Biomechanics of the Back Squat – Part 1

The squat is a well-known exercise for the knee and hip muscles and is commonly used in rehab programmes. Biomechanically, the squat is a close-chain movement, requiring simultaneous extension patterns of the ankle, knee and hip joints. The squat can be performed in many different ways, including variations in foot width (sumo squats), foot position (single leg squats, Bulgarian squats), load position (front squats, sissy squats) and depth (full range squats, shallow squats). However, it is important to keep in mind that each one of these variations will influence knee joint forces and muscle recruitment patterns. For example, there has been much debate about the safety of deep (full range) squats. Coaches often define a full range squat by amount knee range of motion (120-140 degrees), or by observation of the thigh angle (below parallel to the floor).

 

Squat depth has been a topic of much debate, some researchers have raised concerns about the levels of knee compressive forces that occur during a full range squats, however this concern has been rejected by leading strength and conditioning authorities such as the National Strength and Conditioning Association (NCSA). In their Position Paper on the squat exercise 95, the NCSA have rejected these concerns, providing strong scientific evidence that full range squats do not expose the knee to damaging compressive forces. Interestingly, researchers have reported knee joint compression may be higher in the partial range of motion squat, as peak patellofemoral joints experience peak forces at 90-100 degrees of knee flexion during the squat (1). According to the NSCA Position Paper (5), the squat when performed correctly regardless of depth, and with appropriate supervision, not only is safe, but may be a significant deterrent to knee injuries. Importantly however, the authors of this Position Paper also report that bouncing out of the bottom position of a squat, to help initiate the ascent increases mechanical load on the knee joint. This is because the highest forces and muscular tension during a squat typically occur at the transition from descent (eccentric) to ascent (concentric), as muscle lengthening is typically much higher during full range of motion squats (1). It is for this very reason, that squat-style kettlebell swings (not hinge pattern swings) predispose the knee to injury, and should be contraindicated for any client with pre-existing knee injuries. In summary, it appears full range of motion squats when performed with good technique and supervision, are not damaging to the knee.

 

The discussion on full range or partial range squatting also extends into athletic performance. Some researchers believe (1-3) full range squats may be more beneficial for vertical jump performance, than partial range squats. In contrast, other researchers have shown that maximal strength in partial range of motion squats is a strong determinant of sprint and vertical jump performance (4). When compared to a full range of motion squat, Rhea et al. (6) reported partial range of motion squats provide greater transfer to improvements in both sprinting and jumping ability, including partial range of motion squats in workouts can result in greater improvements in sporting skills. However the authors of this study (6) acknowledge that full range of motion squats are useful for general sports conditioning programs. The likely explanation for the observed improvement in sprint and jump performance from Partial range of motion squats likely due to similar hip and knee joint ranges observed in both sprinting and jumping. However, partial range of motion squats are not without there limitations. For example, In their recent case study, Kotani et al. (1) reported a preference for full range of motion squats when training their female volleyball players, stating that the volleyball players when prescribed partial range of motion squats complained that the heavier loads (1-1.2 x body weight) increased the discomfort on their backs and knees (1). Taken together, the results of these studies suggest there are benefits to both full and partial range of motion squats.

 

Full range of motion squats are likely to provide general conditioning benefits, and may also be beneficial for improving vertical jump performance. Whereas partial range of motion squats appear to provide specific benefits to both sprinting and vertical jumping performance, however due to the higher loads that can be utilized with quarter range of motion squats, there may be limitations such as additional hip and knee joint stress. Functional Training instructors are advised to take into consideration the client’s previous training history, current mobility restrictions and training goals before selecting either partial or full range of motion squats.

 

References
1. Kotani, Y & Hori, N. From the Field – Directed Topic – The reasons why athletes squat deep in japan volleyball women’s national team. Journal of Australian Strength and Conditioning. 25(3). 2017.
2. Bloomquist, K., Langberg, H., Karlsen, S., Madsgaard, S., Boesen, M.& Raastad, T. Effect of range of motion in heavy load squatting on muscle
and tendon adaptations. European Journal of Applied Physiology. 113: 2133-2142, 2013.
3. Hartmann, H., Wirth, K., Klusemann, M., Dalic, J., Matuschek, C.& Schmidtbleicher, D. Influence of squatting depth on jumping performance.
Journal of Strength & Conditioning Research. 26: 3243-3261, 2012.
4. Wisloff et al. Strong correlation of maximal squat strength with sprint
performance and vertical jump height in elite soccer players. British Journal of Sports Medicine. 38. 2013.
5. N.S.C.A Position Paper: The Squat Exercise in athletic conditioning: A position statement and review of the literature. Available at:http://journals.lww.com/…/N_S_C_A__POSITION_PAPER__The_Squa…
6. Rhea et al. Joint-angle specific strength adaptations influence improvements in power in highly trained athletes. Human Movement. 2016.17(1).

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