Functional Muscle Power Testing in Young, Middle-Aged, and Community-Dwelling Nonfrail and Prefrail Older Adults

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1 ORIGINAL ARTICLE Functional Muscle Power Testing in Young, Middle-Aged, and Community-Dwelling Nonfrail and Prefrail Older Adults Astrid Zech, PhD, PT, Simon Steib, Dipl Sportwiss, Ellen Freiberger, PhD, Klaus Pfeifer, PhD 967 ABSTRACT. Zech A, Steib S, Freiberger E, Pfeifer K. Functional muscle power testing in young, middle-aged, and community-dwelling nonfrail and prefrail older adults. Arch Phys Med Rehabil 2011;92: Objective: To evaluate the stair climb (SC) and sit-to-stand (STS) transfer test for functional power assessment in young, middle-aged, and community-dwelling nonfrail and prefrail older adults. Design: Cross-sectional study. Setting: Sport science institute providing health-related exercise programs for older people. Participants: Participants (N 60; age, 22 81y) were divided into groups of young (n 15; 20 30y), middle-aged (n 16; 40 60y), nonfrail older (n 16; 65y), and prefrail older adults (n 13; 65y). Interventions: Not applicable. Main Outcome Measures: SC and STS transfer power were measured on 2 separate occasions. Results: Age and height correlated positively (P.001) with both power measures. Multiple linear regression analysis showed that 67.9% (R 2 ) of the variance in SC power and 31.3% (R 2 )ofthe variance in STS transfer power can be attributed to age and height. Significant age-related subgroup differences were found for SC power (P.001). Nonfrail and prefrail older adults differed significantly in both power measures (P.001). Conclusions: The findings indicate that SC and STS transfer power are sensitive enough to distinguish between nonfrailty and prefrailty. This suggests that both tests are relevant clinical measures in older people. Key Words: Older adults; Power; Prefrailty; Rehabilitation; Sitto-stand transfer; Stair climb by the American Congress of Rehabilitation Medicine MUSCLE POWER DECREASES with increasing adult age, 1,2 correlates positively with walking performance, 3 and has been shown to be impaired in older adults with an increased fall risk. 4 Hence, muscle power tests commonly are used to determine changes in function or examine the effect of exercise interventions on functional performance in older people. Lower-extremity power measures include the Nottingham power rig, 1,4,5 cycle ergometry, 6 rapid dynamic contractions on resistance training machines, 7 maximal vertical jump, 8,9 stair climbing, 10 stair sprinting, 11 or sit-to-stand (STS) transfer. 12 However, adequate application of most of these power measures in different populations, especially those with functional limitations, appears problematic. One limitation is that the wide range of task-specific power tests produces uncertainty regarding the generalization of corresponding results and thus their relevance for functional performance in daily living. Furthermore, tests such as stair sprinting or maximum vertical jumping require physical demands that cannot be performed easily by older and/or mobility-limited people. Moreover, because current research increasingly focuses on effective interventions to prevent or delay frailty and disability with aging, power tests also should be able to distinguish between different stages of frailty in older people. In summary, functionality, applicability in all age groups, and sensitivity for functional limitations are crucial for adequate power testing. Little is known about whether the existing power measurements fulfill these requirements. This may contribute to the present lack of a criterionstandard measure for lower-extremity muscle power. Based on the findings of Bean et al, 10 who showed an association between stair climb (SC) power impairments and mobility limitations in older people, it is suggested that the SC test may provide a simple, functional, and valid muscle power assessment. However, it is unclear whether functional power tests such as stair climbing or STS transfer are sensitive enough to detect small changes in function or frailty status. Furthermore, it also is unknown whether these outcome measures are suitable in young and middle-aged people. Clarifying these points would help provide a simple and functional standard power measurement for future research in this field; for example, intervention studies for improving functionality or preventing frailty and disability. The purpose of this study was to evaluate 2 activities of daily living, stair climbing and STS transfer, for their ability to measure lowerextremity power. For this, we examined age-related differences in adulthood and differences between nonfrail and prefrail older adults. METHODS Ostensibly healthy people (N 60; height, cm; body mass, ; BMI, kg/m 2 ; age, 22 81y; mean SD, y) gave written informed consent to participate in this study after the procedures had been fully explained. Young (n 15; 20 30y) and middle-aged adults (n 16; 40 60y) were recruited from students and staff of our institution by using informational fliers distributed within the From the Department of Movement Science, University of Hamburg, Hamburg (Zech); and Department of Sports Science and Sports, University of Erlangen- Nuremberg, Erlangen (Steib, Freiberger, Pfeifer), Germany. No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit on the authors or on any organization with which the authors are associated. Correspondence to Astrid Zech, PhD, PT, University of Hamburg, Dept of Movement Science, Mollerstraße 2, Hamburg, Germany, astrid.zech@unihamburg.de. Reprints are not available from the author. Published online April 22, 2011 at /11/ $36.00/0 doi: /j.apmr ANOVA BMI ICC SE SC STS W List of Abbreviations analysis of variance body mass index intraclass correlation coefficient standard error stair climb sit-to-stand watts

2 968 FUNCTIONAL POWER TESTS IN OLDER PEOPLE, Zech institute inviting students, office and technical workers, and lecturers to participate. Older adults (n 29; age 65y) were recruited from health-related exercise groups provided at our facility. The institutional exercise programs consisted of weekly training sessions for older people during study term and were designed to give students practical experience in teaching health and sports-related sciences. Hence, each exercise session aimed at different health-related purposes (eg, fall prevention, stretching, strengthening) without a systematic increase in training intensity and were led by different instructors (students). Measurements were undertaken at the beginning of the study term and thus included older adults who had not participated in these exercise programs for at least 2 months. Exclusion criteria were any lower-limb injuries or surgeries in the past 12 months and movement disorders. After providing informed consent, eligible older participants underwent frailty screening. According to Fried et al, 13 a phenotype of frailty was identified by the presence of 3 or more of the following 5 components: (1) weight loss ( 4.5kg during the last year), (2) slow walking speed (time to walk 4.5m, adjusted for sex and height), (3) low handgrip strength (men, 29 32kg; women, kg), (4) low physical activity (weighted score of kilocalories expended a week; Minnesota Leisure Time Physical Activity Questionnaire), and (5) self-reported exhaustion (exhaustion 3d in the past week; Center for Epidemiologic Studies-Depression Scale). Participants with 1 or 2 components were identified as prefrail, and those with no symptoms, as nonfrail. 13 All subjects were tested twice in a standardized procedure, separated by 1 week. Participants were asked to perform their usual activities during the week of testing. The study conformed to the standards set by the Declaration of Helsinki. Functional Power Tests Lower-extremity muscle power was measured using the STS transfer and SC tests. All participants were given a 5-minute practice session to become familiar with the tasks. STS power. Participants were seated on the front part of a chair with arms crossed over the chest, eyes fixed straight ahead, and both feet on a forceplate. a They were asked to rise as fast as possible into the standing position and stand quietly for 5 seconds. The test was performed 3 times with 1 minute of rest between trials (the best performance was retained). Failed attempts were excluded from data analysis and the test was repeated. Participants were verbally encouraged by the investigator to ensure the explosive movement. Power was defined as P F*s/t and was calculated by using the changes in vertical ground reaction force (collected at 1000Hz) during the rising phase (time between peak force and end of the rising phase t), vertical ground reaction force during quiet standing (F), and the difference between body height standing and body height sitting (s). 12 SC power. Participants stood ready at a starting line 3m in front of a stairway. On the command ready, go, they rapidly approached the steps and climbed 4 stairs as fast as possible. Participants were instructed to grab the hand rail in case of balance loss. The test was repeated 2 times with 1 minute of rest between trials (the best performance was retained). Failed attempts were excluded from data analysis and the test was repeated. SC times were recorded using 2 pairs of infrared photocells b placed on the first and fourth stairs. SC time, body mass, and height between the first and fourth stair were used to calculate SC power. Statistical Analysis Power data (mean SD) were expressed either in absolute terms (watts [W]) or normalized to body mass (W/kg). Pearson correlation coefficients were used to measure the association between subject characteristics (age, height, body mass, body mass index [BMI]) and normalized SC and STS power, respectively. Subsequently, linear regression analysis with multiple independent variables (subject characteristics) was performed to assess differences in associations between independent variables and normalized SC and STS power. BMI variables were not included in the regression analysis because of their collinearity with height and body mass. Additionally, 1-way analysis of variance (ANOVA) with post hoc comparisons (Bonferroni adjusted) was performed to determine STS power and SC power differences between young, middle-aged, nonfrail, and prefrail older adults. Differences in subject characteristics were analyzed by using ANOVA for continuous variables and chi-square test for nominal variables. Only baseline data were used for ANOVA and regression analyses. For comparison between occasions (intrarater reliability), we used the intraclass correlation coefficient (ICC) based on a 2-way ANOVA with mixed effects and the relative standard error (SE) of measurement. 14 RESULTS All participants were able to complete the power tests without assistance. There were 2 failed SC attempts (1 nonfrail and 1 prefrail older adult) due to balance loss. In 6 cases, subjects (2 nonfrail and 4 prefrail older adults) did not immediately reach an upright standing position during STS transfer. These data were excluded from analysis and the trials were repeated. Table 1: Participant Characteristics of Absolute and Body Mass Normalized SC Power and STS Transfer Power Variable Young Adults Middle-Aged Adults Nonfrail Older Adults Prefrail Older Adults P Age (y) (22 30) (40 59) (66 81) (69 88).001 Sex (women/men) 7/8 7/9 8/8 7/6.955 Height (cm) Weight (kg) BMI (kg/m 2 ) SC power Absolute (W) Normalized (W/kg) STS power Absolute (W) Normalized (W/kg) NOTE. Values expressed as mean SD (range) or n.

3 FUNCTIONAL POWER TESTS IN OLDER PEOPLE, Zech 969 Table 2: Multiple Regression Analysis for Muscle Power Measures (normalized for body mass) and Independent Variables (age, height, mass) Variable Regression Coefficient SE 95% CI SC power (n 60); adjusted R Intercept to Age (P.001) to Height (P.008) to Body mass to STS power (n 60); adjusted R Intercept to 5.87 Age (P.001) to Height (P.008) to Body mass to Abbreviation: CI, confidence interval. Fig 1. Correlation between age and SC power (normalized to body mass) in all participants. Filled triangle, nonfrail; nonfilled triangle, prefrail. Participant characteristics and power data are listed in table 1. Pearson correlation between subject characteristics and power measures showed that age and height significantly (P.001) correlated with body mass normalized SC (age, r.809; height, r.412) and STS power (age, r.493; height, r.451). No significant correlation was found between power and body mass, whereas BMI significantly correlated with SC power (r.367; P.004). The highest association was found between age and SC power (r.804) and age and STS power (r.493) (figs 1 and 2). Multiple regression analysis showed that age and height significantly influenced normalized SC and STS power (table 2). The association between independent variables (age, height, body mass) and normalized SC power was higher (R 2 679) than between independent variables and normalized STS power (R 2.313). Fig 2. Correlation between age and STS transfer power (normalized to body mass) in all participants. Filled triangle, nonfrail; nonfilled triangle, prefrail. All participants were divided further into groups of young adults (n 15), middle-aged adults (n 16), and nonfrail (n 16) and prefrail (n 13) older community-dwelling adults. ANOVA showed a significant group effect for absolute (P.001; Cohen f 1.24) and normalized (P.001; f 1.71) SC power, as well as for absolute (P.002; f.56) and normalized (P.001; f.85) STS power. Post hoc tests with Bonferroni correction showed that absolute and normalized SC power in young adults were significantly higher than in middleaged adults (P.019; f.45; P.001; f.84), who in turn scored higher than nonfrail older adults (P.003; f.59; P.001; f 0.67). No group differences between young, middle-aged, and nonfrail older adults were shown for STS power. Nonfrail and prefrail older adults differed significantly in absolute (P.001; f.77) and normalized (P.001; f.83) SC power, as well as in absolute (P.005; f.59) and normalized (P.001; f.78) STS power. SC and STS power correlated significantly at r.731 (P.001) in all participants. The association between both power tests was significant in all subgroups and did not differ between groups. ICCs for test-retest reliability ranged from.770 to.963 in SC and STS power, and SE of measurements were all less than 3% (table 3). Reliability was similar across age groups and power tests. DISCUSSION In this study, the SC and STS transfer power tests were evaluated in young, middle-aged, and community-dwelling nonfrail and prefrail older adults. A significant association was found between functional lower-limb power and participant characteristics. Age had the strongest association with both power measures, followed by body height. Multiple linear regression analysis suggests that almost 70% (R 2 ) of SC power and approximately 30% (R 2 ) of STS power can be predicted by age and height, respectively. Age-related changes in muscle power previously have been reported by other investigators. 1,2 Bassey and Short 1 examined leg extensor power in all adult age groups up to 93 years and reported a negative correlation between muscle power and age, which was more pronounced in people older than 60 years. Similar findings were shown for upper-extremity muscle power. 2 However, little is known about the age versus muscle power relationship in people younger than 60 years. To our knowledge, the present study was the first that distinguished between power performances in several adult age groups. Our subgroup analyses in young, middle-aged, and older adults support the finding that SC

4 970 FUNCTIONAL POWER TESTS IN OLDER PEOPLE, Zech Variable Table 3: ICCs and Relative SE of Measurement for SC Power and STS Transfer Power Young Adults Middle-Aged Adults Nonfrail Older Adults Prefrail Older Adults ICC SE of Measurement (%) ICC SE of Measurement (%) ICC SE of Measurement (%) ICC SE of Measurement (%) SC STS power in particular is influenced predominantly by age. No age-related subgroup differences were found for STS power, which might be caused in part by a relatively small decrease with increasing age. Based on the size of the multiple regression correlation coefficient between subject characteristics and STS power for all participants, it also is likely that factors other than age or height contribute to STS power differences (eg, mobility limitations). Furthermore, Bean et al 5 reported no difference between strength and power in their relation to chair rise time and speculated that the chair rise movement is related more to force production than to the rate at which that force is produced (power). Thus, although both power measures were significantly related to age, SC power might be exceptionally sensitive for the detection of power changes over adult life span. However, it also should be noted that because of a small sample size in each subgroup, the statistical power (10% and 30%) of age-related subgroup STS power comparisons was less than the required standard. Significant group differences between nonfrail and prefrail older adults were shown for both power measures. In nonfrail adults, absolute SC power was about 30% and absolute STS power about 25% higher than in prefrail adults. Consequently, it might be hypothesized that both functional power measures are sufficient to distinguish between minor differences in frailty status. However, because this was, to our knowledge, the first study to compare functional power between nonfrail and prefrail older adults, additional research is needed to verify this finding. A comparison between mobility limitations and SC power performance in older adults 10 indicated that SC power predicts approximately one-third of short physical performance battery performance. In that study, Bean et al 10 concluded that SC power is a relevant measure of leg power impairments in mobility-limited older adults. Although the association between prefrailty and functional power impairments has not been calculated in the present study, our results suggest that the occurrence of frailty components may have a certain impact on both power measures in older adults. This implies that lowerlimb power performances are not only influenced by age and height, but also may contribute to mobility impairments and frailty. However, other factors that were not measured in this study (eg, history of falls, 4 walking performance, 3,15 health and disease status) also could explain muscle power differences. The high reliability of power outcomes in prefrail and nonfrail older adults emphasizes the applicability of both measures for muscle power testing in older people. Study Limitations Limitations of this study include (1) concerns regarding the generalizability of the findings given that the older participants previously had participated in health-related exercise programs; (2) lack of other standard power measurements, such as the Nottingham power rig; and (3) lack of information regarding health indicators, history of falls, and disease status in older adults. Furthermore, the small number of participants in each subgroup produces uncertainty regarding the statistical power of ANOVA to detect group differences. Despite these concerns, prefrail versus nonfrail comparisons in both power measures and age-related group differences in SC power were statistically significant because of large effect sizes. The statistical power for these comparisons ranged from 95% to 100%. However, the small sample size resulted in insufficient statistical power of age-related STS power subgroup comparisons (10% and 30%) and thus the statistical validity of these results is limited. CONCLUSIONS Results of the study showed that increasing adult age was associated with a decrease in SC and STS power. Almost 70% of SC power and approximately 30% of STS power can be predicted by age and height. These findings were supported by significant differences in SC power among subgroups of young, middle-aged, and older adults. Because of low statistical power, the absence of significant age-related subgroup differences in STS power should be viewed with caution. Nevertheless, results indicate that especially stair climbing is a sensitive test for the detection of agerelated changes in muscle power during the life span, well applicable in all adult age groups. Furthermore, for the first time to our knowledge, it was shown that the outcomes of both power measures differed significantly between nonfrail and prefrail older adults. This suggests that SC and STS tests are clinically relevant outcome measures to assess muscle power in older people. However, future research is needed to verify these findings and clarify the applicability of both measures for muscle power assessment in frail older people. References 1. Bassey EJ, Short AH. A new method for measuring power output in a single leg extension: feasibility, reliability and validity. 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5 FUNCTIONAL POWER TESTS IN OLDER PEOPLE, Zech 971 shortening cycle movements in elderly males and females. Eur J Appl Physiol 2001;84: Bean JF, Kiely DK, LaRose S, et al. Is stair climb power a clinically relevant measure of leg power impairments in at-risk older adults? Arch Phys Med Rehabil 2007;88: Margaria R, Aghemo P, Rovelli E. Measurement of muscular power (anaerobic) in man. J Appl Physiol 1966;21: Lindemann U, Claus H, Stuber M, et al. Measuring power during the sit-to-stand transfer. Eur J Appl Physiol 2003;89: Fried LP, Tangen CM, Walston J, et al. Frailty in older adults: evidence for a phenotype. J Gerontol A Biol Sci Med Sci 2001; 56:M Domholdt E. Rehabilitation research, principles and applications. 3rd ed. Philadelphia: W.B. Saunders Co; Puthoff ML, Janz KF, Nielson D. The relationship between lower extremity strength and power to everyday walking behaviors in older adults with functional limitations. J Geriatr Phys Ther 2008; 31: Suppliers a. Kistler Instrumente GmbH, Daimlerstrasse 6, Ostfildern, Germany. b. TAG Heuer SA Professional TiminG, 6A Rue Louis-Joseph Chevrolet, CH-2300 La Chaux-de-Fonds, Switzerland.