When you contract a muscle, the brain not only sends out signals that stimulate fiber contraction, but inhibitory signals that limit it as well. This protects you from overcontraction, which could cause injury, but limits the amount of muscle fibers being stimulated. Whenever you experience a muscle spasm or cramp, you are getting a taste of what would happen if these inhibitory signals did not exist
Training progress happens in part because you are making your muscle fibers bigger and stronger. and in part because you gradually reedeucate your nervous system so that it will decrease the inhibitory signals involved and allow for a stronger contraction. It takes energy to overcome this inhibition, to overwhelm the protective mechanisms. The more intense the imagery you use, the harder you concentrate and focus the mind into your muscle, the more you break through these inhibitory limitations your brain is creating and the more rapid you progress."