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Neuromuscular Efficiency refers to the method by which muscles strengthen due to changes at the neuromuscular junction. In other words, the muscle strengthens because the connection between the brain and the muscle improve.

The two main improvements that lead to increased neuromuscular efficiency are:
1. Muscle fiber recruitment: The central nervous system (brain and spinal cord) learns to recruit more muscle fibers in response to the challenge of lifting heavier weights.

2. Rate Coding: The central nervous system sends faster signals to the muscle fibers so they contract more quickly and more forcefully.

As neuromuscular efficiency improves, strength goes up. In fact, much of the initial strength gain during any program is due to neuromuscular efficiency and not increased muscle mass.

From - http://novusfitnesstraining.com/neuromuscular-efficiency/