Rethinking Squat Mechanics: could squatting with “knees-in” provide some benefit?

Introduction:

Squatting is a complex exercise involving multiple joints and muscle groups. Historically, the focus of this exercise’s research has been predominantly on how far your knees travel over your toes or how deep you should go. However, movement in other directions are equally important to consider for optimizing squat technique and minimizing injury risk. 

Conventional wisdom often warns against squatting with knee valgus, defined as internal rotation of the femur about a fixed tibia, causing the knee to drift inwards and rotate externally relative to the femur (Neumann & Kelly, 2010). This position is generally avoided due its association with increased injury risk, notably patellofemoral pain and ACL sprains. Interestingly, elite weightlifters frequently employ knee valgus during squats without apparent harm. This background sets the stage for a University of Alberta study, conducted by biomechanics researcher Loren Chiu, which was published in the Journal of Strength and Conditioning Research (Chiu, 2024).This study embarked on a detailed investigation comparing the biomechanical implications of squatting with an inward knee position, a normal knee position, and an outward knee position; and what use they could be of to both athletes and clinicians. 

This study invites a reevaluation of established squatting dogmas. By elucidating the biomechanical effects of knee positioning during squats, this research can provide informed training practices that accommodate the diverse biomechanical landscapes of individuals. As we continue to embrace the evolving science of human movement, let us remain open to integrating new insights into our approach to training and rehabilitation, ensuring that our methodologies are both effective and evidence based. This blog post will review the methodology and results of this study, then discuss the implication for clinician and exercise professionals.

Terms to Know:

Before delving into the nuances of this study, it’s essential be familiar with several terms:

• • Moment: The product of an applied force and the perpendicular distance at which it acts on a joint. This concept can be likened to the challenge of lifting a hammer by its handle versus near its head; the distance from the force application point significantly influences the effort required.
  • Net Joint Moment (NJM): NJM estimates the minimum muscular torque needed at a joint to either initiate a movement or maintain a position against applied moments. For example, the NJM for the biceps during a bicep curl is the minimal amount of muscular force needed to overcome the moment applied from the weight. 
  • Ground Reaction Forces (GRF): GRFs illustrate Newton’s third law in action – it is the force exerted back to our bodies from the ground. For example, When we jump, we exert force on the ground, which is then reciprocated pushing us upwards. Likewise, during sprinting, pushing backwards with our legs results in the ground reciprocating with a forward force, propelling us ahead. This study examines how GRFs can be reciprocated either vertically or horizontally.
  • Planes of Movement: Understanding body movements in the frontal and transverse planes is critical for grasping the study’s outcomes (See figure 1.). These terms can be used to describe a group of muscles that perform a given action at a joint (for example, knee extensors refer to the quadriceps).
    • Frontal Plane: This plane runs from side to side, including movements like abduction (raising limbs straight out to the side) and adduction (drawing limbs back toward the body).
    • Transverse Plane: This plane runs horizontally through our body. Movements in this plane are rotational – called internal/medial rotation (think about rotating your thigh with a straight leg so your toes point inwards) and external/lateral rotation (think about rotating your thigh with a straight leg so your toes point outwards).

• Sagittal Plane: This plane runs from front to back, and encompasses flexion (reducing the angle at a joint, bending your knee for example) and extension (increasing the angle at a joint, straightening your knee for example). 

• Minimum detectable effect (MDE): This is the smallest difference between groups that determines the threshold for meaningful results. The MDE value for the studied variables was calculated using values from other research that resulted in differences in muscle activity or NJM, or from studies comparing injured to non-injured participants. For example, there may be a statistical difference between two values, but it does not meet a threshold required to observe a change in muscle function and is therefore not meaningful.  

Study Goals and Design: 

This study aimed to compare differences between squats performed with experienced squatters’ normal technique, with intentional lateral hip rotation (knees-out), and with intentional medial hip rotation (knees-in). It is an important distinction to make that the knees during these squats drift in or out as a result of medial and lateral rotation of the thigh, respectively. Participants went through two familiarization sessions, which included teaching progressions for both laterally and medially rotated squats. 

For knees-in squats, participants started by sitting and squeezing a small medicine ball between their thighs, just above the knees, by rotating their thigh inwards – not by bringing their thighs together. This initial exercise evolved into a series of progressively challenging movements: first, holding a squat position with the ball in place; next, performing a sit-to-stand motion with the ball; and ultimately, executing a barbell squat without the ball.

Familiarization with knees-out squats began with participants in a seated position, where they pushed against a resistance band placed around their thighs, just above the knees, by rotating their thighs outwards – not by pushing their thighs to the side. This exercise progressed to a squat hold with the band in place, then to a sit-to-stand exercise with the band, and concluding with a barbell squat performed without the band.

After participants demonstrated competency in each of the barbell squat variations with a weight of at least 60% 1 repetition maximum, they were analyzed on a third visit using 3D NJM analysis. The stated research objectives were threefold: to compare hip mechanics and ground reaction forces in the medial-lateral axis across squat variations, to evaluate the stresses on the knee joint, and to investigate the muscular efforts required for each squat variation.

Summary of Study Findings: 

On the third visit, following a warm-up protocol, each participant performed 2 sets of 3 repetitions at 70% 1 repetition maximum of each squat variation for analysis. The study unveiled significant insights across various parameters:

Medial to Lateral GRFs:

GRFs in the frontal plane were consistently directed medially in all squat variations, indicating the ground reciprocated a force that attempted to push the knees inward. Among the squats, knees-in squats showed the highest medial GRF, with a large effect size compared to the other 2 squats. While normal squats had greater medially directed GRFs compared to knees-out squats, the difference did not surpass the MDE.

Hip and Knee NJM at Greatest Squat Depth:

Hip extensor NJM: This was greatest in normal squats, which was greater than knees-out squats (with MDE) and knee-in squats (large effect). This means that the contribution of the muscles performing hip extension – think about pushing your hips forward – was reduced in knees-out and knees-in squats.

Hip adductor NJM: Largest in knees-out squats compared to both normal (minimal effect) and knees-in (large effect) squats. Normal squats exceeded knees-in squats with the minimum detectable effect (MDE). This indicates that the contribution of muscles performing adduction (e.g., squeezing your legs together) was greatest in knees-out squats, followed by normal and knees-in squats.

Hip Lateral and Medial Rotator NJM: Knees-in squats showed the highest lateral rotator NJM compared to normal and knees-out squats, with large effect sizes. Normal squats also exceeded knees-out squats, which approached 0, with a large effect size. Simplified, the muscles contributing to outward thigh rotation were most active in knees-in squats, followed by normal and knees-out squats.

Frontal Plane Knee NJMs: The NJM for knee adductors was greatest in knees-in squats, followed by normal squats, and was smallest in knees-out squats. Conversely, knee abduction NJM was highest in knees-out squats, followed by normal squats, and then knees-in squats. In essence, this implies that the muscles in the hip and lower leg that pull the knee outward were more active in knees-out squats, while those pulling the knee inward were more engaged in knees-in squats.

What Does This All Mean?

This research confronts traditional views on squat mechanics, particularly regarding knees-in or medially rotated squats. Let’s explore these findings and their application:

Joint Angles and Medial-lateral GRFs: 

Although instructed to rotate, both knees-in and knees-out squats resulted in greater adduction and abduction angles, respectively. Greater hip medial rotation causes greater medial GRF, while greater adduction reduces medial GRF. Given the high medial GRF in knees-in squats, adduction simply accompanied the mechanics of knees-in squats. Greater hip lateral rotation causes lower medial GRF, while abduction increases medial GRF. During knees-out squats, it is proposed that the accompanying abduction reduces the lowering effects of greater lateral rotation on GRF – thus, the medial GRF was only slightly lower than normal squats.

Implications for injury risk:

Concerns about knee-in squats often center around potential stress on the anterior cruciate ligament (ACL). For injurious stress on the ACL, a combination of externally applied knee abduction and medial rotation moments are necessary, typically with a slightly bent knee (Shin et al., 2011). 

Knees-in squats did exhibit an increased externally applied knee abduction moment, indicated by a greater adductor NJM – the muscles acting as knee adductors were working to counter the forces pulling the knee outwards as a result of the squat mechanics. Similarly, knees-in squats also showed an externally applied lateral rotation moment at the knee, evidenced by the presence of a medial rotator NJM. Thus, the pattern of applied forces during knees-in squats don’t align with the pattern known to injure the ACL. Furthermore, ACL injuries commonly occur with the knee slightly bent, whereas squats involve significant knee bending – this reduces the loading on the ACL due to joint compressive forces and muscular contribution of the hamstrings and quadriceps. 

These promising findings suggest that knees-in squats may not exert sufficient stress with the right forces to cause injury to the ACL. However, it’s important to note that ACL loading involves triaxial forces, tri-planar moments, and knee flexion, and the data from this study doesn’t conclusively answer this question. Taken from a different angle, perhaps knees-in squats could provide a training stimulus against ACL tears. Squatting in this manner did result in one of the elements in the equation for ACL damage —an increased externally applied knee abduction moment. This raises the possibility that incorporating knees-in squats could strengthen the muscles stabilizing the knee, without imposing sufficient load to damage the ACL. Such an approach could offer a potential benefit for progressively rehabilitating the ACL and preventing tears.

Another concern relates to excessive medial or inward rotation of the thigh, leading to increased stress on patellar cartilage. However, hip medial rotation was similar across all 3 squat variations, and is thus inadequate to induce heightened patellar stress.

Influence Of Squat Variations On Hip Musculature Effort:

Key findings emerge regarding the impact of squatting patterns on hip musculature effort. In normal squats, there’s a demand for hip extensor, adductor, and lateral rotator NJM to perform the lift. Existing research indicates that the gluteus maximus and the adductor magnus fulfill these roles. Despite hamstrings partially acting as hip extensors, their contributions are likely limited due to antagonistic role at the knee counteracting the work of the quadriceps.

These findings resonate with Chiu’s study, where normal squats exhibit extensor, adductor, and lateral rotator NJM. The gluteus maximus functions as a hip extensor and lateral rotator, while the adductor magnus serves as a hip extensor and adductor, matching the muscular demands required at the hip joint. 

In contrast, knees-in squats reduce hip adductor NJM and elevate lateral rotator NJM. This suggests a preferential loading of the hip lateral rotators, such as gluteus maximus, while the demand placed on the adductor muscles, such as adductor magnus, is reduced. Furthermore, co-contraction of lateral and medial hip rotators must occur for the observed squat mechanics, suggesting the lateral rotators are working harder than is estimated from the NJM. 

Conversely, knees-out squats elevated hip adductor NJM and diminished lateral rotator NJM. This suggests a preferential loading of the adductor muscles, such as adductor magnus, while the demand for the hip lateral rotators, such as gluteus maximus, is reduced. 

Both knees-in and knees-out squats witness decreased hip extensor NJM, hinting at a redistribution of contribution between hip extensors that may compromise overall hip extension NJM generated. However, this doesn’t exclude other muscles, such as hip adductors other than adductor magnus, from contributing to hip extension.

Conclusion & Application:

Chiu’s findings have practical implications for athletes and clinicians alike. They suggest that squatting can be biased to leverage an individual’s innate strengths and challenge traditional approaches to squatting. Furthermore, squat variations can be used to target specific muscles. For example, an athlete aiming to prioritize glute strength can intentionally rotate their thighs inward to preferentially load gluteus maximus. While not definitive, the data from this study suggests that the use of these knees-in squats may not damage the ACL as traditional perspective’s propose, and may serve as a way to reinforce vulnerable positions. Conversely, an athlete can target the adductors, especially adductor magnus, by rotating their thigh outwards. Future research should further explore the training effects associated with these different squatting variations to evaluate the long term effects. For more research on squats, see our blog post on the effect of squat depth and loading here.

References:

Chiu L. Z. F. (2024). “Knees Out” or “Knees In”? Volitional Lateral vs. Medial Hip Rotation During Barbell Squats. Journal of strength and conditioning research, 38(3), 435–443.

Neumann, D. A., & Kelly, E. R. (2010). Kinesiology of the musculoskeletal system: Foundations for rehabilitation. Mosby Elsevier. 

Shin, C. S., Chaudhari, A. M., & Andriacchi, T. P. (2011). Valgus plus internal rotation moments increase anterior cruciate ligament strain more than either alone. Medicine & Science in Sports & Exercise, 43(8), 1484–1491.