If I were to pick one muscle to get as strong as possible, I would choose the diaphragm. Many trainers may choose the glutes or maybe the abdominals, however, from my clinical experience no muscle is more important for creating strength and neurological balance than the diaphragm. More specifically it is best to train the left diaphragm more than the right because it is smaller and more mechanically disadvantaged than the right.
From a movement perspective the brain cares about three things more than anything else.
- Can You Breathe
- Can you Swallow
- Can you move Forward in Space
Most muscle imbalances that occur throughout the body are the brain’s way of solving these three problems. The diaphragm is involved in all three of those movement solutions. As a result, improving diaphragm function can improve most muscle and neurological imbalances. 
The importance of the diaphragm
Obviously, the diaphragm is an incredibly important muscle because it is the primary muscle of breathing, yet the majority of people will never specifically train the diaphragm. Other functions of the diaphragm include:
- Postural Stability
- Spinal Decompression
- Fluid Dynamics
- Visceral Health
- Emotional Regulation
- Cervical Pain Regulation
- Body Compartment Pressure Gradient Regulation
- Pelvic Floor Symmetry
Diaphragm Training is Abdominal Training, Abdominal Training is Diaphragm Training
I cheated a little bit when I said that the most important to muscle to train is the diaphragm. Because it is impossible to truly train one muscle without assistance from other muscles and structures.
The primary goal of the abdominal muscles is to act as muscles of respiration. During expiration, when the abdominal wall contracts it forces compression at the lower ribs and forces the diaphragm into an upward direction, both of which decrease lung volume, and air is forced out.
On inspiration the abdominal wall creates an opposition to the diaphragm contraction (it creates and anchor point), therefore preventing abdominal protrusion upon inspiration. As the diaphragm contracts harder, squishing your guts down, the abdominals are then required to maintain the abdominal wall. Therefore, the strength of the diaphragm is directly linked to the strength of the abdominals.
Training the Diaphragm requires an Increase in Intra-Abdominal Pressure
When the Diaphragm Contracts it will apply a downward force on the abdominal viscera. If the abdominal wall does its job the pressure within the abdominal cavity will increase. This is the key to lifting heavy weights.
If you want to lift something heavy you need to take create pressure in your body, meaning you need to take your body which is primarily a container of fluid and turn it into an incompressible fluid. If you do not learn how to create pressure in your abdomen your body will create pressure in other places and more than likely the pressure will go to your back.
The Seven Best Exercises for Training the Diaphragm
Most of the concepts presented in this article are concepts taken from the Postural Restoration Institute. When attempting to train your diaphragm there are five key concepts in order to maximize effectiveness.
- Abdominal wall contraction – as mentioned earlier the abdominal wall is the anchor point for diaphragm contraction therefore there needs to be some form of abdominal wall activity
- Full exhales – the more air you exhale the more your abdomen will contract, in addition the more you exhale the better position your diaphragm will be to perform inhalation
- Slow Inhales – inhales need to be slow, long, and quiet. This will prevent the use of accessory muscles in the neck and low back from getting in to bring in air.
- Pause – To create lasting neurological changes it is best to observe a pause between inhales and exhales. You are essentially letting your body experience a full inhale and create space decompression to a full exhale and creating compression.
- Increase in abdominal pressure – if you adhere to the 4 previous concepts, you should observe an increase in abdominal pressure when performing these exercises. The bigger the increase in intra-abdominal pressure the stronger your diaphragm is getting.