B.1.3 Muscular function Notes

Types of Muscle Contractions

Muscle contractions are the foundation of all movement and stability in the body. Depending on the activity, muscles can contract in different ways, each serving a specific function. These contractions can be categorized into four types: isometric, isotonic concentric, isotonic eccentric, and isokinetic.

Isometric Contractions:

An isometric contraction occurs when a muscle generates force without changing its length. Imagine holding a plank position or trying to push a wall that doesn’t move. In these scenarios, your muscles are active, but there’s no visible movement at the joint.

Isotonic Contractions:

Isotonic contractions occur when a muscle changes length while generating force. This category is further divided into two types:

1. Concentric contractions: The muscle shortens as it contracts. For example, during the upward phase of a bicep curl, your biceps shorten to lift the weight.
2. Eccentric contractions: The muscle lengthens as it contracts. This happens during the downward phase of a bicep curl, as your biceps control the weight’s descent.

Isokinetic Contractions:

An isokinetic contraction involves muscle shortening or lengthening at a constant speed throughout the movement. This type of contraction is less common in everyday activities but is often used in rehabilitation settings with specialized equipment.

Agonists, Antagonists, and Reciprocal Inhibition

Muscle function is rarely isolated. Instead, muscles work in coordinated pairs to produce smooth and controlled movements. These pairs are organized as:

• Agonist: The primary muscle responsible for the movement (e.g., the biceps during a bicep curl).
• Antagonist: The muscle that opposes the movement (e.g., the triceps during a bicep curl).

This coordination relies on a process called reciprocal inhibition, where the nervous system reduces activation of the antagonist muscle while the agonist contracts. This ensures efficient and fluid motion.

Motor Unit Types: Fiber Type and Function

Motor units are classified based on the type of muscle fibers they contain:

1. Type I (Slow-Twitch Fibers):
   • Fatigue-resistant
   • Generate low force
   • Ideal for endurance activities, like long-distance running
2. Type IIa (Fast-Twitch Oxidative Fibers):
   • Moderately fatigue-resistant
   • Generate higher force than Type I fibers
   • Suitable for activities like middle-distance running or swimming
3. Type IIb (Fast-Twitch Glycolytic Fibers):
   • Fatigue quickly
   • Generate the highest force
   • Best for short, explosive activities like sprinting or weightlifting

Hypertrophy, Atrophy, and Changes in Motor Unit Recruitment

Muscle size and strength are not static—they adapt based on use or disuse. These changes directly affect motor unit recruitment patterns.

• Hypertrophy: An increase in muscle size due to consistent resistance training. Larger muscles can recruit more motor units, resulting in greater force production.
• Atrophy: A decrease in muscle size due to inactivity or injury. Reduced muscle mass leads to fewer motor units being available for recruitment.

The Sliding Filament Theory: How Muscles Contract at the Molecular Level

At the microscopic level, muscle contraction is powered by the interaction of proteins within the muscle fibers. This process is explained by the sliding filament theory, which describes how the thick and thin filaments in a sarcomere slide past each other to generate force.

Key Players in Muscle Contraction

1. Actin and Myosin: The primary proteins involved in contraction. Myosin (thick filament) binds to actin (thin filament) to form cross-bridges.
2. Troponin and Tropomyosin: Regulatory proteins on the actin filament. Tropomyosin blocks the binding sites on actin, while troponin controls its position.
3. Calcium (Ca²⁺): Released from the sarcoplasmic reticulum, calcium binds to troponin, causing a conformational change that moves tropomyosin and exposes the binding sites on actin.
4. ATP: Provides the energy for myosin heads to detach from actin and reattach for another power stroke.

The Steps of the Sliding Filament Theory

1. Calcium Release: An action potential triggers the release of calcium ions from the sarcoplasmic reticulum.
2. Cross-Bridge Formation: Calcium binds to troponin, shifting tropomyosin and allowing myosin to bind to actin.
3. Power Stroke: The myosin head pivots, pulling the actin filament toward the center of the sarcomere.
4. Detachment: ATP binds to myosin, causing it to release actin.
5. Reset: ATP is hydrolyzed, re-cocking the myosin head for another cycle.