Most people underestimate the importance of isometric muscle contraction in helping to build muscle. It is very beneficial to learn how to consciously contract a muscle because it will improve the underlying neural function needed to generate the near 100% muscle activation. This improves gains because the more muscle fibers that fired during a lift, the greater the strength and size gain potential.
One thing you should keep in mind when thinking about the body is that all muscle is basically the same. There are speed differences and different ratios of fast and slow twitch fibers, but by in large, if a fiber receives the signal to fire it will fire. If it doesn’t, it won’t. This process is an all or nothing thing so individual fibers will never contract at a force of 50%. Such a binary approach is successful because there are millions of muscle fibers that we can learn to control more or less individually. If you need to lift something with your biceps that requires 25% of its strength, your nervous system will recruit about 25% of your biceps muscle fibers to fire and contract to get the job done. This is a fantastic system because it allows for very precise effort control while eliminating a lot of wasted energy that 100% muscle firing would require.
Control of the motor units requires some adaptive changes from the nervous system. For the purpose of building muscle, the level of change is not extremely high because you are trying to fire as many motor units as possible in an attempt to work the entire muscle. This being said, neural tissues grow very slowly so it takes time to develop the appropriate pathways to allow for increasing numbers of motor units to be triggered. Whether or not we do develop this nervous system control dependents on our need to do so. This is where practice comes into play as it demonstrates this need and it forces the body to grow the enhanced neural pathways to allow for the improved control. Over time and with practice you will develop the ability to fire a larger and larger percentage of the muscle motor units.
Think back to a time when you were learning a new exercise or when you first started working out. If you are like most people, you probably noticed a dramatic increase in strength in the first few weeks of performing the new movements. Many people report up to 100% strength increases in their first 6 weeks of working out. This strength however, is not accompanied by a 100% increase in size, which is what you would expect to see. In fact, you gain a lot of strength before you notice any change in muscle size. These initial strength increases are the result of more motor units firing when the muscle contracts and not because each motor unit is contracting any harder. The practice helped your body learn how to improve muscle fiber recruitment.
The lesson is that you can learn to recruit more motor units for a muscle contraction if you practice. The more you practice, the faster the skill will develop. People who understand this are the ones how make flexing or posing part of their workout routine. They are the ones learning how to recruit as many muscle fibers as they can when they flex because they know that conscious control of the process is only possible if they have the ability to engage all of the motor units. The skill you are working to acquire is to be able to contract all of the muscle fibers when you are lifting a load since that is what is going to make the fibers grow in size and strength. It is only when you are able to engage all of the muscle fibers when you are lifting that you will be working the entire muscle and forcing maximum growth. Isometric muscle contraction during flexing or posing is a great way to learn how to get this control.