Neuromuscular contraction

Before being physical, strength is a nervous phenomenon. Muscle contraction originates in the motor cortex, passes through the alpha motor neurons of the spinal cord, and then reaches the myocytes via the motor end plate in the form of an action potential. It is by optimizing this command chain that maximum strength (Fmax) is developed.

A beginner must imperatively observe a neuromuscular adaptation phase. During the first few weeks, strength gains are spectacular while muscle volume remains stable: this is proof that the "software" (CNS) learns to better utilize the "hardware" (muscles).

1. The Motor Unit (MU): The Quantum of Movement

The motor unit is the basic functional entity of the neuromuscular system. It consists of an alpha motor neuron, its axon, and all the muscle fibers it innervates.

• The All-or-Nothing Law: When a motor neuron sends an impulse, all the fibers of the Motor Unit contract simultaneously and at their maximum capacity.
• Specificity: A precise muscle (like the eye) has small MUs (1 neuron for 10 fibers) for precision, while a force muscle (quadriceps) has large MUs (1 neuron for 2000 fibers).

2. Spatial Recruitment and Strength Deficit

The organism never activates the entirety of its MUs at the same time, as a measure of structural protection (prevention of tendon avulsion). The difference between the theoretical contractile capacity and the maximal voluntary force is the strength deficit.

• The Beginner: Suffers from strong protective inhibition. Their deficit can reach 40%. Training reduces this inhibition through a desensitization process of the Golgi tendon organs.
• The Athlete: Through maximal strength training (loads 85%), they learn to recruit high-threshold MUs, reducing their deficit to less than 10%.

3. Henneman's Law: Graduated Recruitment

The recruitment of MUs follows an immutable hierarchical order based on the size of the motor neuron. This is the Henneman Size Principle.

1. Type I (Slow) Fibers: Small motor neurons, low excitation threshold. They are recruited for daily tasks and endurance.
2. Type II (Fast) Fibers: Large motor neurons, high excitation threshold. They are only activated when the load is heavy or the intent is explosive.

The challenge of adaptation: The beginner lacks "connection" with their Type II fibers. Their first adaptation is to teach the brain to send a powerful enough electrical signal to cross the activation threshold of these explosive fibers.

4. Temporal Summation (Rate Coding)

Beyond the number of recruited fibers (spatial recruitment), strength depends on the discharge rate (temporal recruitment). This is the number of nerve impulses sent per second (measured in Hz).

• The higher the frequency, the more the individual contractions merge to reach the physiological tetanus, generating maximum tension force.
• Strength training increases the CNS's ability to send high-frequency bursts of impulses at the start of the movement (rate of force development).

Conclusion: Resistance Training as Software Update

In summary, initial strength is not a matter of "arm size," but of neural efficiency. The adaptation phase allows a triple adaptation: better synchronization of the MUs, an increase in discharge frequency, and a release of cerebral inhibitions. Once this system is optimized, the muscle can finally enter a productive phase of structural hypertrophy.

1. Duchateau et al. (2006) - Training effects on neuromuscular architecture and function.
2. Aagaard (2003) - Training-induced changes in neural function.
3. Henneman et al. (1965) - Functional significance of cell size in spinal motoneurons.