The influences of foot placement on lumbopelvic rhythm during trunk flexion motion

Author’s Accepted Manuscript The influences of foot placement on lumbopelvic rhythm during trunk flexion motion Jie Zhou, Xiaopeng Ning, Boyi Hu, Boyi Dai

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S0021-9290(16)30401-8 http://dx.doi.org/10.1016/j.jbiomech.2016.03.048 BM7669

To appear in: Journal of Biomechanics Received date: 3 July 2015 Revised date: 5 January 2016 Accepted date: 28 March 2016 Cite this article as: Jie Zhou, Xiaopeng Ning, Boyi Hu and Boyi Dai, The influences of foot placement on lumbopelvic rhythm during trunk flexion motion, Journal of Biomechanics, http://dx.doi.org/10.1016/j.jbiomech.2016.03.048 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

The influences of foot placement on lumbopelvic rhythm during trunk flexion motion Jie Zhoua,b, Xiaopeng Ningb,*, Boyi Hub and Boyi Daic a

b

Department of Biological and Agricultural Engineering, University of California in Davis, Davis, CA 95616, USA Department of Industrial and Management Systems Engineering, West Virginia University, Morgantown, WV 26506, USA

c

Division of Kinesiology and Health, University of Wyoming, Laramie, WY, 82071 USA

Author: Jie Zhou Mailing address: Department of Biological and Agricultural Engineering, University of California in Davis, One shields Avenue, Davis, CA 95616, USA Phone: 530-760-9509 e-mail: [email protected] Corresponding author: Xiaopeng Ning Mailing address: Industrial and Management Systems Engineering, P.O. Box 6070 West Virginia University, Morgantown, WV 26506, USA Phone: 304-293-9436 Fax: 304-293-4970 e-mail: [email protected] Author: Boyi Hu Mailing address: The Ergonomics Lab, Industrial and Management Systems Engineering, West Virginia University, Morgantown, WV 26505, USA Phone: 304-435-9133 Fax: 304-293-4970 e-mail: [email protected] Author: Boyi Dai Mailing address: Division of Kinesiology and Health, University of Wyoming, 1000E University Ave, Laramie, WY 82071, USA Phone: 307-766-5423 Fax: 307-766-4098 e-mail: [email protected]

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Abstract: Different standing postures could potentially influence trunk biomechanics during task performance. The current study investigated how foot placement, especially stance width and foot angle influenced lumbopelvis rhythm during sagittal trunk flexion motion. Ten participants performed pace controlled sagittally symmetric trunk flexion motions while maintaining three different stance widths and two different foot angles. The results showed the narrower stance and angled foot placement conditions generated more in-phase lumbopelvic coordination patterns during trunk flexion motions, possibly due to the reduced base of support and the associated postural stability. Findings of this study provided important information regarding the effects of foot placement on postural control and trunk biomechanics during trunk bending motions; these results suggested that foot placement could alter the motion patterns of spinal segments.

Keywords: foot placement, lumbopelvic rhythm, low back pain, postural stability

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1. Introduction Low back pain (LBP) is one of the most serious health problems with the incidence of 1.39 per 1,000 person-years in the United States (Waterman et al., 2012). A report from National Center for Health Statics demonstrated that more that 28% of the adults had suffered from LBP in the previous 3 months (NCHS 2012). LBP is highly prevalent among elderly population (Hoy et al., 2012; Manchikanti et al., 2009), a more recent study found that a comparatively large percentage of young adults also suffer from LBP (Mallen et al., 2005). In addition to causing personal suffering, LBP also generates tremendous economic costs, which include both direct (e.g. medical treatment cost) and indirect costs (e.g. lost working days) (Druss et al., 2002). In the United States, the annual costs for LBP can be as high as 100 billion dollar (Katz 2006). To mitigate the risk of LBP, the identification of LBP risk factors is critical, so as to develop control strategies for LBP (Kerr et al., 2001). Multi-segmental motion is achieved by the collective activation of individual muscles through neuromuscular control to meet the task requirements and overcome external constrains (Chow et al., 2014; Kurz and Stergiou, 2004. Qu et al., 2012; Schöner et al., 1990). Therefore, the coordination of movement from multiple motion segments may reflect the underlying postural and balance control strategies of human. Previous epidemiological studies have identified prolonged and repetitive trunk flexion as major risk factors that may cause LBP (Hoogendoorn et al., 2000; Punnett et al., 1991), wherein trunk is defined as the main part of the human body excluding the head and limbs. Trunk flexion is mostly accompanied by the forward rotation of pelvis and lumbar spine. The lumbar-pelvic motion coordination, namely lumbopelvic rhythm, has been identified as an important variable that can be used to determine trunk muscle exertion, spinal loading and in thus the risk of LBP during flexion and extension motions

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(Arjmand et al., 2011; Granata and Sanford, 2000; Hu and Ning, 2015). A number of previous studies have studied lumbopelvic rhythms and discovered that LBP patients use more in-phase coordination patterns compared with non-symptomatic population when performing multisegments movements, such as running, walking and sit-to-stand motions (Esola et al., 1996; Porter and Wilkinson, 1997; Seay et al., 2011; Shum et al., 2005). In addition, significant differences in lumbopelvic rhythms between trunk flexion and extension motions have been found in previous studies (Nelson et al., 1995; Zhou et al., 2015a). A more recent study observed a more in-phase lumbopelvic coordination during trunk bending motions with the influence of lumbar muscle fatigue (Hu and Ning, 2015). Previous studies have investigated the influence of different foot placements on standing stability and trunk biomechanics (refers to the structure and movement of body segment and the associated tissue loading) during different task performance. It was revealed that foot placement significantly affected postural stability during static standing, the enlargement of foot placement allowed a greater displacement of center of pressure, which indicates a higher level of stability (Holbein-Jenny et al., 2007; Kirby et al., 1987). The effect of foot stance on trunk kinetics and kinematics has also been investigated when performing manual material handling tasks (Cholewicki et al., 1991; Sorensen et al., 2011); the results of these studies suggested that wider stance was preferred, as it could reduce spinal loading. In addition, a recent study discovered that foot placement significantly influenced the magnitude of trunk forward flexion and spinal loading at the L5/S1 joint when participants experienced sudden loading (Zhou et al., 2013a). Even though foot placement has been found to change trunk and whole body biomechanics during task performance, how it could alter lumbopelvic coordination patterns during trunk flexion motions is still unclear.

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According to the findings from previous studies, it was suspected that different foot placement could alter lumbopelvic coordination patterns. Therefore, the purpose of the current study was to investigate the effects of foot placement, especially stance width and foot angle on lumbopelvic coordinations when performing sagittal trunk flexion motions. Previous findings suggested that narrower foot stance could reduce postural stability (Holbein-Jenny et al., 2007; Kirby et al., 1987), which may be associated with more in-phase motion patterns (Esola et al., 1996; Hu and Ning, 2015; Seay et al., 2011). It was hypothesized that the decrease of stance width and the presence of an unparalleled foot stance will result in a more in-phase lumbopelvic coordination pattern.

2. Methods 2.1. Participants Ten male college students with average (SD) age, body mass and body height of 24.7 (3.7) years, 74.8 (6.3) kg and 175.8 (3.0) cm, respectively, volunteered to participate in this study. Those who have a previous history of LBP were excluded from the current study. The experimental procedure was approved by the Institutional Review Board of West Virginia University. 2.2. Equipment A magnetic field based 3D motion tracking system (Motion Star, Ascension, Burlington, VT, USA) was used to track lumbar and pelvic kinematics. Two magnetic sensors were fitted onto the skin surface of spinous processes at L1 and S1 levels, respectively. The sagittal angular difference between L1 and S1 sensors was defined as the lumbar flexion angle, and the sagittal rotation of S1 sensor was defined as pelvic angle (Figure 1). A metronome was used to help control the pace of the trunk flexion motion. __________________________________ 5

Insert Figure 1 about here __________________________________

2.3 Experiment design The current study has two independent variables: stance width (WIDTH) and foot angle (ANGLE). WIDTH has three levels: narrow (feet together), medium (feet open shoulder width apart) and wide (feet open 150% shoulder width apart); and ANGLE has two levels: parallel (0° between feet) and angled (60° between feet). As demonstrated in Figure 2, the combination of the two independent variables created six different experimental conditions: narrow-parallel, narrow-angled, medium-parallel, medium-angled, wide-parallel and wide-angled. Participants were required to perform three repetitions of trunk flexion motions in each condition (creating a total of 18 experimental trials) and the presentation of experimental trials were completely randomized. __________________________________ Insert Figure 2 about here __________________________________

Five lumbopelvic continuous relative phase (CRP) related dependent variables were investigated in the current study. They include the overall CRP (defined as the CRP during the entire trunk flexion motion) and the CRP in each of the four quarters (evenly divided based on the trunk range of motion) of the trunk flexion motion. The CRP was calculated as the difference in phase plane angles between the lumbar and the pelvis motion segments during trunk flexion

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motions, the details of lumbopelvic CRP calculation procedures are demonstrated in the “Data processing” session. 2.4 Procedure The experimental procedures were first explained to the participants, and the signed informed consents were obtained. Then participants had a five minutes warm-up session, and two motion sensors were then attached to the skin surface over the L1 and S1 spinous processes. After having the sensors attached, participants performed all 18 experimental trials. In each trial, participants were required to bend trunk from an upright posture to a fully flexed posture in seven seconds while keeping knees fully extended and maintaining the pre-assigned foot placement condition, a fully flexed posture is defined as a posture achieved when subjects bend over and relax all trunk muscles. Between trials, at least one minute of rest was provided to avoid muscle fatigue. 2.5. Data processing The process of calculating CRP has already been introduced by several previous studies (Kurz and Stergiou 2002; Lamoth et al., 2002; Seay et al., 2011; Selles et al., 2001; Hu and Ning 2015). In the current study, lumbar and pelvic angular velocities were first derived from the angular position data. The angular position and the angular velocity were then normalized to a range from -1 to +1, in which the minimum and maximum values were set to -1 and +1, respectively. The phase plane plots of the normalized data of the two segments can then be obtained (Figure 3(a)). After normalization, the normalized angular data were transferred to angles (in rad) (Burgess-Limerick et al. 1993); the angular difference between lumbar and pelvic motion segments was defined as the lumbopelvic CRP (Figure 3(b)). A CRP value approaching 0

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indicates a more in-phase motion pattern, and a CRP value close to π indicates a more out-ofphase motion pattern. __________________________________ Insert Figure 3 about here __________________________________

2.6. Statistical analysis The assumptions of the analysis of variance test (ANOVA) (observations independence, constant variance of residuals and normal distribution of residuals) were first examined (Montgomery 2008). Repeated measures ANOVA analysis was then performed to test the effects of WIDTH, ANGLE and their interaction on CRP. Tukey-Kramer post hoc test was finally performed to further investigate the differences in CRP values between different conditions. In all the statistical analyses, participant was set as the blocking factor, and a p-value of 0.05 was set as the criteria. Minitab (Minitab v.15, Minitab Inc., Pennsylvania, USA) was used to perform all the statistical analyses.

3. Result A summary of kinematics data including starting and ending trunk flexion angles (degree), lumbar and pelvis range of motion (ROM) (degree) and average lumbar and pelvis velocities (degree/s) of all subject is shown in Table 1.

__________________________________ Insert Table 1 about here __________________________________ 8

The results of ANOVA showed significant effects of WIDTH and ANGLE on lumbopelvic CRP (Table 2). The follow up post-hoc test demonstrated that compared with medium and wide conditions, narrow WIDTH conditions generated significantly smaller CRP values in the third, fourth quarters and the overall range of trunk flexion motion, which indicated more in-phase lumbopelvic coordination patterns (Figure 4).

__________________________________ Insert Table 2 about here __________________________________

More in-phase lumbopelvic coordination patterns were also observed in angled foot condition during the second, third quarters and the overall range of trunk flexion motion (Figure 5).

__________________________________ Insert Figure 4 and 5 about here __________________________________

4. Discussion The purpose of the current study was to investigate how different foot placements change lumbopelvic rhythms when performing trunk flexion motions. In general, the results of the

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present study supported our initial hypotheses that with the decrease of stance width and the increase of lateral foot rotation, more in-phase lumbopelvic coordination patterns will be observed (Figure 4 and 5). Foot placement is reported to change postural stability, which in turn affects trunk biomechanics: It has been revealed that during powerlifting, compared with conventional stance (shoulder width), wide stance (sumo style) reduced the spinal moment and shear force by 10% and 8%, respectively, at the L4/L5 vertebral level (Cholewicki et al., 1991). Sorensen and colleagues investigated the effects of foot placement on trunk kinetics and kinematics when performing lifting tasks in an occupational setting, the results of this studies suggested that the trunk range of motion and the peak trunk acceleration were 16% and 20% smaller in wide stance (150% of shoulder width) than narrow stance (feet together), respectively (Sorensen et al., 2011). In the current study, the observed changes in lumbopelvic rhythms among different WIDTH conditions may be caused by the changes in postural stability. It has been reported in previous studies that narrow stance could decrease postural stability (Day et al., 1993; Henry et al., 2001; Kirby et al., 1987; Stoffregen et al., 2010; Winter et al., 1996, 1998). On one hand, this is due to the change in the base of support (BOS). BOS refers to the area enclosed by the points of contact that a person makes with the supporting surface (Winter 1995). In order to maintain stability, the projected body center of mass (COM) should fall within the BOS (Shumway-Cook and Woolacott 1995). Kirby et al., (1987) showed that significantly larger postural sway was observed in narrow stance (feet together) than wider stance postures, because of the associated smaller BOS. A reduced BOS could push COM closer to the stability boundary (the magnitude of stability margin) (Kirby et al., 1987; Stoffregen et al., 2010) and result in reduced stability. On the other hand, stance width could also affect the stiffness and neuromuscular control of low

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extremities. Day et al., (1993) demonstrated that increase in stance width elevated the stiffness of hip-ankle coupling, therefore reduced the requirement of active neuromuscular control and resulted in increased postural stability (Day et al., 1993; Winter et al., 1996, 1998). It has also been suggested that a wider stance increased proprioceptive sensitivity of low extremities, which could enhance the detection of postural perturbation, thus improve postural stability (Day et al., 1993; Henry et al., 2001). Therefore, the possibly decreased stability in the narrow stance condition (in comparison to medium and wide stance conditions) could be caused by the reduced BOS and the change in tissue stiffness and neuromuscular control pattern of low extremities. It has also been shown that a more in-phase lumbopelvic coordination pattern was spontaneously adopted by those who suffer from impaired postural stability (Cholewicki and McGill 1996; Lamoth et al., 2002; Seay et al., 2011; Selles et al., 2001), because it serves as a protective strategy to compensate for the compromised stability and increased risk of injury (Ahern et al., 1988; Hu and Ning, 2015; Marras and Wongsam, 1986). Selles et al. found that a more in-phase coordination pattern was observed among LBP patients at higher walking speeds compared with healthy individuals (Selles et al., 2001). This finding was supported by a later study, which demonstrated significantly smaller CRPs among back pain patients when walking at relatively high speeds (Lamoth et al., 2002). Seay and colleagues found that during walking, the frontal plane coordination was approximately 15% smaller for the LBP group compared to the control group; during running, LBP group also showed more in-phase coordination patterns (up to 30% smaller) in the transverse plane (Seay et al., 2011). In addition, one study discovered that patients with Parkinson disease also showed significantly smaller CRPs during walking than healthy subjects (Van Emmerik et al., 1999). A more recent study found that the presence of transient back pain introduced by lumbar extensor muscle fatigue significantly reduced

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lumbopelvic CRP by 12% during weight lifting tasks (Hu and Ning, 2015). In the current study, participants experienced reduced postural stability in narrow stance condition compared with medium and wide conditions. Therefore, as demonstrated in Figure 4 and Table 2, a significantly reduced lumbopelvic CRP (9.5% reduction on average, up to 20% reduction in Q4) was observed, indicating a more in-phase coordination pattern was adopted by participants to compensate for their decreased stability in the narrow stance condition. In addition, the changes of pelvis range of motion (ROM) may also contribute to the changes in lumbopelvic rhythm. As demonstrated by previous studies, pelvic rotation is restrained by the tensions on hip extensors muscles, therefore changes in lower extremity postures could result in changes in tensions on those muscles and further influence the pelvic ROM (Bohannon et al., 1985; Murray et al., 2002; Shin et al., 2004). As shown in Figure 6, we observed significantly larger maximum pelvic angle with the increase of WIDTH. The maximum lumbar angle, however, was not significantly affected by WIDTH. Such changes could have contributed to the observed differences in lumbopelvic coordination patterns among different WIDTH conditions. __________________________________ Insert Figure 6 about here __________________________________

The results of this study also demonstrated significantly more in-phase lumbopelvic coordination in the angled feet condition. According to the definition of BOS, the parallel and angled foot conditions have similar areas of BOS. However, in the current study, the trunk flexion motion was mostly restricted in the sagittal plane, therefore the BOS in the anterior-

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posterior (AP) direction may play a more important role in postural stability. As supported by previous studies, the angle foot posture may provide better support in the medial-lateral (ML) direction but less support in the AP direction as compare to the parallel foot posture (Day et al., 1993; Kirby et al., 1987; Zhou et al., 2013a; Zhou et al., 2013b); therefore, reduced postural stability may be experienced when participants perform sagittal trunk flexion motions with angled foot posture. As a consequence, more in-phase lumbopelvic coordination patterns (CRP reduced by 10.5% on average, up to 15% as demonstrated in Q3) could be adopted in the angled foot posture in order to compensate for the reduced postural stability (Figure 5 and Table 2). This analogy could also explain why the CRP values in the first and fourth quarters of the trunk flexion motion were not significantly affect by ANGLE while the second and the third quarters were influenced (Figure 5). During the beginning and ending phase of the trunk flexion motion, the COM was moving away and into a relatively central position of BOS, respectively. Therefore, the reduced support in the AP direction may not have much influence on the lumbopelvic coordination patterns; however, in the second and third quarters of trunk flexion motion, COM could be approaching the boundary of BOS and causes participants to experience reduced postural stability which result in more in-phase lumbopelvic coordination patterns. Several limitations of the current study need to be noted. First, only sagittal trunk flexion motion was investigated. Previous studies have demonstrated the significant influence of asymmetry on trunk biomechanics (Ning et al. 2011; Ning et al. 2014; Zhou et al. 2015b); therefore, the lumbopelvic coordination pattern during asymmetric trunk flexion motion warrants future investigation. Second, only three WIDTH and two ANGLE conditions were tested in the present study, other foot placements (e.g. staggered stance) (Kirby et al., 1987; Zhou et al., 2013b) should be tested in the future studies. Third, this is a no-load experiment, if holding external

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loads would change lumbopelvic rhythm needs to be examined in the future. Finally, to avoid the possible gender effect, only male participants were recruited, and it is unclear whether females would response in a different trend, since gender difference in spinal stability has been noted by a previous study (Granata and Orishimo. 2001).

Conclusion Findings of the current study provided important knowledge regarding how foot placement, especially stance width and foot postures could influence trunk biomechanics during trunk flexion motions. Based the results of the current study, narrow stance width and angled foot posture generated more in-phase lumbopelvic coordination patterns, possibly due to the reduced postural stability during sagittal trunk flexion motions.

Conflict of interest statement The authors of this manuscript submitted for possible publication in the scientific journal of Journal of Biomechanics do hereby state that none of us have any conflicts of interest to disclose with regard to any financial and personal relationships with other people or organizations that could inappropriately influence this work.

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Figure Legends Figure 1. Definition of lumbar angle and pelvic angles.

Figure 2. A demonstration of the six experimental conditions.

Figure 3. Illustration of CRP calculation: (a) Phase plane for pelvis segment and lumbar segment. (b) Phase angle profile for pelvis and lumbar segment and final CRP profile. Figure 4: Continuous relative phase values of different WIDTH conditions, „N‟, „M‟ and „W‟ indicate narrow, medium and wide stance, respectively. Different letters denote values that are statistically different from one another. Bars indicate the corresponding 95% confidence interval. Figure 5: Continuous relative phase values of different ANGLE conditions, „0‟ and „60‟ indicate foot parallel and foot angled, respectively. Different letters denote values that are statistically different from one another. Bars indicate the corresponding 95% confidence interval. Figure 6: The maximum pelvic and lumbar angles of different WIDTH conditions, „N‟, „M‟ and „W‟ indicate narrow, medium and wide stance, respectively. Different letters denote angles that are statistically different from one another. Bars indicate the corresponding 95% confidence interval.

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L1

α: pelvic angle

β

β: lumbar angle α S1

Figure 1.

21

Parallel

Angled

Narrow

Medium

Wide

Figure 2.

22

1 1

-1

1 -1

1 -1

1 1

-1

(a)

(b) Figure 3.

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Continuous Relative Phase (Rad)

1.2 1.1

B

1.0

A

0.9

B 0.8

B

B

A

0.7

B 0.6

A

0.5

AB

0.4 0.3

N M W Overall

N M W Q1

N M W Q2

N

M W Q3

N

M W Q4

Figure 4.

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Continuous Relative Phase (Rad)

1.2

B 1.1

A

B

1.0

A

0.9 0.8

B A

0.7 0.6 0.5 0.4 0.3 0 60 Overall

0

60 Q1

0

60 Q2

0

60 Q3

0

60 Q4

Figure 5.

25

38 36

B

Angle (degree)

B 34 32

A

30 28 26 24 N

M MaxPelvic

W

N

M MaxLumbar

W

Figure 6.

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Tables:

Table 1: A summary of kinematics data of all ten subjects. Subject Kinematics Trunk starting (o) (SD) Trunk ending (o) (SD) Lumbar ROM (o) (SD) Pelvis ROM (o) (SD) Lumbar velocity (o/s) (SD) Pelvis velocity (o/s) (SD)

1

2

3

4

5

6

7

8

9

10

1.1 -0.2 100.9 -0.5 30.4 -0.2 55.4 -0.4 4.4 -0.04 8.0 -0.07

2.5 -0.4 100.0 -0.6 31.8 -0.2 57.1 -0.6 4.6 -0.03 8.3 -0.11

0.7 -0.1 96.4 -0.6 28.4 -0.2 53.1 -0.5 4.1 -0.03 7.6 -0.08

2.2 -0.3 128.5 -0.8 35.1 -0.2 73.4 -0.8 5.0 -0.04 10.5 -0.13

1.3 -0.3 105.5 -0.6 16.4 -0.3 70.4 -0.6 2.4 -0.05 10.1 -0.10

3.7 -0.3 95.7 -0.6 18.4 -0.1 64.5 -0.4 2.6 -0.03 9.3 -0.06

3.3 -0.3 95.6 -1.4 19.3 -0.2 61.0 -1.1 2.8 -0.03 8.8 -0.15

2.1 -0.3 104.2 -0.8 33.5 -0.2 53.7 -0.6 4.8 -0.04 7.7 -0.10

5.8 -0.3 105.7 -0.6 27.7 -0.1 59.8 -0.4 4.0 -0.02 8.6 -0.06

1.7 -0.2 96.1 -1.0 30.3 -0.2 47.1 -0.7 4.4 -0.04 6.8 -0.11

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Table 2: The results ANOVA tests, bolded p value indicates significant effect. Independent Variables

Width

Angle

Dependent Variables CRP overall CRP Q1 CRP Q2 CRP Q3 CRP Q4 CRP overall CRP Q1 CRP Q2 CRP Q3 CRP Q4

ANOVA Degree of Freedom

F value

p value

Cohen's d

2 2 2 2 2 1 1 1 1 1

4.22 0.24 0.65 5.75 4.78 12.29 0.18 4.66 15.94 2.42

0.016 0.787 0.526 0.004 0.010 0.001 0.674 0.032 <0.001 0.122

1.124 0.252 0.236 1.313 1.085 1.653 0.200 1.018 1.882 0.733

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