Theoretical and experimental indicators of falls during pregnancy as assessed by postural perturbations
Introduction
As pregnancy advances, women undergo various physical, physiological, and hormonal alterations. For example, they typically gain 11–16 kg in weight [1]. These weight gains are primarily concentrated in the abdominal region and can increase lumbar lordosis [2]. Hormonal fluctuations can increase ligamentous laxity [3], [4], and changes in plantar foot pressures are observed [5]. Such alterations can lead to balance problems. According to one study, nearly 27% of pregnant women experienced an accidental fall [6], which is a rate comparable to the 30% rate of falls observed in individuals aged 65 yrs and older [7]. Falls that cause fractures and sprains can contribute to the fear of falling [8], while very serious falls can terminate maternal or fetal life [9], [10].
Several researchers have studied the changes in postural control during pregnancy. Butler et al. reported that the center of pressure (COP) excursion in a pregnant group increased in length compared with a control group during quiet stance, and that the amount of weight gained was not significantly associated with the postural sway measures investigated [11]. Nagai et al. showed an increased area of body sway and length of anterior–posterior (A/P) body sway in a pregnant group compared with nonpregnant controls during quiet stance, and that high anxiety correlated with instability [12]. Oliveira et al. reported that pregnant women exhibited larger elliptical fits to COP trajectories as pregnancy progressed and higher COP frequency content along the A/P direction in the absence of visual inputs [13]. However, none of these studies addressed changes of postural control in response to external perturbations.
McCrory et al. investigated pregnant women's responses to A/P support surface translations [14]. Their main finding was that pregnant fallers, who reported at least one fall during pregnancy, had a truncated COP displacement immediately in response to the perturbation compared with pregnant nonfallers and controls.
Both the COP and center of gravity (COG) variables have been used individually to quantify postural stability in biomechanics studies. In terms of postural control during quiet standing, the COG and COP can be interpreted as the controlled and controlling variables, respectively [15], [16], [17], where the COP is proportional to the ankle torque [18]. COP and COG can also be measured simultaneously and the scalar difference between COP and COG (COP–COG) can be computed as a metric to characterize postural control. The COP–COG has been characterized in both the time and frequency domain. The most common metrics associated with the COP–COG variable are amplitude [18], standard deviation [17], root mean square [19], peak magnitude of displacement [20], latencies of initial and peak displacement [20], and frequency spectra [21]. COP–COG metrics have been applied, for example, to elderly stroke patients [19] and healthy elderly and young subjects [17], but have not yet been used to characterize balance in pregnant women.
In terms of mathematical modeling, the inverted pendulum, a one-link representation capturing a single degree of freedom, is the simplest mathematical model for describing bipedal postural control [22]. Simple one and two degree of freedom models have been used to study the effects of biofeedback on individuals with vestibular loss [23] and the risk of falling due to obesity [24]. However, to the best of our to our knowledge, these models have not been applied to pregnancy.
The specific goals of this study are (1) to investigate whether COP–COG can differentiate pregnant fallers from nonfallers; and (2) to use mathematical models to gain insights into the differences in postural control strategies between pregnant fallers and nonfallers.
Section snippets
Methods
The experimental data were obtained from a prior study of 15 women with a fall history during pregnancy (29.4 ± 4.7 yrs), 14 women without a fall history during pregnancy (30.6 ± 3.8 yrs), and 40 controls (26.5 ± 6.4 yrs) who were not pregnant and had a body mass index that matched the pre-pregnancy indices of the pregnant subjects [14]. The study had originally enrolled 41 pregnant women, however 12 subjects could not complete the study: four delivered pre-term, four had complications (preeclampsia,
Results
For both the forward and backward perturbations, the COP and COG were initially displaced in the opposite direction of the perturbation followed by an overshoot in the direction of the perturbation before a return to an upright position. For a detailed analysis of the COP data, see [14].
Fig. 2 illustrates that the pregnant fallers showed significantly smaller peak COP–COG values compared with the pregnant nonfallers and controls (). Perturbation magnitude was a
Discussion
Our experimental data indicate that the pregnant fallers utilize smaller peak COP–COG values compared with the pregnant nonfallers and controls, while our theoretical data assert that the pregnant nonfallers demonstrate increased ankle stiffness compared with the pregnant fallers and controls.
If the COP–COG variable is considered as an error signal that the balance system senses [18], then the pregnant fallers’ smaller COP–COG values would indicate smaller deviations requiring correction,
Acknowledgements
This work was partially supported by the NIOSH K01 OH8548 and the National Science Foundation's CAREER program (RAPD-0846471, funded under the American Recovery and Reinvestment Act of 2009). The study sponsors had no role in the study design, data collection, analysis, or interpretation, writing of the manuscript, or in the decision to submit the manuscript for publication. We are grateful to the UPMC Womancare Research Registry for allowing us to recruit the pregnant participants from its
References (30)
- et al.
Peripheral joint laxity increases in pregnancy but does not correlate with serum relaxin levels
American Journal of Obstetrics and Gynecology
(1996) - et al.
Postural equilibrium during pregnancy: decreased stability with an increased reliance on visual cues
American Journal of Obstetrics and Gynecology
(2006) - et al.
Characteristics of the control of standing posture during pregnancy
Neuroscience Letters
(2009) - et al.
Postural sway changes during pregnancy: a descriptive study using stabilometry
European Journal of Obstetrics and Gynecology and Reproductive Biology
(2009) - et al.
Balance recovery from medio-lateral perturbations of the upper body during standing
Journal of Biomechanics
(1999) - et al.
Larger center of pressure minus center of gravity in the elderly induces larger body acceleration during quiet standing
Neuroscience Letters
(2007) - et al.
Intrasession reliability of the “center of pressure minus center of mass” variable of postural control in the healthy elderly
Archives of Physical Medicine and Rehabilitation
(2000) - et al.
Decreased limits of stability in response to postural perturbations in subjects with low back pain
Clinical Biomechanics
(2006) - et al.
Changes in the standing posture of stroke patients during rehabilitation
Gait and Posture
(2005) - et al.
Body segment inertial parameter estimation for the general population of older adults
Journal of Biomechanics
(2002)
Body weight is a strong predictor of postural stability
Gait and Posture
Danforth's obstetrics and gynecology
An analysis of posture and back pain in the first and third trimesters of pregnancy
Journal of Orthopaedic and Sports Physical Therapy
Changes in joint laxity occurring during pregnancy
Annals of the Rheumatic Diseases
Planter foot pressures in pregnant women
Israel Journal of Medical Sciences
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