SOURCE: Quara
This question came up on Quara...
How do you build strength without muscle?
The best answer so far is from...
Nate Moore, combatcircuit.com I'm a professional MMA fighter for Strikeforce. Currently 9-3 professionally, I have trained at the American Kickboxing Academy for the past 5 years.
Nate writes,
The answer is motor unit recruitment and refined motor patterns.
Your muscles are controlled by the nerves in your brain. Each nerve is connected to several individual muscle fibers, so just one neuron is needed to get several muscle fibers contract at once. However not all of the motor units contract at once. So you could flex your bicep, but maybe only half of the muscle fibers would be activated. The stronger the contraction, the more motor units that are recruited to produce a stronger force. This is called motor recruitment.
Imagine that you're trying to push your car down the street, but you're having a bit of trouble. You have 7 friends that are just standing there with nothing to do, so you ask them to help you push the car down the street. You and each of your friends could be considered a separate motor unit.
So imagine you ask your friends to help push the car, but only 5 of them decide to help. This happens not because your friends (motor units) are lazy, but because haven't been trained to work together yet.
See the rest of Nate's answer and others here