Human movement is extremely complex. Some muscles are designed to create movement while others to stabilize and hinder movement. Each joint in the body tends to have a specific function and purpose that is required for efficient movement to take place. If that joint doesn’t work as it was supposed to work then you need to “cheat” from somewhere else in order to achieve the desired movement. Below you can see which joints need to be moblile and which stable.

Ankle : mobility

Knee : stability

Hip : mobility

Lumbar spine: stability

Thoracic spine : mobility

Scapula : stability

Shoulder : mobility

As you can see the joints simply alternate mobility and stability. Usually problems at one joint show up as pain in the joint above or below. Loss of function in the joint below seems to affect the joint or joints above. Lets take the squat as an example. Many people suffer from low back pain while squatting. Loss of hip mobility might be the cause because when the hip can’t move, the lumbar spine will. And here is the problem. The lumbar spine is designed for stability and the hip is built for mobility. When the supposedly mobile joint becomes immobile, the stable joint is forced to move as compensation, becoming less stable and subsequently painful.

Important note : Sometimes it is better to look at the big picture and “train movements rather than muscles”