Intrinsic Factors Associated With Pregnancy Falls Xuefang Wu, MSc; Han T. Yeoh, PhD
ABSTRACT Approximately 25% to 27% of women sustain a fall during pregnancy, and falls are associated with serious injuries and can affect pregnancy outcomes. The objective of the current study was to identify intrinsic factors associated with pregnancy that may contribute to women’s increased risk of falls. A literature search (Medline and Pubmed) identified articles published between January 1980 and June 2013 that measured associations between pregnancy and fall risks, using an existing fall accident investigation framework. The results indicated that physiological, biomechanical, and psychological changes associated with pregnancy may influence the initiation, detection, and recovery phases of falls and increase the risk of falls in this population. Considering the logistic difficulties and ethnic concerns in recruiting pregnant women to participate in this investigation of fall risk factors, identification of these factors could establish effective fall prevention and intervention programs for pregnant women and improve birth outcomes. [Workplace Health Saf 2014;62(10):403-408.]
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pproximately 4.4 million women are pregnant annually in the United States (U.S. Census, 2008). It is estimated that during pregnancy, approximately 25% to 27% of these women sustain a fall (Dunning et al., 2003), a rate comparable to older adults age 65 years and older (Rossi-Izquierdo et al., 2014). Falls are the second leading cause of injury among pregnant women (Connolly, Katz, Bash, McMahon, & Hansen, 1997; Crosby, 1983) after motor vehicle accidents. More than 50% of women who fell during pregnancy experienced injuries (Dunning et al., 2003), resulting in approximately 20% of pregnancy-associated injuries (Kuo, Jamieson, McPheters, Meikus, & Posner, 2007; Nannini et al., 2008). Those women who were hospitalized for falls during pregnancy had a higher rate of maternal and fetal morbidity and mortality (Crosby, 1983; Mirza,
ABOUT THE AUTHORS
Ms. Wu is a PhD Candidate, and Dr. Yeoh is Research Associate, Virginia Polytechnic Institute and State University, Blacksburg, Virginia. Submitted: November 4, 2013; Accepted: July 25, 2014; Posted online: September 9, 2014 The authors have disclosed no potential conflicts, financial or otherwise. Correspondence: Han T. Yeoh, PhD, Virginia Polytechnic Institute and State University, Industrial System Engineering, 250 Durham Hall, Blacksburg, VA 24061. E-mail: [email protected] doi:10.3928/21650799-20140902-04
Devine, & Gaddipati, 2010; Weiss, Songer, & Fabio, 2001), and were closely monitored for adverse outcomes (Connolly et al., 1997). Of women who were admitted for a fall, 23% delivered during that hospital visit (Kady, Xing, & Gilbert, 2004). Thus, falls are associated with serious injuries and average pregnancy outcomes. However, despite high fall injury rates and the severity of these injuries, little attention has been paid to investigate the effects of pregnancy on falls or developing fall intervention and prevention programs for pregnant women. To reduce falls among pregnant women, occupational health nurses must identify risk factors that contribute to women’s increased fall risks. In this study, the identification of various fall risk factors for pregnant women will provide a better understanding of the etiology of falls in this population. FALLS Falls are defined as events associated with unintentionally “coming to the ground” or to some lower level (Gibson, 1987), caused by forward, backward, or lateral loss of balance. Both extrinsic and intrinsic factors may affect the risk of falls among pregnant women. The leading extrinsic causes of falls for pregnant women include slippery surfaces, working in a loud environment, and carrying an object or child (Dunning et al., 2003).
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Figure 1. Process of initiation, detection, internal model generation, and recovery of inadvertent fall situation with possible causes.
Although many studies have investigated the effects of extrinsic factors on fall risks (Hisang & Chang, 2002; Park, Lee, Lockhart, & Kim, 2011; Perkins, 1978), intrinsic factors associated with falls among pregnant women are less clear. Considering the difficulties, including ethical concerns, of recruiting pregnant women as study participants, the objective of the current study was to identify possible intrinsic factors associated with pregnancy that may contribute to fall risk by reviewing the literature and presenting evidence of an association between pregnancy and falls using an existing fall framework (Chang & Courtney, 2003; Lockhart, Woldstad, & Smith, 2003). A literature search of Medline and Pubmed databases was conducted; articles published between January 1980 and June 2013 were reviewed for the effects of pregnancy on falls. Additionally, because fall risks may be identified by biomechanical analysis of gait and balance (Hausdorff, Rios, & Edelberg, 2001; Kimura, Kobayashi, Nakayama, & Hanaoaka, 2007), studies evaluating the effects of pregnancy on gait and balance were also reviewed. Key words included “pregnancy,” “falls,” “balance,” “gait,” and key word combinations. The authors believe that knowledge of these factors is essential for establishing effective fall prevention and intervention programs for pregnant women, improving birth outcomes. FACTORS INFLUENCING FALL RISKS AMONG PREGNANT WOMEN Previous studies (Chang & Courtney, 2003; Lockhart, 2008; Lockhart, Smith, & Woldstad, 2005) used several biomechanical models to analyze fall events and define the role of musculoskeletal and neuromuscular systems in these events. A hypothetical unintentional fall situation was modified (isolated internal process) from previous studies (Figure 1, with possible causes in the boxes) to better understand fall risks associated with pregnancy in this study. The processes of fall accidents were divided into four distinct phases (i.e., initiation, detection, internal processing, and recovery). As a consequence of sustaining a perturbation (e.g., slip, trip, or loss of balance), a potential fall can be initiated. Gait and postural changes may contribute to fall initiation risks (Menant, Shone, & Lord, 2014; Wu et al., 2012). During the detection phase, if a potential fall is imminent, proprioception, vision, and vestibular sensory inputs trigger or alert those centers responsible for response selection. At the input stage, any sensory alterations may increase the likelihood of falls. Additionally, inappropriate control schemes (Horak, 1996) using an internal model to alter motor command
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generation can lead to inappropriate recovery response to a postural perturbation, and increase the risk of falls. Furthermore, during the recovery phase, muscular weakness can impair women’s abilities to properly adjust the whole body center-of-mass to prevent falls (Lockhart et al., 2005), and also lead to increased propensity for falls. Pregnant women undergo continuous anatomical and dimensional changes (Chesley, 1994; Jensen, Doucet, & Treitz, 1996; Rutter et al., 1984) including increased body weight, shifted whole body center-of-mass due to joint laxity (Foti, Davis, & Bagley, 2000; Fries & Hellebrandt, 1943), physiological changes (Alvarez, Stokes, Aspirinio, Trevinio, & Braun, 1988; Bryant-Greenwood, 1982; Bursch, 1987; Weiss, 1984; Zarrow, Holmstrom, & Salhanick, 1955), and psychological changes (e.g., anxiety and depression) (Fast, Weiss, Ducommun, Medina, & Butler, 1990). These changes may lead to an increased fall propensity during pregnancy. Weight Gain and Shifted Whole Body Center-of-Mass
Due to the growth and development of the fetus, the mother’s body and its segments change shape and size (Jensen et al., 1996). Pregnant women’s body mass increases an average of 12 to 16 kg (Chesley, 1994; Rutter et al., 1984), which accounts for an average 17% increase over pre-pregnancy body mass (Rutter et al., 1984). The average weight gains by trimester are 1.13, 4.9, and 5.1 kg, respectively, with a variable rate throughout the period (Chesley, 1994). As the fetus grows, pregnant women’s body mass distribution also changes, with most of the weight accumulated on the lower trunk (Jensen et al., 1996). Increased load on the lower anterior trunk results in average increases of 2.8 cm in abdominal extension circumference, 8.2 cm in abdominal extension depth, and 16.1 cm in abdominal circumference (Rutter et al., 1984). As a result, pregnant women’s whole body centerof-mass shifts anteriorly as pregnancy advances (Foti et al., 2000; Fries & Hellebrandt, 1943). To compensate for increased body mass and shifted whole body center-ofmass, biomechanical alterations occur (Foti et al., 2000; Rutter et al., 1984; Diffrient & Tilley, 1979), which can increase pregnant women’s fall initiation risk. First, pregnant women’s increased body mass can lead to increased gravitational movement about the ankle (Corbeil, Simoneau, Rancourt, Tremblay, & Teasdale, 2001) and decreased ankle dorsiflexion (Foti et al., 2000; Hagan & Wong, 2010), increasing trip initiation risk (i.e., forward fall) (Pavol, Owings, Foley, & Grabiner, 2002). Second, shifted whole body center-of-mass and added load on the lower trunk may increase momentum, pulling the whole body center-of-mass forward into the next step. Subsequently, less active force may be needed during the push-off phase of the gait cycle (Hisang & Chang, 2002). Reduced push-off force of the stance leg may reduce the transitional acceleration of the whole body center-of-mass and increase friction demand (Lockhart et al., 2003), resulting in a potential increase in slip initiation risk. Additionally, with increased body mass during pregnancy, increased load on the unsupported (swing) side of the pelvis (Eng & Winter, 1995) must be lifted by the
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stance hip abductor muscles (e.g., shown as increased hip abduction and adduction joint moments) (Branco, Santos-Rocha, Aguiar, Vieria, & Veloso, 2013; Foti et al., 2000). Increased double-support time, when both limbs are in contact with the ground, and decreased singlesupport time, when only one limb is in contact with the ground, may be compensatory mechanisms to minimize the time spent in single support when increased joint effort is required to support increased body mass with only one limb. However, increased use of hip abductors may increase muscle fatigue, which can also lead to fall initiation risk (Parijat & Lockhart, 2008). Increases in body weight throughout pregnancy can also be associated with increases in soft tissue mass, particularly in the thigh, hip, and pelvic regions (Jensen et al., 1996; Rutter et al., 1984). Pregnant women’s hip and thigh depth and circumference increases dramatically during pregnancy and can result in wider step width (Bird, Manz, & Hyde, 1999) and increased lateral force when walking (Branco et al., 2013; Foti et al., 2000; Lymbery & Gilleard, 2005). Pregnant women’s increased mediallateral ground reaction force during walking should be considered in relation to greater demand for side-to-side stability (Enders, Berger, Chambers, Redfern, & McCrory, 2009) due to greater body mass. However, increased medial ground reaction forces (Lymbery & Gilleard, 2005; McCrory, Chambers, Daflary, & Redfern, 2011) may also indicate increased lateral slip-induced fall risks (i.e., increased medial ground reaction force is related to increased medial required coefficient of friction and can lead to lateral slip risk) (Wu et al., 2012). As such, increases in body weight may also increase pregnant women’s lateral fall initiation risks (Troy, Donovan, Marone, Bareither, & Grabiner, 2008). Physiological Changes
Musculoskeletal Alterations. Impaired musculoskeletal functions are known to be strong indicators of fall risks (Close, Lord, Menz, & Sherrington, 2005) during the recovery phase. During pregnancy, a tenfold increase in relaxin (Zarrow, Holmstrom, & Salhanick, 1955) released by the corpus luteum softens ligaments by altering ground substance and digesting collagen fibers (BryantGreenwood, 1982) that affect joint laxity (Dumas & Reid, 1997). Joint relaxation begins in the first half of pregnancy, increases during the next 3 months, and returns to normal soon after delivery (Abramson, Roberts, & Wilson, 1934), with maximum laxity associated with subsequent pregnancies (Calguneri, Bird, & Wright, 1982). Joint laxity can alter joint stability by increasing range of motion of the pelvis (Gutke, Ostgaard, & Oberg, 2008) and peripheral joints (Calguneri et al., 1982; Dumas & Reid, 1997; Marnach et al., 2003) and may lead to less joint stiffness (Crawford & Sauders, 2006). Joint stiffness provides passive support during recovery from postural perturbations (Corbeil et al., 2001). Pregnant women can prevent loss of balance when perturbed by increasing active lower extremity joint movement output to compensate for decreased passive joint support (due to decreased joint stiffness) and increased body mass (Elling, Bryant,
Petersen, Murphy, & Hohmann, 2007). With decreased muscle strength (Morkved, Salvesen, Bo, & Eik-Nes, 2004), joint torque necessary to maintain dynamic balance may exceed joint torque capacity and increase the risk of falls. Additionally, as pregnancy advances, the abdominal muscles lengthen to accommodate the enlarged uterus; thus its function is diminished (Gillard & Brown, 1996). When a large, fast perturbation is experienced, abdominal muscles contribute to balance recovery (Diener, Horak, & Nashner, 1988; Tang & Woollacott, 1998). Therefore, diminished abdominal muscle function may also contribute to pregnant women’s diminished ability to recover from a perturbation-induced loss of balance and may increase the risk of falls. Sensory System Modifications. The lower extremity muscles regulate the center of pressure under the feet, which modulates the whole body center-of-mass movement (Winter, 2009). In a stable system, the lower extremity musculature effectively controls the center of pressure so that the centerof-mass remains within the base of support or the “stable region” (McCollum & Leen, 1989). However, researchers reported that pregnant women demonstrated impaired centers of pressure movement compared with non-pregnant women (Jang, Hsiao, Hsiao-Wecksler, 2008; McCrory et al., 2010; Oliveira, Vieria, Macedo, Simpson, & Nadel, 2009; Poudevigne & O’Connor, 2006). Alterations in center of pressure movement could reflect an altered control system such that leg muscles do not have appropriate response to control joint torques (Foti et al., 2000). This altered control system can reflect inappropriate sensory system inputs. Visual, vestibular, and proprioceptive sensory systems play significant roles in stance and dynamic postural control. During the detection phase of a fall, proprioceptive, visual, and vestibular sensory inputs trigger or alert those centers responsible for response selections. Any sensory function alterations may increase the likelihood of falls. Under normal conditions, the nervous system weighs the priority of proprioceptive information for postural control more heavily than visual and vestibular inputs (Butler, Colon, Druzin, & Rose, 2006). However, a reduction in proprioceptive and vestibular sensory functions can lead to a reliance on the visual system (Choy, Brauer, & Nitz, 2003). Butler et al. (2006) reported increased reliance on the visual system to maintain balance as pregnancy advanced, which may indicate diminished proprioceptive and vestibular functions among pregnant women. Joint laxity and increased interstitial fluid volume (Abrams & Parker, 1990), especially in weight-bearing joints, lead to decreased kinesthetic sensation and diminished coordination among pregnant women (Araujo, 1997), which suggests an alteration in proprioceptive sensory function in weight-bearing joints. Furthermore, compared to non-pregnant women, pregnant women showed increased plantar pressures and larger plantar ground contact area during standing and walking (Nyska et al., 1997). Higher plantar pressures can cause pain (Hodge, Bach, & Carter, 1999) and tissue damage (Kwon & Mueller, 2001), making it harder to sense the need for postural and gait corrections and suggesting a reduction in proprioception within the footpad beneath the bones of the feet. Additionally, Schmidt, Flores, Rossi, and Sil-
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IN SUMMARY
Intrinsic Factors Associated With Pregnancy Falls Wu, X., Yeoh, H. T.
Figure 2. Process of initiation, detection, internal model generation, and recovery of inadvertent fall situation with possible causes associated with pregnancy.
veria (2010) reported that pregnant women had auditory and vestibular complaints, which suggest altered vestibular functions among pregnant women. Therefore, with inaccurate proprioceptive and vestibular sensory inputs (Shumway-Cook & Woollacott, 2001), detection time may increase, leading to increased fall risks among pregnant women. Psychological Factors
Reactive movement in response to postural perturbations is determined by central nervous system mechanisms related to expectations, attention, experience, environmental context, and intention, as well as by preprogrammed muscle synergies (Horak, 1996). If these factors do not coincide with current body status, inappropriate motor commands may be sent, resulting in inadequate or excessive body movements (Shumway-Cook & Woollacott, 2001). During pregnancy, body weight gains are at a non-constant rate (Chesley, 1994). As such, when balance is perturbed, pregnant women’s control scheme may not match their current body weight state, leading to inaccurate control schemes (Shumway-Cook & Woollacott, 2001) and increased risk of falls. Pregnancy is also associated with increased psychological distress for many women, including increased anxiety, depression, and fatigue (Poudevigne & O’Connor, 2006). Psychological explanations for elevated anxiety during the first trimester often focus on chronic worry over miscarriage risk (Leifer, 1977). Anxiety often stabilizes or improves during the second trimester (Leifer, 1977; Power & Parke 1984). During the third trimester, anxiety is more focused on the uncertain birth outcome (Fleming & Corter, 1988). Accordingly, Zib, Lim, and Walters (1999) reported in their controlled study of 117 pregnant women that fatigue was the symptom most often reported during pregnancy, especially during the first trimester (96.6%). Elek, Hudson, and Flek (1997) also reported a 33.1% increase in fatigue during the last trimester. Clearly, anxiety and stress levels in the first and third trimesters are high (Wilson-Evered & Stanely, 1986); arousal levels may be altered from the optimal level (Lader, 1983; Wilson-Evered & Stanely, 1986). Stress, anxiety, and arousal can affect pregnant women’s motor control and readiness to respond to postural perturbations (Yerkes & Dodson, 1908), which can lead to increased fall risk. DISCUSSION The purpose of this study was to identify possible intrinsic factors that may contribute to pregnant women’s increased fall risks. To identify intrinsic factors, a hypo-
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Workplace Health & Safety 2014;62(10):403-408.
1
Pregnant women experience more falls and subsequent injuries. Factors that may lead to the increased fall risks in this population were synthesized by reviewing the literature.
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Factors that may contribute to the increased fall risks in pregnant women were summarized and possible mitigation solutions were suggested.
3
To develop and implement effective fall intervention programs for pregnant women, fall risk factors were analyzed and synthesized from the literature. Suggestions on intervention solutions were also provided to occupational health practitioners.
thetical inadvertent fall accident framework was used to find possible causal effects. Pregnancy can put mothersto-be at increased risk during initiation, detection, internal processing, and recovery phases of a fall (Figure 2). This study identified several factors that may contribute to pregnant women’s fall risk. The growth of the fetus, increased body mass, and shifted whole body centerof-mass may influence gait and posture, which may increase fall risk (Menant et al., 2014; Wu et al., 2012). For fall-initiating risks, decreased ankle dorsiflexion and altered ground reaction forces during normal walking may increase the risks of slip, trip, and fall-initiation risks. Moreover, altered sensory system functions due to physiological changes among pregnant women can elevate their fall risks during the detection phase of a fall because altered sensory system inputs (Araujo, 1997) can increase detection time and reduce detection accuracy prior to fall events. Increased detection time and reduced detection accuracy can lead to inappropriate motor command generation and increase the risk of falls. In addition, inappropriate control schemes (Horak, 1996) during the internal processing phase may also increase pregnant women’s fall risk. Finally, during the recovery phase, pregnant women’s muscular weakness can also result in more falls (Morkved et al., 2004). Although implicated, not enough empirical data are available to generalize the mechanisms associated with falls among pregnant women. As such, this study only presented possible fall mechanisms by synthesizing limited literature. Furthermore, the focus of this study was on normal pregnant women without any pregnancy-associated complications. Minor but normal complications such as low back pain, edema, pregnancy-related pelvic pain, and calf cramping were not discussed in this study
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because all of these complications may cause further anatomical and physiological changes in pregnant women and further increase the risk of falls. Why do pregnant women fall more than nonpregnant women? Based on this literature review, pregnancy can alter characteristics of initiation, detection, internal model generation, and recovery phases of fall accidents, which can contribute to an increased likelihood of falls in pregnant women. Considering barriers to and ethical concerns about recruiting pregnant women to investigate their fall risk, knowledge of these factors can be used to establish fall prevention and intervention programs for pregnant women and improve birth outcomes. Appropriate interventions such as balance training (i.e., yoga), exercise (i.e., swim), use of personal protective equipment (i.e., shoes to increase the shoe-floor coefficient of friction), administrative controls (i.e., work/rest schedules), and engineering controls (i.e., reduce job components during which pregnant workers must carry a load or child) may reduce falls among pregnant women and improve birth outcomes. REFERENCES
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study using stabilometry. European Journal of Obstetrics and Gynecology and Reproductive Biology, 147, 25-28. Park, S. H., Lee, K., Lockhart, T. E., & Kim, S. (2011). Effects of sound on postural stability during quiet standing. Journal of Neuroengineering Rehabilitation, 15, 67. Parijat, P., & Lockhart, T. E. (2008). Effects of quadriceps fatigue on the biomechanics of gait and slip propensity. Gait Posture, 28, 568-573. Pavol, M. J., Owings, T. M., Foley, K. T., & Grabiner, M. D. (2002). Influence of lower extremity strength of healthy older adults on the outcome of an induced trip. Journal of American Geriatrics Society, 50, 256-262. Perkins, P. J. (1978). Measurement of slip between the shoe and ground during walking. American Society for Testing and Material Special Technical Publication, 649, 71-87. Poudevigne, M. S., & O’Connor, P. J. (2006). A review of physical activity patterns in pregnant women and their relationship to psychological health. Sports Medicine, 36, 19-38. Power, T. G., & Parke, R. D. (1984). Social network factors and the transition to parenthood. Sex Roles, 10, 949-972. Rossi-Izquierdo, M., Santos-Perez, S., Del-Rio-Valeiras, M., LirolaDelgado, A., Faraldo-Garcia, A., Vaamonde-Sanchez-Andrade, I., . . . Soto-Varela, A. (2014). Is there a relationship between objective and subjective assessment of balance in elderly patients with instability? European Archives of Otorhinolaryngology. Published ahead of print June 12, 2014. Rutter, B. G., Haager, J. A., Daigle, G. C., Smith, S., Jr., McFarland, N., & Kelsey, N. (1984). Dimensional changes throughout pregnancy: A preliminary report. Charles Selected Papers, 36, 44-52. Schmidt, P. M., Flores, F. T., Rossi, A. G., & Silveria, A. F. (2010). Hearing and vestibular complaints during pregnancy. Brazilian Journal of Otorhinolaryngology, 76, 29-33. Shumway-Cook, A., & Woollacott, M. H. (2001). Motor control theory and practical applications (2nd Ed.). Philadelphia: Lippincott, Williams & Wilkins. Tang, P. F., & Woollacott, M. H. (1998). Inefficient postural responses to unexpected slips during walking in older adults. Journal of Gerontology, 53, M471-M480. Troy, K. L., Donovan, S. J., Marone, J. R., Bareither, M .L., & Grabiner, M. D. (2008). Modifiable performance domain risk-factors that cause slip-related falls. Gait Posture, 28, 461-465. U.S. Census. (2008). Fertility of American women-2008. Retrieved from http://www.census.gov/hhes/fertility/data/cps/2008.html Weiss, G. (1984). Relaxin. Annual Review of Physiology, 46, 43. Weiss, H. B., Songer, T. J., & Fabio, A. (2001). Fetal deaths related to maternal injury. Journal of American Medical Association, 286, 1863-1868. Wilson-Evered, E., & Stanely, G. (1986). Stress and arousal during pregnancy and childbirth. British Journal of Medical Psychology, 59, 57-60. Winter, D. A. (2009). Biomechanics and motor control of human movement (4th Ed.). New York: Wiley. Wu, X., Lockhart, T. E., & Yeoh, H. T. (2012). Effects of obesity on slip-induced fall risks among young male adults. Journal of Biomechanics, 45, 1042-1047. Yerkes, R. M., & Dodson, J. D. (1908). The relation of strength of stimulus to rapidity of habit-formation. Journal of Comparative Neurology and Psychology, 18, 459-482. Zarrow, M. X., Holmstrom, E. G., & Salhanick, H. A. (1955). The concentration of relaxin in the blood serum and other tissues of women during pregnancy. Journal of Clinical Endocrinology and Metabolism, 15, 22-27. Zib, M., Lim, L., & Walters, A. W. (1999). Symptoms during normal pregnancy: A prospective controlled study. Australian and New Zealand Journal of Obstetrics and Gynecology, 39, 401-410.
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ABSTRACT Approximately 25% to 27% of women sustain a fall during pregnancy, and falls are associated with serious injuries and can affect pregnancy outcomes. The objective of the current study was to identify intrinsic factors associated with pregnancy that may contribute to women’s increased risk of falls. A literature search (Medline and Pubmed) identified articles published between January 1980 and June 2013 that measured associations between pregnancy and fall risks, using an existing fall accident investigation framework. The results indicated that physiological, biomechanical, and psychological changes associated with pregnancy may influence the initiation, detection, and recovery phases of falls and increase the risk of falls in this population. Considering the logistic difficulties and ethnic concerns in recruiting pregnant women to participate in this investigation of fall risk factors, identification of these factors could establish effective fall prevention and intervention programs for pregnant women and improve birth outcomes. [Workplace Health Saf 2014;62(10):403-408.]
A
pproximately 4.4 million women are pregnant annually in the United States (U.S. Census, 2008). It is estimated that during pregnancy, approximately 25% to 27% of these women sustain a fall (Dunning et al., 2003), a rate comparable to older adults age 65 years and older (Rossi-Izquierdo et al., 2014). Falls are the second leading cause of injury among pregnant women (Connolly, Katz, Bash, McMahon, & Hansen, 1997; Crosby, 1983) after motor vehicle accidents. More than 50% of women who fell during pregnancy experienced injuries (Dunning et al., 2003), resulting in approximately 20% of pregnancy-associated injuries (Kuo, Jamieson, McPheters, Meikus, & Posner, 2007; Nannini et al., 2008). Those women who were hospitalized for falls during pregnancy had a higher rate of maternal and fetal morbidity and mortality (Crosby, 1983; Mirza,
ABOUT THE AUTHORS
Ms. Wu is a PhD Candidate, and Dr. Yeoh is Research Associate, Virginia Polytechnic Institute and State University, Blacksburg, Virginia. Submitted: November 4, 2013; Accepted: July 25, 2014; Posted online: September 9, 2014 The authors have disclosed no potential conflicts, financial or otherwise. Correspondence: Han T. Yeoh, PhD, Virginia Polytechnic Institute and State University, Industrial System Engineering, 250 Durham Hall, Blacksburg, VA 24061. E-mail: [email protected] doi:10.3928/21650799-20140902-04
Devine, & Gaddipati, 2010; Weiss, Songer, & Fabio, 2001), and were closely monitored for adverse outcomes (Connolly et al., 1997). Of women who were admitted for a fall, 23% delivered during that hospital visit (Kady, Xing, & Gilbert, 2004). Thus, falls are associated with serious injuries and average pregnancy outcomes. However, despite high fall injury rates and the severity of these injuries, little attention has been paid to investigate the effects of pregnancy on falls or developing fall intervention and prevention programs for pregnant women. To reduce falls among pregnant women, occupational health nurses must identify risk factors that contribute to women’s increased fall risks. In this study, the identification of various fall risk factors for pregnant women will provide a better understanding of the etiology of falls in this population. FALLS Falls are defined as events associated with unintentionally “coming to the ground” or to some lower level (Gibson, 1987), caused by forward, backward, or lateral loss of balance. Both extrinsic and intrinsic factors may affect the risk of falls among pregnant women. The leading extrinsic causes of falls for pregnant women include slippery surfaces, working in a loud environment, and carrying an object or child (Dunning et al., 2003).
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Figure 1. Process of initiation, detection, internal model generation, and recovery of inadvertent fall situation with possible causes.
Although many studies have investigated the effects of extrinsic factors on fall risks (Hisang & Chang, 2002; Park, Lee, Lockhart, & Kim, 2011; Perkins, 1978), intrinsic factors associated with falls among pregnant women are less clear. Considering the difficulties, including ethical concerns, of recruiting pregnant women as study participants, the objective of the current study was to identify possible intrinsic factors associated with pregnancy that may contribute to fall risk by reviewing the literature and presenting evidence of an association between pregnancy and falls using an existing fall framework (Chang & Courtney, 2003; Lockhart, Woldstad, & Smith, 2003). A literature search of Medline and Pubmed databases was conducted; articles published between January 1980 and June 2013 were reviewed for the effects of pregnancy on falls. Additionally, because fall risks may be identified by biomechanical analysis of gait and balance (Hausdorff, Rios, & Edelberg, 2001; Kimura, Kobayashi, Nakayama, & Hanaoaka, 2007), studies evaluating the effects of pregnancy on gait and balance were also reviewed. Key words included “pregnancy,” “falls,” “balance,” “gait,” and key word combinations. The authors believe that knowledge of these factors is essential for establishing effective fall prevention and intervention programs for pregnant women, improving birth outcomes. FACTORS INFLUENCING FALL RISKS AMONG PREGNANT WOMEN Previous studies (Chang & Courtney, 2003; Lockhart, 2008; Lockhart, Smith, & Woldstad, 2005) used several biomechanical models to analyze fall events and define the role of musculoskeletal and neuromuscular systems in these events. A hypothetical unintentional fall situation was modified (isolated internal process) from previous studies (Figure 1, with possible causes in the boxes) to better understand fall risks associated with pregnancy in this study. The processes of fall accidents were divided into four distinct phases (i.e., initiation, detection, internal processing, and recovery). As a consequence of sustaining a perturbation (e.g., slip, trip, or loss of balance), a potential fall can be initiated. Gait and postural changes may contribute to fall initiation risks (Menant, Shone, & Lord, 2014; Wu et al., 2012). During the detection phase, if a potential fall is imminent, proprioception, vision, and vestibular sensory inputs trigger or alert those centers responsible for response selection. At the input stage, any sensory alterations may increase the likelihood of falls. Additionally, inappropriate control schemes (Horak, 1996) using an internal model to alter motor command
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generation can lead to inappropriate recovery response to a postural perturbation, and increase the risk of falls. Furthermore, during the recovery phase, muscular weakness can impair women’s abilities to properly adjust the whole body center-of-mass to prevent falls (Lockhart et al., 2005), and also lead to increased propensity for falls. Pregnant women undergo continuous anatomical and dimensional changes (Chesley, 1994; Jensen, Doucet, & Treitz, 1996; Rutter et al., 1984) including increased body weight, shifted whole body center-of-mass due to joint laxity (Foti, Davis, & Bagley, 2000; Fries & Hellebrandt, 1943), physiological changes (Alvarez, Stokes, Aspirinio, Trevinio, & Braun, 1988; Bryant-Greenwood, 1982; Bursch, 1987; Weiss, 1984; Zarrow, Holmstrom, & Salhanick, 1955), and psychological changes (e.g., anxiety and depression) (Fast, Weiss, Ducommun, Medina, & Butler, 1990). These changes may lead to an increased fall propensity during pregnancy. Weight Gain and Shifted Whole Body Center-of-Mass
Due to the growth and development of the fetus, the mother’s body and its segments change shape and size (Jensen et al., 1996). Pregnant women’s body mass increases an average of 12 to 16 kg (Chesley, 1994; Rutter et al., 1984), which accounts for an average 17% increase over pre-pregnancy body mass (Rutter et al., 1984). The average weight gains by trimester are 1.13, 4.9, and 5.1 kg, respectively, with a variable rate throughout the period (Chesley, 1994). As the fetus grows, pregnant women’s body mass distribution also changes, with most of the weight accumulated on the lower trunk (Jensen et al., 1996). Increased load on the lower anterior trunk results in average increases of 2.8 cm in abdominal extension circumference, 8.2 cm in abdominal extension depth, and 16.1 cm in abdominal circumference (Rutter et al., 1984). As a result, pregnant women’s whole body centerof-mass shifts anteriorly as pregnancy advances (Foti et al., 2000; Fries & Hellebrandt, 1943). To compensate for increased body mass and shifted whole body center-ofmass, biomechanical alterations occur (Foti et al., 2000; Rutter et al., 1984; Diffrient & Tilley, 1979), which can increase pregnant women’s fall initiation risk. First, pregnant women’s increased body mass can lead to increased gravitational movement about the ankle (Corbeil, Simoneau, Rancourt, Tremblay, & Teasdale, 2001) and decreased ankle dorsiflexion (Foti et al., 2000; Hagan & Wong, 2010), increasing trip initiation risk (i.e., forward fall) (Pavol, Owings, Foley, & Grabiner, 2002). Second, shifted whole body center-of-mass and added load on the lower trunk may increase momentum, pulling the whole body center-of-mass forward into the next step. Subsequently, less active force may be needed during the push-off phase of the gait cycle (Hisang & Chang, 2002). Reduced push-off force of the stance leg may reduce the transitional acceleration of the whole body center-of-mass and increase friction demand (Lockhart et al., 2003), resulting in a potential increase in slip initiation risk. Additionally, with increased body mass during pregnancy, increased load on the unsupported (swing) side of the pelvis (Eng & Winter, 1995) must be lifted by the
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stance hip abductor muscles (e.g., shown as increased hip abduction and adduction joint moments) (Branco, Santos-Rocha, Aguiar, Vieria, & Veloso, 2013; Foti et al., 2000). Increased double-support time, when both limbs are in contact with the ground, and decreased singlesupport time, when only one limb is in contact with the ground, may be compensatory mechanisms to minimize the time spent in single support when increased joint effort is required to support increased body mass with only one limb. However, increased use of hip abductors may increase muscle fatigue, which can also lead to fall initiation risk (Parijat & Lockhart, 2008). Increases in body weight throughout pregnancy can also be associated with increases in soft tissue mass, particularly in the thigh, hip, and pelvic regions (Jensen et al., 1996; Rutter et al., 1984). Pregnant women’s hip and thigh depth and circumference increases dramatically during pregnancy and can result in wider step width (Bird, Manz, & Hyde, 1999) and increased lateral force when walking (Branco et al., 2013; Foti et al., 2000; Lymbery & Gilleard, 2005). Pregnant women’s increased mediallateral ground reaction force during walking should be considered in relation to greater demand for side-to-side stability (Enders, Berger, Chambers, Redfern, & McCrory, 2009) due to greater body mass. However, increased medial ground reaction forces (Lymbery & Gilleard, 2005; McCrory, Chambers, Daflary, & Redfern, 2011) may also indicate increased lateral slip-induced fall risks (i.e., increased medial ground reaction force is related to increased medial required coefficient of friction and can lead to lateral slip risk) (Wu et al., 2012). As such, increases in body weight may also increase pregnant women’s lateral fall initiation risks (Troy, Donovan, Marone, Bareither, & Grabiner, 2008). Physiological Changes
Musculoskeletal Alterations. Impaired musculoskeletal functions are known to be strong indicators of fall risks (Close, Lord, Menz, & Sherrington, 2005) during the recovery phase. During pregnancy, a tenfold increase in relaxin (Zarrow, Holmstrom, & Salhanick, 1955) released by the corpus luteum softens ligaments by altering ground substance and digesting collagen fibers (BryantGreenwood, 1982) that affect joint laxity (Dumas & Reid, 1997). Joint relaxation begins in the first half of pregnancy, increases during the next 3 months, and returns to normal soon after delivery (Abramson, Roberts, & Wilson, 1934), with maximum laxity associated with subsequent pregnancies (Calguneri, Bird, & Wright, 1982). Joint laxity can alter joint stability by increasing range of motion of the pelvis (Gutke, Ostgaard, & Oberg, 2008) and peripheral joints (Calguneri et al., 1982; Dumas & Reid, 1997; Marnach et al., 2003) and may lead to less joint stiffness (Crawford & Sauders, 2006). Joint stiffness provides passive support during recovery from postural perturbations (Corbeil et al., 2001). Pregnant women can prevent loss of balance when perturbed by increasing active lower extremity joint movement output to compensate for decreased passive joint support (due to decreased joint stiffness) and increased body mass (Elling, Bryant,
Petersen, Murphy, & Hohmann, 2007). With decreased muscle strength (Morkved, Salvesen, Bo, & Eik-Nes, 2004), joint torque necessary to maintain dynamic balance may exceed joint torque capacity and increase the risk of falls. Additionally, as pregnancy advances, the abdominal muscles lengthen to accommodate the enlarged uterus; thus its function is diminished (Gillard & Brown, 1996). When a large, fast perturbation is experienced, abdominal muscles contribute to balance recovery (Diener, Horak, & Nashner, 1988; Tang & Woollacott, 1998). Therefore, diminished abdominal muscle function may also contribute to pregnant women’s diminished ability to recover from a perturbation-induced loss of balance and may increase the risk of falls. Sensory System Modifications. The lower extremity muscles regulate the center of pressure under the feet, which modulates the whole body center-of-mass movement (Winter, 2009). In a stable system, the lower extremity musculature effectively controls the center of pressure so that the centerof-mass remains within the base of support or the “stable region” (McCollum & Leen, 1989). However, researchers reported that pregnant women demonstrated impaired centers of pressure movement compared with non-pregnant women (Jang, Hsiao, Hsiao-Wecksler, 2008; McCrory et al., 2010; Oliveira, Vieria, Macedo, Simpson, & Nadel, 2009; Poudevigne & O’Connor, 2006). Alterations in center of pressure movement could reflect an altered control system such that leg muscles do not have appropriate response to control joint torques (Foti et al., 2000). This altered control system can reflect inappropriate sensory system inputs. Visual, vestibular, and proprioceptive sensory systems play significant roles in stance and dynamic postural control. During the detection phase of a fall, proprioceptive, visual, and vestibular sensory inputs trigger or alert those centers responsible for response selections. Any sensory function alterations may increase the likelihood of falls. Under normal conditions, the nervous system weighs the priority of proprioceptive information for postural control more heavily than visual and vestibular inputs (Butler, Colon, Druzin, & Rose, 2006). However, a reduction in proprioceptive and vestibular sensory functions can lead to a reliance on the visual system (Choy, Brauer, & Nitz, 2003). Butler et al. (2006) reported increased reliance on the visual system to maintain balance as pregnancy advanced, which may indicate diminished proprioceptive and vestibular functions among pregnant women. Joint laxity and increased interstitial fluid volume (Abrams & Parker, 1990), especially in weight-bearing joints, lead to decreased kinesthetic sensation and diminished coordination among pregnant women (Araujo, 1997), which suggests an alteration in proprioceptive sensory function in weight-bearing joints. Furthermore, compared to non-pregnant women, pregnant women showed increased plantar pressures and larger plantar ground contact area during standing and walking (Nyska et al., 1997). Higher plantar pressures can cause pain (Hodge, Bach, & Carter, 1999) and tissue damage (Kwon & Mueller, 2001), making it harder to sense the need for postural and gait corrections and suggesting a reduction in proprioception within the footpad beneath the bones of the feet. Additionally, Schmidt, Flores, Rossi, and Sil-
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IN SUMMARY
Intrinsic Factors Associated With Pregnancy Falls Wu, X., Yeoh, H. T.
Figure 2. Process of initiation, detection, internal model generation, and recovery of inadvertent fall situation with possible causes associated with pregnancy.
veria (2010) reported that pregnant women had auditory and vestibular complaints, which suggest altered vestibular functions among pregnant women. Therefore, with inaccurate proprioceptive and vestibular sensory inputs (Shumway-Cook & Woollacott, 2001), detection time may increase, leading to increased fall risks among pregnant women. Psychological Factors
Reactive movement in response to postural perturbations is determined by central nervous system mechanisms related to expectations, attention, experience, environmental context, and intention, as well as by preprogrammed muscle synergies (Horak, 1996). If these factors do not coincide with current body status, inappropriate motor commands may be sent, resulting in inadequate or excessive body movements (Shumway-Cook & Woollacott, 2001). During pregnancy, body weight gains are at a non-constant rate (Chesley, 1994). As such, when balance is perturbed, pregnant women’s control scheme may not match their current body weight state, leading to inaccurate control schemes (Shumway-Cook & Woollacott, 2001) and increased risk of falls. Pregnancy is also associated with increased psychological distress for many women, including increased anxiety, depression, and fatigue (Poudevigne & O’Connor, 2006). Psychological explanations for elevated anxiety during the first trimester often focus on chronic worry over miscarriage risk (Leifer, 1977). Anxiety often stabilizes or improves during the second trimester (Leifer, 1977; Power & Parke 1984). During the third trimester, anxiety is more focused on the uncertain birth outcome (Fleming & Corter, 1988). Accordingly, Zib, Lim, and Walters (1999) reported in their controlled study of 117 pregnant women that fatigue was the symptom most often reported during pregnancy, especially during the first trimester (96.6%). Elek, Hudson, and Flek (1997) also reported a 33.1% increase in fatigue during the last trimester. Clearly, anxiety and stress levels in the first and third trimesters are high (Wilson-Evered & Stanely, 1986); arousal levels may be altered from the optimal level (Lader, 1983; Wilson-Evered & Stanely, 1986). Stress, anxiety, and arousal can affect pregnant women’s motor control and readiness to respond to postural perturbations (Yerkes & Dodson, 1908), which can lead to increased fall risk. DISCUSSION The purpose of this study was to identify possible intrinsic factors that may contribute to pregnant women’s increased fall risks. To identify intrinsic factors, a hypo-
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Workplace Health & Safety 2014;62(10):403-408.
1
Pregnant women experience more falls and subsequent injuries. Factors that may lead to the increased fall risks in this population were synthesized by reviewing the literature.
2
Factors that may contribute to the increased fall risks in pregnant women were summarized and possible mitigation solutions were suggested.
3
To develop and implement effective fall intervention programs for pregnant women, fall risk factors were analyzed and synthesized from the literature. Suggestions on intervention solutions were also provided to occupational health practitioners.
thetical inadvertent fall accident framework was used to find possible causal effects. Pregnancy can put mothersto-be at increased risk during initiation, detection, internal processing, and recovery phases of a fall (Figure 2). This study identified several factors that may contribute to pregnant women’s fall risk. The growth of the fetus, increased body mass, and shifted whole body centerof-mass may influence gait and posture, which may increase fall risk (Menant et al., 2014; Wu et al., 2012). For fall-initiating risks, decreased ankle dorsiflexion and altered ground reaction forces during normal walking may increase the risks of slip, trip, and fall-initiation risks. Moreover, altered sensory system functions due to physiological changes among pregnant women can elevate their fall risks during the detection phase of a fall because altered sensory system inputs (Araujo, 1997) can increase detection time and reduce detection accuracy prior to fall events. Increased detection time and reduced detection accuracy can lead to inappropriate motor command generation and increase the risk of falls. In addition, inappropriate control schemes (Horak, 1996) during the internal processing phase may also increase pregnant women’s fall risk. Finally, during the recovery phase, pregnant women’s muscular weakness can also result in more falls (Morkved et al., 2004). Although implicated, not enough empirical data are available to generalize the mechanisms associated with falls among pregnant women. As such, this study only presented possible fall mechanisms by synthesizing limited literature. Furthermore, the focus of this study was on normal pregnant women without any pregnancy-associated complications. Minor but normal complications such as low back pain, edema, pregnancy-related pelvic pain, and calf cramping were not discussed in this study
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because all of these complications may cause further anatomical and physiological changes in pregnant women and further increase the risk of falls. Why do pregnant women fall more than nonpregnant women? Based on this literature review, pregnancy can alter characteristics of initiation, detection, internal model generation, and recovery phases of fall accidents, which can contribute to an increased likelihood of falls in pregnant women. Considering barriers to and ethical concerns about recruiting pregnant women to investigate their fall risk, knowledge of these factors can be used to establish fall prevention and intervention programs for pregnant women and improve birth outcomes. Appropriate interventions such as balance training (i.e., yoga), exercise (i.e., swim), use of personal protective equipment (i.e., shoes to increase the shoe-floor coefficient of friction), administrative controls (i.e., work/rest schedules), and engineering controls (i.e., reduce job components during which pregnant workers must carry a load or child) may reduce falls among pregnant women and improve birth outcomes. REFERENCES
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