Energy systems in and neural adaptations to anaerobic training

Anaerobic training uses high-intensity, intermittent bouts of exercise that primarily rely on the phosphagen and glycolytic energy systems. It includes activities such as resistance training, plyometrics, sprinting, and agility drills. Over time, anaerobic training leads to adaptations in muscular strength, power, endurance, neuromuscular function, and metabolic efficiency.

Anaerobic training relies heavily on two key energy pathways:

• Phosphagen system (ATP-PCr system): Provides immediate energy for short-duration, high-intensity activities (≤10 seconds), such as sprints and maximal lifts.
• Glycolytic system: Supplies energy for moderate-duration, high-intensity activities (10-60 seconds), such as 400m sprints or interval-based resistance training.

The aerobic system still contributes, mainly by helping you recover between bouts. However, in explosive sports, performance is dominated by the anaerobic energy systems.

Primary metabolic demands of various sports

Different sports require different contributions from the phosphagen, glycolytic, and aerobic systems. The following table summarizes these demands:

This table highlights why training should match the sport’s metabolic demands. Keep in mind that these contributions overlap - no sport relies exclusively on a single energy system.

Neural adaptations

Neural adaptations are a major driver of improvements in strength, power, and motor unit recruitment. They occur throughout the neuromuscular system, from the central nervous system (CNS) down to individual muscle fibers.

Key neural adaptations:

1. Increased motor unit recruitment: More motor units are activated, allowing for greater force production.
2. Enhanced rate of force development: Contractions become faster and more forceful due to improved neural drive.
3. Synchronization of motor units: Better coordination among motor units improves performance.
4. Reduced inhibitory mechanisms: Less influence from inhibitory structures (such as the Golgi tendon organ) allows for greater force expression.

Neural adaptations typically occur before structural changes in muscle, which is why they’re often the first improvements you see with anaerobic training.

Central adaptations

The CNS supports anaerobic performance by improving motor unit activation and coordination. Key adaptations include:

• Greater cortical activity: Increased neural output from the brain enhances muscle activation.
• Descending corticospinal tract efficiency: More effective transmission of signals from the brain to the muscles improves reaction time and coordination.
• Selective recruitment of high-threshold motor units: Advanced athletes can bypass lower-threshold motor units and activate fast-twitch fibers more efficiently, which supports power and speed.

Together, these changes help athletes produce maximal force more efficiently, improving overall performance.

Adaptations of motor units

Motor unit recruitment follows the size principle: low-threshold motor units are recruited first, and higher-threshold, fast-twitch motor units are recruited as force demands increase.

Selective recruitment in explosive movements

• Highly trained athletes can selectively recruit high-threshold motor units earlier, which supports faster and more powerful movements.
• This is crucial for Olympic weightlifting, plyometrics, and sprinting.

The ability to activate fast-twitch fibers quickly is a key determinant of performance in power-based sports.

Neuromuscular adaptations

Anaerobic training produces adaptations within the neuromuscular system that improve force production and efficiency. These include changes at the neuromuscular junction (NMJ), increased motor unit recruitment, and enhanced neuromuscular reflex potential.

Neuromuscular junction (NMJ) adaptations

The NMJ is the connection point between the nervous system and skeletal muscle fibers, and it plays a central role in initiating muscle contraction. After anaerobic training, NMJ adaptations include:

• Increased total area of the NMJ, improving neural transmission.
• More dispersed synapses with longer branching, enhancing signal efficiency.
• A greater number of acetylcholine receptors, leading to faster neuromuscular communication.

These changes support quicker and more forceful contractions, contributing to improved strength and power.

Neuromuscular reflex potentiation

Anaerobic training can also increase reflex potentiation, especially through the muscle spindle and stretch reflex mechanisms. This leads to:

• Faster rate of force development (RFD).
• Greater reflex activation, amplifying power output.
• Increased synchronization of motor unit firing, improving efficiency in ballistic movements.

Resistance-trained individuals show significantly higher reflex potentiation than untrained individuals. This matters most in activities that require rapid force application, such as sprinting and weightlifting.

Electromyography (EMG) and neural activity

Electromyography (EMG) measures muscle activation during movement. Research indicates:

• Early neural adaptations (0-10 weeks) are associated with increased EMG activity.
• Strength gains early in training are primarily neurological, before significant hypertrophy occurs.
• Cross-education effect: Training one limb can improve strength in the untrained limb due to neural carryover.
• Bilateral deficit: Untrained individuals generate less force when using both limbs simultaneously compared to unilateral contractions.

These findings emphasize the role of neural factors in early strength development. Increases in EMG activity generally reflect improved neural efficiency rather than structural muscle changes.