We thank Dr. vanWingerden for the thoughtful comments regarding our article, “A Biomechanical Review of the Squat Exercise: Implications for Clinical Practice.” We appreciate the opportunity to clarify the context of several statements raised.
It is important to emphasize that our article was intended as a clinical commentary focused on squat prescription rather than a comprehensive review of spinal biomechanics. Our goal was to synthesize available evidence and provide practical guidance for clinicians prescribing squat exercises. Throughout the commentary, we aimed to prioritize studies examining human squatting whenever possible. Accordingly, our discussion of lumbar spine loading was framed within the context of loaded functional tasks such as the squat.
First, Dr. vanWingerden states: “In the paragraph SQUAT DEPTH it is argued that lumbar spine flexion is coupled with an increase of compressive and shear forces at the lumbar spine. This is contradicted by literature.” In the referenced paragraph, we state: “Given that posterior pelvic tilt is coupled with lumbar spine flexion,1,2 compressive and shear forces occur at the lumbar spine.3–5” References 1–2 examined lumbopelvic motion during human squatting (one loaded and one bodyweight condition). References 3–5 were cited to provide biomechanical context regarding lumbar posture, trunk muscle function, and spinal loading during functional tasks, including lifting.3–5 Several of the studies cited by Dr. vanWingerden examine passive trunk flexion, cadaveric motion segments, or simplified lifting models. While these studies provide useful insight into spinal biomechanics, they differ substantially from the dynamic externally loaded conditions of the squat. As such, their findings cannot be directly extrapolated to the multi-joint, loaded task examined in our article. Our intent was not to suggest that lumbar flexion alone is the primary determinant of compressive or shear forces, but rather to highlight that, within the context of loaded squatting, posterior pelvic tilt and associated lumbar flexion occur alongside conditions that increase spinal loading.
Second, Dr. vanWingerden states: “Figure 2 of the paper shows a more flexed spine and a spine with more lordosis. It is stated that the flexed posture (A) has less tolerance to compressive loads. This is in contrast with the findings of Adam and Dolan. According to their studies, lumbar flexion provides a better distribution of force within the discs, leading to improved compressive resistance.6” In the figure 2 caption, we state: “Forward inclination of the trunk that is achieved by spine flexion (A) results in decreased tolerance to compressive loads and less control of anterior shear forces as compared to when forward trunk inclination is achieved with a neutral spine position (B).” This interpretation was informed by biomechanical work by McGill and colleagues examining the influence of lumbar posture on trunk muscle function and spinal tissue loading, including studies involving human participants.4,5 From a mechanical perspective, maintaining a more neutral lumbar posture during loaded tasks may allow for more effective contribution of active muscular control, reduced reliance on passive tissues, and a more favorable distribution of forces across spinal structures. In contrast, Adams et al.6 examined compressive behavior of cadaveric lumbar motion segments. While both lines of research contribute to our understanding of spinal biomechanics, they address related but distinct conditions that should be interpreted within their respective contexts.
Third, Dr. vanWingerden states: “There will be situations where the squat applies. This is clearly not the case in lower back pain … Better advice would therefore be not to use the squat when recovering from lower back pain.” With respect to clinical recommendations, our intent was not to advocate universal use of the squat exercise for all individuals with low back pain. Rather, the purpose of the review was to summarize biomechanical considerations and discuss potential clinical applications. Specifically, for patients with low back pain, we recommended maintaining a relatively neutral lumbar posture, limiting excessive trunk inclination, and selecting squat depth appropriate to the individual patient. These recommendations were intended to provide conservative, individualized guidance and reflect modification of the squat to reduce lumbar spine demand rather than endorsement of the squat as a universal intervention for all patients with low back pain. Squatting is also an essential activity of daily living (i.e., sitting, toileting, and transitional movements), underscoring the importance of understanding how this movement can be modified rather than avoided in clinical populations.
Finally, as with any clinical commentary/narrative review, study selection reflects the intended clinical scope and focus on squat-specific biomechanics. We agree that continued discussion of spinal loading mechanisms is valuable and welcome the perspectives raised. We appreciate the opportunity to clarify these points and hope that this exchange contributes to a broader understanding of the biomechanical considerations associated with squat performance in clinical practice.
Sincerely,
Rachel K. Straub, University of Southern California
Christopher M. Powers, University of Southern California