| | Is Stair Climb Power a Clinically Relevant Measure of Leg Power Impairments in At-Risk Older Adults?Abstract Bean JF, Kiely DK, LaRose S, Alian J, Frontera WR. Is stair climb power a clinically relevant measure of leg power impairments in at-risk older adults? ObjectiveTo test the clinical relevance of the stair climb power test (SCPT) as a measure of leg power impairments in mobility-limited older adults. DesignCross-sectional analysis of baseline data from participants within a randomized controlled trial. SettingRehabilitation research gym. ParticipantsCommunity-dwelling older adults (N=138; mean age, 75.4y) with mobility limitations as defined by the Short Physical Performance Battery (SPPB). InterventionsNot applicable. Main Outcome MeasuresLeg power measures included the SCPT and double leg press power measured at 40% (DLP40) and 70% (DLP70) of the 1 repetition maximum. Mobility performance tests included the SPPB and its 3 components: gait speed, chair stand time, and standing balance. ResultsStair climb power per kilogram (SCP/kg) had correlations of moderate strength (r=.47, r=.52) with DLP40/kg and DLP70/kg, respectively. All 3 leg power measures correlated with each of the mobility performance measures with the exception of DLP40/kg (r=.11, P=.27) and DLP70/kg (r=.11, P=.18) with standing balance. Magnitudes of association, as described by the Pearson correlation coefficient, did not differ substantively among the separate power measures as they related to SPPB performance overall. Separate adjusted multivariate models evaluating the relationship between leg power and SPPB performance were all statistically significant and described equivalent amounts of the total variance (R2) in SPPB performance (SCP/kg, R2=.30; DLP40, R2=.32; DLP70, R2=.31). Analyses of the components of the SPPB show that the SCPT had stronger associations than the other leg power impairment measures with models predicting chair stand (SCP/kg, R2=.25; DLP40, R2=.12; DLP70, R2=.13), whereas both types of leg press power testing had stronger associations with models predicting gait speed (SCP/kg, R2=.16; DLP40, R2=.34; DLP70, R2=.34). Stair climb power was the only power measure that was a significant component of models predicting standing balance (SCP/kg R2=.20). ConclusionsThe SCPT is a clinically relevant measure of leg power impairments. It is associated with more complex modes of testing leg power impairments and is meaningfully associated with mobility performance, making it suitable for clinical settings in which impairment-mobility relationships are of interest. LEG MUSCLE POWER has gained increased recognition as a modifiable impairment limiting the mobility performance of older adults.1 Power is the product of force and velocity of movement, and leg power is a more influential factor than strength in the mobility performance of older adults, especially among those with the greatest limitations.2, 3 In an analysis of more than 800 older adults, subjects with low leg power were more likely to exhibit significant mobility limitations than those with low strength.3 The importance of leg power is further emphasized by the facts that almost 25% of adults 65 years or older have mobility limitations4 and that mobility performance is predictive of subsequent morbidity, disability, and mortality.5, 6, 7 In fact, one mobility performance test, the Short Physical Performance Battery (SPPB), has been advocated as a vital sign to be used for screening purposes in primary care settings because of its ability to identify those people at risk for adverse outcomes.5, 8 As a result, recent investigations from both the United States and Europe have evaluated exercise interventions designed to treat leg power impairments among mobility-limited older adults.9, 10, 11, 12, 13 As physicians and rehabilitation professionals attempt to address muscle power impairments within their clinical practices, challenges arise surrounding measurement methodology. Leg muscle power has generally been measured with large, expensive, and specialized equipment.11, 14, 15 Others have used alternative methods that may be clinically challenging or even unsuitable for mobility-limited older adults.16, 17, 18, 19 The ideal leg power measure would be simple, low cost, and appropriate for frail older adults. One potential leg power measure is stair climb power. The stair climb power test (SCPT) is an inexpensive test that is simple to perform. This test can be completed in less than 1 minute and requires only access to a flight of stairs, a scale, and a stopwatch. Both community-dwelling and institutionalized older adults can successfully perform the SCPT.12, 20 The clinical relevance of the SCPT as it relates to other measurements of leg power and its relationship to established mobility performance measures have yet to be investigated. We hypothesized that among mobility-limited older adults, the SCPT would be a clinically relevant measurement of leg power. Therefore, we conducted a study among mobility-limited, community-dwelling older adults comparing (1) stair climb power versus leg power impairments measured with pneumatic resistance machines and (2) the separate respective associations of each leg power measure with the SPPB and its component measures of mobility performance. Methods  This study was a cross-sectional analysis of baseline data from a randomized controlled trial of exercise among mobility-limited older adults. Conceptual Paradigm For the purpose of this investigation, we evaluated disablement outcomes using the paradigm originally established by Nagi21 and currently accepted by the Institute of Medicine. In this paradigm (1) impairments are defined as deficits at the level of the organ system (eg, loss of muscle power); (2) functional limitations are defined as alterations in the performance of a functional task (eg, lower mobility performance); and (3) disability is defined as the inability for a person to perform his/her social and environmental roles because of impairments and functional limitations.22 Recruitment of Participants Initially, 590 inquiries were solicited via advertising in newspapers, direct mailings, referrals from primary care providers, and telephone screenings. Of these, 260 people were identified as potentially eligible and attended an initial screening assessment at our facility. Outcomes testing for the intervention study was completed over 1 to 2 subsequent visits depending on participant availability. Measures used for this analysis were all completed within the first 2 visits, which were scheduled within 1 week of each other. On completion of the initial screening, 92 people could not participate in the study because of exclusion criteria, and 30 chose not to commit to the study, leaving 138 participants. Screening Process Subjects included in the study were community-dwelling older adults (age ≥ 65y) with SPPB scores between 4 and 10 who were able to climb a flight of stairs independently with a device such as a cane. Exclusion criteria were unstable acute or chronic disease, a score of less than 23 on the Folstein Mini-Mental State Examination,23 a neuromusculoskeletal impairment limiting participation in further outcomes testing, or an exercise tolerance test with positive findings for cardiovascular disease. After providing informed consent, participants underwent a comprehensive history and physical examination that was conducted by the principal investigator. At the completion of the physical examination, the total number of active medical conditions was recorded for each participant. Active medical conditions were defined as either (1) any condition for which a participant was actively receiving treatment or (2) a condition requiring medical treatment within the past 1 year. Medical records were requested from participants’ primary care physicians to corroborate these findings. The total number of prescription and over-the-counter medications was recorded. Impairment Measures Lower-limb strength and power was measured with a pneumatic double leg press resistance machinea as previously described.24 Briefly, the 1 repetition maximum (1-RM) was determined by progressively increasing the resistance for successive repetitions until each participant could no longer move the lever arm 1 time through the full range of motion. Peak power was measured as the best of 5 repetitions performed at 40% and 70% of the 1-RM, in which participants performed the concentric action of 1 repetition as quickly as possible. These 2 intensities were chosen to represent double leg press muscle power production at relatively high force and low velocity—70% 1-RM (DLP70)—and low force and high velocity—40% 1-RM (DLP40). Stair climb power was calculated with the following formula: power equals force times velocity. Stair climb time (see below) and vertical height of the stairs were used to calculate velocity (distance/time), and body mass and acceleration due to gravity were used to calculate force. Stair climb time was measured by the following procedures. The examiner stood with each subject at the base of a well-lighted, 10-stair flight of stairs. Subjects were instructed to safely ascend the stairs as fast as they could. They were further instructed that they could use the handrail if they thought it was necessary for safety purposes and to begin when the examiner said, “Ready, set, go.” Timing began after the examiner said “go” and once each subject began moving. When both feet of a subject reached the top step, the timing stopped. Time was recorded to the nearest .01 second, and the average of 2 trials was taken. Test-retest reliability for this measure within our clinical lab is excellent (R=.99). Stair climb power testing was initiated after the inception of the study, and therefore only 124 of the 138 subjects underwent testing. Mobility Performance Testing Short Physical Performance Battery The SPPB is a well-established, reliable, and valid measure of lower-extremity performance.6, 25 Testing involves an assessment of standing balance, the timed 4.0-m walk, and a timed test of 5 repetitions of rising from a chair and sitting down. All times are measured to the nearest .01 second with a stopwatch. Each of the aforementioned tests is scored between 0 and 4 and summed (maximum score, 12). SPPB scores have been found to predict disability over 1 to 6 years in several older populations.6, 25 Testing was completed over a subsequent visit at which point a second SPPB was performed. Although the initial SPPB was the test that determined eligibility, the average of the 2 SPPB scores was used in this analysis. In addition, to better understand the respective associations between each leg power measure and the SPPB, each of its subcomponents (gait speed, chair stand time, standing balance score) were evaluated as unique secondary outcomes. Adjustment Variables Age, mass, height, sex, number of chronic conditions, and number of medications were assessed as part of the screening examination and served as potential adjustment variables. Statistical Analysis Descriptive statistics were calculated, including frequencies and proportions for categoric variables and mean and standard deviations for continuous variables. To account for body size and sex differences among participants, we normalized all power measures by mass. On inspection of the primary and secondary outcomes, to ensure that chair stand time was normally distributed, 1 value that was an outlier (69.9s) was recoded to the whole number value (37s) immediately above the next longest value. In addition, 4 subjects who attempted the chair stand test but were unable to complete it were converted to values of 37 seconds as well. Inclusion of these observations through this transformation did not materially alter the analyses. To compare the bivariate associations between each leg power measure (aim 1) and their respective association with the SPPB and its components (aim 2), simple correlations were calculated between each of the impairment measures and each of the physical performance measures. Next, we evaluated the bivariate relationships between each of the potential adjustment variables and the mobility measures, ensuring a statistically significant association (P<.05). In addition, we inspected adjustment variables for colinearity, eliminating those colinear covariates that had the weaker association with the outcome. Prior reports show that impairment-function relationships may best be characterized as curvilinear.3, 26 We therefore inspected plots of the bivariate relationships to determine whether linear or curvilinear models would best characterize our data. Next, we constructed 12 separate multivariate regression models, controlling for those covariates that were included in the final model. For all models, we inspected the distribution of the predictors and outcomes to ensure normality and inspected residuals to evaluate for influential data points. To ensure normality and ease of comparison between the secondary outcomes, chair stand time values were allowed a maximum value of 37 seconds. Last, the results were reanalyzed without those subjects who used the handrail during stair climb testing to ascertain whether this in any way altered the findings. Results Participants had a mean age of 75.4 years and were predominately women (69%), predominantly white (85%) (15% black), and overweight, with a mean body mass index of 27.5kg/cm2 (table 1). On average, participants reported 5.6 chronic medical conditions and were prescribed 4.3 medications. Participants had a mean SPPB of 8.7, which has been characterized as mobility limitations of moderate severity.25 Consistent with this, performance measures included the following mean values: gait speed of .93m/s, chair stand time of 15.94 seconds, and average standing balance score of 3.15 out of 4. Twenty-one of the 124 participants completing the SCPT (17%) used the handrail during stair climb testing because of self-perceived safety concerns. Normalized values of stair climb power per kilogram (SCP/kg) were significantly associated (P<.001) with both normalized values of leg press power (DLP40/kg, R=.47; DLP70/kg, R=.52). Both leg press power values were highly associated (r=.93). With respect to the SCPT, through the course of the study there were no falls or adverse events, and no subjects refused to attempt the test. The bivariate associations between each normalized value of leg power and the SPPB and each of its components are presented in table 2. Pearson correlation coefficients between mean SPPB score and SCP/kg, DLP40/kg, and DLP70/kg were all statistically significant (P<.001) with magnitudes of .51, .42, and .44, respectively. In evaluating gait speed, both DLP40 (r=.54) and DLP70 (r=.56) had greater associations than those seen with the SCPT (r=.29). The pattern of association was opposite with measures of chair stand, with r values being −.43, −.32, and −.31 for SCP/kg, DLP40/kg, and DLP70/kg, respectively (P<.001). Last, bivariate associations with standing balance were greater for SCP/kg (r=.31, P=.004) and were of lesser magnitude with both leg press measures, never reaching statistical significance (DLP40/kg, r=.11, P=.20; DLP70/kg r=.11, P=.18). Inspection of the bivariate plots of the respective leg power–mobility measure associations did not suggest that curvilinear models were superior in characterizing these relationships. Among the potential adjustment variables, age and height were included within the multivariate models. The number of chronic medical conditions and number of regular medications are both established measures characterizing health status. They were sufficiently colinear (r=.67, P<.001) to justify using only the number of regular medications as a covariate, given that it had a stronger association with the outcomes than did chronic medical conditions. The resulting 12 separate multivariate linear regression models are presented in table 3. Within the 3 models predicting SPPB, the magnitude of the estimate measured in watts per kilogram was greater for SCP/kg (.59 vs .22–.23) compared with the other leg power measures; however, the respective models of leg power impairments described equivalent levels of the variance in SPPB performance, ranging between 30% and 32%. To better understand these relationships, separate models were constructed predicting performance on each component of the SPPB. With respect to gait speed, although the magnitude of effect (parameter estimates) in watts per kilogram was similar (between .03 and .04) for all 3 power measures, models of DLP40/kg (R2=.34) and DLP70/kg (R2=.34) described a larger proportion of the variance than did SCP/kg (R2=.16). Compared with the bivariate associations, the magnitude of the effect of SCP/kg on gait speed was not statistically significant (P=.13). In the evaluation of chair stand performance, SCP/kg had a much greater effect (estimate, −2.62W/kg), compared with an estimate of −.71W/kg for both DLP40/kg and DLP70/kg. Although all 3 models were statistically significant, SCP/kg described a larger proportion of the variance in chair stand performance, having an R2 value of .25 compared with an R2 value of .12 for DLP40/kg and an R2 value of .13 for DLP70/kg. Using a chair stand ceiling value of 37 seconds, as mentioned in the Methods section, did not materially influence the results. For standing balance, the magnitude of the estimate was greatest for SCP/kg (SCP/kg estimate, .14; DLP40/kg estimate, .03; DLP70/kg estimate, −.03). Although the total variance in standing balance performance was similar for all 3 separate models, with R2 ranging between .19 and .20, SCP/kg was the only power impairment measure that had statistical significance (SCP/kg, P=.04; DLP40/kg, P=.33; DLP70/kg, P=.67). When we re-evaluated the 12 statistical models excluding those subjects who used the handrail during stair climb (data not shown), we found that handrail use did not weaken the association between SCP/kg and each of the performance measures. Similarly, these multivariate relationships were not materially different when evaluated using curvilinear models (data not shown). Discussion Our investigation is the first to attempt to formally evaluate the SCPT as a potential measure of lower-extremity power and a predictor of important outcomes of mobility performance. The major finding of our study is that models using the SCPT predict approximately one third of SPPB performance, which is comparable with that predicted by leg power measured using pneumatic isotonic resistance machines. In addressing the 3 components of the SPPB, compared with both DLP40 and DLP70, the SCPT had a weaker association with gait speed performance and a stronger association with chair stand performance, and it was the only impairment measure to achieve statistical significance with standing balance performance. The importance of these findings should be interpreted within the larger understanding of the clinical relevance of mobility performance testing. In a recent editorial,27 mobility performance testing was advocated as an important clinical test, potentially a vital sign, for use in the primary care of older adults. For rehabilitation providers and clinical researchers considering the importance of muscle power impairments that underlie mobility performance, the SCPT may be an optimal means of measuring the magnitude of impairment and the possible effects of therapy. Similarly, for clinical research projects looking at SPPB performance and associated impairment measures, the SCPT may be a reasonable, low-cost measure of leg power. The mechanistic differences among SCP/kg, DLP40/kg, and DLP70/kg with respect to gait speed and the other components of SPPB performance may best be seen in figure 1, which presents plots of mean recorded maximal power and subsequently derived mean maximal velocities (see fig 1 legend for formulae). Although the derived percentage of the 1-RM at which SCP/kg production occurred was 52%, clearly in the middle between 40% and 70%, both leg press impairment measures generated higher velocities of movement compared with the stair climb power. The mean gait speed velocity of participants (.93m/s) is indicated on figure 1. The stronger association between leg press measures of power and gait speed seen in table 3 may be mechanistically linked to the similarity between velocity and power generated during these activities relative to stair climb power testing. Also, prior reports evaluating gait among functionally limited older adults have reported combined power generation from the hip and leg extensors to be in the range of 6 to 8W/kg, more closely approximating the power output seen with leg press testing.28 This line of thinking may also clarify the other findings in table 3, in which models with the SCPT explained greater variances in chair stand and standing balance performance. Although not measured in our investigation, it may be that the power output and velocities of movement to maintain standing balance and to perform the chair stand are more consistent with those generated during stair climb power testing than with leg press power measurements. Cuoco et al29 have suggested that walking tasks may be better characterized by power output at lower resistance and higher velocities. Although our findings with regard to double leg press power are not fully consistent with this supposition, this may reflect different methodologies used to measure gait speed. The overall message of figure 1 supports a general conclusion that can be derived from the report by Cuoco.29 When considering rehabilitation methods, impairment and functional measures in general should be regarded in terms of their mechanistic similarities. The weaker associations observed between the stair climb power and gait speed should be considered if gait speed is a sole outcome of interest. Gait speed has been shown to be almost as strong a predictor of mortality and disability as the SPPB.6 However, leg power generation at more distal segments of the leg, such as the ankle, play a larger role in gait.30 In addition, stair climb activity emphasizes more proximal lower-extremity muscle power generation.31, 32 If gait speed is the only outcome of interest, the SCPT may be a less optimal power measurement. One potential solution to this concern would be to consider future measurement of leg power while walking up a ramp. Although not a focus of the current study, this other means of leg power measurement may more favorably evaluate leg power generation through the more distal lower-extremity muscle groups. This option may also limit the confounding effect of height observed between stair climb power and gait speed, which was likely related to the relative disadvantage with stair climbing for people of short stature. Study Limitations Our study has limitations. It might be suggested that use of the handrail could allow upper-extremity power output to be included within this lower-extremity power measure. However, the association between the SCPT and these lower-extremity mobility performance measures was strongest when people who used the handrail for safety purposes were included in the analyses. Our own observations suggest that use of the handrail primarily served the purpose of mitigating balance concerns and ensuring confidence to perform optimally. This allows people with balance impairments to more closely approximate the testing experience of leg press testing, in which they are seated. Furthermore, although not measured, upper-extremity weight bearing and push off from the handrail appeared to be minimal. In addition, our study was not able to compare stair climb power with other methods of leg power generation (eg, isokinetic testing). Although pneumatic resistance equipment is in our opinion superior, our investigation would have been strengthened with a direct comparison with these other leg power measures. Also, this was a cross-sectional analysis among a relatively homogenous cohort. A longitudinal investigation among a more racially diverse population would better characterize the temporal relationships between our leg power and performance measures, allowing clinicians to determine the sensitivity of the SCPT to detect training-induced changes and, in turn, to identify critical and clinically relevant values of the SCPT. Beyond the mechanistic considerations and potential limitations, it is important to reconsider the primary aim of the study: to determine the clinical usefulness of the SCPT as a measure of leg power impairments among older adults with mobility problems. It is not uncommon for sound clinical measures to lack the precision and accuracy of their sophisticated and expensive lab-based counterparts. Conclusions Our study has shown that the associations between the SCPT and the SPPB are sufficiently strong to consider the SCPT a relevant clinical measure of leg muscle power impairments. The true test of the feasibility of the SCPT as an impairment measure would be the evaluation of its use within clinical settings that use mobility performance testing as a means of screening for those at risk for disability. Most clinicians caring for older adults do not have access to lab-based power measures. We recognize that the SPPB and similar mobility tests will serve as clinical screening tools in the future; therefore, for older adults with mobility limitations, the SCPT may be a useful impairment measure guiding care. Supplier References 1. 1Evans WJ. Exercise strategies should be designed to increase muscle power. 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No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the author(s) or upon any organization with which the author(s) is/are associated. PII: S0003-9993(07)00101-3 doi:10.1016/j.apmr.2007.02.004 © 2007 American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation. Published by Elsevier Inc. All rights reserved. | |
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