Age-related changes in speed and accuracy during rapid targeted center of pressure movements near the posterior limit of the base of support
Introduction
Falls are among the most significant causes of mortality and serious injury in adults over 65 years of age (Riley, 1992, Rubenstein and Josephson, 2002), particularly for older women (Norton et al., 1997, Tinetti et al., 1995). Backward falls are of particular significance, as they are likely to lead to wrist fractures among older women (Nevitt and Cummings, 1993). When recovering balance from large posterior perturbations, older adults are more apt to take multiple steps than young adults (Luchies et al., 1994, Schulz et al., 2005). While leaning maximally, older adults exhibited increased center of pressure (COP) variability and reduced spatiotemporal stability margins in the anteroposterior direction, when compared to young adults (van Wegen et al., 2002). The use of more frequent and variable COP movements by older adults when maintaining balance near the anteroposterior limits of stability is undesirable, as it would present a greater risk for a loss of balance, particularly in response to large perturbations.
To prevent a loss of balance while leaning when standing bipedally, the COP must often be moved rapidly in the sagittal plane to keep the center of mass (COM) from straying beyond the functional base of support. During rapid untargeted continuous COP movements, decreases in speed have been found with increased age and fall-risk (Tucker et al., 2008, Tucker et al., 2009). Considering the speed–accuracy tradeoffs that occur when the COP is moved rapidly (Duarte and Freitas, 2005), limitations on the accuracy or maximal speed of compensatory COP movements while leaning, reaching, or bending down to the floor would be expected as we age, due to neuromuscular changes (Galganski et al., 1993, Roos et al., 1997, Tracy and Enoka, 2002). Decreases in the number of large amplitude weight transfers have been observed among older adults versus young (Prado et al., 2011). As proposed by Lafond et al., postural changes (i.e., weight transfers) can be viewed as a physiological response to reduce musculoskeletal fatigue and discomfort (Brantingham et al., 1970, Cavanagh et al., 1987, Lafond et al., 2009, Zhang et al., 1991). Given the need for somatosensory information to trigger weight transfers, a decrease in weight transfers may be indicative of impairments in the somatosensory system (Lafond et al., 2009). As it is the position and change in position of the COP that controls the center of gravity, the evaluation of COP control strategies should provide insight into the mechanisms underlying age-related changes the postural control of functional movements.
Speed–accuracy tradeoffs have been observed in a wide range of tasks (Danion et al., 1999, Duarte and Freitas, 2005, Fitts, 1954, Plamondon and Alimi, 1997). The stochastic optimized submovement model proposes the use of a ballistic, primary submovement (PSM) or a series of secondary, corrective submovements (Meyer et al., 1988). Depending on the operational demands of a task, individuals can employ low force, slow submovements to achieve a high level of accuracy or high forces so as to generate rapid movements. However, to compensate for the increased motor noise, rapid movements would necessitate an increased number of submovements to accurately reach a desired target. Thus, the underlying motor control noise is expected to directly compromise primary submovement characteristics. The optimized submovement model facilitates predictions about age-related changes in the underlying submovement structure of targeted movements, as increases in motor noise due to age (Galganski et al., 1993, Roos et al., 1997, Tracy and Enoka, 2002) would be expected to result in a higher number of corrective submovements during targeted movements.
Older adults use slower movements, than young adults, to achieve a similar level of accuracy (Goggin and Meeuwsen, 1992, Hernandez et al., 2012, Ketcham et al., 2002, Salthouse, 1988). However, few data exist regarding COP speed–accuracy changes with age under challenging balance conditions, such as those that might precede a loss of balance. During undisturbed upright stance, older adults have demonstrated greater delays before deploying feedback control of COP movements (Collins et al., 1995), which is consistent with findings of increased distal muscle latency in older adults during postural perturbations (Woollacott et al., 1986). Older adults have shown a higher variability in postural responses to more challenging balance conditions (Schultz et al., 1992). Thus, we would expect older adults, who tend to have greater variability during upright stance (Pyykkö et al., 1990) and slower voluntary movements when correcting postural perturbations (Stelmach et al., 1989), to use more conservative COP movements near the limits of stability, in contrast to young. On the other hand, the use of hand support has been shown to reduce postural sway with just a light touch (Jeka, 1997, Jeka and Lackner, 1994) and would be expected to reduce age-related changes in postural control. Thus, the provision of hand support would provide us with further insight into the benefits of additional support in the COP control of large amplitude movements.
The primary goal of this study was to examine the effects of age on the speed and accuracy of PSMs in large and small posterior leaning tasks. We hypothesized that in comparison to young women, older women would display a disproportionate decrease of speed and accuracy (e.g., increased number of submovements and increased incidence of COP undershooting) in their COP PSMs, as movement amplitude increased. We expect that the analysis of COP primary and secondary submovements may better describe the extent of the increasingly conservative strategy used by older adults near the limits of the functional base of support (FBOS), a quasi-static limit of stability (King et al., 1994), to maintain safe upright stance. These data may ultimately provide insight into mechanisms underlying fall avoidance in older adults, particularly in the backward direction.
Section snippets
Participants
Thirteen young and twelve older healthy, community-dwelling, women were recruited for this study. Young women were 19–29 years (mean (SD) age 23 (3) years), 155–175 cm tall (164 (6) cm), and weighted 49.1–92.7 kg (63 (11) kg). Older women were 68–84 years (76 (6) years), 150–169 cm tall (159 (5) cm), and weighted 51.4–85.9 kg (63 (11) kg). All young participants completed a medical history questionnaire and older participants were physically screened by a nurse practitioner, so as to exclude those
Results
Older women demonstrated no significant differences in their baseline postural sway (i.e., COP RMS error) with eyes open in an upright stance, or in the amount of body weight they supported through the use of hand support, when compared to younger women (P > 0.05). However, older women were shorter and had a shorter mean (SD) functional base of support (FBOS) than young women (17(2) cm versus 20(1) cm, P < 0.05). Table 1 presents the mean and standard deviation of primary submovement (PSM) speed,
Discussion
To our knowledge, this is the first study to examine the effects of age and movement amplitude on discrete and large accuracy-constrained COP movements in upright bilateral stance. A novel finding was that primary submovement (PSM) speed was disproportionately decreased in older women when compared to young as movement amplitude increased. Even though older women achieved similar endpoint accuracy, they demonstrated a 2 to 5-fold increase in the incidence rate of the PSM undershooting the
Conclusions
The increased incidence rate of undershooting by the PSM and increased secondary submovements are indicative of an increasingly conservative strategy used by older adults near the limits of the FBOS that may explain their slower speeds during whole body movements to maintain upright balance. These data may ultimately provide insight into mechanisms underlying fall avoidance in older adults, particularly in the backward direction.
Acknowledgments
The authors would like to acknowledge the support of National Institute on Aging (NIA) grant AG024824 (University of Michigan Claude D. Pepper Older Americans Independence Center), NIA grant F31AG024689, the Office of Research and Development, Medical Service and Rehabilitation Research, the Development Service of the Department of Veterans Affairs, and the Dorothy and Herman Miller Fund for Mobility Research in Older Adults. Dr. Alexander is also a recipient of the K24 Mid-Career Investigator
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