Effects of lower-limb joint mobility and core muscle activation on dynamic lumbar kyphosis during deep squats

Background: Deep squats improves core stability and lower-limb strength. However, dynamic lumbar kyphosis during this movement may increase spinal injury risk. We quantified the dynamic lumbar kyphosis angle during deep squats and examined its association with lower-limb joint angles and muscle activation in the rectus abdominis, external oblique, erector spinae, latissimus dorsi, rectus femoris, gluteus maximus and biceps femoris. Our main aim was to identify strategies for mitigating lumbar kyphosis. Methods: Thirty adult men proficient in resistance training participated in the study. Reflective markers were affixed to joints, and surface electromyography (sEMG) electrodes were placed on muscles involved in lumbar flexion and extension. Participants performed deep squats at 70% of their one-repetition maximum, with motion data captured using eight infrared cameras (sampling rate: 250 Hz). Muscle activity was recorded via a wireless sEMG system (sampling rate: 2000 Hz). Peak joint angles and maximum lumbar flexion were analyzed using correlation and stepwise multiple linear regression to identify factors contributing to dynamic lumbar kyphosis (α = 0.05). Results: Significant positive correlations were observed between maximum hip flexion angle (r = 0.590, p = 0.001), erector spinae activity (r = 0.403, p = 0.027), and maximum lumbar flexion angle. Multiple regression analysis identified a model including hip flexion and erector spinae activity (r = 0.682, adj. R2 = 0.426, p < 0.001), with a hip joint peak flexion angle (HJ_PFA) (t = 3.910, β = 0.553, p = 0.001) as the strongest predictor of lumbar flexion angle, followed by erector spinae muscle activity (ES_MA) (t = 2.436, β = 0.345, p = 0.022). Conclusions: Enhancing hip mobility and erector spinae strengthening prevent dynamic lumbar kyphosis during deep squats. Training programs should incorporate hip mobility exercises, stretching and range of motion routines with resistance exercises targeting the erector spinae.

Deep squat; Dynamic lumbar kyphosis; Butt wink; Lumbar injury; Electromyography

[1] Straub RK, Powers CM. A biomechanical review of the squat exercise: implications for clinical practice. International Journal of Sports Physical Therapy. 2024; 19: 490–501.

[2] Escamilla RF. Knee biomechanics of the dynamic squat exercise. Medicine & Science in Sports & Exercise. 2001; 33: 127–141.

[3] Soyuer F, Koku M. Core muscle activity in exercise. International Journal of Family & Community Medicine. 2024; 8: 73–75.

[4] Stastny P, Kolinger D, Pisz A, Wilk M, Petruzela J, Krzysztofik M. Effects of eccentric speed during front squat conditioning activity on post-activation performance enhancement of hip and thigh muscles. Journal of Human Kinetics. 2024; 91: 5–18.

[5] Butler RJ, Plisky PJ, Southers C, Scoma C, Kiesel KB. Biomechanical analysis of the different classifications of the functional movement screen deep squat test. Sports Biomechanics. 2010; 9: 270–279.

[6] Chan CK, Azah HN, Yeow CH, Goh SK, Ting HN, Salmah K. Effects of squatting speed and depth on lower extremity kinematics, kinetics and energetics. Journal of Mechanics in Medicine and Biology. 2022; 22: 2250032.

[7] Zawadka M, Smolka J, Skublewska-Paszkowska M, Lukasik E, Gawda P. How are squat timing and kinematics in the sagittal plane related to squat depth? Journal of Sports Science and Medicine. 2020; 19: 500–507.

[8] Papadakis Z, Stamatis A, Almajid R, Appiah-Kubi K, Smith ML, Parnes N, et al. Addressing biomechanical errors in the back squat for older adults: a clinical perspective for maintaining neutral spine and knee alignment. Journal of Functional Morphology and Kinesiology. 2024; 9: 224.

[9] Schoenfeld BJ. Squatting kinematics and kinetics and their application to exercise performance. Journal of Strength and Conditioning Research. 2010; 24: 3497–3506.

[10] Enoki S, Shiba J, Hakozaki T, Suzuki Y, Kuzuhara K. Correlations between one-repetition maximum weights of different back squat depths. Isokinetics and Exercise Science. 2023; 31: 97–102.

[11] Patterson C S, Asavasopon S, Gharibvand L, Powers CM. Influence of hip and lumbar extensor strength on lumbar moments during a squat lift: an evaluation of persons with and without low back pain. Clinical Biomechanics. 2025; 122: 106444.

[12] Ribeiro AS, Santos ED, Nunes JP, Nascimento MA, Graça Á, Bezerra ES, et al. A brief review on the effects of the squat exercise on lower-limb muscle hypertrophy. Strength & Conditioning Journal. 2023; 45: 58–66.

[13] Wang X, Liu W, Zhao Y, Ma P. The impact of disc degeneration on the dynamic characteristics of the lumbar spine: a finite element analysis. Frontiers in Bioengineering and Biotechnology. 2024; 12: 1384187.

[14] Stone MH, Hornsby WG, Mizuguchi S, Sato K, Gahreman D, Duca M, et al. The use of free weight squats in sports: a narrative review—terminology and biomechanics. Applied Sciences. 2024; 14: 1977.

[15] Lee JW, Lim YT, Kwon MS. The effect of dynamic lumbar kyphosis on the biomechanical factors of the lumbar joints during deep squats. Korean Journal of Sport Science. 2024; 35: 296–305.

[16] McKean MR, Dunn PK, Burkett BJ. The lumbar and sacrum movement pattern during the back squat exercise. Journal of Strength and Conditioning Research. 2010; 24: 2731–2741.

[17] Schäfer R, Trompeter K, Fett D, Heinrich K, Funken J, Willwacher S, et al. The mechanical loading of the spine in physical activities. European Spine Journal. 2023; 32: 2991–3001.

[18] Yanagisawa O, Oshikawa T, Adachi G, Matsunaga N, Kaneoka K. Acute effects of varying squat depths on lumbar intervertebral disks during high‐load barbell back squat exercise. Scandinavian Journal of Medicine & Science in Sports. 2021; 31: 350–357.

[19] Maduri A, Pearson BL, Wilson SE. Lumbar-pelvic range and coordination during lifting tasks. Journal of Electromyography and Kinesiology. 2008; 18: 807–814.

[20] Tafazzol A, Arjmand N, Shirazi-Adl A, Parnianpour M. Lumbopelvic rhythm during forward and backward sagittal trunk rotations: combined in vivo measurement with inertial tracking device and biomechanical modeling. Clinical Biomechanics. 2014; 29: 7–13.

[21] Bengtsson V, Berglund L, Öhberg F, Aasa U. Thoracolumbar and lumbopelvic spinal alignment during the barbell back squat: a comparison between men and women. International Journal of Sports Physical Therapy. 2023; 18: 820–830.

[22] Zawadka M, Smolka J, Skublewska-Paszkowska M, Lukasik E, Jablonski M, Gawda P. The influence of sedentary behaviour on lumbar-pelvic kinematics during squatting and forward bending among physically active students. Ergonomics. 2023; 66: 101–112.

[23] Berglund L, Öhberg F, Strömbäck E, Papacosta D. Are anthropometric measures, range of motion, or movement control tests associated with lumbopelvic flexion during barbell back squats? International Journal of Sports Physical Therapy. 2024; 19: 1097.

[24] Faul F, Erdfelder E, Lang AG, Buchner A. G*Power 3: a flexible statistical power analysis program for the social, behavioral, and biomedical sciences. Behavior Research Methods. 2007; 39: 175–191.

[25] Faul F, Erdfelder E, Buchner A, Lang AG. Statistical power analyses using G* Power 3.1: tests for correlation and regression analyses. Behavior Research Methods. 2009; 41: 1149–1160.

[26] Haff GG, Triplett NT. Essentials of strength training and conditioning. 4th edn. Human Kinetics: Champaign, USA. 2015.

[27] Vicon Motion Systems. Plug-in gait reference guide. 2022. Available at: https://help.vicon.com/download/attachments/11378719/Plug-in%20Gait%20Reference%20Guide.pdf (Accessed: 03 June 2022).

[28] Escamilla RF, Fleisig GS, Lowry TM, Barrentine SW, Andrews JR. A three-dimensional biomechanical analysis of the squat during varying stance widths. Medicine and Science in Sports and Exercise. 2001; 33: 984–998.

[29] Price D, Ginn KA, Halaki M, Reed D. What is the contribution of latissimus dorsi to trunk movement and control? A systematic review and meta-analysis. Physical Therapy Reviews. 2024; 29: 47–63.

[30] Chen YL, Lin WC. Enhancing understanding: back muscle strength and individual flexibility impact on the flexion-relaxation phenomenon in the lumbar erector spinae. Journal of Electromyography and Kinesiology. 2024; 79: 102949.

[31] McGill SM, Grenier S, Kavcic N, Cholewicki J. Coordination of muscle activity to assure stability of the lumbar spine. Journal of Electromyography and Kinesiology. 2003; 13: 353–359.

[32] Norris CM, Matthews M. Correlation between hamstring muscle length and pelvic tilt range during forward bending in healthy individuals: an initial evaluation. Journal of Bodywork and Movement Therapies. 2006; 10: 122–126.

[33] Stegeman D, Hermens H. Standards for surface electromyography: the European project Surface EMG for non-invasive assessment of muscles (SENIAM). Enschede: Roessingh Research and Development. 2007; 10: 108–112.

[34] Saeterbakken AH, Stien N, Pedersen H, Andersen V. Core muscle activation in three lower extremity exercises with different stability requirements. The Journal of Strength & Conditioning Research. 2022; 36: 304–309.

[35] Stambolian D, Asfour S, Eltoukhy M. Using Vicon bodybuilder and plug-in-gait to generate L5/S1 angles, forces and moments. IEEE Aerospace Conference. Big Sky, MT, USA. March 2014.

[36] Eltoukhy M, Travascio F, Asfour S, Elmasry S, Heredia-Vargas H, Signorile J. Examination of a lumbar spine biomechanical model for assessing axial compression, shear, and bending moment using selected Olympic lifts. Journal of Orthopaedics. 2016; 13: 210–219.

[37] Dolan P, Adams MA. Repetitive lifting tasks fatigue the back muscles and increase the bending moment acting on the lumbar spine. Journal of Biomechanics. 1998; 31: 713–721.

[38] Kothurkar R, Lekurwale R, Gad M, Rathod CM. Estimation and comparison of knee joint contact forces during heel contact and heel rise deep squatting. Indian Journal of Orthopaedics. 2023; 57: 310–318.

[39] Endo Y, Miura M, Sakamoto M. The relationship between the deep squat movement and the hip, knee and ankle range of motion and muscle strength. Journal of Physical Therapy Science. 2020; 32: 391–394.

[40] Patterson CS, Lohman E, Asavasopon S, Dudley R, Gharibvand L, Powers CM. The influence of hip flexion mobility and lumbar spine extensor strength on lumbar spine flexion during a squat lift. Musculoskeletal Science and Practice. 2022; 58: 102501.

[41] Takaki S, Kaneoka K, Okubo Y, Otsuka S, Tatsumura M, Shiina I. Analysis of muscle activity during active pelvic tilting in sagittal plane. Physical Therapy Research. 2016; 19: 50–57.

[42] Hashemirad F, Talebian S, Hatef B, Kahlaee AH. The relationship between flexibility and EMG activity pattern of the erector spinae muscles during trunk flexion-extension. Journal of Electromyography and Kinesiology. 2009; 19: 746–753.

[43] McGill SM, Kippers V. Transfer of loads between lumbar tissues during the flexion-relaxation phenomenon. Spine. 1994; 19: 2190–2196.

[44] Kang SH, Mirka GA. Load transfer between active and passive lumbar tissues and its implications in time-dependent EMG-assisted biomechanical modeling. Journal of Biomechanics. 2025; 183: 112600.

[45] Qi K, Yin Z, Li C, Zhang J, Song J. Effects of a lumbar exoskeleton that provides two traction forces on spinal loading and muscles. Frontiers in Bioengineering and Biotechnology. 2025; 13: 1530034.