The Biomechanical Effect of Knee Flexion Angles on Squat Lifting with a Flat Back Position

Purpose: This study aimed to evaluate the biomechanical effects of different knee flexion angles on internal joint forces at the knee and lumbosacral (L۵/S۱) joints during squat lifting with a flat back posture. There is a lack of integrated studies examining how varying knee angles combined with maintained lumbar lordosis affect load distribution, especially under lifting tasks. Understanding these interactions is critical for designing safer resistance training programs and minimizing injury risk, particularly for athletes and individuals with lower back vulnerability. Methods: In this semi-experimental study, ten healthy young males (mean age: ۲۳.۵ ± ۱.۹ years) performed squat lifts at three knee flexion angles: full flexion (-۱۰°), semi-flexion (۳۵°), and low flexion (۷۰°), lifting an ۸.۴ kg standardized box. Kinematic data were captured using a ۵۶-marker setup and a VICON motion capture system, while kinetic data were recorded via AMTI force plates. A ۳D linked-segment model and inverse dynamics approach were applied to calculate shear forces (ML and AP), compressive forces, and time-to-peak (TTP) for both the knee and L۵/S۱ joints. Results: A significant increase was observed in L۵/S۱ maximum ML shear force from ۲.۶۲ to ۴.۹۵ N/kg as knee flexion increased (p = ۰.۰۰۱). Time-to-peak compressive force at L۵/S۱ decreased significantly from ۵۶% (FF) to ۴۳% (SF) (p = ۰.۰۰۳). In contrast, maximum ML shear force at the knee decreased from ۴.۱۳ to ۲.۱۳ N/kg (p = ۰.۰۰۳) with increasing flexion. Knee compressive force rose from ۳.۳۲ to ۴.۱۸ N/kg, although not significantly. Conclusion: Increased knee flexion with a flat back posture leads to higher lumbar shear forces but reduces knee shear loading while increasing compressive forces at the knee. These insights emphasize the importance of adjusting knee angles and trunk posture in resistance training to optimize joint safety and efficiency. Keywords: Squat, Knee Flexion Angles, Inverse Dynamics, Joint Forces, Lumbar Spine, Time-to-Peak (TTP) Introduction Squat lifting is a key exercise in resistance training that involves significant biomechanical interactions between the knee, hip, and lumbar spine. Proper analysis of these forces is crucial to prevent injury and optimize performance. Previous studies, such as those by Biscarini et al. (۲۰۲۰) [۲], have emphasized the importance of joint reaction forces in squat exercises, while others, such as Yavuz et al. (۲۰۱۵) [۱۰], have focused on how knee flexion affects muscle activation during squats. Additionally, spine posture, particularly the flat back position, plays a major role in distributing forces during lifting tasks, which has been explored by Kim et al. (۲۰۲۱) [۵] in their analysis of joint forces. Despite its importance, biomechanical analysis in squat lifting often overlooks the combined effects of knee position and spine posture. Most studies isolate knee flexion angles or spine posture, but the interaction between these factors is rarely explored. Von Arx et al. (۲۰۲۱) [۸] showed that lumbar spine loading varies across lifting styles, highlighting the need for further research on the interplay between knee flexion and lumbar spine posture. Furthermore, research by Whitting and Meir (۲۰۱۶) [۹] demonstrates how varying squat depths affect lower limb forces, emphasizing the need for a more comprehensive biomechanical approach. Although there has been substantial progress in biomechanical modeling and squat analysis, several gaps remain. A major gap is the limited exploration of how flat back postures, commonly recommended in resistance training, interact with knee flexion variations during squat lifting. Some studies have analyzed the biomechanical impact of squat movements in specific knee flexion angles or lumbar postures, but few have combined these factors comprehensively. For instance, Yavuz et al. (۲۰۱۵) [۱۰] explored the kinematics and muscle activity in different squat variations, but the interaction between knee angles and lumbar posture was not addressed. Similarly, Kim et al. (۲۰۲۱) [۵] focused on the muscle activation patterns in

Keywords:

Squat , Knee Flexion Angles , Inverse Dynamics , Joint Forces , Lumbar Spine , Time-to-Peak (TTP)