By Jordan Feigenbaum MS, CSCS, HFS, USAW Club Coach
DFM Director of Strength and Conditioning
This article is all about force and it’s relationship to the body during a squat. While you might not ever think about what kinds of forces are going on when you squat (or should be squatting), understanding how force is acting upon your joints as well as how to maximize your efficiency as a lifter is part of increasing your prowess in the iron game. By the end of this article you should be comfortable discussing vectors, levers, moment arms, and torque as they apply the the squat. This should keep your physical therapy and chiropractic friends happy if they tell you squats are bad for your _________.
The forces operating on the barbell and the body in barbell training include compression, tension, moment force (or torque), and possibly momentum. In the squat for instance, the load is carried on the shoulders, which elicits compression throughout the lifter’s skeletal components as gravity acts upon the barbell vertically downwards. Additionally, the muscles of the lifter create tension when the weight is walked out of the rack to keep the skeleton more or less upright. A failure of enough tension to be generated by the lifter’s muscles will result in a swift fall to the floor of the lifter and barbell in the case of the squat. The last force left to discuss is the moment force. The force generated by gravity on the barbell is a vector force with magnitude and direction. In the case of the squat this vector points straight downwards. Any time there is a horizontal distance between this vertical vector and a point of rotation or joint there will be a moment arm created. Momentum may also be present during barbell training if the barbell is moving very quickly. A lifter who drops too quickly into the bottom of the squat is actually adding force on top of the barbell’s load and this may cause a lift to be missed by increasing the effective load the lifter is trying to lift.
In the fully upright position of a squat (at the top), the moment forces are minimal, if any, as there is no horizontal distance between the load and the possible points of rotation in the back, hips, and knees (fulcrums). In this upright position the bar is perfectly balanced over the lifters mid-foot, which eliminates any moment arms from being present so long as the lifter is fully upright and the skeletal components are aligned. Additionally, the lack of moment arms results in a lack of torque on these joints. Whenever horizontal distance is created between these joints and the barbell’s vector, levers will result in both the spine/back and femoral segments.
Upon initiating the descent of the squat with both the hips and knees entering flexion, levers, moment arms, and forces subsequently develop along the back/spine and femur. We will now discuss three relevant levers, moment arms, and moment forces that develop during the descent (eccentric portion) of the squat.
The back/spine lever is the distance from where the barbell is carried (hopefully in the low bar position- below the spine of the scapula) to the point of rotation, which is where the lumbar vertebrae articulate with the sacrum and one another. There is now a horizontal distance between the barbell’s vector and the lumbar spine, which yields a moment arm. The moment arm is defined as this horizontal distance while the lever is the actual vertebral column that is between the load and the hips. The force acting over the moment arm is the moment force, which is a torque equivalent to the weight of the barbell multiplied by the moment arm (horizontal distance). In a properly executed low-bar back squat the lifter’s torso will move towards horizontal, to what degree is determined by the lifter’s anthropometry, and this lever must remain rigid to ensure no force is dissipated through an unlocked spine. This is why the erectors’ isometric function is so vital in the squat. These muscles must lock the spine into its normal anatomical position, which is a neutral lordosis in the lumbar region and thoracic extension, and keep it there to prevent a dissipation of force that will be created by the lifter. By making sure this lever is rigid we can be confident that the force the lifter applies to the barbell will be transmitted efficiently along this segment to the load to drive it upwards during the concentric portion of the squat.
As the descent continues downward towards the bottom of the squat the length of the lever and subsequently, the moment arm, increases as the center of the barbell moves further away from the fulcrum of the lumbar vertebrae, sacrum, and hips. This occurs as the barbell must remain balanced over the lifter’s mid-foot during the squat. In the event that the barbell deviates forwards or backwards from this mid-foot position another moment arm will develop between the lifter’s mid-foot and the barbell’s vector. This will most likely cause either a failed rep from falling forwards or backwards or a decreased level of efficiency in the system for applying force to the barbell by the lifter. During the concentric contraction of the muscles responsible for hip and knee extension apply a force upward at the inferior end of the spinal lever. Since the lever’s length is maximized with respect to the bar’s positioning over the mid-foot this force is multiplied and more effective than the same force acting over a shorter lever.
The next lever we are concerned with is that of the proximal femur. During the descent of the squat the barbell’s vector crosses the femur somewhere along its diaphysis and thus creates a lever from the point in which it crosses the femur to where the femur articulates in the acetabulum of the hip. The resulting moment arm is the horizontal distance from the point at which the barbell’s vector crosses the femur to a vertical line extending downward from the middle of the hip joint. The moment force or torque then, is the weight of the barbell multiplied by the length (distance) of the moment arm. It is important to note that in the low-bar back squat the proximal femoral lever is greater in distance relative to the distal femoral lever. This transmits a greater percentage of the load to the hips rather than the knees. In a front squat, which has a shorter proximal femoral segment and longer distal femoral segment, more of the load is transmitted to the knee joint as the lever, moment arm, and resulting torque is greater.
Again, during the descent of the squat the barbell’s vector must remain in balance over the lifter’s mid-foot. To maximize the leverage of the muscles acting on the femur-hip interface the length of this segment must increase during the eccentric phase (down) of the squat. In a properly executed low-bar back squat the proximal femoral lever’s length increases as the distance between the barbell’s vector and the center of the femur’s articulation with the hip increase. This longer lever allows for the force that is to be applied to the hips to be magnified by this increased distance. As Archimedes said, “Give me a place to stand and I shall move the Earth with a lever.” Again, torque is quantitatively the product of the lever’s length and the force acting on it. Given a certain force, a longer lever will increase the torque and thus allow for heavier weights to be moved.
As discussed previously, the third lever of importance is that of the distal femur. From the point that the barbell’s vector crosses the shaft of the femur to the center of the knee joint represents the length of this distal femoral lever. The moment arm is then the horizontal distance between the barbell’s vector (straight downward) and the center of the knee joint. Finally, the moment force or torque acting on this segment can be calculated by multiplying the length of the moment arm by the barbell’s load.
Similar to the other levers discussed, the distal femur’s lever length increases during the descent of the squat provided that proper form is executed during the squat. During the descent the knee joint travels slightly forward in-line with the feet to a point that is determined by the lifter’s anthropometry. As the lifter descends into the bottom of the squat the horizontal distance between the barbell’s vector and the center of the knee joint increases, making this longer lever available. Out of the bottom of the squat the quadriceps muscles act to produce force to extend the knee and this slightly longer lever facilitates force magnification in the same ways previously discussed.
During the ascent of the squat the three aforementioned lever lengths start to decrease as the back becomes more vertical from hip extension, the hip angle opens up due to hip extension, and the knee angle also opens up due to knee extension. Despite the decreased length of the levers, and thus less torque being applied to the joints, the muscles are now at a better length for maximal contractility due to the force-length relationship of the muscle and the increased prevalence of actin-myosin cross-linking that can occur at these specific lengths. Additionally the momentum of the weight being moved upwards aids in successfully completing the rep.