I was treating a patient recently who was getting back pain while stretching his hip flexors. He said that always happens to him. I told him to squeeze his core and glutes. The pain immediately went away. Of course he wanted to understand why that was. I explained it, and then said we need to stabilize, strengthen, then dynamically strengthen. This is a general principle in physical therapy, and can be seen as a progression. What does this mean? What is the difference between these terms? Let me explain: 

Someone once said (I think it was Gray Cook?):

Strength enhances our ability to create movement, stability enhances our ability to prevent movement.

I’d like to add to it:

Strength enhances our ability to create movement, stability enhances our ability to prevent movement, dynamic strength enhances our ability to do both at the same time. 

Think of stability as the ability to keep a position when challenged. If someone throws a heavy medicine ball at you and you are able to catch it and create the exact counterforce to prevent yourself from falling, you have adequate stability for that task. Any good coach can usually find these ‘holes’ in your stability. You either apply enough load or speed and any stability issues will show themselves. Think of someone benching under heavy load. Once it gets too heavy for that individual, inevitably the elbows flare. The shoulder is losing its position and is ‘tension hunting’. For the shoulder, internal rotation twists the rotator cuff and creates a layer of stability, and that’s why the elbows flare. The problem is, this can sometimes lead to pain or injury. 

I won’t bother explaining strength, this is self evident. So I’ll get directly into dynamic strength. In a gym setting, most of the ‘machines’ are for isolation exercises which are great for making single joint motions stronger. The problem is we don’t move that way in real life. It is much more dynamic. If you are unloading a heavy box from your trunk, you have to be able to keep a stable leg position while picking up the object. Then the arms have to stay stable while you move your legs to wherever you are going. This is dynamic strength. We have to be able to simultaneously and/or alternate between stability and strength. 

It turns out that the majority of the stability muscles are slow twitch (postural) muscles, and the majority of the strength muscles are fast twitch muscles. We see decline in performance, and sometimes injury, when there is a large mismatch between these muscle groups. Imagine having a ferrari without power steering. When you drive fast and hit corners, you’ll likely lose control. 

So let’s go back to the athlete with back pain while stretching his hip flexors. What was going on there? I see this as two problems. First off, his hip flexors were stiff. They attach to the lumbar vertebrae. The other problem is that the muscles that line the spine and hold their position were weak, allowing the hip flexor to pull on the spine. When the nervous system senses a threat, it will send a pain signal to that area to get you to try to change your position. In this case, flexors the core muscles help keep the spine in its happy place. So for this athlete, we had to strengthen the muscles of the spine with some antirotation:

And then stretch the hip flexors with the couch stretch:

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