Biomechanics of the Back Squat – Part 2

In our last article we looked at the difference between partial range and full range of motion squats, in this article we will look at the impact squatting has on joint forces in the lower limbs. In a review of the biomechanics of squat, Escamilla 2001 describes the forces and muscle activity involved in the variations of the squat exercise. Understanding the effect squatting has on the knee joint, can assist FTI instructors and practitioners make informed decisions about the use of the squat for rehabilitation purposes.

Shear Forces

There are two main types of shear forces that directly affect the knee, they are anterior and posterior shear forces. A posterior shear force draws the tibia back, placing strain upon the posterior cruciate ligament (PCL) and an anterior shear force draws the tibia forward, placing strain upon the anterior cruciate ligament (ACL) (Abernethy 2013). A range of studies have calculated the shear forces acting on the tibiofemoral joint (Lutz et al. 2003, Wilk et al. 1996). Importantly, excessive shear forces can be injurious to the ligaments of the knee (Escamilla 2001). Research suggests there is a moderate load placed on the PCL during the squat, which increases as the knee flexes. The PCL force occurs after 60 degrees of knee flexion – when the quadriceps force exerts a posterior force on the joint – with a magnitude in the range of 1000-2000 newtons (Escamilla 2001. The maximum load of the PCL has been estimated at 4000 newtons (Race et al. 1994), and so the squat exercise should be well tolerated for clients with a healthy PCL. Those recovering from PCL injury should restrict the range of movement to no greater than 60 degrees of knee flexion, as this is when PCL loading begins (Escamilla 2001). In contrast, to the PCL, ACL forces were generated between 0 – 60 degrees of knee flexion – when the quadriceps force exerts an anterior force on the joint. However the ACL loads are found to be low, at a peak of 500 newtons. Considering that the maximum load of the ACL is around 2000 newtons, it would seem squats involve little strain on the ACL and should be safe for ACL patients to include in their programmes. By increasing the forward lean of the trunk during the squat (this is why incline board squats are often used in knee rehabilitation programs), ACL stress can be significantly reduced due to the increased hamstring activity providing extra posterior force on the joint. However, by increasing the forward movement of the knees the shear forces can increase and so rehabilitation clients should avoid this position by keeping the knees behind the toes. Finally, the exercise speed can also affect shear forces. One study (Hattin 1989) showed that a fast cadence squat (one second ascent and one second descent) produced up to 30% greater shear forces than slower cadence squats (two seconds each phase). Therefore slow and controlled technique will safeguard the cruciate ligaments.

Interestingly, increases in weight lifted do not effect the ACL or PCL forces in the same way as compression forces. For example, Nisell (1986) examined knee joint loads in powerlifters during the ascent portion of the squat involved with loads of up to 250kg, and although the compression and quadriceps muscle forces were very high (8000 N) the shear forces for ACL and PCL were in the normal range described above. This data suggests that a greater compression force may be important for the stability of the joint, helping to control shear forces. The width of stance during the squat was also shown to increase compression forces, with wider stance increasing compression force by 15% (Escamilla 2001). Stance width had no effects on shear forces, but shear forces are greater during the ascent phase of the squat exercise. Importantly, on a practical level, as fatigue increases so do the shear and compression forces in the tibiofemoral joint. Most biomechanical studies (Lutz et al. 2003, Wilk et al. 1996, Dahlkvist et al. 1982) will analyse a few repetitions; however, in reality patients and athletes will complete a few sets of a number of repetitions, e.g., 4 x 8. Previously, Hattin (1989) conducted a study which investigated the effect of cadence on knee joint forces. The study involved performing up to 50 repetitions of the squat at different loads (15-30% of 1RM). The results showed that shear and compression forces increased from 25-85% from the first to last repetitions. This suggests that knee joint stress may be much greater towards the end of the last few sets. This is why it is important to gradually increase the volume of squats prescribed to a new or functionally untrained client.

Compression forces

Researchers have also calculated the compression forces that occur during a squat, in the tibiofemoral joint (Dahlkvist et al. 1982, Escamilla 1998). Patellofemoral compression force is caused by the contact between the underside of the patella and its articulation with the femur. This is the load due to the articulating surfaces of the tibia and femur (Escamilla 2001). These compressive forces calculations range from 500 – 8000 newtons, where compression increases as load increases during a squat. Calculations of the patellofemoral compression forces during the barbell squat with a weight of around 70% of 1RM, show that the joint force is 4-7 times bodyweight, which is equivalent to approximately 4000 – 5000 N (Escamilla 1998).

These are generally greater loads than many patients will lift in the initial stages of a rehab programme, and so patients are unlikely to load the joint as much in a rehab context until the injury is healed and strength is regained. The peak compression force occurs at the greatest knee flexion angles, generally around 90 degrees and beyond. Patellofemoral patients (eg chrondomalacia patella) need to perform squats in the 0-50 degree range as the loads are moderate in this range. It can be inferred from these data that narrow stance may be preferred over the wide stance when the objective is to minimize compressive forces. Furthermore, compression forces increase with stance width. For instance, Escamilla (1997) reported that the wide-stance squat increased patellofemoral compression force by 15% during the descent. Additionally, if the squat is performed in the low bar position, with the barbell held below the acromion, then greater trunk and hip flexion occurs in the movement and the patellofemoral forces are reduced. This data suggests, clients with knee pain/injuries should use a narrow-stance low-bar technique to minimise patellofemoral compression. Finally, the effect of bar position during the squat can also affect knee compression forces. Escamilla (1997) reports a low-bar position during the squat reduces knee compressive forces and therefore would be a preferential method of squatting for clients with a history of knee pain/injury. In addition, the low-bar position, which increases forward lean of the trunk may also decrease strain on the ACL, in part due to greater hamstring activity and less quadriceps activity , (Ohkoshi et al. 1991), making it further applicable to clients with recovering from an ACL injury.

Practical Implications

For FTI instructors and practitioners, the practical implications of these studies are:

• Low shear forces occur between 0 and 60° of knee flexion during the squat. Therefore, when prescribing a squat exercise, for a client with a history of ACL injury; it is important to keep squat depth to a maximum of 60 degrees of knee flexion.
• Knee compression forces progressively increase with squat depth, therefore when prescribing the squat exercise for clients with a history of knee pain/injury, it is recommended to keep squat depth to no greater than 50 degrees of knee flexion.
• Squatting with a narrow stance may further reduce knee compression forces and is therefore also recommended when prescribing the squat for clients with a history of knee pain/injury.
• A low bar position (below the level of the acromion) when squatting reduces knee compressive forces and ACL strain. It is also therefore recommended to use a low-bar position when prescribing the squat to a client with a history of knee pain or ACL injury.

References:

1. Escamilla, R. F. (2001). Knee biomechanics of the dynamic squat exercise. Medicine & Science in Sports & Exercise, 33(1), 127-141.
   2. Lutz, G. E., Palmitier, R. A., An, K. N., & Chao, E. Y. (1993). Comparison of tibiofemoral joint forces during open-kinetic-chain and closed-kinetic-chain exercises. JBJS, 75(5), 732-739.
   3. Wilk, K. E., Escamilla, R. F., Fleisig, G. S., Barrentine, S. W., Andrews, J. R., & Boyd, M. L. (1996). A comparison of tibiofemoral joint forces and electromyographic activity during open and closed kinetic chain exercises. The American journal of sports medicine, 24(4), 518-527.
   4. Abernethy, B., Kippers, V., & Hanrahan, S. (2013). Biophysical foundations of human movement. Human Kinetics.
   5. Race, A., & Amis, A. A. (1994). The mechanical properties of the two bundles of the human posterior cruciate ligament. Journal of biomechanics, 27(1), 13-24.
   6. Nisell, R. (1986). Joint load during the parallel squat in powerlifting and forces analysis of in vivo bilateral quadriceps tendon rupture. Scand J. Sports Sci, 8, 63-70.
   7. Dahlkvist, N. J., Mayo, P., & Seedhom, B. B. (1982). Forces during squatting and rising from a deep squat. Engineering in medicine, 11(2), 69-76.
   8. Escamilla, R. F., Fleisig, G. S., Zheng, N., Barrentine, S. W., Wilk, K. E., & Andrews, J. R. (1998). Biomechanics of the knee during closed kinetic chain and open kinetic chain exercises. Medicine and science in sports and exercise, 30, 556-559.
   9. Hattin, H. C., Pierrynowski, M. R., & Ball, K. A. (1989). Effect of load, cadence, and fatigue on tibio-femoral joint force during a half squat. Medicine and science in sports and exercise, 21(5), 613-618.
   10. Escamilla, R. F., Zheng, N., Fleisig, G. S., Lander, J. E., Barrentine, S. W., Cutter, G. R., & Andrews, J. R. (1997). The Effects Of Technique Variations On Knee Biomechanics During The Squat And Leg Press 887. Medicine & Science in Sports & Exercise, 29(5), 156.
   11. Ohkoshi, Y., Yasuda, K., Kaneda, K., Wada, T., & Yamanaka, M. (1991). Biomechanical analysis of rehabilitation in the standing position. The American journal of sports medicine, 19(6), 605-611.