A Comparison of the Center of Pressure during Stair Descent in Young and Healthy Elderly Adults

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1 Original Article A Comparison of the Center of Pressure during Stair Descent in Young and Healthy Elderly Adults J. Phys. Ther. Sci. 21: , 2009 HYEONG-DONG KIM 1) 1) Catholic University of Daegu, College of Health Science, Department of Physical Therapy: 330 Geumnak 1-ri, Hayang-eup, Gyeongsan-si, Gyeongbuk, Republic of Korea TEL: , FAX: Abstract.[Purpose] The purpose of this study was to investigate the center of pressure (COP) trajectory during stair descent in young and healthy elderly adults. [Subjects] Fifteen community-dwelling healthy elderly adults (mean age, 73.1 ± 4.3 years) and 15 healthy young adults (mean age, 23.6 ± 2.4 years) volunteered to participate in this study. [Methods] Subjects stood in a predetermined position on the top of a staircase. Participants were then instructed to descend the staircase with three steps at a self-paced speed and continue for several strides after striking the force platform on the floor. Participants were required to place only one foot on each step (foot-over-foot) at a time. Performance was assessed by recording changes in the displacement of COP in the anteroposterior (A-P) and mediolateral (M-L) directions as well as recording the average velocity of COP by the use of two force platforms. [Results] The A-P and M-L displacements of COP as well as the average velocity of COP of the elderly were significantly lower than those of the young adults for both the right and the left feet (p<0.01). [Conclusion] The COP shift and velocity during stair descent might be appropriate parameters for identifying risk factors associated with stair-based accidents or falling. Key words: Center of pressure, Falls, Stair descent (This article was submitted Oct. 30, 2008, and was accepted Dec. 11, 2008) INTRODUCTION Negotiation of stairs is a common activity frequently encountered in daily life and the ability to negotiate stairs with relative ease is important to quality of life. Stair negotiation is considered more physically demanding than level walking and it is among the top five tasks listed as being difficult for the elderly 1). A fall on stairs is responsible for approximately 10% of total accident cases 2,3) and the prevalence of a fall on stairs is much higher in the elderly 2,4,5). Furthermore, the likelihood of the risk of serious injury in the elderly is much greater for a fall on stairs than for a fall occurring on level ground 4). A fall on stairs is also the leading cause of accidental death in the United States, accounting for approximately 10% of fall-related deaths 3). Although stair negotiation is unlikely to be a challenging task for the healthy elderly, the task may be difficult for subjects with reduced motor function and lower extremity disabilities. Such examples of reduced motor function and lower extremity disabilities are found in the frail elderly and in individuals with muscle or joint diseases as well as subjects with joint or limb replacements. The greater demand of physical ability required during the negotiation of stairs is likely to challenge the musculoskeletal system of the lower extremities for these populations and increase the risk of falling associated with this task. Although both stair ascent

2 130 J. Phys. Ther. Sci. Vol. 21, No. 2, 2009 and descent are quite demanding for the elderly, stair descent is the most challenging aspect of stair negotiation 3,6) and it has been attributed to threequarters of all staircase accidents 2). Previous studies 7-9) have demonstrated that a large range of movement at the knee and ankle and relatively high moment generation at these joints are needed during the descent of stairs by young adults. Furthermore, the peak joint moment at the knee joint during the negotiation of stairs was three times as high as for level walking 9,10). Compared to young adults, the elderly show a similar peak vertical ground reaction force 11,12) and joint moment at the knee, as well as similar changes of the plantarflexion-dorsiflexion angle of the ankle joint 13) during the descent of stairs. However, the elderly demonstrate a lower joint moment at the ankle joint during the descent of stairs 13). A maximum knee joint angle of degrees, a maximum ankle joint angle of greater than 30 degrees and greater pelvis and hip motion in the frontal and transverse planes during the descent of stairs have also been reported for the elderly, as compared to young adults 7,8,14,15). These observations indicate that the elderly may use less effective stair descent strategies, which could result in difficulties in maintaining balance during stair negotiation, as compared to young adults. However, previous reports have only focused on changes in the joint moment and kinematic parameters of the lower extremities associated with aging during stair descent. To investigate further the impact of aging on stair negotiation strategies in the elderly, an analysis of the potential underlying mechanisms for the abilities of stair negotiation by the elderly is necessary. The displacement of the center of motion (COM) and its position relative to the center of pressure (COP) are highly regulated during locomotive tasks 16). The maintenance of stability during gait is dependent on the ability of the control of the horizontal distance of COM from COP beneath the feet within appropriate limits. Thus, COP is commonly used as an indicator of balance and postural control 17) and changes in COP reflect the response of the central nervous system (CNS) to movement of the whole body COM 18). Although the COM displacement and the COM-COP separation during the descent of stairs in both young and older adults have previously been reported 19), age-related changes of the COP trajectory have never been investigated. Moreover, knowledge of the changes in the COP trajectory during stair negotiation may be useful for understanding the factors behind the increased incidence of falls on stairs among the elderly. The purpose of this study was to investigate the COP trajectory during stair descent in young and healthy elderly adults. SUBJECTS AND METHODS Fifteen community-dwelling healthy elderly adults (mean age, 73.1 ± 4.3 years; age range, years) and 15 healthy young adults (mean age, 23.6 ± 2.4 years; age range, years) volunteered to participate in this study. Inclusion criteria for the elderly participants were a Berg Functional Balance Scale 20,21) score > 50, a Frenchay Instrumental Activities of Daily Living 22) score > 50 and a Physical Function 23) score > 20. All participants scored more than 25 on the Mini Mental Status Examination (MMSE) 24). The reliability and validity of these tests have previously been demonstrated as satisfactory 25 27). All elderly participants were living independently in the local community and most of the subjects were recreationally active, participating in activities such as walking and gardening. All participants had visual acuity, with the use of corrective eyewear if needed, that was greater than 20/40 on a standard visual acuity test. No subject presented with neurological or orthopedic problems that precluded the subjects from participation in the study. None of the elderly participants had a history of falls in the previous 12 months. All participants provided their informed consent and the University Institutional Review Board approved this study. The subject information is summarized in Table 1. All measurements were performed on a custombuilt standard dimension staircase consisting of three steps (rise height, 17 cm; tread breadth, 28 cm; width, 90 cm). Two force platforms (AMTI, Newton, MA, USA), embedded in the first step of the staircase and in the level walkway (2 m in length and 1.22 m in width) directly in front of the staircase, were used to measure the ground reaction forces during stair descent. Each step was independently constructed using a solid steel frame (the vertical steel frames had a width and depth of 8 and 4 cm, respectively) securely bolted to the ground. This arrangement ensured a mechanically

3 131 Table 1. Subject information Group N Age Height Weight Sex (male/female) (years) (cm) (kg) Young adults (2.4) (4.1) 64.7 (5.4) 5/10 Old adults (4.3) (4.4) 52.0 (2.7) 6/9 Values are means ± standard deviations. Table 2. Mean values (± SD) for the COP parameters (cm) during stair descent in younger and older adults Dependent variables Young adults Elderly Right foot A-P displacement (cm) * 14.1 (4.1) 10.0 (3.7) M-L displacement (cm) * 9.4 (2.2) 6.8 (2.8) COP velocity (cm/sec) * 108 (32) 72.9 (15) Left foot A-P displacement (cm) * 14.4 (4.3) 10.0 (2.4) M-L displacement (cm) * 8.7 (2.6) 5.4 (2.1) COP velocity (cm/sec) * 131 (29) 74.4 (12) * Significant difference (p<0.01) for the two subject groups. SD: Standard deviation; COP: center of pressure; A-P: anteroposterior; M-L: mediolateral. stiff structure that enabled forces to be recorded from the first step and the ground. Amplified force platform signals were sampled online at a rate of 1000 Hz for 30 seconds (AMTI, Watertown, MA, USA). COP was defined as the point of application of the ground reaction force (GRF) vector in three directions on the force platform. The COP data were analyzed using BioAnalysis v2.0 software (AMTI, Watertown, MA, USA). For each trial, subjects stood in a predetermined position on the top of the staircase. Participants were then instructed to descend the staircase at a self-paced speed, with the left limb in response to auditory cues and continue for several strides after striking the force platform on the floor. Participants were required to place only one foot on each step (foot-over-foot). The analysis began with the initial ground contact of the right foot on the second step down (first force platform) and ended at the initial ground contact of the left foot on the floor (second force platform). This part of the analysis was chosen to represent a steady-state step. Subjects completed two practice trials and approximately five successful experimental trials. Differences between young and elderly adults for the COP trajectory were tested using an independent samples t-test. Statistical significance was indicated at p<0.05 and p<0.01. The dependent variables included the A-P and M-L displacements of COP as well as the average velocity of COP. The A-P (or M-L) displacement of COP was defined as the total distance (or difference) between the minimum and maximum A-P (or M-L) COP location for the length of time that either the right or the left foot was in contact with the force platform. The average velocity of COP was defined as the average velocity traveled by COP for the length of time that either the right foot or left foot was in contact with the force platform. Statistical software SPSS 14.0 KO (SPSS, Chicago, IL, USA) was used for the statistical analyses. RESULTS The differences between the young adults and the elderly for the COP trajectory during stair descent were analyzed. There were significant differences between the young and elderly subjects in respect of the A-P and M-L displacements of COP and the average velocity of COP for both the right and the left feet. The use of statistical analysis demonstrated that the A-P and M-L displacements of COP for the elderly were significantly lower than the A-P and M-L displacements of COP for the young adults for both the right and the left feet (p<0.01) (Table 2). Furthermore, the average velocities of COP for both the right and the left feet of the elderly were significantly lower than those of young adults (p<0.01). The mean values of the COP data for both groups (young adults and the elderly) are summarized in Table 2. DISCUSSION The aim of this study was to compare the COP trajectory of young adults and the elderly descending a staircase. The A-P and M-L displacements of COP as well as the average

4 132 J. Phys. Ther. Sci. Vol. 21, No. 2, 2009 velocity of COP were significantly less for the elderly in both the right foot and the left foot, as compared to the young adults. The mean A-P displacement of COP for young adults was approximately 143% greater than the mean A-P displacement of COP for the elderly for both the right foot and the left foot. These observations are similar to observations made during level walking and while stepping over an obstacle on the ground 28,29). Polyn et al. 28) reported a decreased backward displacement of COP shown by the elderly during gait initiation (GI), which diminished the forward momentum necessary to initiate gait. Such a reduction in forward momentum would generate a decreased forward COM in the elderly during the initial stage of GI. This reduction in the magnitude of the displacement of the A-P COP and in forward momentum shown by the elderly has been interpreted as a less efficient strategy resulting from reduced inhibition of soleus and gastrocnemius activity and increased ankle joint stiffness that is related to aging 30). When descending from a step to the level ground, greater forward and downward moments are expected. This task requires a greater demand of the neuromuscular recruitments from the supporting limbs to control whole body movement appropriately 31). In the current study, age-related declines in muscle strength and joint flexibility of the elderly (particularly at the ankle joint) 32-34) might compromise their ability to generate the momentum necessary to descend the stairs. It has been demonstrated that the eccentric muscle strength of the knee extensors that are used for stair descent declines with age 8,9,32,35). Furthermore, increased stiffness at the ankle joint associated with aging 30) may be further magnified by a much higher demand of ankle dorsiflexion during stair descent than during stair ascent 36). It has been shown that the maximum dorsiflexion angle during stair descent (34 degrees) is much higher than the maximum dorsiflexion angle during stair ascent (22 degrees) 36). Displacement of COP or its relationship with COM provides an indication of dynamic stability during gait. It has been demonstrated that the elderly show a smaller COM-COP separation in the frontal plane during stair ascent, as compared to young adults 36). A previous study 18) demonstrated that the elderly had a significantly decreased M-L displacement of COP during obstacle crossing as compared to young adults. A greater displacement of COM in the frontal plane has also been reported for healthy elderly persons during obstacle crossing, as compared to elderly persons with balance impairments 37). In the present study, there were significant differences in the displacement of COP in the frontal plane between the elderly and young adults during stair descent for both the right and the left feet (Table 2). The elderly showed a significantly lower displacement of COP in the frontal plane as compared to young adults. This finding agrees with previous findings for obstacle crossing that showed significant age differences in the frontal plane of COP displacement 18). Moreover, in the current study, the demands of stair descent seem to have presented problems of dynamic balance control for the elderly. It is reasonable to expect that instability of dynamic balance control in the frontal plane may become magnified in a more sedentary group of elderly persons and/or when there is an increase in task demands, for example, such as an increase in step height. Furthermore, previous studies 38 40) have reported that the control of whole body COM in the M-L direction through manipulation of COP during walking is closely related to the maintenance of lateral balance, which is highly related to increased risks of lateral falls in the elderly. Chou et al. 37) have suggested that M-L COM motion during obstacle crossing is an important functional indicator to identify persons who are at a risk of lateral falls or imbalance. The velocity of COP provides valuable important information about how individuals modulate gait when negotiating stairs. A greater COP velocity may present a challenge to the maintenance of balance while a slower speed may be suggestive of a more stable position. The mean velocity of COP shown by the elderly was significantly lower than that shown by young adults for both the right foot and the left foot. The slower speed of COP may be potentially beneficial for maintaining the dynamic balance that is necessary for supporting an upright posture during stair descent. In a previous study 41), the elderly decreased the frontal plane separation between COM and COP to avoid losing balance when they climbed stairs with their vision experimentally blurred as compared to the same task with vision fully functional. These observations may indicate that the elderly participants displayed a more cautious strategy for

5 133 optimizing postural stability during stair negotiation. There are several limitations to the current study. The current study had a relatively low sample size of elderly and young adults. A further investigation with a larger sample size is needed. Furthermore, the elderly participants in this study had a high functional level; thus, the study population may not be representative of community dwelling older adults who have mobility or cognitive deficits. Finally, the current study used a staircase containing three steps. Thus, the staircase used in this study may not have allowed for true stair descent of a typical staircase of a much longer length. In conclusion, there were age-related differences in the A-P and M-L displacement of COP and COP velocity during stair descent. The elderly showed significantly less COP shift and velocity than young adults. The decreased magnitude of the COP shift diminishes the ability of the elderly to generate forward momentum and to maintain lateral stability. The reduction of COP velocity may indicate a control strategy that has been adopted by the elderly to maintain a more cautious strategy for optimizing postural stability, as compared to young adults, during stair descent. Given these circumstances, the COP shift and velocity during stair descent might be appropriate parameters for evaluating dynamic balance and motor control strategies shown by the elderly. Furthermore, the findings of this study may provide normative data for the COP trajectory during stair descent and may help to identify risk factors associated with stair-based accidents or falling. Finally, it is necessary to explore the COP shift and velocity behavior shown by the elderly who are at risk of falls and in populations with CNS diseases associated with aging such as Parkinson s disease and other degenerative disorders including Alzheimer s disease. REFERENCES 1) Williamson JD, Fried LP: Characterization of older adults who attribute functional decrements to old age. J Am Geriatr Soc, 1996, 44: ) Svanström L: Falls on stairs: an epidemiological accident study. Scand J Soc Med, 1974, 2: ) Startzell JK, Owens DA, Mulfinger LM, et al.: Stair negotiation in older people: a review. J Am Geriatr Soc, 2000, 48: ) Nevitt MC, Cummings SR, Hudes ES: Risk factors for injurious falls: a prospective study. J Gerontol, 1991, 46: M164 M170. 5) Hemenway D, Solnick SJ, Koeck C, et al.: The incidence of stairway injuries in Austria. Accid Anal Prev, 1994, 26: ) Pauls JL: Safety standards, requirements and litigation in relation to building use and safety, especially safety from falls involving stairs. Safety Sci, 1991, 14: ) Andriacchi TP, Andersson GB, Fermier RW, et al.: A study of lower-limb mechanics during stair-climbing. J Bone Joint Surg Am, 1980, 62: ) McFadyen BJ, Winter DA: An integrated biomechanical analysis of normal stair ascent and descent. J Biomech, 1988, 21: ) Riener R, Rabuffetti M, Frigo C: Stair ascent and descent at different inclinations. Gait Posture, 2002, 15: ) Nadeau S, McFadyen BJ, Malouin F: Frontal and sagittal plane analyses of the stair climbing task in healthy adults aged over 40 years: what are the challenges compared to level walking? Clin Biomech (Bristol, Avon), 2003, 18: ) Christina KA, Cavanagh PR: Ground reaction forces and frictional demands during stair descent: effects of age and illumination. Gait Posture, 2002, 15: ) Hamel KA, Okita N, Bus SA, et al.: A comparison of foot/ground interaction during stair negotiation and level walking in young and older women. Ergonomics, 2005, 48: ) Reeves ND, Spanjaard M, Mohagheghi AA, et al.: The demands of stair descent relative to maximum capacities in elderly and young adults. J Electromyogr Kinesiol, 2008, 18: ) Livingston LA, Stevenson JM, Olney SJ: Stairclimbing kinematics on stairs of differing dimensions. Arch Phys Med Rehabil, 1991, 72: ) Mian OS, Thom JM, Narici MV, et al.: Kinematics of stair descent in young and older adults and the impact of exercise training. Gait Posture, 2007, 25: ) Winter DA: Human balance and posture control during standing and walking. Gait Posture, 1995, 3: ) Doyle RJ, Hsiao Wecksler ET, Ragan BG, et al.: Generalizability of center of pressure measures of quiet standing. Gait Posture, 2007, 25: ) Martin M, Shinberg M, Kuchibhatla M, et al.: Gait initiation in community-dwelling adults with Parkinson disease: comparison with older and younger adults without the disease. Phys Ther, 2002, 82: ) Mian OS, Narici MV, Minetti AE, et al.: Centre of mass motion during stair negotiation in young and older men. Gait Posture, 2007, 26: ) Berg KO, Wood-Dauphinee SL, Williams JI, et al.: Measuring balance in the elderly: preliminary development of an instrument. Physiother Can, 1989, 41:

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