When doing an exercise, the muscles that contract to allow for movement are known as agonists. During squats, your hips and legs act as agonists, while your back acts as a stabilizer. The co-contraction of muscles in those areas permits for a smooth and efficient range of motion. If there is a weakness in the chain of agonists, then your form will suffer and you will risk hurting yourself while squatting.
You can perform squats with any piece of free weight equipment, with resistance machines and even with just your body weight. The important things to keep in mind while squatting are to have your feet shoulder-width apart on the ground, your heels planted firmly on the ground and your back straight at all times. To do the exercise, stand upright and then bend at your hips and at your knees until your legs form a right angle to the ground. Then, raise your body to the upright position by extending at your hips and knees.
The main agonist during the squat is a four-headed muscle group at the front of your thighs called the quadriceps femoris. The four muscles are the vastus intermedius, vastus medialis obliquus, vastus lateralis and rectus femoris. These agonistic muscles all contract to extend your knees during the upward phase of the squat.
There are other muscles that also come into play while you squat. The gluteus maximus, or buttocks, and the hamstrings of your back thighs are agonists muscles that extend your hips during the downward phase of the squat. The adductor magnus posterior head at the inside of your thighs is yet another hip-extending agonist in the squat. Furthermore, the soleus muscle of your calves acts as a secondary agonist by extending your ankles during the up phase.
Your back muscles, particularly your erector spinae, play a large role during the squat. The muscles of the erector spinae are the iliocostalis, longissimus and spinalis. Although they don't act as agonists to produce movement while you squat, they are responsible for keeping your spine erect, hence the name spinal erectors. You never want to round or hyperextend your spine when you squat, especially when putting weight over your upper back, as doing so may cause serious spinal injury such as a herniated disc. The contraction of the erector spinae muscles helps to stabilize your back and keep it straight. The rectus abdominis of your abs also plays a role in keeping your back straight, so it is important to have balance between the two.
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