In order to execute your barbell squat properly, you need to make sure that your form is right even before you touch the barbell. If you are unsure of your form, do the following exercises step by step and analyze if the ideal result is achieved.
The first thing to do is to do a wall sit. This is executed by placing your back flat against a solid wall and bending your knees so that the top of your thigh is parallel to the floor. This is the maximum range of motion for your quadriceps and your hamstrings so make sure you are squatting all the way down to this point every time you hit the squat rack or smith machine. The next thing is to slowly shift your feet forward until your knees are directly above your ankles. This ensures that the weight is transported by the back and your legs and the kneecaps are free from the pressure of the weights you carry. Do this exercise for two minutes each day for a week before moving on to the next exercise so that your body is used to squatting deep enough so you can get the maximum gain and safety when you do the barbell squat.
The next exercise to do after week one is to get into the proper position for an unsupported squat. Instead of leaving your back on the wall, use your hands to push against the wall and shift your body off the wall. Now your hands are the only things that are resting on the wall. You will feel the strain now as your legs and back muscles are enrolled in order to keep yourself upright. Once you achieve stability in this position, lower your back towards your legs while keeping your chest and head up, creating a straight diagonal line that runs down your spine to your hip. Slowly lift your hands off the wall so that the only thing supporting you is your legs in the squat position. This is the position to adopt when you are doing your squat. The reason why you need to keep your back in this position is because you will transfer the weights that you are carrying on your barbell to your legs only and protect the spinal cord from taking any unnecessary pressure. If you find that your back is curved upwards, that is the posture that will cause you to hurt your back. When you are in this position, your back muscles brace around the spine so that it will remain stationary which is the basis of good form.
Lastly, let us talk about the holding position of the barbell. Lower the barbell on the smith machine or the squat rack until it is slightly below your shoulder level with your legs standing shoulder width apart. If the catch for the barbell is lower, make sure you open your legs wider rather than closing your legs in order to be taller to reach the barbell. The reason is because when your center of gravity is higher, this could result in an unstable standing position when you mount on the weights. The next thing is to place your hands around the bar at roughly 1 palm spacing from your shoulder on each side. This will ensure that you have a firm grip on the bar compared to having your hands holding the bar further away from the body. The last thing to take note is where your barbell rest. Ideally, your barbell should rest on your Trapezius which is located between your shoulder and your neck. It should never rest on the bone which connotes the neck to the body as this could potentially lead to unnecessary pressure on the spinal column.