[This post is written by Justin Hardy who is currently working as a Sport Performance Intern at Athletic Lab]

One of the best, if not the best, exercises to do for gaining strength, muscle, and losing fat, is the squat. However, there are many people who simply do not do the exercise correctly. This is often due to the sedentary lifestyle that many people live, which results in tight and weak muscles. If the squat is performed incorrectly over a long period of time it can lead to muscle compensations and chronic conditions, such as lower back pain. As Americans already suffer heavily from this malady let’s look at two common ways squatting can cause low back pain.
Squatting can cause lower back pain when the neutral curve in our back is not maintained throughout the movement. A telltale sign of this is a rounding of the back and a loss of a curve in the lower back, often seen towards the bottom of the squat. As Mike Robertson discusses, a major cause of low back pain during squats is when a participant “exceeds their current level of hip mobility, and places stress onto their lumbar spine.”
It can also be harmful to have an excessive curve in the back during squats. According to Kritz, Cronin, and Patria (2009):

When an athlete performs a squat and does not stabilize the lumbar spine and fails to maintain a straight or slightly extended thoracic spine position, an increase in compressive and shear forces of the lumbar spine has been observed. Squatting with an external load with excessive lumbar extension (curved back) dramatically increases compressive forces.

Thus, to avoid putting excessive strain on the back it is crucial to keep a slight, but not dramatic, curve in the back. The Squat should be first attempted with only the body as weight and should be learned correctly, before adding weight. Learning improper movement patterns, such as a nonexistent or an excessive curve in the back can lead to chronic pain.
In order to avoid a rounded back it is important to keep an upright chest throughout the squat. Mike Robertson suggests that to avoid letting the chest cave in athletes should “move your hands in closer to your shoulders, drive your elbows underneath the bar, or to adjust the bar placement on your back”. However, a lot of the times, as Robertson discusses, excessive or nonexistent curving of the back can be caused by a weakness in the stabilizing muscles of the lower back. He suggests using Good Mornings to help strengthen the erector spinae muscles while also helping you to avoid a bent over position during squats. When done correctly squats are a tremendous exercise that should be incorporated in almost all strength training workouts.

Mike Robertson, MS, CSCS, USAW, is the President of Robertson Training Systems and the Director of Custom Athletics in Indianapolis, Indiana.https://robertsontrainingsystems.com/blog/squat/
Kritz, Matthew MSc, CSCS ; Cronin, John PhD ; Hume, Patria PhD. The Bodyweight Squat: A movement Screen for the Squat Pattern. Strength and Conditioning Journal. 31.1 (February 2009), pp 76-85.