

Thin Filament Structure

The thin filament is composed of three types of proteins:

• Actin: Forms helical rolls.
• Troponin: Binds to actin and tropomyosin. It is formed by three subunits:
  • C: Calcium-binding unit.
  • I: Calcium-inhibiting unit.
  • T: Troponin-tropomyosin union unit.
• Tropomyosin: Covers the active binding sites on actin for myosin.

Muscle Adaptation Mechanisms

Hypertrophy

An increase in the size of muscle fibers, therefore increasing muscle size.

Hyperplasia

An increase in the number of muscle fibers, not their size. This has been observed in animals like cats.

Atrophy

A reduction in the size of the muscle. It is typically caused by lack of muscle use and protein loss.

Autogenic Inhibition Explained

Autogenic inhibition is a reflex that causes muscles to relax. When a muscle undergoes maximum contraction, this reflex is triggered, leading to muscle relaxation. Stretching the muscle can also induce this relaxation, eliminating tension and allowing the muscle to become unconstrained.

Muscle Spindle: Function and Types

Muscle spindles are sensory structures found within muscle fibers, typically near the tendon. Their primary function is to detect changes in muscle length and the rate at which these changes occur.

The central region of the muscle spindle lacks actin and myosin, which prevents it from collapsing at the ends. The neuron that innervates the spindle fibers is the gamma motor neuron.

Types of Muscle Spindle Fibers:

• Nuclear Bag Fibers: Characterized by central nuclei clustered in a "bag."
• Nuclear Chain Fibers: Characterized by nuclei arranged in a chain along the fiber.

Sensory Innervation:

• Primary (Annulospiral) Endings: Higher speed conduction; innervate both nuclear bag and chain fibers.
• Secondary (Flower-Spray or Rose Window) Endings: Lower speed conduction; primarily innervate nuclear chain fibers.

Types of Response:

• Static Response: The spindle continues to send signals as long as the stretch is maintained.
• Dynamic Response: Responds to sudden stretches; once the stretch is completed, the spindle stops sending signals.

Engrams: Stored Motor Patterns

Engrams are memorized motor patterns stored in the brain, also known as motor programs. These can be stored in:

• Position Sensors (Sensory Memory): Generally slower access.
• Motor Cortex (Motor Memory): Generally faster access.

Oxygen Deficit and Debt in Exercise

During submaximal exercise, VO2 (oxygen consumption) stabilizes after approximately 3 minutes. Between the start of the exercise and this 3-minute mark, an oxygen deficit occurs because the aerobic system cannot immediately supply enough energy, leading to reliance on anaerobic pathways.

Oxygen debt (also known as EPOC - Excess Post-exercise Oxygen Consumption) occurs after exercise finishes. During this period, the body consumes more oxygen than at rest. This excess oxygen is used to replenish depleted phosphagen stores and metabolize lactate accumulated during the oxygen deficit phase.

Autonomic Nervous System Effects

Sympathetic System (SIMPATICO)

The sympathetic nervous system typically:

• Increases heart rate and the force of the heartbeat.
• Dilates veins and arteries, allowing for more oxygen delivery.
• Increases arterial tension and venous return.
• Improves breathing (e.g., bronchodilation).
• Increases metabolic rate.

Parasympathetic System (PARAS)

The parasympathetic nervous system typically:

• Decreases heart rate (FC).
• Causes constriction of coronary vessels.
• Causes bronchoconstriction.

Protein Metabolism and Storage

Proteins cannot be stored in the body in the same way as carbohydrates or fats; they primarily form structures. Excess amino acids from protein breakdown are not stored. Instead, their carbon skeletons can be converted into fat or glucose, and the nitrogenous waste (remainder) is processed and eliminated, primarily through the liver and kidneys (e.g., as urea).

Pachymeter (Skinfold Caliper)

A pachymeter, also known as a skinfold caliper, is an instrument used to measure small thicknesses or diameters. In anthropometric measurements, it is specifically used as a clamp to measure skinfold thickness, which is an indicator of subcutaneous fat.

Motor Neuron Function

Motor neurons are integral to the motor system. They carry information that has been analyzed and processed by the Central Nervous System (CNS) to effector organs. A single motor neuron supplies a set of muscle fibers (forming a motor unit), transmitting signals to the neuromuscular junction. This stimulation leads to muscle contraction by enabling the interaction of actin and myosin filaments.

Fat Oxidation and Glycogen Sparing

When individuals, particularly endurance athletes, consume lipids (fats), their bodies can adapt to produce ATP (energy) more efficiently from fat oxidation. This process helps to spare glycogen stores. Preserving glycogen is beneficial as these stores can then be utilized for high-intensity activities like sprints. This adaptation is more pronounced in endurance athletes, who typically have a higher density of mitochondria, the cellular sites of aerobic metabolism including fat oxidation.