Done correctly with the proper form, studies and numerous reports state that squats do not pose any significant risk to the knees. Any discomfort while performing the exercise is usually an indicator of incorrect form.
While generally regarded as safe, caution should be advised for people with conditions that might be exacerbated by the exercise (e.g., osteoarthritis of the knees).
The basic movements involved in performing a squat is simple.
The individual begins in a standing position, with the feet slightly wider than the hip width. While maintaining the chest up and the core engaged, the individual begins to push the hips back into a seated position – all the while shifting the weight onto the heels. The hips are further lowered until the thighs are parallel to the ground. Pausing with the knees directly on top of the toes, the single repetition is ended as the individual begins to push back into the starting position.
While ultimately simple, the squat is said to be a highly effective exercise. It is considered a compound exercise because unlike other isolated exercises that specifically work one muscle group, this exercise engages multiple muscle groups – namely, the buttocks (gluteus maximus, gluteus minimus, and gluteus medius), quadriceps, hamstrings, adductor, hip flexors, and calves.
Over the years, numerous variations of the squat have been developed – with each variant emphasizing a different muscle group. The most popular variant of the squat is the back squat.
The back squat uses the same motions as the traditional squat but with added resistance from holding a barbell on the individual’s shoulders. Overhead squats also use the traditional squat form however the trainee holds their arms straight up over their head. This variation emphasizes engagement of the core and the lower back, while still activating the other muscle groups involved with the squatting motion.
Back squats and overhead squats target the erector muscles of the lower back, which aids in keeping the back straight throughout the movement.
While there are some reports or rumors that state squats are bad for your knees, studies do not agree with this statement.
Even with Olympic weightlifters, athletes who perform squats regularly, it has been observed that the prevalence of injury is very low.
A 1999 paper published in the Journal of Athletic Training observed 27 Olympic weightlifters for a period of six years. The prevalence of injury occurring was calculated to be around 3.3 events per 1000 hours of training. Injury events observed by the researchers required a day or less of missed training for 87.3% of the cases, and less than a week for the rest.
Other studies have been conducted to assess the load of squats (and some of its parameters such as weight and depth) on the knees.
A 1989 paper published in Medicine and Science in Sports and Exercise studied the effects of squats on knee stability. The study had a hundred male and female volunteers participate in an eight-week training program with multiple groups (control group, full squat group, and half squat group). Using a knee ligament arthrometer to measure knee stability, the study concluded that both full and half squats did not have any effect on the knee stability of the participants.
A 2013 paper published in Sports Medicine reviewed over 164 published articles in PubMed to analyze the effect of squatting depth and weight load on the knee joint. According to the literature review and calculations, deep squats confer more stress on the knee and spinal joints compared to half and quarter squats. However, neither anterior and posterior shear forces in deep squats can reach magnitudes that pose risks to the posterior cruciate ligament (PCL) and anterior cruciate ligament (ACL), ligaments that keep the knee stabilized.
Thus, it is concluded that with proper form and technique, the exercise should not pose any risks for the knees of people performing squats.
However, there may be some exceptions. While safe, it is true that the exercise puts a load on the joints of the knees (i.e., tibiofemoral joint and patellofemoral joint). This means that caution should be advised for individuals with recent knee injuries, osteoarthritis of the knees, and other knee or bone conditions.
Furthermore, squats are considered low impact exercises (unless you're squatting very heavy weight) which means that the exercise generally exerts less force on the body - like with bodyweight squats. Examples of high impact exercises that pose more threat to the knees include jumping motions and running.
If an individual’s knees do hurt when performing squats, there are common reasons as to why this happens. These are usually associated with form and technique.
For example, a common mistake when performing squats is shifting your weight forward. This usually occurs when people are either afraid of falling backward or they forget to keep their chest up.
Another common mistake would be when the knees and toes are not aligned. This is often a case of muscular compensation when a certain muscle group must compensate for the weakness of another.
For people who are unable to perform the traditional squat due to certain factors (e.g., pre-existing condition, inability to balance, skill level, etc.), there are variations that are easier to perform.
A lighter version of the squat can be done by not performing the movement until the thighs are parallel to the ground. For example, a quarter or a half squat can be done if a full squat is difficult or dangerous for a person to do.
One mild variation of the squat requires a wall and an exercise ball. This version of the squat begins by placing the exercise ball between the individual and the wall.
The individual rests their back on the exercise ball, stabilizing themselves. The exercise ball should be rested on the small of the individual’s back, with their feet slightly in front of their body and apart at shoulder width.
At this point, the body of the trainee is leaning at around 70-80 degrees against the ball. The individual then begins to perform the squat position until the thighs are parallel to the ground. The repetition ends as the individual returns to the starting position.