Muscular and Neuromuscular Systems

Quick Answer: Muscle fibers are classified as Type I (slow-twitch, oxidative, fatigue-resistant) or Type II (fast-twitch, glycolytic, high force). Type II subdivides into IIa (moderate endurance) and IIx (highest power, lowest endurance). The sliding filament theory describes how actin and myosin interact to produce force, with motor units recruited based on the size principle.

The muscular and neuromuscular systems form the foundation of human movement and athletic performance. A thorough understanding of muscle physiology is essential for designing training programs that target specific adaptations.

Muscle Fiber Types

Skeletal muscle fibers are classified based on their contractile and metabolic properties:

Type I (Slow-Twitch) Fibers

• High oxidative capacity with abundant mitochondria
• Rich capillary supply for oxygen delivery
• High myoglobin content (red color)
• Low glycogen stores relative to Type II
• Slow myosin ATPase activity
• Suited for: marathon running, cycling, swimming long distances

Type II (Fast-Twitch) Fibers

Type IIa (Intermediate)

• Moderate oxidative and glycolytic capacity
• Can adapt toward Type I or IIx characteristics with training
• Important for sports requiring both power and endurance (soccer, basketball)

Type IIx (Fastest)

• Highest glycolytic capacity
• Largest fiber diameter (greatest force production)
• Fastest contraction velocity
• Lowest fatigue resistance
• Critical for: sprinting, jumping, throwing, Olympic lifts

Exam Tip: Fiber type distribution is largely genetically determined, but training can shift Type IIx fibers toward Type IIa characteristics (and vice versa with detraining).

Muscle Fiber Type Distribution in Athletes

Motor Units and the Size Principle

A motor unit consists of a single motor neuron and all the muscle fibers it innervates. Motor units vary in size:

The Size Principle (Henneman's Size Principle)

Motor units are recruited in order from smallest to largest based on force requirements:

1. Low force requirements (walking, light lifting): Only small motor units activate
2. Moderate force (jogging, moderate loads): Small and medium units recruit
3. High force (sprinting, heavy lifting): All motor units including large ones recruit

Key Concept: To recruit high-threshold (large) motor units, you must either lift heavy loads (>80% 1RM) or move lighter loads with maximal intent/velocity.

Rate Coding

Beyond recruitment, force is also modulated by rate coding—the frequency of action potentials sent to motor units. Higher firing rates produce greater force through:

• Temporal summation of muscle twitches
• Achieving tetanus (sustained contraction) at high frequencies

The Sliding Filament Theory

The sliding filament theory explains the molecular mechanism of muscle contraction:

Key Structures

The Contraction Cycle

1. Action potential travels down motor neuron to neuromuscular junction
2. Acetylcholine (ACh) releases and binds to motor end plate
3. Action potential propagates along sarcolemma and down T-tubules
4. Calcium release from sarcoplasmic reticulum
5. Calcium binds troponin, moving tropomyosin to expose actin binding sites
6. Myosin heads bind to actin forming cross-bridges
7. Power stroke: Myosin heads pivot, pulling actin toward center (requires ATP)
8. ATP binds to myosin, causing detachment from actin
9. ATP hydrolysis re-energizes myosin head for another cycle
10. Process repeats as long as calcium and ATP are present

Exam Tip: Without ATP, myosin cannot detach from actin, resulting in rigor mortis. This is why death causes muscle stiffness.

Types of Muscle Actions

Force Production Comparison

Eccentric > Isometric > Concentric

Eccentric contractions can produce approximately 20-50% more force than concentric contractions because:

• Cross-bridges are forcibly detached rather than actively releasing
• Elastic components (titin, connective tissue) contribute passive tension
• Fewer motor units needed for same force output

Eccentric Training Considerations

Exam Tip: Eccentric training is particularly valuable for injury prevention (e.g., Nordic hamstring curls) and rehabilitation settings.

The Neuromuscular Junction

The neuromuscular junction (NMJ) is where the motor neuron communicates with muscle fibers:

Signal Transmission Process

1. Action potential reaches axon terminal
2. Calcium influx triggers ACh release
3. ACh crosses synaptic cleft and binds receptors
4. Depolarization of motor end plate
5. Acetylcholinesterase degrades ACh
6. Muscle returns to resting state

All-or-None Principle

Individual muscle fibers follow the all-or-none principle:

• A fiber either contracts fully or not at all when stimulated
• Force is graded by:
  • Number of motor units recruited
  • Firing rate of those motor units
• There is no partial contraction of a single fiber