Review: Lower Extremity Strength and the Range of Motion in Relation to Squat Depth
Squats are a relatively common exercise, often used to increase lower body strength (Kim, et al, 2015). When used in weightlifting and powerlifting, high flexion angles are required in the hips, knees, and ankles, which may increase musculoskeletal stress or knee injury (Kim, et al, 2015). Mediolateral rotation of the hip, knee alignment inside or outside of the hip during the squat can also increase the compressive and shear forces occurring at the ankle, knee, and hip (Kim, et al, 2015).
Decreased hip and ankle musculature strength have been reported to reduce stability of the lower extremity, which can in turn result in faulty alignment of the lower extremities (adduction and rotation of the hip, knee valgus) (Kim, et al, 2015). Activation of the tibialis anterior muscle is needed to initiate the squat, and the rectus femoris serves as both a hip flexor and a stabilizer of the hip and knee (Kim, et al, 2015).
Because squatting involves multiple joints, performance of the squat requires both strength and mobility of the hip, knee, and ankle (Kim, et al, 2015). Previous studies have shown deficits in dorsiflexion led to decreased peak knee flexion and increased knee valgus, while lifters with medial knee displacement during a squat exhibit tight and weak ankle musculature (Kim, et al, 2015).
One hundred and one participants were assessed by the same examiner. Both passive and active range of motion were measured at the hip, knee, and ankle with a universal goniometer. The examiner measured hip flexion, hip internal and external rotation, ankle dorsiflexion (while the knee was both flexed and extended). Strength of the hip flexors and dorsiflexors were measured with a hand-held dynamometer. Additionally, participants performed a bodyweight squat to their maximum depth (with heels flat on the floor) with their hands behind their heads, and this position was photographed.
In males, hip flexion, internal rotation, and ankle dorsiflexion in both knee positions were negatively correlated with squat depth. In females, Dorsiflexion in both knee positions was negatively correlated with squat depth, but dorsiflexor strength was positively correlated with squat depth. For males, ankle dorsiflexion with a flexed knee and hip flexion ROM were predictor variables for squat depth. For females, ankle dorsiflexion with an extended knee and dorsiflexor strength were predictor variables for squat depth.
Ankle dorsiflexion ROM with a flexed knee and hip flexion were important squat depth predictors in males, and knee-extended dorsiflexion and dorsiflexor strength were predictive in females. Lower extremity mobility, stability, postural control, and pelvic and core stability are required for safe squatting performance, and weakness or limited mobility limit squat depth (Kim, et al. 2015). When individuals with limited mobility perform squats, faulty movement patterns or compensations must be used to complete the movement pattern. The ankle is a crucial component of the squat, mean limited mobility at the ankle can inhibit movement of the more proximal knee and hip joints. People with increased dorsiflexion demonstrate more medial knee displacement compared to those without mobility limitations in single leg squatting, and these differences diminish when limited dorsiflexion is compensated for by raising the heel, as with specialized lifting shoes (Kim, et al, 2015). Those with decreased strength of the ankle musculature are also prone to excessive knee valgus (Kim, et al, 2015).
The hip joint serves to transfer force between the pelvis and lower extremities during the squat, so it follows that when hip flexion is limited, trunk flexion may used to compensate and complete the movement pattern (Kim, et al, 2015). This study found that limited hip mobility correlated negatively with male squat depth, so increasing hip flexion ability may be an important factor for safe squatting. The same was found to be true of hip internal rotation (Kim, et al, 2015).
The reviewed study does not mention the subjects training history. This would be an interesting methodology to repeat with subjects with training experience and resistance added to the squat. The study also looks at internal and external rotation of the hip, but required the participants to squat with the femurs and feet parallel (sagittal plane movement only). The researchers determined that internal rotation was a limiting factor of squat depth, but I would argue that most resistance-trained individuals squat with some degree of abduction and/or external rotation, which could potentially reduce the amount of internal rotation necessary to reach the same squat depth, and possibly modify these results slightly. Additionally, it would be interesting to learn how each participant’s anthropometry affected the ROM required at each join to achieve a deep squat.
Kim, S., Kwon, O., Park, K., Jeon, I., & Weon, J. (2015). Lower Extremity Strength and the Range of Motion in Relation to Squat Depth, Journal of Human Kinetics, 45(1), 59-69. doi: https://doi.org/10.1515/hukin-2015-0007