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Balance (perceived and actual) and preferred stance width during pregnancy
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Clinical Biomechanics 23 (2008) 468–476 www.elsevier.com/locate/clinbiomech
Balance (perceived and actual) and preferred stance width during pregnancy John Jang a, Katherine T. Hsiao b, Elizabeth T. Hsiao-Wecksler a,* a
Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, 124 Mechanical Engineering Building, MC-244, 1206 West Green Street, Urbana, IL 61801, USA b Department of Obstetrics and Gynecology, California Pacific Medical Center, San Francisco, CA, USA Received 1 August 2007; accepted 19 November 2007
Abstract Background. Pregnant women often remark that their balance degrades during pregnancy; however, it appears that no studies have documented the gravida’s perception of her balance nor measured direction-specific changes in balance throughout pregnancy or after delivery. Methods. Thirty women, fifteen pregnant and fifteen non-pregnant controls, were tested monthly and through 6-month postpartum. For each session, perceived degradation in sense of balance, laboratory-based balance measures, stance width, and the number of falls since the previous session were recorded. Laboratory-based balance measures, quantified by direction-specific measures of postural sway, were computed from ten 30 s quiet-standing trials on a stationary force platform. Repeated-measures analysis of variance, paired t-tests, and Pearson correlations were use to examine group and time effects. Findings. For the pregnant group, perceived balance degradation and stance width were highly correlated (r = 0.94). Both increased during pregnancy (P 6 0:016) and dropped to near-control levels after delivery (P 6 0:004). Compared to the control group, pregnant subjects displayed increased sway, especially in the anterior–posterior and radial directions (P 6 0:039). Anterior–posterior sway measures strongly correlated with perceived balance (0.82 > r > 0.72) and also decreased significantly between the third trimester and postpartum (P 6 0:029). Interestingly, medial–lateral balance measures varied little during pregnancy, but increased after delivery. Contrary to recent work suggesting fall rates of 25%, only 13% of our subjects (n = 2) fell during pregnancy. Interpretation. Perceived degradation in balance during pregnancy was strongly related to increasing postural sway instability in the anterior–posterior direction. Lateral stability was maintained during pregnancy and likely accomplished by increasing stance width. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Balance; Postural control; Stance width; Pregnancy
1. Introduction Physiological changes throughout pregnancy may result in unique modifications in a gravida’s biomechanics that can alter both perceived and actual changes in balance and postural control. During pregnancy, a woman will gain between 25 and 35 lbs (11–16 kg) (Gabbe et al., 2002). This weight gain is unique because it is primarily localized to the
*
Corresponding author. E-mail address: [email protected] (E.T. Hsiao-Wecksler).
0268-0033/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.clinbiomech.2007.11.011
anterior abdominal region, which results in a change in the position of the body’s center of mass (Fries and Hellebrandt, 1946). Furthermore, it has been suggested that the hormones that permit joint laxity for the development of the fetus may also play a role in global joint laxity (e.g., Marnach et al., 2003). These factors may contribute toward the perception of increasing postural instability throughout pregnancy. Although numerous studies have examined how a woman’s physical posture changes during pregnancy (e.g., Moore et al., 1990; Bullock et al., 1987; Dumas et al., 1995; Gilleard et al., 2002; Franklin and Conner-Kerr,
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The current study was a prospective case–control study of 30 women that examined postural sway parameters, which included direction-specific measures. These parameters have been found to be effective at assessing balance performance (Prieto et al., 1996; Rocchi et al., 2004) and interpreting the behavior of the postural control system (Collins and De Luca, 1993). Additionally, monthly measurements of perceived sense of balance, preferred stance width, and fall incidences were collected. The primary goal of this study was to track balance and stance width in pregnant women throughout pregnancy and the postpartum period. As a secondary aim, we also tracked monthly incidences of falls. 2. Methods 2.1. Participants
PG
The pregnancy group (PG) consisted of 15 gravida; start of testing mean (and standard deviation) age of 31 (4) yrs, range 25–38 yrs, height 168 (7) cm, mass 77.4 (21.1) kg, body mass index (BMI) 27.4 (6.8) kg/m2; five primigravida, all singleton gestations. PG subjects were included in the study if enrolled by the 16th week of gestation. Fifteen
CG
1998), few studies have examined how balance, and postural control that moderates balance, may actually vary due to pregnancy (Butler et al., 2006; Davies et al., 2002; Fries and Hellebrandt, 1946; Pickering et al., 1999). Pickering et al. (1999) and Davies et al. (2002) examined differences in balance; however, their studies only examined women while in labor and focused on the effect of low-dose analgesia (spinal-epidural). Fries and Hellebrandt (1946) presented a prospective case study for a single individual over nine sessions (third month of pregnancy to 6 weeks postpartum). They recorded postural sway using a kymograph (an instrument that recorded movement using a stylus and rotating drum) and qualitatively noted increased sway throughout pregnancy that persisted throughout puerperium. Recently, Butler et al. (2006) performed a prospective case–control study with 12 gravida, tested over four sessions (11–14 weeks gestation through 6–8 weeks postpartum). They reported that, compared to non-pregnant control subjects, postural sway increased throughout the assessment period. That study used measures of gross, planar sway (path length and average radial displacement). It is not known, however, whether direction-specific (anterior–posterior (AP) or medial–lateral (ML)) changes in balance and postural control may occur with pregnancy, especially given the extreme AP changes in body posture. Minimal research has been published on how pregnancy may change the gravida’s choice in base of support, which is defined by the positioning of the feet (Bird et al., 1999; Foti et al., 2000; Lymbery and Gilleard, 2005). Bird et al. (1999) found that step width while walking increased during pregnancy. This change in gait function was suggested to be a compensatory mechanism to improve locomotor stability. Assessments of quiet standing, with healthy non-pregnant subjects, have shown that balance is affected by stance width, i.e., increasing stance width decreases ML postural sway (Day et al., 1993; Kirby et al., 1987). It is not known, however, whether stance width during upright standing changes throughout pregnancy and how such changes may affect a pregnant woman’s balance. Injurious falls account for 17–39% of maternal trauma cases needing medical attention (Dunning et al., 2003). Serious falls after a loss of balance can result in maternal and/or fetal complications including 3–7% of fetal deaths (Connolly et al., 1997; Weiss et al., 2001). Thus, determining fall rates among pregnant women is important. Recent studies suggest that a rather high percentage of pregnant women may experience a fall. Butler et al. found that 25% of a subset of their pregnant subjects fell during pregnancy, while no control subjects reported falling during the 12 months before testing (Butler et al., 2006). Similar fall rates were reported for a large cohort of women employed during pregnancy, i.e., 26.6% experienced a fall (Dunning et al., 2003). Both studies collected retrospective data six or more weeks postpartum, requiring subjects to recall fall incidences during pregnancy. Tracking fall incidences prospectively during pregnancy may provide additional insight into this seemingly high fall rate.
469
subject 0 P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 P11 P12 P13 P14 P15 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15
4
Week 8 12 16 20 24 28 32 36 40 46 52 64
Fig. 1. Pregnant (PG) and control (CG) subject enrollment and attendance record (gray: absent session). Eight PG subjects were absent for week 40 testing due to delivery.
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non-pregnant women matched by age, within 1 yr, were recruited as a control group (CG; mean age 31 (4) yrs, range 24–39 yrs, height 166 (6) cm, mass 69.7 (12.7) kg, and BMI 25.4 (4.2) kg/m2). Since this study was a longitudinal, repeated-measures design, no additional inclusion/ exclusion criteria were identified for either group. Subjects were recruited from university personnel and the local community. Informed consent was given by all subjects and the study was approved by the University of Illinois Institutional Review Board. Each pregnant subject was tested at 4 week intervals until delivery. Three follow-up sessions after delivery were scheduled at 6 weeks, 12 weeks, and 6-month postpartum. To match PG testing, CG subjects were tested at 4-week intervals for 40 weeks, and then 6 weeks, 12 weeks, and 6 months after the 40th week. For data tracking, these last three sessions were defined as weeks 46, 52, and 64, respectively. Three PG and two CG subjects withdrew before the completion of the study due to disinterest or relocation (Fig. 1). 3. Experimental procedure At the beginning of each session, the subject completed a self-evaluation questionnaire. To quantify perceived sense of balance (SB), we developed our own scale which asked the subject ‘‘How would you rank your balance from 0 (normal) to 10 (extremely unstable)”. The previous session’s SB score was provided as reference. To track fall incidences, the subject recorded whether she had fallen (‘‘impacted the ground with any part of the body”) since the previous session. Based on a written description of the fall, the fall event was later classified by a researcher (ETHW) into one of five categories: trip or slip while walking or running on level ground in everyday activities (Trip, Slip), fall during stair ascent or descent (SA, SD), or fall during athletic or sporting activity (Ath). Injury severity was also classified as no injury, minor injury (required home treatment), and major injury (required medical attention). Postural sway and stance width were then recorded. The subject stood unshod with both feet on a large force plate (model BP600900, AMTI, Watertown, MA). The force plate was embedded in a surrounding walkway and sampled at 100 Hz. (Force plate data were filtered using a 10th order forward–backward low pass Butterworth filter with 7 Hz cut-off frequency.) The subject was instructed to stand quietly with arms at her side and eyes open, looking at a stationary picture placed at eye level and 3 m away. She was instructed to place the tips of her toes directly behind a marked line on the force plate, but could otherwise select her preferred stance width for each trial. During short rest breaks between each trial, the subject stepped off and on the force plate. Ten 30 s trials were conducted for each session. After the completion of the 10th trial, the distance between heel centers was measured to determine preferred stance width (SW).
4. Balance measures From force plate data, the movement of the center of pressure under the feet (CoP) was computed. Planar representation of CoP fluctuations is often called a stabilogram. A number of measures can be obtained from these stabilograms. Traditionally, general variability of the CoP is assessed using summary statistical parameters, such as mean sway velocity and standard deviation of the displacement about the mean (Prieto et al., 1996). A newer analysis technique, referred to as stabilogram diffusion analysis (SDA), produces a set of measures that provide insight into the (open- and closed-loop) postural control system during upright stance (Collins and De Luca, 1993). For this study, balance measures were based on the average over ten trials. Reliability of these traditional and SDA measures have been found to be fair to excellent, ranging from 0.56 to 0.95 (Doyle et al., 2007a, b). 4.1. Traditional parameters Based on recommendations from Rocchi et al. (2004), a set of nine traditional summary statistic CoP parameters was examined. These included the standard deviation of the displacement about the mean (Stdev) in the AP, ML, and combined radial (RAD) directions; mean sway velocity (Vel) in the three directions; 95% power frequency (95Freq) in the AP and RAD directions; and angular deviation (AngDev) of the principal sway direction from the AP axis. 4.2. Stabilogram diffusion analysis parameters SDA models fluctuations of the CoP as a system of coupled, correlated random walks and provides descriptors of the CoP movement within the context of a stochastic process. This process is clinically advantageous because it provides insight into the ‘‘steady state behavior and functional interaction of the neuromuscular system” (Collins and De Luca, 1993). SDA has been used to study, for example, individuals with Parkinson’s disease and profound vestibular loss (Mitchell et al., 1995; Dozza et al., 2005). It has been shown that SDA may be more sensitive than traditional parameters at detecting subtle age-related differences in postural control (Collins et al., 1995). The SDA procedure suggests the existence of two regions of behavior; a short-term region (typically below 1–2 s) and a long-term region of larger time intervals. The short- and long-term time intervals have been interpreted as representing the behavior of the postural control system during periods of open-loop and closed-loop control, respectively (Collins and De Luca, 1993). SDA parameters reported in this study are short-term and long-term diffusion coefficients (DS, DL) and scaling exponents (HS, HL) in the AP, ML, and RAD directions. Diffusion coefficients reflect the level of stochastic activity of the CoP (Collins and De Luca, 1993). Larger coefficients imply postural
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instability. Scaling exponents indicate the tendency of the CoP to drift away from a relative equilibrium point.
Table 1 P-values for effect of group (PG versus CG) and time (weeks 16–64, excluding week 40) on each parameter using mixed models repeatedmeasures ANOVA
5. Statistical analysis
Measure
Week
Sense of balance Stance width Traditional parameters StdevAP StdevML StdevRAD VelAP VelML VelRAD 95FreqAP 95FreqRAD AngDev SDA parameters DS,AP DL,AP DS,ML DL,ML DS,RAD DL,RAD HS,AP HL,AP HS,ML HL,ML HS,RAD HL,RAD
<0.001 0.028
0.012 0.21
<0.001 <0.001
0.17 0.018 0.43 0.039 0.005 0.034 0.53 0.62 0.005
<0.001 0.17 <0.001 0.18 0.91 0.27 0.14 0.13 0.88
0.009 0.10 0.016 0.006 0.36 0.11 0.37 0.47 0.26
0.15 0.95 0.24 0.005 0.34 0.95 0.21 0.85 0.16 0.08 0.46 0.62
0.027 0.001 0.55 0.018 0.07 <0.001 0.23 0.039 0.15 0.08 0.18 0.019
0.014 0.84 0.86 0.013 0.32 0.52 0.41 0.37 0.32 0.11 0.54 0.35
Statistical analyses were based on antepartum testing weeks 16–36 and postpartum testing weeks 46–64. These data-analysis limits were necessary since nine PG subjects enrolled in the study at 16 weeks gestation and eight PG subjects did not attend week 40 testing due to delivery (Fig. 1). Statistical analyses were performed on measures recorded for SB, SW, and balance (traditional and SDA parameters). To determine whether group (PG versus CG) and/or time effects (over weeks 16–64 excluding 40) were significant for a given measure, mixed models repeated-measures analysis of variance (RM ANOVA) was employed. A number of additional analyses were conducted only on the PG data to understand the effect of pregnancy. One-way RM ANOVA tests were conducted for the antepartum weeks in the second through third trimesters (weeks 16–36). Paired t-tests examined before and after delivery changes, i.e., third trimester versus postpartum comparison using the averages for weeks 28–36 versus 46–64. Pearson correlations tested whether average values for SW and SB were correlated with each other or any balance measures. Significant ANOVAs were analyzed by LSD post-hoc comparisons. The level of significance was set to a = 0.05 (Perneger, 1998; Rothman, 1990). Statistical analyses were run on SPSS (SPSS Inc., Chicago, IL; v13). SB scores were found to be non-normally distributed. The normality assumption for the mixed models RM ANOVA residuals for SB, however, was satisfied. Nonparametric Friedman test and Wilcoxon Signed Ranks test were used instead of the one-way RM ANOVA and paired t-test, respectively, for the SB score analyses within the pregnant group.
Week Group
Group
6.1. Perceived sense of balance
6.2. Laboratory-based balance measures
Sense of balance scores (SB) indicated that pregnant subjects perceived that their balance degraded during preg-
Postural sway in the AP and radial directions appeared to increase during pregnancy and then decrease after
6
25
5
23
Stance Width (cm)
Balance (0-stable,10-unstable)
6. Results
nancy and improved substantially by 6 weeks postpartum (Table 1 and Fig. 2). SB scores were found to differ significantly between PG and CG starting from week 20 to delivery (P < 0.001). For pregnant subjects, average SB score increased from 1.7 (Stdev of 0.7) to 3.9 (0.6) during the antepartum analysis period (P = 0.004, Friedman). The SB score increased further to 4.7 (0.5) for the remaining seven PG subjects tested at week 40. SB then dropped after delivery with an average score of 1.4 (0.2) over the 6-month postpartum period (P = 0.008, Wilcoxon). For CG, average SB was 0.7 (0.1) over the 64 weeks of testing.
4 3 2
Controls Pregnant
21 19 17
1 15 0 16 20 24 28 32 36 40
Week
46
52
64
16 20 24 28 32 36 40
46
52
Week
Fig. 2. Average values for perceived sense of balance and preferred stance width. Bars = standard error.
64
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delivery; whereas ML sway remained relatively consistent during pregnancy, but tended to increase during the postpartum period (Table 1, Fig. 3). Compared to control subjects, pregnant subjects had elevated values for a number of measures, especially in the AP and RAD directions (P 6 0:039; Table 1). Group week interaction effects also suggest group differences over time. For example, post-hoc
tests found that StdevAP and StdevRAD differed between groups for all analyzed weeks and DL,AP and DL,RAD differed for all but week 20; whereas, DS,AP differed through the third trimester and 6 weeks postpartum. Although average values for the AP and RAD measures increased for the PG during pregnancy (Fig. 3), large variability resulted in no statistically significant differences in these
12
8
10
StDev (mm)
Vel (mm/s)
6 8 6 4
4
2 2 0
0
27 2.2
95Freq (Hz)
17 12
1.7 1.2
7
0.7
2
0.2
20 18 16 14 12 10 8 6 4 2 0
0.88 0.86
HS (mm 2/s)
DS (mm 2/s)
AngDev (deg)
22
0.84 0.82 0.8 0.78 AP-C ML-C
3.5 0.38
3
HL (mm 2 /s)
2.5
DL (mm 2 /s)
RAD-C AP-P
0.33
2 1.5 1 0.5
ML-P
0.28
RAD-P
0.23 0.18 0.13 0.08
0
0.03 16 20 24 28 32 36 40
Week
46
52
64
16 20 24 28 32 36 40
46
52
64
Week
Fig. 3. Average balance measures for pregnant (P) and control (C) subjects. Traditional measures: mean sway velocity (Vel), standard deviation about the mean (Stdev), angular deviation from the AP axis (AngDev), and 95% power frequency (95Freq). Stabiliogram diffusion analysis (SDA) measures: diffusion coefficients (D) and scaling exponents (H) in the short (S) and long (L) term regions, for anterior–posterior (AP), medial–lateral (ML) and radial (RAD) directions. Bars = standard error.
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measures over the second through third trimesters (P > 0.05, one-way RM ANOVA). Comparisons of average values before and after delivery, however, suggest reductions in StdevAP, VelAP, and DS,AP from the third trimester to postpartum (0:010 6 P 6 0:029, paired t-test). Interestingly, ML measures appeared to increase only during the postpartum period. VelML increased from the third trimester to postpartum (P = 0.027), while other ML parameters also displayed non-statistically significant increasing trends after delivery (P > 0.05). DL,ML was also found to be different between PG and CG for weeks 32, 52, and 64 (P = 0.013). 6.3. Preferred stance width Preferred stance width (SW) was found to increase during pregnancy and dropped to control levels after delivery (Table 1 and Fig. 2). SW was found to differ between groups during the third trimester (P < 0.001). During pregnancy, average SW increased from 17.9 (0.8) cm to 20.6 (0.9) cm from weeks 16–36 (P = 0.016). SW further increased to 21.9 (1.9) cm at week 40 for the remaining PG subjects. SW dropped to an average of 17.6 (0.5) cm during the postpartum period (P < 0.001). For CG, average SW was 17.7 (0.6) cm over the 64 weeks of testing. Among pregnant subjects, strong correlations were found between stance width, perceived balance, and actual balance (Table 2). The steady antepartum increase and precipitous postpartum drop in sense of balance scores and preferred stance width were found to be highly correlated Table 2 Pearson correlation values (r) of balance parameters compared to sense of balance and stance width for the pregnant group Measure Traditional parameters StdevAP StdevML StdevRAD VelAP VelML VelRAD 95FreqAP 95FreqRAD AngDev SDA parameters DS,AP DL,AP DS,ML DL,ML DS,RAD DL,RAD HS,AP HL,AP HS,ML HL,ML HS,RAD HL,RAD Denotes P < 0.05. ** Denotes P < 0.01. *
Balance
Stance width
0.82** 0.46 0.63 0.72* 0.80** 0.10 0.19 0.31 0.56
0.82** 0.62 0.54 0.63 0.88** 0.05 0.32 0.35 0.74*
0.80** 0.37 0.45 0.54 0.47 0.04 0.29 0.12 0.10 0.20 0.39 0.12
0.72* 0.32 0.40 0.63 0.42 0.16 0.38 0.03 0.20 0.30 0.44 0.07
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Table 3 Fall incidence(s) reported since last testing session Subject
Week
Pregnant group (PG) P1 52 P8 28 P14 24 P14 32 P14 40 Control group (CG) C2 8 C2 64 C3 46 C3 52 C4 12 C4 64 C6 32 C6 40 C6 46 C6 64 C8 0 C8 20 C8 28 C8 32 C10 0 C10 16 C10 52 C15 16
Type
# Falls
Injury
SD SA Slip Slip Slip
1 1 1 1 1
No No No No No
Trip SD Ath SD SD Ath Slip Slip Slip Ath Slip Ath Ath Ath Slip Ath Ath Trip
1 1 2 1 1 1 1 1 1 2 1 2 3 1 1 1 2 1
No Minor No No Minor No No No No Minor No No No Major Minor Minor No No
A fall was defined as ‘‘impacted the ground with any part of the body”. Fall type: Trip/Slip (tripped or slipped during walking or running on level ground), SA/SD (fall during stair ascent/descent), and Ath (fall during athletic or sport activity). Injury classifications: no injury, minor injury (required home treatment), and major injury (required medical attention).
(r = 0.94, P < 0.001). SB scores and SW both demonstrated positive correlations with AP balance measures, but negative correlations with ML measures and angular deviation from the AP sway axis. 6.4. Falls incidences Pregnant subjects reported less falls than controls and compared to recent studies (Butler et al., 2006; Dunning et al., 2003). Two PG subjects (13%) fell during pregnancy; one multiple times (Table 3). Another PG subject fell during the postpartum follow-up period. PG incidences were due to slipping, or falling while ascending or descending stairs; they experienced no athletic/sporting-related falls. Seven CG subjects (47%) fell over the 64 weeks of testing; all seven fell at least once during the first 40 weeks. Six fell two or more times. All CG fallers experienced at least one fall due to slipping, tripping, or stairs. Five fell during athletic or sporting activities, which ranged from 25% to 75% of falls for these individuals. 7. Discussion This study appears to be the first to document changes in perceived balance and stance width during pregnancy. Using a simple, noninvasive testing protocol, we were also
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able to assess standing balance by examining postural sway. These parameters along with fall incidences were tracked from 16 weeks gestation through 6-month postpartum. Significant changes in these parameters were observed over time and as compared to a non-pregnant age-matched control group. Among pregnant subjects, perceived and actual standing balance were found to decline while stance width increased during pregnancy. Strong correlations were found among these parameters. Compared to non-pregnant controls, the pregnant group (PG) had significantly elevated balance measures in the radial and AP directions during pregnancy and these measures decreased during the postpartum period (Table 1, Fig. 3). Greater gross (or radial) postural sway and reduced stability by the PG during the antepartum period had been observed previously (Butler et al., 2006). By using direction-specific measures, however, we further distinguished that this increased instability was predominately directed along the sagittal plane, i.e., AP direction. Potential increases in lateral sway due to pregnancy may have been mitigated by increasing stance width. In normal healthy individuals, increasing stance width has been shown to reduce postural sway, especially lateral sway (Day et al., 1993; Kirby et al., 1987). This relationship between stance width and lateral sway may explain why, compared to controls, ML balance measures maintain similar levels during pregnancy, while AP measures increased. It is possible that postural sway would increase in all directions if the gravida was to retain her pre-pregnancy stance width; however, by increasing stance width as pregnancy progresses, she is able to minimize lateral sway and maintain lateral stability. Interestingly, increases in lateral sway occurred after delivery (Fig. 3). Even though the postpartum patient returned to her pre-/early-pregnancy stance width, these findings may suggest that there is a residual effect of stance width reductions that resulted in increased lateral sway. This increase in postpartum lateral sway may also explain why postpartum SB scores for the pregnant group remained slightly higher than the control group. Stabilogram diffusion analysis results suggest differences in postural control mechanisms due to pregnancy, in that greater postural sway activity was noted among the pregnant group. Group differences were particularly apparent for postural sway during long time intervals in the AP and radial directions (DL,AP, DL,RAD; Table 1, Fig. 3). These results suggest that, compared to controls, pregnant subjects had increased instability during periods of closedloop control. Short-term postural sway in the AP direction (characterized by DS,AP) was found to increase during the latter stages of pregnancy and drop postpartum. DS,AP was the only SDA parameter to correlate with perceived sense of balance and stance width (Table 2). This result suggests strong relationships exist between open-loop AP postural control, balance perception and base of support size. Long-term postural sway in the ML direction (DL,ML)
was found to increase in pregnant subjects especially postpartum when compared to controls. This suggests increasing ML instability during periods of closed-loop control particularly after delivery. Little to no changes in shortterm and long-term scaling exponents (HS, HL) suggest little effect of pregnancy on the CoP drift tendencies about a relative equilibrium point. With regard to fall incidences, we found conflicting results compared to prior work. First, we found that only 13% (2 of 15) of our pregnant subjects fell during pregnancy. Others suggest a fall rate closer to 25% (Butler et al., 2006; Dunning et al., 2003). Butler et al. (2006) report a fall rate of 25% (2 of 8) based on assessment during the postpartum session; however, they report that only 8 of 12 gravida participated in this final test session. From their report, it is not known whether the other four subjects experienced a fall during pregnancy. Second, we found an unexpectedly high percent of our controls (47%) experienced at least one fall. Control subjects in Butler et al. retrospectively reported that they did not experience a fall in the 12 months prior to testing. Given that only one majorinjury fall (i.e., required medical attention) occurred with our CG subjects, it is possible that, retrospectively, our subjects would fail to recall the no- or minor-injury fall occurrences after a 12-month periods. A recent meta-analysis of fall history recall in older adults (Ganz et al., 2005) found that individuals with injurious falls were more likely to recall their falls. Additionally, the authors suggest that to improve accuracy of fall data, fall information should be collected on a regular basis (weekly or monthly) throughout a study. A strength of this study was that we examined directionspecific balance measures using both traditional descriptive and SDA parameters. Butler et al. (2006) identified gross planar changes in postural sway due to pregnancy. Similar to our study, the authors allowed the subject to use her own preferred stance width and foot position; however, these data were not recorded (personal communication). Thus, our results suggest that their observed gross changes were actually related to changes in AP, not lateral sway. Furthermore, these changes appear to particularly increase postural instability during periods of closed-loop control. Our study also provides documentation of changes in preferred stance width and perceived sense of balance degradation. Both increased substantially as pregnancy progessed and dropped to near-control levels within six weeks postpartum. Additionally, we found that perceived balance, actual balance and preferred stance width were strongly coupled during pregnancy. Finally, this study prospectively tracked fall incidences in both pregnant and nonpregnant healthy young women over extended periods. These results found lower fall rates among the pregnant women and higher rates among controls than previous studies (Butler et al., 2006; Dunning et al., 2003). Weaknesses of this study include early withdrawal of two CG subjects and loss of all postpartum data for one PG subject due to study withdrawal. Small sample size is
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a limitation of this study; however, even with this small sample size, we are able to make some generalized conclusions about the effect of physical changes on perceived and actual balance and preferred stance width. Future studies could examine changes in balance measures relative to constrained stance based on preferred pre-conception stance width. That study could assess whether the observed increase in stance width was responsible for the maintenance of lateral sway throughout pregnancy. Measures of foot sensation, ankle range of motion, and/or ankle muscle activity may provide insight into postural control mechanisms and should be considered in future studies. Since the current study only examined postural sway behavior during quiet, unperturbed stance, future studies should assess standing balance when the gravida is subjected to mild balance disturbances, which may be more aligned with real-world situations (Baloh et al., 1994; Hsiao-Wecksler et al., 2003; Ishida et al., 1997). Investigations of balance exercises and fall prevention training programs may also be warranted. 8. Conclusions Preferred stance width, perceived sense of balance degradation, and actual balance measures were strongly coupled during pregnancy. Perceived balance degradation during pregnancy was related to increasing postural sway in the AP direction. Increasing stance width may be used to compensate for perceived and actual reductions in stability. Additionally, lateral balance in pregnant women may be effectively preserved by this increase in stance width. Clinical application of these results suggest patient recommendations to use a comfortable wide and slightly staggered foot placement while standing. This foot placement would increase both the lateral and AP base of support, which may lead to improved postural stability in all directions. Acknowledgements We thank Molly Hathaway, Susan Shah, Nicholas Wills, Todd Mayer, and James Jackson Potter for their assistance with data collection. Partial funding provided by the Abbott Laboratories and Jeff Olson Fund at the University of Illinois. References Baloh, R.W., Fife, T.D., Zwerling, L., Socotch, T., Jacobson, K., Bell, T., Beykirch, K., 1994. Comparison of static and dynamic posturography in young and older normal people. J. Am. Geriatr. Soc. 42, 405– 412. Bird, A.R., Menz, H.B., Hyde, C.C., 1999. The effect of pregnancy on footprint parameters. A prospective investigation. J. Am. Podiatr. Med. Assoc. 89, 405–409. Bullock, J.E., Jull, G.A., Bullock, M.I., 1987. The relationship of low back pain to postural changes during pregnancy. Aust. J. Physiother. 33, 10–17.
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Butler, E.E., Colon, I., Druzin, M.L., Rose, J., 2006. Postural equilibrium during pregnancy: decreased stability with an increased reliance on visual cues. Am. J. Obstet. Gynecol. 195, 1104–1108. Collins, J.J., De Luca, C.J., 1993. Open-loop and closed-loop control of posture: a random-walk analysis of center-of-pressure trajectories. Exp. Brain Res. 95, 308–318. Collins, J.J., De Luca, C.J., Burrows, A., Lipsitz, L.A., 1995. Age-related changes in open-loop and closed-loop postural control mechanisms. Exp. Brain Res. 104, 480–492. Connolly, A.M., Katz, V.L., Bash, K.L., McMahon, M.J., Hansen, W.F., 1997. Trauma and pregnancy. Am. J. Perinatol. 14, 331–336. Davies, J., Fernando, R., McLeod, A., Verma, S., Found, P., 2002. Postural stability following ambulatory regional analgesia for labor. Anesthesiology 97, 1576–1581. Day, B.L., Steiger, M.J., Thompson, P.D., Marsden, C.D., 1993. Effect of vision and stance width on human body motion when standing: implications for afferent control of lateral sway. J. Physiol. 469, 479– 499. Doyle, R.J., Hsiao-Wecksler, E.T., Ragan, B.G., Rosengren, K.S., 2007a. Generalizability of center of pressure measures of quiet standing. Gait Posture 25, 166–171. Doyle, R.J., Ragan, B.G., Rajendran, K., Rosengren, K.S., HsiaoWecksler, E.T., 2007b. Generalizability of stabiliogram diffusion analysis of center of pressure measures. Gait Posture [Epub ahead of print]. Dozza, M., Chiari, L., Chan, B., Rocchi, L., Horak, F.B., Cappello, A., 2005. Influence of a portable audio-biofeedback device on structural properties of postural sway. J. Neuroeng. Rehabil. 2, 13. Dumas, G.A., Reid, J.G., Wolfe, L.A., Griffin, M.P., McGrath, M.J., 1995. Exercise, posture, and back pain during pregnancy. Part 1. Exercise and posture. Clin. Biomech. (Bristol, Avon) 10, 98–103. Dunning, K., LeMasters, G., Levin, L., Bhattacharya, A., Alterman, T., Lordo, K., 2003. Falls in workers during pregnancy: risk factors, job hazards, and high risk occupations. Am. J. Ind. Med. 44, 664–672. Foti, T., Davids, J.R., Bagley, A., 2000. A biomechanical analysis of gait during pregnancy. J. Bone Joint Surg. Am. 82, 625–632. Franklin, M.E., Conner-Kerr, T., 1998. An analysis of posture and back pain in the first and third trimesters of pregnancy. J. Orthop. Sports Phys. Ther. 28, 133–138. Fries, C.E., Hellebrandt, F.A., 1946. The influence of pregnancy on the location of the center of gravity, postural stability, and postural alignment. Am. J. Obstet. Gynecol. 46, 374–380. Gabbe, S.G., Niebyl, J.R., Simpson, J.L. (Eds.), 2002. Obstetrics: Normal and Problem Pregnancies. Churchill Livingstone, New York. Ganz, D.A., Higashi, T., Rubenstein, L.Z., 2005. Monitoring falls in cohort studies of community-dwelling older people: effect of the recall interval. J. Am. Geriatr. Soc. 53, 2190–2194. Gilleard, W.L., Crosbie, J., Smith, R., 2002. Static trunk posture in sitting and standing during pregnancy and early postpartum. Arch. Phys. Med. Rehabil. 83, 1739–1744. Hsiao-Wecksler, E.T., Katdare, K., Matson, J., Liu, W., Lipsitz, L.A., Collins, J.J., 2003. Predicting the dynamic postural control response from quiet-stance behavior in elderly adults. J. Biomech. 36, 1327– 1333. Ishida, A., Imai, S., Fukuoka, Y., 1997. Analysis of the posture control system under fixed and sway-referenced support conditions. IEEE Trans. Biomed. Eng. 44, 331–336. Kirby, R.L., Price, N.A., Macleod, D.A., 1987. The influence of foot position on standing balance. J. Biomech. 20, 423–437. Lymbery, J.K., Gilleard, W., 2005. The stance phase of walking during late pregnancy: temporospatial and ground reaction force variables. J. Am. Podiatr. Med. Assoc. 95, 247–253. Marnach, M.L., Ramin, K.D., Ramsey, P.S., Song, S.W., Stensland, J.J., An, K.N., 2003. Characterization of the relationship between joint laxity and maternal hormones in pregnancy. Obstet. Gynecol. 101, 331–335. Mitchell, S.L., Collins, J.J., De Luca, C.J., Burrows, A., Lipsitz, L.A., 1995. Open-loop and closed-loop postural control mechanisms in
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Parkinson’s disease: increased mediolateral activity during quiet standing. Neurosci. Lett. 197, 133–136. Moore, K., Dumas, G.A., Reid, J.G., 1990. Postural changes associated with pregnancy and their relationships with low back pain. Clin. Biomech. (Bristol, Avon) 5, 169–174. Perneger, T.V., 1998. What’s wrong with Bonferroni adjustments. BMJ 316, 1236–1238. Pickering, A.E., Parry, M.G., Ousta, B., Fernando, R., 1999. Effect of combined spinal-epidural ambulatory labor analgesia on balance. Anesthesiology 91, 436–441.
Prieto, T.E., Myklebust, J.B., Hoffman, R.G., Lovett, E.G., Myklebust, B.M., 1996. Measures of postural steadiness: differences between healthy young and elderly adults. IEEE Trans. Biomed. Eng. 43, 956–966. Rocchi, L., Chiari, L., Cappello, A., 2004. Feature selection of stabilometric parameters based on principal component analysis. Med. Biol. Eng. Comput. 42, 71–79. Rothman, K.J., 1990. No adjustments are needed for multiple comparisons. Epidemiology 1, 43–46. Weiss, H.B., Songer, T.J., Fabio, A., 2001. Fetal deaths related to maternal injury. JAMA 286, 1863–1868.
Clinical Biomechanics 23 (2008) 468–476 www.elsevier.com/locate/clinbiomech
Balance (perceived and actual) and preferred stance width during pregnancy John Jang a, Katherine T. Hsiao b, Elizabeth T. Hsiao-Wecksler a,* a
Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, 124 Mechanical Engineering Building, MC-244, 1206 West Green Street, Urbana, IL 61801, USA b Department of Obstetrics and Gynecology, California Pacific Medical Center, San Francisco, CA, USA Received 1 August 2007; accepted 19 November 2007
Abstract Background. Pregnant women often remark that their balance degrades during pregnancy; however, it appears that no studies have documented the gravida’s perception of her balance nor measured direction-specific changes in balance throughout pregnancy or after delivery. Methods. Thirty women, fifteen pregnant and fifteen non-pregnant controls, were tested monthly and through 6-month postpartum. For each session, perceived degradation in sense of balance, laboratory-based balance measures, stance width, and the number of falls since the previous session were recorded. Laboratory-based balance measures, quantified by direction-specific measures of postural sway, were computed from ten 30 s quiet-standing trials on a stationary force platform. Repeated-measures analysis of variance, paired t-tests, and Pearson correlations were use to examine group and time effects. Findings. For the pregnant group, perceived balance degradation and stance width were highly correlated (r = 0.94). Both increased during pregnancy (P 6 0:016) and dropped to near-control levels after delivery (P 6 0:004). Compared to the control group, pregnant subjects displayed increased sway, especially in the anterior–posterior and radial directions (P 6 0:039). Anterior–posterior sway measures strongly correlated with perceived balance (0.82 > r > 0.72) and also decreased significantly between the third trimester and postpartum (P 6 0:029). Interestingly, medial–lateral balance measures varied little during pregnancy, but increased after delivery. Contrary to recent work suggesting fall rates of 25%, only 13% of our subjects (n = 2) fell during pregnancy. Interpretation. Perceived degradation in balance during pregnancy was strongly related to increasing postural sway instability in the anterior–posterior direction. Lateral stability was maintained during pregnancy and likely accomplished by increasing stance width. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Balance; Postural control; Stance width; Pregnancy
1. Introduction Physiological changes throughout pregnancy may result in unique modifications in a gravida’s biomechanics that can alter both perceived and actual changes in balance and postural control. During pregnancy, a woman will gain between 25 and 35 lbs (11–16 kg) (Gabbe et al., 2002). This weight gain is unique because it is primarily localized to the
*
Corresponding author. E-mail address: [email protected] (E.T. Hsiao-Wecksler).
0268-0033/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.clinbiomech.2007.11.011
anterior abdominal region, which results in a change in the position of the body’s center of mass (Fries and Hellebrandt, 1946). Furthermore, it has been suggested that the hormones that permit joint laxity for the development of the fetus may also play a role in global joint laxity (e.g., Marnach et al., 2003). These factors may contribute toward the perception of increasing postural instability throughout pregnancy. Although numerous studies have examined how a woman’s physical posture changes during pregnancy (e.g., Moore et al., 1990; Bullock et al., 1987; Dumas et al., 1995; Gilleard et al., 2002; Franklin and Conner-Kerr,
J. Jang et al. / Clinical Biomechanics 23 (2008) 468–476
The current study was a prospective case–control study of 30 women that examined postural sway parameters, which included direction-specific measures. These parameters have been found to be effective at assessing balance performance (Prieto et al., 1996; Rocchi et al., 2004) and interpreting the behavior of the postural control system (Collins and De Luca, 1993). Additionally, monthly measurements of perceived sense of balance, preferred stance width, and fall incidences were collected. The primary goal of this study was to track balance and stance width in pregnant women throughout pregnancy and the postpartum period. As a secondary aim, we also tracked monthly incidences of falls. 2. Methods 2.1. Participants
PG
The pregnancy group (PG) consisted of 15 gravida; start of testing mean (and standard deviation) age of 31 (4) yrs, range 25–38 yrs, height 168 (7) cm, mass 77.4 (21.1) kg, body mass index (BMI) 27.4 (6.8) kg/m2; five primigravida, all singleton gestations. PG subjects were included in the study if enrolled by the 16th week of gestation. Fifteen
CG
1998), few studies have examined how balance, and postural control that moderates balance, may actually vary due to pregnancy (Butler et al., 2006; Davies et al., 2002; Fries and Hellebrandt, 1946; Pickering et al., 1999). Pickering et al. (1999) and Davies et al. (2002) examined differences in balance; however, their studies only examined women while in labor and focused on the effect of low-dose analgesia (spinal-epidural). Fries and Hellebrandt (1946) presented a prospective case study for a single individual over nine sessions (third month of pregnancy to 6 weeks postpartum). They recorded postural sway using a kymograph (an instrument that recorded movement using a stylus and rotating drum) and qualitatively noted increased sway throughout pregnancy that persisted throughout puerperium. Recently, Butler et al. (2006) performed a prospective case–control study with 12 gravida, tested over four sessions (11–14 weeks gestation through 6–8 weeks postpartum). They reported that, compared to non-pregnant control subjects, postural sway increased throughout the assessment period. That study used measures of gross, planar sway (path length and average radial displacement). It is not known, however, whether direction-specific (anterior–posterior (AP) or medial–lateral (ML)) changes in balance and postural control may occur with pregnancy, especially given the extreme AP changes in body posture. Minimal research has been published on how pregnancy may change the gravida’s choice in base of support, which is defined by the positioning of the feet (Bird et al., 1999; Foti et al., 2000; Lymbery and Gilleard, 2005). Bird et al. (1999) found that step width while walking increased during pregnancy. This change in gait function was suggested to be a compensatory mechanism to improve locomotor stability. Assessments of quiet standing, with healthy non-pregnant subjects, have shown that balance is affected by stance width, i.e., increasing stance width decreases ML postural sway (Day et al., 1993; Kirby et al., 1987). It is not known, however, whether stance width during upright standing changes throughout pregnancy and how such changes may affect a pregnant woman’s balance. Injurious falls account for 17–39% of maternal trauma cases needing medical attention (Dunning et al., 2003). Serious falls after a loss of balance can result in maternal and/or fetal complications including 3–7% of fetal deaths (Connolly et al., 1997; Weiss et al., 2001). Thus, determining fall rates among pregnant women is important. Recent studies suggest that a rather high percentage of pregnant women may experience a fall. Butler et al. found that 25% of a subset of their pregnant subjects fell during pregnancy, while no control subjects reported falling during the 12 months before testing (Butler et al., 2006). Similar fall rates were reported for a large cohort of women employed during pregnancy, i.e., 26.6% experienced a fall (Dunning et al., 2003). Both studies collected retrospective data six or more weeks postpartum, requiring subjects to recall fall incidences during pregnancy. Tracking fall incidences prospectively during pregnancy may provide additional insight into this seemingly high fall rate.
469
subject 0 P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 P11 P12 P13 P14 P15 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15
4
Week 8 12 16 20 24 28 32 36 40 46 52 64
Fig. 1. Pregnant (PG) and control (CG) subject enrollment and attendance record (gray: absent session). Eight PG subjects were absent for week 40 testing due to delivery.
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non-pregnant women matched by age, within 1 yr, were recruited as a control group (CG; mean age 31 (4) yrs, range 24–39 yrs, height 166 (6) cm, mass 69.7 (12.7) kg, and BMI 25.4 (4.2) kg/m2). Since this study was a longitudinal, repeated-measures design, no additional inclusion/ exclusion criteria were identified for either group. Subjects were recruited from university personnel and the local community. Informed consent was given by all subjects and the study was approved by the University of Illinois Institutional Review Board. Each pregnant subject was tested at 4 week intervals until delivery. Three follow-up sessions after delivery were scheduled at 6 weeks, 12 weeks, and 6-month postpartum. To match PG testing, CG subjects were tested at 4-week intervals for 40 weeks, and then 6 weeks, 12 weeks, and 6 months after the 40th week. For data tracking, these last three sessions were defined as weeks 46, 52, and 64, respectively. Three PG and two CG subjects withdrew before the completion of the study due to disinterest or relocation (Fig. 1). 3. Experimental procedure At the beginning of each session, the subject completed a self-evaluation questionnaire. To quantify perceived sense of balance (SB), we developed our own scale which asked the subject ‘‘How would you rank your balance from 0 (normal) to 10 (extremely unstable)”. The previous session’s SB score was provided as reference. To track fall incidences, the subject recorded whether she had fallen (‘‘impacted the ground with any part of the body”) since the previous session. Based on a written description of the fall, the fall event was later classified by a researcher (ETHW) into one of five categories: trip or slip while walking or running on level ground in everyday activities (Trip, Slip), fall during stair ascent or descent (SA, SD), or fall during athletic or sporting activity (Ath). Injury severity was also classified as no injury, minor injury (required home treatment), and major injury (required medical attention). Postural sway and stance width were then recorded. The subject stood unshod with both feet on a large force plate (model BP600900, AMTI, Watertown, MA). The force plate was embedded in a surrounding walkway and sampled at 100 Hz. (Force plate data were filtered using a 10th order forward–backward low pass Butterworth filter with 7 Hz cut-off frequency.) The subject was instructed to stand quietly with arms at her side and eyes open, looking at a stationary picture placed at eye level and 3 m away. She was instructed to place the tips of her toes directly behind a marked line on the force plate, but could otherwise select her preferred stance width for each trial. During short rest breaks between each trial, the subject stepped off and on the force plate. Ten 30 s trials were conducted for each session. After the completion of the 10th trial, the distance between heel centers was measured to determine preferred stance width (SW).
4. Balance measures From force plate data, the movement of the center of pressure under the feet (CoP) was computed. Planar representation of CoP fluctuations is often called a stabilogram. A number of measures can be obtained from these stabilograms. Traditionally, general variability of the CoP is assessed using summary statistical parameters, such as mean sway velocity and standard deviation of the displacement about the mean (Prieto et al., 1996). A newer analysis technique, referred to as stabilogram diffusion analysis (SDA), produces a set of measures that provide insight into the (open- and closed-loop) postural control system during upright stance (Collins and De Luca, 1993). For this study, balance measures were based on the average over ten trials. Reliability of these traditional and SDA measures have been found to be fair to excellent, ranging from 0.56 to 0.95 (Doyle et al., 2007a, b). 4.1. Traditional parameters Based on recommendations from Rocchi et al. (2004), a set of nine traditional summary statistic CoP parameters was examined. These included the standard deviation of the displacement about the mean (Stdev) in the AP, ML, and combined radial (RAD) directions; mean sway velocity (Vel) in the three directions; 95% power frequency (95Freq) in the AP and RAD directions; and angular deviation (AngDev) of the principal sway direction from the AP axis. 4.2. Stabilogram diffusion analysis parameters SDA models fluctuations of the CoP as a system of coupled, correlated random walks and provides descriptors of the CoP movement within the context of a stochastic process. This process is clinically advantageous because it provides insight into the ‘‘steady state behavior and functional interaction of the neuromuscular system” (Collins and De Luca, 1993). SDA has been used to study, for example, individuals with Parkinson’s disease and profound vestibular loss (Mitchell et al., 1995; Dozza et al., 2005). It has been shown that SDA may be more sensitive than traditional parameters at detecting subtle age-related differences in postural control (Collins et al., 1995). The SDA procedure suggests the existence of two regions of behavior; a short-term region (typically below 1–2 s) and a long-term region of larger time intervals. The short- and long-term time intervals have been interpreted as representing the behavior of the postural control system during periods of open-loop and closed-loop control, respectively (Collins and De Luca, 1993). SDA parameters reported in this study are short-term and long-term diffusion coefficients (DS, DL) and scaling exponents (HS, HL) in the AP, ML, and RAD directions. Diffusion coefficients reflect the level of stochastic activity of the CoP (Collins and De Luca, 1993). Larger coefficients imply postural
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instability. Scaling exponents indicate the tendency of the CoP to drift away from a relative equilibrium point.
Table 1 P-values for effect of group (PG versus CG) and time (weeks 16–64, excluding week 40) on each parameter using mixed models repeatedmeasures ANOVA
5. Statistical analysis
Measure
Week
Sense of balance Stance width Traditional parameters StdevAP StdevML StdevRAD VelAP VelML VelRAD 95FreqAP 95FreqRAD AngDev SDA parameters DS,AP DL,AP DS,ML DL,ML DS,RAD DL,RAD HS,AP HL,AP HS,ML HL,ML HS,RAD HL,RAD
<0.001 0.028
0.012 0.21
<0.001 <0.001
0.17 0.018 0.43 0.039 0.005 0.034 0.53 0.62 0.005
<0.001 0.17 <0.001 0.18 0.91 0.27 0.14 0.13 0.88
0.009 0.10 0.016 0.006 0.36 0.11 0.37 0.47 0.26
0.15 0.95 0.24 0.005 0.34 0.95 0.21 0.85 0.16 0.08 0.46 0.62
0.027 0.001 0.55 0.018 0.07 <0.001 0.23 0.039 0.15 0.08 0.18 0.019
0.014 0.84 0.86 0.013 0.32 0.52 0.41 0.37 0.32 0.11 0.54 0.35
Statistical analyses were based on antepartum testing weeks 16–36 and postpartum testing weeks 46–64. These data-analysis limits were necessary since nine PG subjects enrolled in the study at 16 weeks gestation and eight PG subjects did not attend week 40 testing due to delivery (Fig. 1). Statistical analyses were performed on measures recorded for SB, SW, and balance (traditional and SDA parameters). To determine whether group (PG versus CG) and/or time effects (over weeks 16–64 excluding 40) were significant for a given measure, mixed models repeated-measures analysis of variance (RM ANOVA) was employed. A number of additional analyses were conducted only on the PG data to understand the effect of pregnancy. One-way RM ANOVA tests were conducted for the antepartum weeks in the second through third trimesters (weeks 16–36). Paired t-tests examined before and after delivery changes, i.e., third trimester versus postpartum comparison using the averages for weeks 28–36 versus 46–64. Pearson correlations tested whether average values for SW and SB were correlated with each other or any balance measures. Significant ANOVAs were analyzed by LSD post-hoc comparisons. The level of significance was set to a = 0.05 (Perneger, 1998; Rothman, 1990). Statistical analyses were run on SPSS (SPSS Inc., Chicago, IL; v13). SB scores were found to be non-normally distributed. The normality assumption for the mixed models RM ANOVA residuals for SB, however, was satisfied. Nonparametric Friedman test and Wilcoxon Signed Ranks test were used instead of the one-way RM ANOVA and paired t-test, respectively, for the SB score analyses within the pregnant group.
Week Group
Group
6.1. Perceived sense of balance
6.2. Laboratory-based balance measures
Sense of balance scores (SB) indicated that pregnant subjects perceived that their balance degraded during preg-
Postural sway in the AP and radial directions appeared to increase during pregnancy and then decrease after
6
25
5
23
Stance Width (cm)
Balance (0-stable,10-unstable)
6. Results
nancy and improved substantially by 6 weeks postpartum (Table 1 and Fig. 2). SB scores were found to differ significantly between PG and CG starting from week 20 to delivery (P < 0.001). For pregnant subjects, average SB score increased from 1.7 (Stdev of 0.7) to 3.9 (0.6) during the antepartum analysis period (P = 0.004, Friedman). The SB score increased further to 4.7 (0.5) for the remaining seven PG subjects tested at week 40. SB then dropped after delivery with an average score of 1.4 (0.2) over the 6-month postpartum period (P = 0.008, Wilcoxon). For CG, average SB was 0.7 (0.1) over the 64 weeks of testing.
4 3 2
Controls Pregnant
21 19 17
1 15 0 16 20 24 28 32 36 40
Week
46
52
64
16 20 24 28 32 36 40
46
52
Week
Fig. 2. Average values for perceived sense of balance and preferred stance width. Bars = standard error.
64
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delivery; whereas ML sway remained relatively consistent during pregnancy, but tended to increase during the postpartum period (Table 1, Fig. 3). Compared to control subjects, pregnant subjects had elevated values for a number of measures, especially in the AP and RAD directions (P 6 0:039; Table 1). Group week interaction effects also suggest group differences over time. For example, post-hoc
tests found that StdevAP and StdevRAD differed between groups for all analyzed weeks and DL,AP and DL,RAD differed for all but week 20; whereas, DS,AP differed through the third trimester and 6 weeks postpartum. Although average values for the AP and RAD measures increased for the PG during pregnancy (Fig. 3), large variability resulted in no statistically significant differences in these
12
8
10
StDev (mm)
Vel (mm/s)
6 8 6 4
4
2 2 0
0
27 2.2
95Freq (Hz)
17 12
1.7 1.2
7
0.7
2
0.2
20 18 16 14 12 10 8 6 4 2 0
0.88 0.86
HS (mm 2/s)
DS (mm 2/s)
AngDev (deg)
22
0.84 0.82 0.8 0.78 AP-C ML-C
3.5 0.38
3
HL (mm 2 /s)
2.5
DL (mm 2 /s)
RAD-C AP-P
0.33
2 1.5 1 0.5
ML-P
0.28
RAD-P
0.23 0.18 0.13 0.08
0
0.03 16 20 24 28 32 36 40
Week
46
52
64
16 20 24 28 32 36 40
46
52
64
Week
Fig. 3. Average balance measures for pregnant (P) and control (C) subjects. Traditional measures: mean sway velocity (Vel), standard deviation about the mean (Stdev), angular deviation from the AP axis (AngDev), and 95% power frequency (95Freq). Stabiliogram diffusion analysis (SDA) measures: diffusion coefficients (D) and scaling exponents (H) in the short (S) and long (L) term regions, for anterior–posterior (AP), medial–lateral (ML) and radial (RAD) directions. Bars = standard error.
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measures over the second through third trimesters (P > 0.05, one-way RM ANOVA). Comparisons of average values before and after delivery, however, suggest reductions in StdevAP, VelAP, and DS,AP from the third trimester to postpartum (0:010 6 P 6 0:029, paired t-test). Interestingly, ML measures appeared to increase only during the postpartum period. VelML increased from the third trimester to postpartum (P = 0.027), while other ML parameters also displayed non-statistically significant increasing trends after delivery (P > 0.05). DL,ML was also found to be different between PG and CG for weeks 32, 52, and 64 (P = 0.013). 6.3. Preferred stance width Preferred stance width (SW) was found to increase during pregnancy and dropped to control levels after delivery (Table 1 and Fig. 2). SW was found to differ between groups during the third trimester (P < 0.001). During pregnancy, average SW increased from 17.9 (0.8) cm to 20.6 (0.9) cm from weeks 16–36 (P = 0.016). SW further increased to 21.9 (1.9) cm at week 40 for the remaining PG subjects. SW dropped to an average of 17.6 (0.5) cm during the postpartum period (P < 0.001). For CG, average SW was 17.7 (0.6) cm over the 64 weeks of testing. Among pregnant subjects, strong correlations were found between stance width, perceived balance, and actual balance (Table 2). The steady antepartum increase and precipitous postpartum drop in sense of balance scores and preferred stance width were found to be highly correlated Table 2 Pearson correlation values (r) of balance parameters compared to sense of balance and stance width for the pregnant group Measure Traditional parameters StdevAP StdevML StdevRAD VelAP VelML VelRAD 95FreqAP 95FreqRAD AngDev SDA parameters DS,AP DL,AP DS,ML DL,ML DS,RAD DL,RAD HS,AP HL,AP HS,ML HL,ML HS,RAD HL,RAD Denotes P < 0.05. ** Denotes P < 0.01. *
Balance
Stance width
0.82** 0.46 0.63 0.72* 0.80** 0.10 0.19 0.31 0.56
0.82** 0.62 0.54 0.63 0.88** 0.05 0.32 0.35 0.74*
0.80** 0.37 0.45 0.54 0.47 0.04 0.29 0.12 0.10 0.20 0.39 0.12
0.72* 0.32 0.40 0.63 0.42 0.16 0.38 0.03 0.20 0.30 0.44 0.07
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Table 3 Fall incidence(s) reported since last testing session Subject
Week
Pregnant group (PG) P1 52 P8 28 P14 24 P14 32 P14 40 Control group (CG) C2 8 C2 64 C3 46 C3 52 C4 12 C4 64 C6 32 C6 40 C6 46 C6 64 C8 0 C8 20 C8 28 C8 32 C10 0 C10 16 C10 52 C15 16
Type
# Falls
Injury
SD SA Slip Slip Slip
1 1 1 1 1
No No No No No
Trip SD Ath SD SD Ath Slip Slip Slip Ath Slip Ath Ath Ath Slip Ath Ath Trip
1 1 2 1 1 1 1 1 1 2 1 2 3 1 1 1 2 1
No Minor No No Minor No No No No Minor No No No Major Minor Minor No No
A fall was defined as ‘‘impacted the ground with any part of the body”. Fall type: Trip/Slip (tripped or slipped during walking or running on level ground), SA/SD (fall during stair ascent/descent), and Ath (fall during athletic or sport activity). Injury classifications: no injury, minor injury (required home treatment), and major injury (required medical attention).
(r = 0.94, P < 0.001). SB scores and SW both demonstrated positive correlations with AP balance measures, but negative correlations with ML measures and angular deviation from the AP sway axis. 6.4. Falls incidences Pregnant subjects reported less falls than controls and compared to recent studies (Butler et al., 2006; Dunning et al., 2003). Two PG subjects (13%) fell during pregnancy; one multiple times (Table 3). Another PG subject fell during the postpartum follow-up period. PG incidences were due to slipping, or falling while ascending or descending stairs; they experienced no athletic/sporting-related falls. Seven CG subjects (47%) fell over the 64 weeks of testing; all seven fell at least once during the first 40 weeks. Six fell two or more times. All CG fallers experienced at least one fall due to slipping, tripping, or stairs. Five fell during athletic or sporting activities, which ranged from 25% to 75% of falls for these individuals. 7. Discussion This study appears to be the first to document changes in perceived balance and stance width during pregnancy. Using a simple, noninvasive testing protocol, we were also
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able to assess standing balance by examining postural sway. These parameters along with fall incidences were tracked from 16 weeks gestation through 6-month postpartum. Significant changes in these parameters were observed over time and as compared to a non-pregnant age-matched control group. Among pregnant subjects, perceived and actual standing balance were found to decline while stance width increased during pregnancy. Strong correlations were found among these parameters. Compared to non-pregnant controls, the pregnant group (PG) had significantly elevated balance measures in the radial and AP directions during pregnancy and these measures decreased during the postpartum period (Table 1, Fig. 3). Greater gross (or radial) postural sway and reduced stability by the PG during the antepartum period had been observed previously (Butler et al., 2006). By using direction-specific measures, however, we further distinguished that this increased instability was predominately directed along the sagittal plane, i.e., AP direction. Potential increases in lateral sway due to pregnancy may have been mitigated by increasing stance width. In normal healthy individuals, increasing stance width has been shown to reduce postural sway, especially lateral sway (Day et al., 1993; Kirby et al., 1987). This relationship between stance width and lateral sway may explain why, compared to controls, ML balance measures maintain similar levels during pregnancy, while AP measures increased. It is possible that postural sway would increase in all directions if the gravida was to retain her pre-pregnancy stance width; however, by increasing stance width as pregnancy progresses, she is able to minimize lateral sway and maintain lateral stability. Interestingly, increases in lateral sway occurred after delivery (Fig. 3). Even though the postpartum patient returned to her pre-/early-pregnancy stance width, these findings may suggest that there is a residual effect of stance width reductions that resulted in increased lateral sway. This increase in postpartum lateral sway may also explain why postpartum SB scores for the pregnant group remained slightly higher than the control group. Stabilogram diffusion analysis results suggest differences in postural control mechanisms due to pregnancy, in that greater postural sway activity was noted among the pregnant group. Group differences were particularly apparent for postural sway during long time intervals in the AP and radial directions (DL,AP, DL,RAD; Table 1, Fig. 3). These results suggest that, compared to controls, pregnant subjects had increased instability during periods of closedloop control. Short-term postural sway in the AP direction (characterized by DS,AP) was found to increase during the latter stages of pregnancy and drop postpartum. DS,AP was the only SDA parameter to correlate with perceived sense of balance and stance width (Table 2). This result suggests strong relationships exist between open-loop AP postural control, balance perception and base of support size. Long-term postural sway in the ML direction (DL,ML)
was found to increase in pregnant subjects especially postpartum when compared to controls. This suggests increasing ML instability during periods of closed-loop control particularly after delivery. Little to no changes in shortterm and long-term scaling exponents (HS, HL) suggest little effect of pregnancy on the CoP drift tendencies about a relative equilibrium point. With regard to fall incidences, we found conflicting results compared to prior work. First, we found that only 13% (2 of 15) of our pregnant subjects fell during pregnancy. Others suggest a fall rate closer to 25% (Butler et al., 2006; Dunning et al., 2003). Butler et al. (2006) report a fall rate of 25% (2 of 8) based on assessment during the postpartum session; however, they report that only 8 of 12 gravida participated in this final test session. From their report, it is not known whether the other four subjects experienced a fall during pregnancy. Second, we found an unexpectedly high percent of our controls (47%) experienced at least one fall. Control subjects in Butler et al. retrospectively reported that they did not experience a fall in the 12 months prior to testing. Given that only one majorinjury fall (i.e., required medical attention) occurred with our CG subjects, it is possible that, retrospectively, our subjects would fail to recall the no- or minor-injury fall occurrences after a 12-month periods. A recent meta-analysis of fall history recall in older adults (Ganz et al., 2005) found that individuals with injurious falls were more likely to recall their falls. Additionally, the authors suggest that to improve accuracy of fall data, fall information should be collected on a regular basis (weekly or monthly) throughout a study. A strength of this study was that we examined directionspecific balance measures using both traditional descriptive and SDA parameters. Butler et al. (2006) identified gross planar changes in postural sway due to pregnancy. Similar to our study, the authors allowed the subject to use her own preferred stance width and foot position; however, these data were not recorded (personal communication). Thus, our results suggest that their observed gross changes were actually related to changes in AP, not lateral sway. Furthermore, these changes appear to particularly increase postural instability during periods of closed-loop control. Our study also provides documentation of changes in preferred stance width and perceived sense of balance degradation. Both increased substantially as pregnancy progessed and dropped to near-control levels within six weeks postpartum. Additionally, we found that perceived balance, actual balance and preferred stance width were strongly coupled during pregnancy. Finally, this study prospectively tracked fall incidences in both pregnant and nonpregnant healthy young women over extended periods. These results found lower fall rates among the pregnant women and higher rates among controls than previous studies (Butler et al., 2006; Dunning et al., 2003). Weaknesses of this study include early withdrawal of two CG subjects and loss of all postpartum data for one PG subject due to study withdrawal. Small sample size is
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a limitation of this study; however, even with this small sample size, we are able to make some generalized conclusions about the effect of physical changes on perceived and actual balance and preferred stance width. Future studies could examine changes in balance measures relative to constrained stance based on preferred pre-conception stance width. That study could assess whether the observed increase in stance width was responsible for the maintenance of lateral sway throughout pregnancy. Measures of foot sensation, ankle range of motion, and/or ankle muscle activity may provide insight into postural control mechanisms and should be considered in future studies. Since the current study only examined postural sway behavior during quiet, unperturbed stance, future studies should assess standing balance when the gravida is subjected to mild balance disturbances, which may be more aligned with real-world situations (Baloh et al., 1994; Hsiao-Wecksler et al., 2003; Ishida et al., 1997). Investigations of balance exercises and fall prevention training programs may also be warranted. 8. Conclusions Preferred stance width, perceived sense of balance degradation, and actual balance measures were strongly coupled during pregnancy. Perceived balance degradation during pregnancy was related to increasing postural sway in the AP direction. Increasing stance width may be used to compensate for perceived and actual reductions in stability. Additionally, lateral balance in pregnant women may be effectively preserved by this increase in stance width. Clinical application of these results suggest patient recommendations to use a comfortable wide and slightly staggered foot placement while standing. This foot placement would increase both the lateral and AP base of support, which may lead to improved postural stability in all directions. Acknowledgements We thank Molly Hathaway, Susan Shah, Nicholas Wills, Todd Mayer, and James Jackson Potter for their assistance with data collection. Partial funding provided by the Abbott Laboratories and Jeff Olson Fund at the University of Illinois. References Baloh, R.W., Fife, T.D., Zwerling, L., Socotch, T., Jacobson, K., Bell, T., Beykirch, K., 1994. Comparison of static and dynamic posturography in young and older normal people. J. Am. Geriatr. Soc. 42, 405– 412. Bird, A.R., Menz, H.B., Hyde, C.C., 1999. The effect of pregnancy on footprint parameters. A prospective investigation. J. Am. Podiatr. Med. Assoc. 89, 405–409. Bullock, J.E., Jull, G.A., Bullock, M.I., 1987. The relationship of low back pain to postural changes during pregnancy. Aust. J. Physiother. 33, 10–17.
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