AMERICAN SOCIETY OF BIOMECHANICS
Presented at the Twenty-First Annual Meeting
of the American Society of Biomechanics
Clemson University, South Carolina
September 24-27, 1997
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| AMERICAN SOCIETY OF BIOMECHANICS
Presented at the Twenty-First Annual Meeting |
Older adults experience an increased difficulty with the performance of activities of daily living (ADL), a decreased ability to maintain postural balance, and an increased rate of falls (Tinetti, 1986; Schultz, 1992). With age, elderly first experience difficulty with the more challenging activities of daily living, rather than the easier tasks. One of the ADL tasks they view as most challenging is standing on a stool or step ladder to reach for something (Powell 1995). This study was undertaken to examine how age and biomechanical factors interact to affect performance when healthy adults are confronted with having to reach forward as far as possible while standing on a base of support of different heights and sizes.
The maximal distance a person can reach forward while maintaining a fixed base of support in the standing position is a recognized clinical measure of balance. For example, Duncan et al. (1990) showed that forward reach correlates with the center of pressure excursion in a precise and repeatable manner. In addition, they reported that forward reach decreased with age and hypothesized that this decrease might serve as a protective mechanism to minimize the displacement of the center of gravity in order to prevent falling. The challenging nature of reaching is supported by the fact that falls resulting in injuries most commonly occur during daily activities requiring reaching (Nevitt et al., 1991).
In its simplest terms the maintenance of balance involves maintaining the center of gravity within the existing base of support (BOS), usually defined by the area and position of the feet. If the center of gravity moves outside the BOS, a rapid motor response must be initiated to restore balance and prevent a fall. King et al. (1994) have found that the functional base of support, defined in their study as the proportion of the anterior-posterior dimension of the base of support utilized during sustained maximal forward and backward leaning, is decreased in older persons. Their results confirmed those of Lee and Deming (1988) who also found that the effective standing BOS decreased with age for maximal forward, backward, right and left leans. This decrease in BOS may make a person more likely to lose their balance and fall during activities such as reaching, which can potentially require full BOS usage in certain directions.
The purpose of this study was to examine the functional BOS used by both young and older subjects during the performance of a maximal reach and to examine the effects of experimentally manipulating the physical BOS (PBOS) available. We tested the hypothesis that experimentally manipulating the PBOS will neither affect reach distance nor center of reaction (COR) parameters.
Ten young females (YA) (mean age 22 years) and ten community-dwelling healthy older females (OA) (mean age 72 years) were tested while wearing a ceiling-suspended full body safety harness. The OA were examined by a physician-geriatrician and cleared for participation in this experiment. After 30s of quiet stance, subjects were instructed to reach towards, and touch, a target with a finger from each hand. Maximum reach achieved in 10 trials, normalized by outstretched arm length, was recorded. Subjects performed the reaching protocol standing both on a 10 cm "low" platform and on a 60 cm "high" rigid platform. To study the effect of manipulating PBOS, the protocol was repeated using three different foot positions: 100% of the length of each foot supported by a horizontal platform, the rear 75% of each foot supported (toes off), and the rear 65% of each foot supported on the platform, much as could be the case on a step ladder. Each foot was supported by an AMTI force platform which was used to record at 100 Hz the ground reaction forces under their feet with 12 bit resolution. COR data were then calculated. Data were analyzed using a 2 (Platform Height) x 2 (Age) x 3 (PBOS) repeated measures analysis of variance.
The results showed that OA reached 22% less far than the YA. The reach of the YA and OA was significantly decreased by 8% and 14% respectively (p<.001) when the subjects stood on the high platform. However, there were no significant interactions between height of platform and age. Decreasing PBOS from 100% to 65% foot length decreased maximum reach distance by approximately 20% (p <.0001) in both groups of subjects. However, YA reach was decreased by a greater absolute distance (8cm) from their larger initial reach than the OA reach (5.6 cm) (p < .018). The measurement of maximum reach was found to be reliable, with a high correlation (r >.9) between two successive maximum reaches.
The physical 'safety margin' may be defined as the distance between the edge of the force platform and the greatest anterior displacement of the COR during forward reach. For both the 100% PBOS and 65% PBOS conditions the OA had a safety margin approximately twice that of the YA (p < .05). Decreasing PBOS by 35% from 100% to 65% resulted in approximately a 70% decrease in safety margin in both YA and OA (p < .0001). The interaction between age group and PBOS manipulation was significant (p < .016).
The maximum anterior displacement of the COR measured from the ankle, a measure of the anterior boundary of the functional BOS (F-BOS), was about 15% larger for the YA than for the OA (p < .05). Decreasing PBOS from 100% to 65% decreased the maximum COR displacement by approximately 40% in both YA and OA (p < .0001). For the 100% PBOS condition the OA F-BOS extended 13.2 cm anteriorly from the ankle while that of YA extended 15.8 cm. Similarly, for the 65% PBOS condition, the F-BOS extended 8.0 and 9.3 cm anterior to the ankle joint, respectively. The interaction between age group and PBOS manipulation was significant (p < .021). Neither the maximum displacement of the COR nor the safety margin were significantly affected by platform height.
The mean initial position of the COR for a period of quiet stance showed that the OA maintained their COR significantly more posterior to that of the YA (p < .05). When foot position was changed from 100% PBOS to 65% PBOS, subjects in both groups moved their COR from a mean location anterior to the ankle to a mean location posterior to the ankle (p > .05).
Fig. 1. Changes in reach distance and COR parameters with BOS manipulations and age.
This study indicates that increasing platform height decreased maximum reach distance. The significant increase in perceived task difficulty measured on the high platform (quantified by a Likert scale questionnaire, but not reported here for brevity) correlated with that reduced maximum reach distance, even though the biomechanical task demands were identical on the low and high platforms. Because platform height neither affected the safety margin nor the F-BOS used, our results show that reach on the high platform was not limited due to a change in how foot reaction forces were controlled in either young or old subjects. In fact it is interesting that, despite their perceptions of the challenging nature of this task, these healthy OA responded no differently than the YA in terms of how much they elected to increase their safety margin on the high platform.
The significant age x PBOS interaction suggests that, unlike their reach distance, the OA safety margin was more affected by the PBOS manipulation than the YA safety margin.
Decreasing the PBOS and increasing the platform height increased the perceived challenge of the maximum forward reach. This perception of increased challenge led to the observed reduction in maximum reach distance. Because the OA did not respond differently to the platform height manipulation in terms of their COR behavior, kinematic analyses of body segment control will be required to explain the age differences in reach distance.
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PHS Grant P01 AG 08808.
