The simple answer is your knees should go where they naturally want to go. The key caveat being they should not go excessively beyond the toes. What does that even mean and how do we define or measure excessively? It is a two part answer:
A) The distance the knee travels beyond the toe is based on individual anthropometrics and what activities/sport (Olympic lifting for example) they regularly participate in.
B) Is the knee travel “normal” or is it a compensatory mechanism for a lack of range at another joint.
At DTS we believe this recommendation was made with the intention of preventing knee injuries. Unfortunately, restricting a natural range of motion may actually do the opposite in many cases and is generally not a good recommendation. The recommendation is analgous to suggesting that you only do ½ a bowel movement because people will use less toilet paper and that will save the environment.
Let’s take a look at the research and summarize four things that restricting forward range of the knee during squatting does:
- Restricting forward movement of the knees may minimize stress on the knees, it is likely that forces are inappropriately transferred to the hips and low-back region.
Key takeaway: Stress on the low back increases when you restrict natural knee movement.
- The higher moments in the hip during restricted squats suggest a higher load of the lower back. Athletes who aim to strengthen their quadriceps should consider unrestricted squats because of the larger knee load and smaller back load.
Key takeaway: Stress on the lower back increases when you restrict natural knee movement.
- Restricted squats also produced more anterior lean of the trunk and shank and a greater internal angle at the knees and ankles
Key takeaway: Greater forward trunk lean (stress on the lower back) whe you restrict natural knee movement
- In an unrestricted squat, the angle of the knee is larger and the range of motion (ROM) between the lumbar and the thoracic segments is significantly smaller compared with a restricted squat (p < 0.05). The studied subjects showed significantly increased ROM for thoracic curvature during restricted squats. The unrestricted execution of a squat leads to a larger ROM in the knee and smaller changes in the curvature of the thoracic spine and the range of smaller segmental motions within the trunk. This execution in turn leads to lower stresses in the back. To strengthen the muscles of the leg, the unrestricted squat may be the best option for most people. Thus, practitioners should not be overly strict in coaching against anterior knee displacement during performance of the squat.
Key takeway: Stress on the lower back increases when you restrict natural knee movement.
In summary, if you restrict the natural forward motion of the knee, there will be a corresponding increase in stress on the lower back. Key takeaway, is to allow natural knee motion.
To achieve this coach and execute excellent squat technique (neutral spine and centre of pressure maintained on the feet) and allow their knees to move beyond the toes if that is your clients natural position. The squat pattern is one of the primal patterns movements covered in the DTS Level 1 Personal Training Certification.