Effect of pregnancy on trunk range of motion when sitting and standing

Abstract

Background.  During pregnancy, apposition of body segments and changes in trunk mobility and motion control due to increased mass and dimensions may reduce the functional range of motion of the trunk segments. Although static postures have been investigated, dynamic trunk motion during pregnancy in sitting and standing has had very limited investigation.

Methods.  Included in the study was a volunteer sample of convenience of nine primiparous and multiparous women. Twelve nulliparous females formed a control group. The subjects were filmed while performing maximum seated and standing trunk forward flexion, side-to-side flexion and seated axial rotation. A repeated measures anova was used to investigate systematic changes in the motion of the pelvic and thoracic segments and the thoracolumbar spines during pregnancy.

Results.  As pregnancy progressed, there was no significant decrease in the range of side-to-side flexion. For forward flexion and axial rotation, motion of the thoracic segment and the thoracolumbar spine was significantly reduced. Movement of the pelvis was less affected. Base of support width was increased for forward flexion and side-to-side flexion.

Conclusions.  In late pregnancy, strategies such as increasing the width of the base of support and reducing obstruction from other body parts were used to minimize the effects of increased trunk mass and girths. Not all trunk segment motion was affected equally. The differing effect on individual trunk segment motion may lead to altered movement patterns during functional tasks.

Abbreviations:

ICC

intra-class correlations

SE

standard error of the mean

SEM

standard error of the measurement

SD

standard deviation.

During pregnancy women report difficulties in everyday tasks (1), which may be related to changes in trunk mobility and motion control due to increased mass and dimensions. Apposition of body segments and also difficulties in controlling increased angular momentum (2) may reduce the functional range of motion of the trunk segments. Increasing the size of the base of support by changing foot placement may also be necessary to maintain balance. Although static standing and seated postures have been investigated in previous studies (3–5), dynamic trunk motion during pregnancy has had very limited investigation (6). In addition, more complex tasks such as rising from a chair may be affected by limited motion of the trunk segments (7). The aim of this study was to investigate the effects of pregnancy on the kinematics of the trunk segments during seated and standing forward flexion, side-to-side flexion and seated axial rotation. The effect of pregnancy on the mediolateral width of the base of support adopted for these tasks was also investigated.

Methods

A volunteer sample of convenience of nine maternal primiparous and multiparous subjects (aged 28–40 years) and 12 nulliparous subjects (aged 21–35 years) were included in the study. The maternal group was tested at 18 weeks or less, 24 weeks, 32 weeks and 38 weeks gestation and again at 8 weeks postbirth (Maternal sessions 1–5). The control group was tested initially then retested after 16 weeks and 32 weeks (Control sessions 1–3). The study was approved by The University of Sydney Ethics Committee (Reference 94/8/20) and the subjects gave their informed consent to participate.

Data were collected using an Expert Vision™ Motion Analysis System (Eva HiRes 5.00, Motion Analysis Corporation, Santa Rosa, California, USA) with eight 8 mm video cameras and a height adjustable chair. Seat height was set to 110% of fibular head-to-floor distance while standing (8). Cameras were mounted on tripods and positioned in a circle around the subject so that each marker was visible to at least two cameras throughout the movement. Tripod height and angle were adjusted to ensure camera views were not convergent. The subjects were clothed in closely fitted underwear and 2 cm diameter non-colinear retro-reflective markers were used to define the thoracic and pelvic segments. The thoracic segment was defined by markers on T₄ and T₈ spinous processes, and the angle of the 8th rib on the left and right. The pelvic segment consisted of markers on the left and right posterior superior iliac spines, and the S₄ spinous process. Markers were also placed on the right and left lateral malleolus. Motion data were processed using Kintrak™ version 5.7 (Motion Analysis Corporation). For the pelvic and thoracic segments, angular motion of each segment in space was calculated using the segment co-ordinate system with respect to the laboratory co-ordinate system. The relative rotation between the thoracic and pelvic segments was defined as the motion of the thoracolumbar spine. The mediolateral width of the base of support was defined by the placement of the feet and was represented by the mean horizontal distance between the right and left malleolus markers.

After practice trials, the subject performed two trials of each seated and standing stylized anatomical trunk movements detailed below at their preferred speed. The starting and finishing position for each movement was an upright posture. The feet remained flat on the floor throughout the movements and the position was self-selected.

Seated and standing forward flexion

For both tasks, the subjects reached down towards the floor in front of their feet as far as possible and then brought their straight arms above their head looking upwards. This movement was then repeated. For standing forward flexion, the knees were maintained in an extended position.

Seated and standing side-to-side flexion

The subjects bent to the side by sliding the right hand down towards the floor as far as possible, letting the head follow the line of the shoulders. They then returned to the upright position, continuing the same movement towards the left side. The cycle of movements was then repeated. Subjects were asked to avoid flexing or extending and to look forwards throughout the movement.

Seated axial rotation

The subjects turned to the right side while looking over the right shoulder as far as possible with the arms crossed over the chest. They then returned to the front, visually contacted a fixed marker, and repeated the rotation to the left side as far as possible returning to the center. The right and left movements were then repeated.

The start and end of forward flexion and side-to-side flexion movements were defined as an angular velocity in the principal direction greater than 6?s for the two-point segment defined by thoracic markers T₄ and T₈. The same criteria were used for seated axial rotation using the two-point segment defined by markers on the right and left angle of the 8th rib.

Data analysis

Variables included the mediolateral width of the base of support, the angular displacement of the thoracic and pelvic segments from the start to the end of the motion, and the thoracolumbar spine range of motion for each movement. The consistency of performance of control data for each variable over the three test sessions was established using intra-class correlations (ICC) (2,1) (9). ICC values were calculated according to the formula proposed by Shrout and Fleiss (10), and interpreted as: poor (0 < 0.39), fair (0.4 − < 0.59), good (0.6–0.75) and excellent (> 0.75 11). The variability associated with retesting was investigated using the standard error of measurement (12) over 32 weeks between control sessions 1 and 3 for each variable. Repeated measures anova (13) with planned orthogonal contrasts were used to investigate the existence of linear and quadratic trends, which would show any systematic change within the control group over the three test sessions for each variable. If the trend was not significant or the magnitude of the change was smaller than the natural variation, then each of the three test sessions was assumed to be similar. Control data over the three test sessions were then pooled to create an average of the means (control mean) and average of the standard error of the mean (control SE).

Pregnancy is characterized by continuous changes over time, which may be expected to show systematic trends as the pregnancy progressed and be greater than the variability associated with retesting. A repeated measures ANOVA was used to investigate the existence of linear and quadratic trends which would show any systematic change within the maternal group over the four test sessions during pregnancy for each variable. For each of the four test sessions where an effect of pregnancy occurred, the mean and standard error of the mean (SE) were calculated and the maternal test sessions inspected graphically with the control mean and control SE. If there was no overlap between the maternal test session mean ± 2 SE and the control mean ± 2 SE, then a two-tailed Student's t-test assuming unequal variances was used to confirm that a significant difference had occurred for the point. Two-tailed Student's t-tests assuming unequal variance (9) were used to compare the maternal postbirth (session 5) with the control session 3.

Initial statistical analysis of side-to-side flexion and axial rotation was performed on the total motion from the right to the left. In order to investigate the symmetry of the movements, the total motion was then divided into motion to the right and left. Initial quiet posture defined the central neutral position. For each session, a repeated measure anova simple contrast was used to test for significant differences between sides. Linear and quadratic contrasts were used to test for the effect of practice within each side.

The amplitude of the lumbar spine movement may correlate with the velocity of the trunk movements (2). As the subjects self-selected their movement speed, it was possible that any changes in amplitude may have been due to differences in velocity. Therefore the correlations between amplitude and time of the movement for the thoracolumbar spine were analyzed using a Pearson product moment correlation coefficient (9).

Missing data for two maternal subjects occurred due to late entry for one subject with her first test at 24 weeks gestation and early delivery at 38 weeks gestation for another subject. Linear interpolation on the remaining three pregnancy test session data points was used to predict missing data. These data represented 5.6% of the total maternal data set for each variable.

Results

In early pregnancy the thoracic and pelvic segment displacements and the thoracolumbar spine range of motion were similar to the control subjects. The effect of pregnancy on trunk motion varied as pregnancy progressed.

Trunk forward flexion

During seated forward flexion of the trunk there were significant decreasing trends as pregnancy progressed for the thoracic (F_linear= 21.16, p = 0.002) and pelvic (F_linear= 11.02, p = 0.01) segment displacement and thoracolumbar (F_linear= 9.08, p = 0.02) spine range of motion. The maternal group thoracic segment displacement was significantly less than the control mean at sessions 3 and 4 (Fig. 1a) and the pelvic segment displacement was significantly less than the control mean at session 4 (Fig. 1b). The range of motion of the maternal group thoracolumbar spine was not significantly different from the control mean at any session (Fig. 1c). The mediolateral width of the base of support during seated forward flexion showed a significant trend (F_linear= 7.60, p = 0.02) for increase. The width was significantly greater in the maternal group compared with the control mean at sessions 3 and 4 (Fig. 2).

Figure 1

Seated forward flexion (mean ± 2 SE) for maternal sessions 1–4 and postbirth (session 5) and the control group mean ± 2 SE: (a) thoracic segment displacement, (b) pelvic segment displacement, (c) thoracolumbar spine range of motion.

Figure 2

Base of support mediolateral width during forward flexion (mean ± 2 SE) for maternal sessions 1–4 and postbirth (session 5) and the control group mean ± 2 SE: (a) seated, (b) standing.

Forward flexion of the trunk in standing displayed a significant decreasing trend as pregnancy progressed for the thoracic segment displacement (F_linear= 13.36, p= 0.006) and thoracolumbar spine range of motion (F_linear= 9.97, p= 0.01). There was no significant effect of pregnancy on pelvic segment displacement (F_linear= 2.99, p = 0.12). The thoracic segment displacement and thoracolumbar spine range of motion were significantly less than the control mean at session 4 as shown in Fig. 3. The width of the base of support during standing forward trunk flexion showed a significant increasing trend (F_linear= 7.36, p = 0.03) as pregnancy progressed. The maternal group base of support was significantly larger than the control mean at session 4 (Fig. 2). Maternal subjects showed little to low correlation (−0.020 ≤r≥−0.481) between thoracolumbar spine forward flexion range of motion and movement time for both seated and standing postures and this did not change as pregnancy progressed.

Figure 3

Standing forward flexion (mean ± 2 SE) for maternal sessions 1–4 and postbirth (session 5) and the control group mean ± 2 SE: (a) thoracic segment displacement, (b) pelvic segment displacement, (c) thoracolumbar spine range of motion.

Postbirth, seated and standing forward flexion showed no significant difference between the maternal group and control group session 3 (0.05 < p > 0.24) (1, 3). There was also no significant difference in the base of support width in either seated (p= 0.51) or standing (p= 0.90) forward flexion.

Trunk side-to-side flexion

As pregnancy progressed there were no significant trends for thoracic or pelvic segment displacement or for the thoracolumbar spine range of motion during seated and standing side-to-side flexion (Table I). The width of the base of support showed a significant trend for increase as pregnancy progressed during seated (F_linear= 5.51, p = 0.046) and standing (F_linear= 25.10, p = 0.001) side-to-side flexion. The base of support widths for side-to-side flexion were significantly greater than the control mean at sessions 3 and 4 for seated and session 4 for standing motion (Fig. 4).

Table I. Maternal subjects' mean (SD) thoracic and pelvic segment displacement (°) and thoracolumbar spine range of motion (°) during seated and standing side-to-side flexion for sessions 1–4 and postbirth

Segment	Session 1	Session 2	Session 3	Session 4	Flinear	Fquad.	Postbirth
Seated
Thorax	86.5 (12.7)	82.0 (12.1)	84.5 (9.5)	80.0 (12.4)	0.84	0.28	80.5 (12.3)
Pelvis	15.5 (8.7)	12.5 (7.7°)	15.0 (5.5)	13.5 (5.8)	0.22	0.57	13.5 (7.9)
Thoracolumbar	72.0 (9.3)	70.5 (8.5°)	70.5 (7.8)	68.0 (8.4)	1.03	0.07	69.0 (10.3)
Standing
Thorax	91.0 (9.2)	89.5 (8.8)	87.0 (7.5)	86.5 (13.7)	1.72	0.05	84.5 (10.5)
Pelvis	13.5 (5.4)	13.5 (4.7)	14.5 (4.7)	15.0 (6.3)	2.29	0.19	12.0 (5.5)
Thoracolumbar	78.5 (9.1)	77.5 (8.9)	73.5 (7.7)	72.5 (12.9)	3.87	0.02	74.0 (10.1)

• F_(1,8) critical = 5.32

Figure 4

Base of support mediolateral width during side-to-side flexion (mean ± 2 SE) for maternal sessions 1–4 and postbirth (session 5) and the control group mean ± 2 SE: (a) seated, (b) standing.

Postbirth, there was no significant difference between the groups for side-to-side motion (0.34 < p > 0.68). There was also no significant difference between the groups for base of support width during either seated (p= 0.41) or standing (p= 0.86) side-to-side flexion.

When left and right side thoracic segment and thoracolumbar spine motions were compared, there were generally no significant differences between the sides for either seated or standing side flexion. Therefore there was no significant effect of pregnancy on the right to left side symmetry of trunk side flexion. Pelvic segment individual right and left side motion were not investigated as the range of movement of the pelvic segment was found to be relatively small and it was considered unreasonable to further divide the motion into right and left components. For the maternal subjects' sessions 1 to 3 there was generally no effect of fatigue or practice as the trials progressed within the session. At session 4 in both seated and standing postures there were significant linear orthogonal contrasts for increase of thoracic segment [seated F_(1,8)= 7.68, p = 0.02; standing F_(1,8)= 10.54, p = 0.01) and thoracolumbar spine segment (seated F_(1,8)= 5.57, p = 0.045; standing F_(1,8)= 9.48, p = 0.02) side motion.

For the maternal subjects, little to low level correlation (0.031 ≤r≥ 0.475) with varying positive and negative sign was seen between thoracolumbar spine range of motion and movement time for both seated and standing side flexion. The relationship between right to left range of motion and movement time did not change as pregnancy progressed.

Trunk axial rotation

As pregnancy progressed there were significant decreasing trends for the thoracic segment displacement (F_linear= 9.03, p = 0.02), pelvic segment displacement (F_quadratic= 11.4, p = 0.009) and thoracolumbar spine range of motion (F_linear= 5.73, p = 0.04). For the thoracic and pelvic segment displacements, the differences between the means at sessions 1 and 4 were less than the variability associated with retesting. The pelvic segment displacement was not significantly different to the control mean at any session (Fig. 5). The thoracic segment displacement and the thoracolumbar spine range of motion were significantly different to the control mean at sessions 3 and 4 (Fig. 5). The base of support width was not significantly affected as pregnancy progressed (F_linear= 3.18, p = 0.11).

Figure 5

Seated axial rotation (mean ± 2 SE) for maternal sessions 1–4 and postbirth (session 5) and the control group mean ± 2 SE: (a) thoracic segment displacement, (b) pelvic segment displacement, (c) thoracolumbar spine range of motion.

Postbirth, there were no significant differences in motion between the groups (0.30 < p > 0.76) (Fig. 5). There was also no significant difference (p= 0.33) for the width of the base of support between the groups. Maternal subjects showed no correlation (0.001 ≤r≥ 0.263) between thoracolumbar spine seated axial rotation range of motion and movement time for both right and left motions, which did not change as pregnancy progressed.

Thoracic segment axial rotation was asymmetric with motion to the right being consistently larger. For the pelvic segment and thoracolumbar spine the motion was symmetrical. The intrasubject variation for all segments, as indicated by the standard deviation, was relatively high. As the trials progressed within each of session 1 to session 3 there was no effect of practice or fatigue. Within session 4, however, there was a significant linear orthogonal contrast (F_linear= 9.36, p= 0.02) for increase in the thoracolumbar spine side motion.

Variability of trunk motion with retesting

The control subjects' descriptive data and the variability associated with retesting for trunk forward flexion, side-to-side flexion and axial rotation are listed in Table II. There were no significant trends over the three sessions for the segment displacements and thoracolumbar spine range of motion where the difference between the means was greater than the variability associated with retesting. The consistency of performance, as shown by the ICC(2,1), for the thoracolumbar spine was good to excellent for all movements. For the thoracic segment, the consistency of performance was generally fair to good while for the pelvic segment the consistency was generally poor to fair. For all movements, the variability from test to test, as indicated by the standard error of the mean (SEM), was a small proportion of the mean for the thorax segment and thoracolumbar spine. For the pelvic segment, however, the variability was larger (Table II).

Table II. Control subjects' mean (SD) thoracic and pelvic segment displacement (°) and thoracolumbar spine range of motion (°) during seated and standing forward flexion, seated and standing side-to-side flexion and seated axial rotation

Segment	Session 1	Session 2	Session 3	Flinear	Fquad.	SEM	ICC(2,1)
Seated forward flexion
Thorax	105.0 (7.9)	104.0 (6.7)	100.0 (7.0)	8.12*^†	0.60	± 4.3	0.594
Pelvis	39.0 (7.5)	35.0 (8.2)	33.0 (7.9)	4.22	0.34	± 6.6	0.482
Thoracolumbar	69.0 (12.7)	71.5 (11.0)	69.0 (10.1)	0.00	2.92	± 6.3	0.745
Standing forward flexion
Thorax	136.5 (10.2)	139.0 (7.9)	136.0 (7.9)	0.04	2.69	± 6.6	0.554
Pelvis	53.5 (7.9)	51.0 (7.9)	51.5 (7.9)	0.78	1.43	± 5.1	0.583
Thoracolumbar	88.5 (10.8)	94.0 (11.9)	88.5 (12.3)	0.01	10.94*^†	± 6.1	0.743
Seated side-to-side flexion
Thorax	76.5 (12.5)	78.5 (9.1)	76.0 (7.3)	0.04	1.85	± 6.6	0.584
Pelvis	11.0 (4.9)	12.0 (5.3)	11.0 (5.3)	0.00	0.86	± 3.2	0.663
Thoracolumbar	66.5 (10.1)	68.0 (9.2)	66.0 (7.4)	0.10	1.89	± 4.7	0.759
Standing side-to-side flexion
Thorax	85.5 (13.5)	88.5 (11.7)	87.5 (12.5)	1.07	1.38	± 4.5	0.853
Pelvis	11.0 (3.1)	13.0 (2.5)	13.5 (2.7)	4.04	1.76	± 3.1	0.118
Thoracolumbar	75.0 (13.3)	77.0 (11.9)	76.5 (11.6)	0.01	0.33	± 3.8	0.893
Seated axial rotation
Thorax	86.0 (17.1)	86.0 (17.3)	88.5 (16.6)	0.35	0.20	± 11.5	0.687
Pelvis	21.0 (9.0)	21.0 (5.4)	21.0 (6.3)	0.00	0.01	± 7.0	0.396
Thoracolumbar	66.5 (13.6)	66.5 (13.9)	69.0 (13.1)	0.61	0.35	± 7.9	0.731

• SEM: standard error of the measurement.
• F_(1,11) critical = 4.84.
• * Significant at 0.05.
• ^† Difference between means within variability indicated by the SEM.

Discussion

Effect of pregnancy on stylized trunk anatomical movements

As pregnancy progressed the forward flexion motion of the trunk was restricted, although not all trunk segments and postures were equally affected. The effect of pregnancy on trunk forward flexion was greater and seen earlier in the pregnancy for the seated posture versus the standing posture. It may be expected that greater and earlier restriction of seated movement would occur in comparison to standing as the pregnant abdomen is in very close proximity to the thighs in the seated position and compensation by the hip joints was restricted by the chair. Standing forward flexion may have been difficult to perform in late pregnancy as forward stability may be decreased. The threat of falling forward (1) may have made the subjects more cautious and less willing to move to the physical end of range in late pregnancy.

It may have been expected that as pregnancy progressed and the trunk dimensions increased that the forward movement of the trunk would be obstructed by apposition of the anterior lower trunk on the thighs. As pregnancy progressed, however, the width of foot placement increased significantly indicating that the distance between the thighs increased as pregnancy progressed. A wider lower limb position may have minimized the obstruction of the pelvis motion during forward flexion until the end stages of pregnancy. Forward flexion of the upper trunk, however, is partly achieved by reducing the distance between the thoracic cage and the pelvis. As pregnancy progresses the gravid uterus provides an increasing physical obstruction to the motion of the thoracic segment, which may have reduced thoracic segment motion.

The effect of decreased forward flexion due to physical obstruction, as seen in pregnancy, on the inter relationship between trunk segment motions during functional tasks is unknown. People with low back pain have been shown to compensate for limited lumbar motion by increasing the contribution of the thoracic spine during forward flexion (14). The effect of pregnancy on maximum forward flexion differed for each segment and posture. Therefore although the overall reduction in flexion may not greatly affect some functional tasks such as rising to stand from a chair, differing effects on individual segments may lead to altered movement patterns as pregnancy progresses. Altered movement patterns may also affect the musculoskeletal demands on the segment.

Postbirth, the segment displacements and the thoracolumbar spine range of motion during seated and standing forward flexion were similar to the control group. The width of the foot placement also was similar. Dumas et al. (6) also reported a return to prepregnancy values for pelvis segment displacement during standing forward flexion. Six weeks postbirth values for lumbar spine range of motion were similar to prepregnant values; however, the range of motion at 16 weeks postbirth was significantly increased (6). Unfortunately Dumas and colleagues did not report the effect of repeated testing over a similar time frame and it was possible that the postbirth increase was within retest variability.

There was no effect of pregnancy on the range of motion of the trunk segments during seated and standing side-to-side flexion; however, the width of the base of support significantly increased and there was a practice effect in late pregnancy. Dumas et al. (6) also reported no significant differences as pregnancy progressed in lumbar spine range of motion for standing side flexion. It may be expected that the increased trunk circumferences might have little effect on the side-to-side trunk motion as the increases were more in the anterior rather than lateral direction. The increased mass, however, was likely to have caused decreased stability when moving in the lateral direction, which was compensated for by increasing the width of the base of support.

The segment displacements and thoracolumbar spine range of motion was generally symmetrical to the right and left during pregnancy. Right to left symmetry of motion for lumbar spine and the thoracic segment for both healthy and low back pain subjects have also been previously reported (14,15). Symmetry of side flexion motion may be expected to continue during pregnancy as the changes in mass and dimensions are also generally symmetrical in the frontal plane.

As pregnancy progressed the axial rotation motion of the trunk was restricted, although not all trunk segments were similarly affected. During seated axial rotation, the ischial tuberosities and hence the pelvis would be restricted from rotating by contact with the seat and pregnancy would not be expected to alter the contact. Therefore no change in pelvic segment displacement as pregnancy progressed would be expected. For the thoracic segment and thoracolumbar spine, it may have been possible that as pregnancy progressed and the trunk dimensions increased, the apposition of the lateral lower abdomen and the ipsilateral thigh increased as pregnancy progressed. The movement restriction was not thought to be related to stability, as the width of foot placement did not change.

The overall restriction of seated axial rotation motion may make tasks involving this motion more difficult to perform. In addition the principal reduction of motion was seen in the thoracic segment. Therefore as pregnancy progresses, the change in demands on the musculoskeletal system of the trunk would not be consistent throughout.

Axial rotation symmetry of motion also differed between the segments. The pelvic segment displacement was symmetrical to the right and left. As the movement of the pelvis is restricted by contact with the seat, the ability to move more to one side than the other was also restricted, therefore symmetry may be expected. The thoracic segment displacement was asymmetrical and the pattern was not affected by pregnancy. The pattern may represent the norm for these particular subjects. Hindle et al. (15) noted that there were intersubject differences for symmetry of movement although when results for all subjects were pooled symmetry was seen.

As pregnancy progresses and stability decreases, an increased movement time relative to the range of motion may have been a possible strategy to improve stability or an increased trunk mass may have reduced trunk acceleration. No correlation for movement time versus range of motion was noted in early pregnancy and this relationship was unchanged as pregnancy progressed. Stability may have been increased by the combined or individual use of self-selected velocities, wider base of support or decreased range of forward motion. Therefore no additional strategy such as reduced velocity was needed.

For some variables a significant trend as pregnancy progressed indicated a consistent pattern of change. The differences between the means in early pregnancy at session 1 and late pregnancy at session 4, however, were smaller than the variability associated with retesting. The maternal group data at each session also may not have been significantly different to the control mean. Thus for some variables there was no definite difference between the maternal and control groups although a significant trend existed. However, as there were low subject numbers and the variability associated with retesting was large for some variables there was also a possibility of a Type II error (9). All variables with a significant linear trend as pregnancy progressed were concluded to have been affected by pregnancy. Care, however, must be taken with conclusions drawn from variables where the magnitude of the change over pregnancy was less than the variability associated with retesting or there was no significant difference between the maternal group at any session and the control mean.

Stability of variables associated with retesting

The trunk segment motions were found to be stable with retesting over the time periods used for investigating the effects of pregnancy. A slightly larger variability was seen from session to session in comparison to previous reports (16–19). As range of motion is affected by habitual activity, any retest time period that allows the possibility of changes in habitual activity would necessarily have greater variation.

Differences in consistency of the data with retesting were also seen between the segments tested. The high variability in pelvic segment displacement from session to session may reflect the many degrees of freedom in the overall task. There may be a range of options in the pelvic segment motion or redundancy may have occurred.

Conclusions

The maternal subjects were similar to the control subjects in early pregnancy and at 8 weeks postbirth. In late pregnancy, the maternal subjects used strategies, such as increasing the width of the base of support and reducing obstruction to movements from other body parts, to attempt to minimize the effects of anatomical changes due to pregnancy. For seated and standing side-to-side flexion, the strategies were successful and no significant decreases in range of motion were seen. For seated and standing forward flexion and seated axial rotation, the motion of both the thoracic segment and the thoracolumbar spine were significantly reduced. Movement of the pelvis was less affected due to the partial success of minimizing obstruction from the thighs by placing the feet further apart during seated and standing forward flexion. The differing effect on individual trunk segment motion may lead to altered movement patterns during functional tasks.

Acknowledgments

The authors wish to acknowledge the assistance of Ray Patton, School of Exercise and Sport Science, University of Sydney for technical assistance and Dr Roger Adams, School of Physiotherapy, University of Sydney for advice concerning statistical analysis.