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The Squat: Hip vs Knee


The concept of balancing the hip musculature through exercise in an effort to improve lumbo-pelvic rhythm, decrease injuries and improve posture is certainly not a revolutionary concept. This philosophy has been presented by many including King in his “Hip Versus Quadriceps Dominant” theory. The theory suggests that certain movements involving hip and knee extension (e.g. squat, deadlift, lunge etc.) will have a bias toward anterior or posterior muscular recruitment and development. For example, it has been proposed that the squat is a quadriceps dominant movement associated with greater input from the quadriceps and hip flexors, and the deadlift is a hip dominant movement associated with glute and hamstring dominance.

While the classification system of hip dominant and quad dominant movements has great theoretical relevance, the practical application is not as simple as many theorists would suggest. When classifying compound lower body movements, it is important to recognize that these movements will involve the quadriceps and hip extensors, and the relative distribution is highly related to the movement mechanics. In other words, it is the execution of the exercise and the conscious activation patterns that will largely dictate the contribution from the flexor and extensor chains. In this article, we will focus on the squat movement and demonstrate how modifications in the mechanics of this lift can affect the relative distribution of forces in the muscles involved.

This information can be extrapolated in the coaching and modification of lower body compound movements to increase the emphasis on focal muscles. The ability to alter load distribution through modifications in movement mechanics in these compound movements serves several key purposes including:

  1. Increased joint and muscle balance by placing greater loads on the positionally weak muscles.
  2. Increased power output through improvements in inter and intramuscular coordination.
  3. Improved activation patterns of the associated muscles, allowing functional movements to be performed with ease.
  4. Decreased risk of injury secondary to faulty motor sequencing.

The Squat: Motor Strategies and Load Distribution

Although often considered a quadriceps dominant movement, the squat can be modified to increase the hip extensor moment (i.e., box squats), requiring greater input from the glutes and hamstrings. Alternatively, the conventional back squat can be modified to increase the emphasis on the knee extensor moment such as in front squats, which requires greater input from the quadriceps. Therefore, when classifying lower body compound exercises, it is important that we consider the mechanics of the movement first and foremost. The relative input from the hip musculature in these compound movements is contingent on the following variables.

  1. Trunk and Hip Angle
  2. Angle of the Tibia in Relation to the Vertical Axis
  3. Direction of Propulsive Force
  4. Muscle Activation Patterns
  5. Mind-Muscle Connection
  6. Foot Position
  7. Bar Placement

Trunk and Hip Angle

The angle of the trunk and hip is a critical factor in the distribution of load in compound lower body exercises. As a general rule, the more acute the trunk angle, the greater the potential input from the hip extensors. This is due to the fact that hip flexion is increased as the trunk is forward flexed. A similar rule applies to hip angle: as hip flexion increases, so too does the potential for hip extensor recruitment such as in the deadlift or box step ups performed on a high box. Deep hip flexion and a forward trunk angle increases the mechanical advantage of the hip extensors and stimulates receptor feedback within these muscles due to the deeper stretch position. Therefore, input from the hip extensors in the squat movement depends greatly on the trunk and hip angle. To increase the hip extensor involvement in the squat, the subject can be instructed to maintain a mild forward lean throughout the movement, to increase the depth of the movement and/or to exaggerate the posterior sway of the hips in the sit back motion as is demonstrated in the box squat below.

Images A and B above demonstrate the slight forward trunk angle associated with the conventional back squat. The thick red line bisects the vertebral bodies and helps demonstrate how the trunk angle can affect the range of motion and force potential about the hips.
Images C and D demonstrate the forward trunk angle associated with a box squat or powerlifting style squat. The thick red line bisects the vertebral bodies and clearly demonstrates a more acute hip angle. The images also clearly demonstrate a much greater hip moment relative to the conventional back squat above and the front squat below. In this position, the moment force about the hips must be greater than that at the knee to keep the body within its base of support throughout the movement. Failure to do so would result in the trunk collapsing forward. 
Images E and F demonstrate an upright trunk angle associated with the front squat. The thick red line bisects the vertebral bodies and clearly demonstrates a more obtuse hip angle. The images also clearly demonstrate a much greater knee moment relative to the conventional back squat and box squat above. In this position, the moment force about the knee must be greater than that at the hips to keep the trunk upright and the body within its base of support throughout the movement.

Angle of the Tibia in Relation to the Vertical Axis (ATRVA)

The angle of the tibia in relation to the vertical axis (ATRVA) is also an important factor to consider when programming lower body exercises. Increases in the ATRVA (see picture below) as demonstrated by forward displacement of the knee joint will increase the degree of knee flexion, subsequently enhancing the moment force produced by the quadriceps, particularly the rectus femoris. The RF assists the uni-articular quadriceps with the knee extension moment in the squat. In lieu of proper stabilization from the lower abdominals and glutes, the RF can pull the pelvis into anterior rotation and limit the hip extension moment. This is the reason squats are often classified as a “quadriceps dominant” movement and have the potential to lead to aberrant pelvis positions.

Fry et al. found that restricting forward movement of the knees (i.e., decreased ATRVA) will transfer forces to the hips and lower back in the squat movement. Anecdotally, I have seen much to support this finding; however, provided a stable trunk position is maintained via input from the core musculature, stress on the lower back is reduced and the action of the hip extensors is maximized. It is important to note that reducing knee flexion in the squat will require compensatory posterior sway of the hips and a mild forward lean of the trunk to keep the body within its base of support, a fact that adds further support to the importance of the trunk angle position to the load distribution.

A practical example of the effect of ATRVA can be seen when comparatively analyzing the front squat versus the powerlifting squat. The front squat will increase forward displacement of the knees and encourage deep knee flexion, increased dorsi flexion, reduced posterior sway of the hips and an upward trunk angle. These moments all have the effect of increasing the input from the quadriceps muscles. Alternatively, the powerlifiting squat reduces forward displacement of the knees and encourages less knee flexion, reduced ankle dorsi flexion, increased posterior sway of the hips and a slight forward lean of the trunk. These moments in turn have the effect of increasing the input from the hip extensors.

Images G and H above demonstrate a significant difference in the ATRVA in a powerlifiting squat relative to a front squat. The thick red line bisects the tibia and demonstrates the angle of the tibia relative to the vertical axis (blue line). Notice the relative trunk angles associated with each motion. The angle of the ATRVA coupled with the angle of the trunk clearly demonstrates a greater hip moment in the powerlifting squat and a more pronounced knee moment in the front squat. 
The vertical blue line in images I and J bisect the lateral epicondyle of the knee, while the red line bisects the tibia. The images clearly demonstrate the relative difference in forward displacement of the knees and the impact this moment has on the ATRVA.

Direction of Propulsive Force

The direction of propulsive force will significantly impact the recruitment and activation patterns of the lower body musculature. A great example of this is seen in the contrast between the vertical jump and the horizontal long jump. The long jump motion will require the hips to move “forward and through” in a horizontal direction. This is supported by Ridderikhoff et al. who found that peak angular velocity in the hip joint was higher and the joint was more extended at take off in the long jump compared to the vertical jump. These findings demonstrate that horizontal propulsive forces require significantly greater input from the hip extensors.

This philosophy can be extrapolated in the analysis of many lower body movements. While angular velocity will be relatively slower, lower body movements requiring horizontal displacement of the body such as a pawback lunge or deadlift will require greater input from the hip extensors. The direction of propulsive force theory can be employed to increase the demand on the hip extensors when coaching the squat by encouraging the subject to extend the hips forward and through to the neutral finish position. This coaching cue will encourage the subject to alter the activation patterns by triggering the hip extensors out of the bottom and throughout the motion.

The thick blue lines in images K and L demonstrate the approximate finish position of the lift associated with upright posture (i.e., head, hips and ankle in vertical alignment). Note where the vertical line bisects the femur. The distance of the hip joint from the vertical line clearly demonstrates a greater need for horizontal force in the powerlifting style squat relative to the conventional squat.

Muscle Activation Patterns

There is more and more evidence to support the notion that the biomechanics of a particular movement is the critical factor in muscle activation patterns above and beyond the exercise itself. From this perspective, minor adjustments to the external factors affecting the biomechanics of the lift can have a significant impact on the activation patterns. Fry et al. state that “the squat technique used can affect the distribution of forces between the knees and hips and the kinematic properties of the exercise.” This is not only true of the squat motion but of many compound lower body movements. The above statement acknowledges the fact that optimal activation patterns and subsequent movement outcomes can and must be coached. From a practical standpoint, lower body movements can be coached in a manner that emphasizes hip extensor recruitment or hip flexor/quadriceps recruitment.

An example of this is demonstrated in the works of Burgess-Limmerick et al. who found a delay in the extension of the hip and lumbar vertebrae after a lift initiated by knee extension in subjects that self selected their movement pattern (i.e., were not coached) in an intermediate stoop/squat movement. When analyzing the mechanics of a seasoned lifter, it is clear that the self-selected movement pattern in the subjects above contradicts that of these efficient lifters. Despite this glaring fact, Scholtz and McMillan have suggested that knee extension should lead hip extension in the squat and freestyle lifting postures. I strongly oppose this viewpoint based on the above analysis and further suggest that the findings of Burgess-Limmerick et al. may help explain the incredible rise in lower back injuries in today’s society. The self-selected movement pattern in their subjects delayed the hip extension moment, a motor strategy that lengthens the lumbar lever and places considerable stress on the structures in the lower back. Therefore, it is recommended that the client be coached to initiate strong hip and knee extension in the squat and other lower body lifts.

Mind-muscle Connection

Burgess-Limmerick’s study demonstrates how suboptimal motor programs can become ingrained if subjects have not been taught proper lifting technique. Thankfully, these individuals can be taught to alter recruitment patterns via minor mechanical adjustments and stimulation of the mind-muscle connection. For example, when performing a step up, the individual can be set up in a manner that restricts the starting knee angle to 90 degrees and further cued to extend their hip and drive the heel into the box, focusing intensely on squeezing the glutes in an effort to increase the input from the hip extensors in this movement. They can further be cued to focus on triggering the hip extensors in the bottom of the motion to voluntarily stimulate the muscles through conscious awareness.

An effective technique is the use of isometric contractions of the focal muscles prior to the onset of phasic movement. This will ensure that these muscles are turned on prior to initiating the lift. Over time, these recruitment patterns will become more reflexive in dynamic actions, requiring little input from the conscious brain. Trunk angle, foot position and the direction of the propulsive forces can further be adjusted to increase the input from the hip extensors. Alternatively, these modifications can be made to increase input from the hip flexors and quadriceps.

While this may take considerable focus initially, over time these neural patterns will become ingrained and the individual will instinctively recruit the appropriate muscles in the appropriate sequence in compound lower body movements. The outcome of this is threefold:

  1. Increased recruitment and development of positionally weak muscles, leading to improvements in structural integrity.
  2. Improved recruitment and activation patterns, resulting in improved dynamic posture and decreased stress on the joints and tissues.
  3. Increased force and power output, leading to increases in performance.

Foot Position

Escamilla et al. found that a wider stance squat and high position of the feet on the leg press increased the hamstring activity compared to a narrower squat stance and lower leg press foot position; unfortunately, gluteus maximus input was not measured. Still, this and other studies serve to reinforce that modifications of foot position can effectively transfer the recruitment strategies of the body. In this case, the wider foot position will increase the hip flexion/extension moment, placing more stress on the extensor tissues. The fitness professional should be encouraged to modify foot position to achieve specific results in the training. 

Images M and N clearly demonstrate deeper hip flexion and more acute trunk angle in the wide stance squat position relative to the narrow stance. These movement variations will alter the recruitment strategies, placing greater emphasis on the hip extensors in the wide stance and greater emphasis on the knee extensors in the narrow stance.

Bar Placement

Wretenberg et al. compared the movement patterns of powerlifters in the “low bar” squat where the bar was placed on the posterior deltoid or the spine of the scapula (see picture below) to that of weightlifters in the “high bar” squat where the bar was placed at the level of C7-T1. The results showed that the high bar squatters tended to distribute the load more evenly across the knee and hip while the low bar lifters put more load on the hip joint. They also found that the low bar squatters had greater hamstring activity than the high bar squatters. While caution must be exercised when interpreting this data due to the movement patterns associated with previous training (i.e., weightlifters versus powerlifters), the study suggests that placement of the bar will impact the movement mechanics and the load distribution.

Images O and P demonstrate the relative difference in bar placement between the high bar position and low bar position. The images clearly show that maintaining the bar on the back in the low bar position will require a more acute trunk angle to stabilize the bar in this position throughout the movement. This may help explain the activation patterns associated with this position.

The squat, as many other lower body lifts, can be modified to emphasize many different muscle groups. Therefore, we must get away from the rigid hip versus knee dominant classification systems presently being presented. When evaluating the muscular input from lower body movements such as the squat, the fitness professional is encouraged to analyze the movement mechanics and conscious activation strategies first and foremost. This philosophy recognizes effective techniques that encourage active recruitment of target muscles, strategies that ingrain optimal firing patterns associated with a high level of functional efficiency.

As discussed above, the trunk angle, angle of the tibia relative to the vertical axis, the direction of propulsive force, conscious muscle activation patterns, foot position and bar placement can all be modified to alter load distribution. As the activation patterns become more ingrained, these movements can be further modified to meet functional or sport demands. For example, the coach may decrease stance width in the squat and suggest a more upright trunk angle while encouraging the subject to maintain similar activation patterns. At this stage, the strong hip extension moment has been learned and will be more easily reproduced in the new squat motion. From this perspective, modifications in technique and of the acute training variables thereof (i.e., reps/sets/lifting speed etc.) should be planned and periodized in a manner that recognizes the individual needs of clients at all stages of their development.

While the squat and other compound lower body movements can be modified in countless ways, the subject should always be encouraged to:

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