Lower extremity joint kinetics and lumbar curvature during squat and stoop lifting
© Hwang et al; licensee BioMed Central Ltd. 2009
Received: 21 April 2008
Accepted: 02 February 2009
Published: 02 February 2009
In this study, kinematics and kinetics of the lower extremity joint and the lumbar lordosis during two different symmetrical lifting techniques(squat and stoop) were examined using the three-dimensional motion analysis.
Twenty-six young male volunteers were selected for the subjects in this study. While they lifted boxes weighing 5, 10 and 15 kg by both squat and stoop lifting techniques, their motions were captured and analyzed using the 3D motion analysis system which was synchronized with two forceplates and the electromyographic system. Joint kinematics was determined by the forty-three reflective markers which were attached on the anatomical locations based on the VICON Plug-in-Gait marker placement protocol. Joint kinetics was analyzed by using the inverse dynamics. Paired t-test and Kruskal-Wallis test was used to compare the differences of variables between two techniques, and among three different weights. Correlation coefficient was calculated to explain the role of lower limb joint motion in relation to the lumbar lordosis.
There were not significant differences in maximum lumbar joint moments between two techniques. The hip and ankle contributed the most part of the support moment during squat lifting, and the knee flexion moment played an important role in stoop lifting. The hip, ankle and lumbar joints generated power and only the knee joint absorbed power in the squat lifting. The knee and ankle joints absorbed power, the hip and lumbar joints generated power in the stoop lifting. The bi-articular antagonist muscles' co-contraction around the knee joint during the squat lifting and the eccentric co-contraction of the gastrocnemius and the biceps femoris were found important for maintaining the straight leg during the stoop lifting. At the time of lordotic curvature appearance in the squat lifting, there were significant correlations in all three lower extremity joint moments with the lumbar joint. Differently, only the hip moment had significant correlation with the lumbar joint in the stoop lifting.
In conclusion, the knee extension which is prominent kinematics during the squat lifting was produced by the contributions of the kinetic factors from the hip and ankle joints(extensor moment and power generation) and the lumbar extension which is prominent kinematics during the stoop lifting could be produced by the contributions of the knee joint kinetic factors(flexor moment, power absorption, bi-articular muscle function).
Low back pain(LBP) is a prevalent problem which causes human suffering and cost for workers and their employers. 60~80% of the adult population have experiences of LBP at least once in their lifetimes [1–5]. Despite improved working conditions, including progress due to automation, many objects in the industry are still handled manually. Among basic manual material handling (MMH) activities, lifting has most frequently been associated with LBP[6, 7]. Recently, there have been many researches about lifting such as three-dimensional motion analyses, musculoskeletal simulations and medical imaging studies. The most commonly advised lifting technique is the squat technique, in which the knees are flexed . It can easily be understood that compliance with this advice is often low, given the high energetic cost of this technique [9–11]. Van Dieen et al. conducted a comprehensive review on 27 biomechanical studies, comparing stoop and squat techniques, and concluded that no justification existed for advocating squat technique. Jager and Luttman used a three-dimensional dynamic model to estimate lumbar compression and found that compression was barely influenced by lumbar curvature. By observations of physiologic, psychologic, biomechanical and clinical evidence on three lifting techniques; squat, semi-squat, and stoop, Leon Straker reported that all those lifting techniques had both advantages and disadvantages depending on circumstances. Burgess-Limerick presented a general guideline on the method to lift with less damage. Besides, recent studies have shown that many variables exist depending on different lifting methods [16–18].
In this study, lumbar, hip, knee, and ankle joint motions and lumbar spine curvatures during squat and stoop lifting of three different weights were analyzed using the 3-D motion analysis to find out the function of lower limb motions contributing to the lumbar joint.
Subject information (N = 26)
Mean ± S.D
23.5 ± 0.76
66.5 ± 6.37
172.1 ± 6.03
Two forceplates(Kistler Instrumente AG, Switzerland) and a surface EMG system(MA 300, Motion Lab Systems Inc., USA) were synchronized with the 3D motion analysis system(VICON Motion System Ltd., UK). A total of 31 reflective markers were attached on the anatomical locations according to the VICON Plug-in-Gait marker placement protocol. Besides that, additional four markers(V1~V4) were mounted on the back along the spinous processes to define the spinal curvature. The boxes (34 × 34 × 27.5 cm) weighed 5, 10 and 15 kg, and had the same sized handles. Subjects were asked to lift those boxes using two different techniques (squat and stoop) in their comfortable speed. Joint moments and joint powers in the lower extremities were calculated using the inverse dynamics and the support moment was also determined as the summation of all lower extremity joint moments[19, 20].
Paired t-test was used to determine the statistical difference of the maximum lumbar joint moments between the squat and stoop liftings, and the Wilcoxon test was used to compare the maximum joint power among three different weights. The Kruskal-Wallis test was used to compare the joint angles and moments with respect to the increase of weights when the lumbar lordosis appears.
In the stoop lifting, the hip and lumbar joint generated power (concentric contraction) in contrast with the ankle and knee joint which absorbed power (eccentric contraction) for the most part.
Co-contraction of the bi-articular knee antagonists (rectus femoris and biceps femoris) were observed markedly during the squat lifting. The concentric contraction of the tibialis anterior and the simultaneous eccentric contraction of the gastrocnemius during the stoop lifting also observed in the ankle joint.
Joint angles and moments when the lumbar lordosis appears during lifting (Kruskal Wallis)
Joint angles (deg)
Joint moments (Nm/kg)
Correlation coefficients between lumbar and lower extremity joint
Correlation coefficient (p)
Lower limb joint angle (deg)
Lower limb joint moment (Nm/kg)
Without limitation of the assumption of biomechanical model used for calculation of kinematic and kinetic results, the limitations of this study were summarized as follows. We just analyzed the representative two lifting techniques on the assumption that they were symmetrical movements. In addition, the movements in the coronal/horizontal plane were not included in this study. Under the in-vitro examination, it was inevitable to keep the subject's motion under control – initial foot position, upper extremity position, knee flexion angle(during squat lifting). The objects were not placed close enough to the body because the reflective markers could be hidden. In fact, lumbar could be often damaged mechanically due to the asymmetric or unbalanced lifting movement.
The heavy weight of object is also critical factor to the lumbar damage but 15 kg was assumed as heavy weight in this study for the safety of the subjects.
The result of the maximum lumbar joint moment comparison between the squat and stoop lifting corresponded to the previous study that there was no conclusive evidence for advocating the squat lifting.
The support moment calculated by the summation of the extension moments in the previous study, however all moments including flexion moments were summated for support moment in this study because the knee flexors could act as supporters.
It was expected that the joint moment results could be supported by the EMG results. However, the normalized EMG data had large variation among subjects and a lot of data excluded for analysis because of its failure of detection therefore we were focused on the activation patterns to interpret EMG data.
The correlation coefficients between the lumbar and the lower extremity were investigated which were calculated by using the angles and moments at the time of lumbar lordosis appearance. The knee angle had the strong correlation with the lumbar angle in the squat lifting, and the hip and ankle angle had the correlation with the lumbar angle in the stoop lifting. These results showed representative kinematic characteristics of each lifting technique. All three lower extremity joint moments had the correlation with the lumbar joint in the squat lifting, and only the hip joint moment had the correlation with the lumbar joint in the stoop lifting.
In addition, the squat lifting as well as the stoop lifting is the typical closed kinetic chain motion [27–29] so that the bi-articular muscle function is more complex(Lombard paradox:[31, 32]). The EMG analysis and the calculation of individual muscle force change using simulation software could be helpful to determine these bi-articular muscle functions.
1) There were not significant differences in maximum lumbar joint moments between two techniques. Rather, the maximum lumbar extension moment was larger in squat than in stoop when 15 kg was lifted (p < 0.05). This result advocates the previous study.
2) The hip and ankle joint contributed to the most part of the support moment during the squat lifting, and the knee flexion moment played an important role in the stoop lifting.
3) The ankle, hip and lumbar joints generated power and only the knee joint absorbed power in the squat lifting. The ankle and knee joints absorbed power and the hip and lumbar joints generated power in the stoop lifting.
4) The EMG results summarized that the co-contraction of the antagonists was observed markedly in the both lifting techniques; the tibialis anterior and the gastrocnemius in the ankle joint, the rectus femoris and the biceps femoris in the knee joint.
5) At the time of lordotic curvature appearance in the squat lifting, strong correlations were found in all three lower extremity joint moments with the lumbar joint. On the other hand, in the stoop lifting, strong correlations existed in the hip moment with the lumbar joint.
In conclusion, considering the correlation with the lumbar joint, the kinetic factors generated by the ankle and hip joints (the extensor moment and the power generation) mostly lead the knee extension which is the remarkable kinematics in the squat lifting. The lumbar joint's kinematics (ROM) was the largest in stoop lifting. However, this movement couldn't be done safely without the knee joint's kinetic factors (the flexor moment, the antagonistic co-contraction of bi-articular muscles and the power absorption).
This work was supported by the Korea Science and Engineering Foundation(KOSEF) grant funded by the Korea government(MOST) (No. R01-2006-000-10257-0), and the Ministry of Education, Science Technology (MEST) and Korea Industrial Technology Foundation (KOTEF) through the Human Resource Training Project for Regional Innovation
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