Squats and Lunges are the well-known compound exercises to improve lower body size and strength. These exercises work on muscles such as quadriceps, hamstrings and gluteus maximus. Alongside, these exercises work on multiple joints like knee and hip joints as well.

In the fitness industry, it is often taught or known that “your knee should not cross the toe when you squat or lunge”. The idea has been around for a long time without any proper research support. On the other hand, an individual’s knee crosses his/her toe line while performing a certain movement on daily basis. For example, walking down the stairs or walking up the stairs, the knee is constantly passing the toe line. In such cases, individuals never suffer from any knee injuries.


Figure 1: Proper depth of squat (> 90 degree) maintained through knee crossing the toe-line

Where Did the Idea Come From?

The idea of “knee should not pass the toe line during squats or lunges” started after a research that was conducted by Ariel (1974), which found that subject with the most knee forward movement had the greatest sheer force on the knee joint [1]. Thus, the researcher states that if the shear force and load is too high then it can cause knee injuries that including anterior cruciate ligament (ACL) tear. However, a recent article has taken a opposing stance against the study that was conducted by Ariel, B. Hartmann et al. (2013) counters that the research by Ariel did not discuss squat technique and the depth of squat that can affect the knee joint [2]. Ariel B. only explains the shear force when the hip is parallel to the ground. However, it does not state the effect of the depth when hip to knee angle is about 70 degrees. According to Hartmann H. et al. (2013) the range of motion of quadriceps muscles increases when the knee crosses the toe line [2].

On the other hand, squat and lunges are compound exercises that include multiple joints. A research was conducted by Smith S. et al. (2003), in which one group of subjects’ knees were restricted to pass the toe-line and the second group of subjects was unrestricted. The results showed that the restricted group of people had a massive increase of 1000% shear forces in lower back than knee (28% increase of shear force) because they leaned forward to maintain the knee restriction[3]. The researchers concluded that the increased load on the back can cause hip and spinal injuries because of the massive torque increase on the hip.

Figure 2: Subject performing squat with ‌‌restricted knee movement

For example, the subject on Figure 2 shows the posture of squat as he is restricting his knees to cross the toe line. It is physically impossible to go down to 90 degree without knee crossing the toe line. Furthermore, since the knee flexion is restricted, the subject is more reliable on hip hinge movement. However, the extensive backward movement of hip joint causes increased pressure on lower back that can lead to potential lower back injuries.

The Squat Principles

The range of motion is much increased when the knee crosses the toe line while performing a squat or lunges. The main principles to follow while performing squats are:

  • Heels should be always on the floor
Figure 3: Side view of the squat. The subject hits proper depth as his knees cross the toe line

Figure 3: Side view of the squat. The subject hits proper depth as his knees cross the toe line

  • Knees should follow the toe direction avoiding caving in
  • The bar should directly go down on the mid-foot line as the leaning forward movement should be avoided

The knee joint is safe regardless of the load and knee crossing the toe-line, as long the heels are on the floor. In such case it is important have ankle flexion. It is important to note that, calves mobility is needed in order to have enough ankle flexion, which will allow the heels to stay on the floor throughout the whole movement. Figure 1 and 3 portray good example of importance of ankle flexion and calve' mobility. Subject's heels are on the floor as his knees pass the toe line due to efficient calves mobility. The subject also shows good technique with hip joint by maintaining straight back posture

Furthermore, the Olympic lifters are squatting with more than double or triple times more than their bodyweight and their knees go way pass the toe line while performing clean and jerk or snatch. In such interest, a research has been done on powerlifting and rate of knee injuries. The results showed significantly low rate of knee injuries in Olympic lifting.

Conclusion

In the end, squat and lunges do not only focus on knee joint. Hip joint also play a massive role to maintain a proper and safe posture. Based on the discussion above, it is clear that knee crossing the toe line is safe for both knee and hip joint as the shear force on both joints is much balanced. Alongside, every person is unique with their physique biomechanics. Based on the individual's ability and preference he/she should perform the compound exercises as long as they are following the stated principles.



References:  

1. Ariel B. G. Biomechanics Analysis of The Knee Joint During Deep Knee Bend with Heavy Load. Biomechanics IV. 1974; 44 - 52.

2. Hartmann H., Wirth K., Klusemann M. Analysis of the load on the knee joint and vertebral column with changes in squatting depth and weight load. Sports Medicine. 2013; 43(10): 993-1008

3. Smith, JC, S., Fry, AC., Schilling, BK. (2003, November). Effect of knee position on hip and knee torques during the barbell squat. Journal Of Strength And Conditioning Research / National Strength & Conditioning Association, 17(4), 629-633.