Relationships among impairments in lower-extremity strength and power, functional limitations, and disability in older adults.

Throughout the aging process, people demonstrate an overall decline in muscle mass, with specific atrophy of type II muscle fibers. (1-4) This generalized loss of skeletal muscle has been termed "sarcopenia" and is considered a major factor leading to the development of impairments in muscle strength and power for older adults. (5,6) Muscle strength is defined as the ability of a muscle or muscle group to exert maximal force or torque at a specific velocity during a contraction. (7) Muscle power is characterized by the product of force production and the velocity at which the force is produced. (7) Muscle strength and muscle power both have been shown to decline during the aging process, with power declining at a greater rate than strength. (1,8)

According to the disablement model, impairments can lead directly to functional limitations, defined as restrictions in the performance of activities that are essential to daffy living. (9) How impairments in muscle strength and power relate to functional limitations in older adults has been examined in various studies. Deficits in lower-extremity strength relate to walking, the ability to get in and out of a chair, the speed of climbing steps, and the rate of falls in older adults. (10-12) Peak muscle power also relates to functional limitations, with power consistently demonstrating a stronger relationship to functional limitations than strength. (13,14) Evans (15) hypothesized that power has a stronger relationship to functional limitations because power involves both force production and contraction velocity and because many daffy activities are force and speed dependent.

One limitation of these findings is that functional limitations typically have been assessed with tools that are speed dependent, are completed in short periods, and measure only small aspects of daily activities. (13,16) Examples include gait speed over a short duration, climbing stairs, and getting out of a chair quickly. Although short-term, high-velocity activities are vital to function, these activities do not reflect all aspects of the daily activities of older adults. Understanding how impairments in strength and power relate to other activities, such as walking over a long duration, preparing meals, performing housework tasks, or taking part in community activities, would help guide therapists in examining older adults and planning interventions to help limit or decrease functional limitations.

Investigators (14,17) have hypothesized that the importance of power measurements may be based on the activities being performed. Power production at a lower relative intensity may have more impact on activities that have a lower force production demand, such as walking at a comfortable pace. For activities that require a higher percentage of maximal force production than walking at a comfortable pace, such as getting in and out of a chair or walking fast, (18,19) power production at a high relative intensity may be more vital. Additionally, because sarcopenia may lead to older adults losing their physiological reserve of strength (maximal voluntary performance that is in excess of what is needed to complete a task (20)), they may need nearly maximal force production to complete some daffy activities, especially high-intensity activities. The hypothesis that performance at nearly maximal force production may have a stronger relationship to high-intensity activity than to low-intensity activity in older adults was demonstrated in a study by Salem et al. (21) For a group of 62 community-dwelling older adults, strength, measured through peak extension torque of the knee extensor muscles, had stronger relationships to activities such as getting out of a chair and climbing steps than to lower-intensity activities, such as walking. (21) These findings suggest that maximal force production is important but that the level of importance varies on the basis of the intensities of the activities being performed.

In one study to date, the investigators (17) contrasted how power at a high relative intensity and power at a low relative intensity related to measures of functional limitations. For 48 older adults with physical disabilities, power at a low relative intensity (40% 1 repetition maximum [1-RM]) explained more of the variance in habitual gait speed than did power at a high relative intensity (70% 1-RM) and explained similar amounts of the variance in stair climbing and rising from a chair. (17) One shortcoming of that study was the use of power at 70% 1-RM as a measure of power at a high intensity instead of the use of an intensity closer to maximal force production, such as power at 90% 1-RM. Examining the importance of power at an almost maximal load would be more representative of power at a high relative intensity and would have more meaningful implications for older adults who may need nearly maximal efforts to complete some daffy activities.

According to the disablement model, once impairments lead to functional limitations, the functional limitations can progress to difficulty in performing activities essential to the completion of expected or specific societal tasks, known as disability. (9) Limited studies have examined the link between disability and impairments in muscle strength and power. In a cross-sectional study of 1,002 women who were 65 years of age or older and were involved in the Women's Health and Aging Study, the strength of the upper and lower extremities was associated with the presence of motor disabilities. (22) Rantanen et al (12) found that, for 567 community-dwelling older adults, low levels of strength were associated with dependency in activities of daily living. No studies to date have examined how muscle power is related to disability.

Limitations of the studies that have been completed are that the relationship between impairments and disability has been viewed as direct and that the mediating effect of functional limitations has been neglected. (23) An analysis that would allow the examination of how the intermediary step of functional limitations affects the relationship between impairments and disability would provide a better understanding of factors leading to disability in older adults.

Although research on the relationships of strength and power to functional limitations and disability already has been completed, there are still aspects that need further exploration and clarification. The specific questions that still need answering include the following. How do strength and power relate to functional limitations when functional limitations are classified through longer-duration activities and tools that include a range of daily activities? How does power at a high relative intensity relate to functional limitations and disability? How do strength and power relate to disability when the indirect relationship between impairments and disability is taken into consideration? Answering these questions could lead to an improved ability to screen older adults for the risk of developing future functional limitations and disability and provide a better understanding of how strength and power at a range of relative intensities relate to functional limitations and disability.

The purpose of this study was to examine the relationships among impairments in lower-extremity strength, peak power, power at a low relative intensity, and power at a high relative intensity, functional limitations, and disability in community-dwelling older adults.

Method

Subjects

Previous examinations of the relationships of impairments in strength and power to functional limitations in community-dwelling older adults revealed Pearson correlation coefficients (r) approximating. (60,16,24,25) On the basis of these findings, this study was designed to detect correlation coefficients of similar magnitudes between measures of impairments and functional limitations. To detect a correlation coefficient of .60 as significant, assuming a significance level of [alpha]=.05, a sample size of 20 subjects would be needed to provide 80% power, and a sample size of 25 subjects would be needed to provide 90% power.

On the basis of the results of the power analysis, 30 community-dwelling older adults who were 65 years of age or older ([bar.X] [+ or -] SD= 77.3 [+ or -] 7.0 years; 25 women and 5 men) and had self-reported mild to moderate functional limitations were recruited to participate in this study. Subjects were recruited from local agencies that interact with older adults and from senior living complexes in the eastern Iowa and western Illinois regions. Self-reported mild to moderate limitations were defined as a self-report of at least one limitation on the physical function subscale of the Medical Outcomes Study 36-Item Health Survey Questionnaire (SF-36). (26,27) Subjects with acute or terminal illnesses, myocardial infarction in the preceding 6 months, moderate or severe chronic obstructive pulmonary disease, uncontrolled hypertension, uncontrolled metabolic disease, acute orthopedic injuries, recent unhealed fractures, neurological disease, muscular disease, or significant cognitive impairments (score of <23 on the Folstein Mini-Mental State Examination (28)) were excluded from participation in this study. In compliance with the University of Iowa Human Subjects Review Committee and Genesis Health System Institutional Review Committee, written informed consent was obtained from each subject prior to participation in the study. Table 1 provides further demographic information on the subjects.

Instrumentation and Measures

Impairments in strength and power. Figure 1 provides an overview of the outcome measures used for impairments, functional limitations, and disability. A Keiser 420 leg press * was used to assess lower-extremity strength and power. This leg press is a piston pneumatic system that uses air pressure to create resistance. As the piston moves inside a closed cylinder, pressure builds and resistance increases. Resistance can be adjusted by adding or removing air from the cylinder prior to movement. Inside the cylinder, a transducer sensor samples pressure changes, and another transducer measures the positional changes of the piston at a rate of 400 Hz to allow an assessment of force and power production.

In order to use the leg press, subjects sit in an adjustable seat and place each foot on a pedal. The seat can be adjusted to align the hips and knees in the desired position, and the foot pedals can move independently to allow testing of one extremity or can be connected to allow testing of bilateral lower-extremity performance. This line of equipment has demonstrated high reliability and validity for the assessment of lower-extremity muscle performance in older adults. (16,29,30)

Lower-extremity strength was assessed with 1-RM testing. One repetition maximum is defined as the maximum amount of weight lifted one time with proper form during a standard exercise and has been shown to be a safe and valid measure of strength in older adults. (31-35) Power measurements were taken at 40%, 50%, 60%, 70%, 80%, and 90% of the measured 1-RM. This method of testing allowed the examination of power across a range of relative intensities. Three measures of power were used for analysis: peak power, power at 40% 1-RM, and power at 90% 1-RM. Peak power was defined as the highest power output regardless of the external load at which it was achieved. This method of defining peak power was used to account for individual differences that might lead to subjects achieving peak power at different percentages of 1-RM. In contrast, in other studies, peak power was defined as power at 70% 1-RM for all subjects. (36) This method was justified in other studies because it was shown that most older adults achieved peak power at 70% 1-RM. (16,37) Power at 40% 1-RM and power at 90% 1-RM were used as outcome measures to represent power at a low relative intensity and power at a high relative intensity, respectively.

[FIGURE 1 OMITTED]

Functional limitations and disability. The Short Physical Performance Battery (SPPB) was used as a measure of functional limitations. The SPPB assesses 3 areas: ability to maintain static balance in feet-together, semitandem, and tandem postures for up to 10 seconds; time to walk a 4-m distance at a normal pace; and time to complete 5 sit-to-stand transfers as quickly as possible. (26) Each subscale is rated on a scale from 0 to 4, and scores can be added to create a summary score between 0 and 12. The SPPB has high predictive validity in identifying people at greater risk for mortality, nursing home admission, and incidence of disability over 1-year and 4-year time periods and has been shown to be sensitive to changes in health status for people with moderate functional limitations. (26,38,39) The SPPB total score and the subscale scores were used as outcome measures.

The Six-Minute Walk Test (SMWT) also was used as a measure of functional limitations. In accordance with guidelines established by the American Thoracic Society, subjects were instructed to cover as much distance as possible during a 6-minute time period. (40) The SMWT has been shown to have relationships to self-report measures of disability, quality of life, and limitations in activities of dally living for older adults. (41,42) The SMWT was included in this study to reflect daily activities that require extended periods of walking.

The Late Life Function and Disability Instrument (LLFDI) was used as a measure of functional limitations and disability. (43,44) The LLFDI is a self-report questionnaire specifically designed to evaluate the functional limitation and disability components of the disablement model. To assess functional limitations, subjects are asked how much difficulty they have in performing 32 daily activities, such as pouring from a large pitcher, picking up and moving a chair, washing dishes, getting dressed, and entering and exiting a car. Responses are scored on a ratio scale of 0 to 100, with scores approaching 100 signifying high levels in the ability to perform actions and scores approaching 0 indicating low levels in the ability to perform actions. The disability component of the LLFDI assesses how frequently older adults perform 16 socially defined life tasks (frequency category) and how limited they feel in completing these tasks (limitation category). Examples of these tasks include caring for one's health, traveling, and running errands. Subjects receive one score in the disability frequency category and another score in the disability limitation category. Scores approaching 100 signify high levels in the frequency of performing life tasks or a high capability of performing life tasks, and scores approaching 0 indicate low levels in the frequency of performing life tasks or a low capability of performing life tasks.

The score for the functional limitation component of the LLFDI was used as a measure of functional limitations. This outcome measure was chosen to reflect common daily activities that are difficult to assess in physical performance tests. The score for the limitation category of the LLFDI disability component was chosen as the measure of disability instead of the score for the frequency category for the following reasons. The limitation category mimics other measures of disability, has higher reliability than the frequency category, and has been chosen in earlier research as a measure of disability. (43,45,46)

Procedure

This study involved a cross-sectional design. Subjects attended 3 sessions over a 2- to 3-week time period. During the first session, subjects completed the SPPB, SMWT, and LLFDI. At the completion of the LLFDI, subjects took part in an orientation session for the Keiser 420 leg press and practiced high-velocity contractions. The seat of the leg press was adjusted so that the subjects' hips and knees were at a 90-degree angle, and the foot pedals were connected to allow testing of bilateral lower-extremity performance.

The second session took place 7 days following the first session. Lower-extremity strength was assessed with 1-RM testing. Subjects completed 2 submaximal sets of 5 to 10 repetitions as a warm-up and then began 1-RM testing. Subjects were given 3 to 4 minutes of rest between attempts, and the goal was to determine 1-RM in less than 5 attempts. To ensure safe and proper form in completing the motion, the subjects' buttocks and back had to remain against the seat, they had to use a proper breathing pattern (exhale during the concentric phase, no breath holding), and they could not use their hands to assist in extending their lower extremities. At the completion of lower-extremity strength testing, 20 minutes of rest was provided.

Next, lower-extremity power was assessed. Power measurements were taken at 40%, 50%, 60%, 70%, 80%, and 90% of the measured 1-RM. Subjects were instructed to extend their legs as quickly as possible against the set resistance to a point of almost full knee extension and then slowly return the pedals to the start position. Three attempts were made at each external resistance, and the highest value was recorded for analysis. A 30- to 60-second rest period was given between attempts.

During the third session (2-7 days after the second session), lower-extremity strength and power testing was repeated with the same protocol as that used during the second session. This repeat session of muscle testing was performed to account for any possible learning effect associated with muscle performance testing in older adults. (47)

Data Analysis

All statistical analyses were completed with SPSS 12.0 for Windows. ([dagger]) Intraclass correlation coefficients (ICC[2,1]) and the standard error of the measurement were used to examine the reliability of strength and power testing between sessions.

The higher of the 2 values for strength assessment was used for analysis. The power results associated with the higher strength measurement were used for analysis. If a subject had the same strength measurements in the 2 sessions, then the power testing session with the overall highest value was used for analysis. These steps were taken to ensure that the subjects' best performance was used for analysis. Because the amount of strength or power needed to perform functional activities, such as walking or getting out of a chair, is related to movement and control of the body, strength and power data were normalized to body mass for analysis. This process created a more normal distribution for strength and power values and provided a means for making intersubject comparisons. This method of analysis was recommended in earlier investigations and is commonly used in this line of research. (8,48-50)

The relationships of impairments in lower-extremity strength and power to functional limitations were examined with regression analyses. Separate linear regression analyses were used to determine the relationship of each measure of impairment (strength, peak power, power at 40% 1-RM, and power at 90% 1-RM) to each measure of functional limitation (SPPB total score, SPPB subscale scores, SWMT distance, and LLFDI functional limitation component score). Regression coefficients, standard errors, and coefficients of determination ([R.sup.2]) were calculated for each analysis. Age and sex were considered as possible covariates. If age or sex was significantly associated with the measures of functional limitations, then it was entered into the model prior to the measures of strength and power. Previous studies demonstrated curvilinear relationships between strength and functional limitations and between power and functional limitations. (16,50) Analyses were performed to determine whether a curvilinear relationship between each impairment and each functional limitation was present.

Mediation analysis was used to examine the relationships of impairments in lower-extremity strength and power to disability and was completed with an SPSS macro developed by Preacher and Hayes. (51) Mediation analysis is a statistical procedure that allows the examination of how an independent variable indirectly influences a dependent variable through an intervening or a mediator variable. (52) A series of regression analyses are completed to estimate the indirect effect of the independent variable on the dependent variable. (53) In this study, impairments in muscle strength and power were the independent variables, disability was the dependent variable, and functional limitations were the mediators. It was determined a priori that only the SPPB total score and SMWT distance would be used as mediators because both have been shown to have strong relationships to disability.

One criticism of mediation analysis is that there is a high rate of type II errors, especially when the number of subjects is smaller than 50. (54) A recommended technique that was used in this study to increase the statistical power of mediation analysis is called resampling with replacement, or bootstrapping. (54) Bootstrapping involves creating a sample size of n by choosing 1 subject's data from the original data pool, returning that subject's data to the original data pool, and then repeating the process until n observations have been selected. The sample size of n is usually set at the size of the original data pool. For example, if 30 subjects are in the original data pool, then the process of randomly selecting 1 subject's data is repeated 30 times. The mediation analysis then is carried out on the n subjects, and the indirect effect is estimated. This process then is repeated anywhere from 1,000 to 10,000 times to create a pseudo-population of indirect effects. From the estimated pseudo-population of indirect effects, 95% confidence intervals (CIs) are created with a bias-corrected method to determine whether 0 is included in the range. If 0 is not within the 95% CIs, then the indirect effect of the independent variable on the dependent variable acting through the mediator is considered significant.

Recommendations in the literature regarding the number of times to repeat bootstrapping vary and are somewhat dependent on the ability of statistical software to complete the analysis. (53,55,56) Efron and Tibshirani (56) recommended that at least 1,000 samples be used in attempts to estimate CIs. Bootstrapping was repeated with 5,000 samples in this study on the basis of the recommendations of Efron and Tibshirani (56) and the abilities of the SPSS software and the SPSS macro to run 5,000 samples in a timely manner.

Results

Impairments in Muscle Strength and Power

All subjects completed strength and power testing without major complications. Three subjects chose not to complete the second strength and power assessment because of discomfort that they experienced after the first assessment. A subject's data from one session were lost because of a computer error. For the 26 subjects who completed both sessions, the reliability of strength and power measurements was high (Tab. 2).

Strength and power measurements normalized to body mass are shown in Table 3. Four subjects did not have data at 40% 1-RM, and 1 subject did not have data at 50% 1-RM, because the smallest amount of resistance that the leg press could provide was greater than 40% or 50% the 1-RM for the subjects. The distribution of power results across relative intensities is shown in Figure 2. Subjects achieved peak power across a range of relative intensities; in contrast, in previous reports, peak power consistently occurred at 70% 1-RM. (17) The mean relative intensity at which peak power occurred was 62% 1-RM, with the following distribution: peak power was achieved by 2 subjects at 40% 1-RM, by 9 subjects at 50% 1-RM, by 9 subjects at 60% 1-RM, by 4 subjects at 70% 1-RM, by 2 subjects at 80% 1-RM, and by 4 subjects at 90% 1-RM.

[FIGURE 2 OMITTED]

Functional Limitations and Disability Measures

Subjects had an average SPPB total score of 8.9 [+ or -] 2.5 ([bar.X] [+ or -] SD). Six subjects were unable to complete 5 sit-to-stand transfers and therefore had no values for time to complete this aspect of the SPPB. Rank transformation of sit-to-stand times was used so that data for these 6 subjects could be included in the SPPB subscale analysis. Time to complete the 4-m walk was converted to gait speed; subjects had an average speed of 0.97 [+ or -] 0.23 m/s. Seventeen of the 30 subjects scored 4 of 4 on the SPPB balance subscale. For SPPB balance subscale analysis, subjects were divided into 2 groups: those who scored 4 of 4 and those who did not. Subjects walked an average distance of 418.2 [+ or -] 83.6 m during the SMWT. The LLFDI functional limitation component score was 54.71 [+ or -] 6.47, and the disability component limitation category score was 65.19 [+ or -] 6.38. One subject's disability score was considered an outlier, and her disability data were excluded from further analysis of disability. This subject consistently communicated difficulty with daily activities in conversation with the primary investigator (MLP), but would not score herself as being limited on the LLFDI and therefore had a score of 100. Even after the primary investigator repeated the directions for the LLFDI to ensure that she understood the test, she still had a score of 100.

[FIGURE 3 OMITTED]

Relationships of Impairments in Muscle Strength and Power to Functional Limitations

The results of the regression analyses of relationships of strength and power measurements to measurements of functional limitations are shown in Tables 4 and 5. Sex was significantly related to gait speed and was entered into the regression equation prior to strength or power data. Overall, strength and all 3 power measurements were significantly related to measures of functional limitations. One exception was that power at 40% 1-RM was not significantly related to the SPPB total score. As determined with a Mann-Whitney U test, strength or power was not related to the SPPB balance subscale score (results not shown in tables). As shown in Figure 3, peak power explained more of the variance in measures of functional limitations that involved walking, over both short and long distances (SPPB gait speed and SMWT distance), than did other measures of power and strength. At no time did power at a low relative intensity (power at 40% 1-RM) explain more of the variance in functional limitations than peak power.

Power at 90% 1-RM explained more of the variance in the SPPB total score than did other measures of power. Additionally, power at 90% 1-RM had the strongest relationship to the SPPB sit-to-stand subscale score. Peak power explained more of the variance in the functional limitation component of the LLFDI than did other measures. No curvilinear relationship of any measures of strength and power to functional limitations was demonstrated.

Relationships of Impairments in Strength and Power to Disability

Table 6 shows the indirect effect and 95% bias-corrected CI for each mediation analysis performed. The 95% CIs demonstrated that measures of impairments in strength and power had significant indirect effects on disability by acting through functional limitations. Power measurements tended to have stronger indirect effects than the strength measurement. Power at 40% 1-RM and peak power had the strongest indirect effects when the SMWT was the mediator, and power at 90% 1-RM had the strongest indirect effect when the SPPB total score was the mediator. All mediation analyses were repeated with the bootstrapping technique 1,000 times, with no significant changes in the results.

Discussion

Measures of Muscle Strength and Power

The results of this study revealed high reliability of strength and power measurements between sessions. Although a learning effect has previously been demonstrated in testing muscle strength in older adults, (47) the present study revealed little change between sessions. One factor that may have accounted for these findings was the practice session provided at the end of the first session. The practice session allowed subjects the chance to use the leg press and practice high-velocity contractions and thus may have lessened the learning effect. These findings would support the need for only one session of testing strength and power in future studies if a practice session is provided beforehand.

Subjects in this study tended to have higher power values and to obtain peak power at a lower relative intensity than subjects in previous studies. (16,17,25,29,37,57) The highest average peak power occurred at 50% 1-RM (Fig. 3), with subjects reaching peak power at an average relative intensity of 62% 1-RM, whereas previous studies reported peak power occurring at 70% 1-RM or higher. (16,17,37,58) Although there are similarities between the testing protocols used in the present study and previous investigations, the slight variations may have accounted for the power value differences. In previous studies, subjects' lower-extremity power was measured in one session, and subjects made only one attempt at each external load (percent 1-RM). (17,37,58) In contrast, subjects in the present study completed a practice session and then performed 2 testing sessions on separate days, with 3 attempts at each load, to account for possible learning effects with high-velocity muscle contractions. This adjustment in the testing protocol may have led to the overall higher power values achieved and to the peak power achieved at a lower external resistance. These findings should be considered in future studies, and investigators should be cautious in using power at a predetermined relative intensity to represent peak power for all subjects.

Relationships of Impairments in Muscle Strength and Power to Functional Limitations

Lower-extremity strength and power were significantly related to almost every measure of functional limitations, with power consistently explaining more of the variance in functional limitations than strength. On the basis of these findings, older adults who possess larger amounts of strength and power should have less difficulty in completing daily tasks. An explanation for the stronger relationship of power than of strength to functional limitations may be related to the facts that many daily activities are dynamic in nature and that dynamic measures of functional limitations were used in the present study. Because power involves not just force production but also the speed at which the force is produced, deficits in power may be related to functional limitations to a greater extent than deficits in strength.

The [R.sup.2] values for strength ranged from .28 to .38, and the Re values for power ranged from .26 to .48. These [R.sup.2] values are similar to those obtained in other studies in which the relationships of strength and power to functional limitations were examined. (14,17,36) The findings in the present study are significant, but they still leave a large part of the variance in functional limitations unexplained. Although various studies have shown that older adults can decrease functional limitations through improvements in lower-extremity strength and power, (59-61) other factors, such as aerobic capacity, (62) balance, (63) and fear of falling, (64) also have been shown to have relationships to functional limitations. Therapists should think beyond strength and power and consider other physiological factors when completing an examination and establishing a plan of care for older adults.

One major difference between the results of the present study and the results of previous studies is that power at a low relative intensity did not have the strongest relationship to walking activities. Peak power had the strongest relationship to walking activities. There are several possible explanations for these findings. First, walking may not be a low-intensity activity for all people. Cress and Meyer (20) presented the idea that over time, older adults lose their physiological reserve and may need to apply a larger percentage of their maximal performance to daily activities. Although subjects in the present study tended to have more lower-extremity power than subjects in previous studies, this finding likely was attributable to testing protocol differences. Subjects in the present study were older, walked more slowly, and had lower physical performance scores than subjects in previous studies. (14,16,17) These data could be an indication that subjects in the present study may have lost a large amount of their physiological reserve and depended more on power production at higher relative intensities for walking than subjects in previous studies. Another explanation for these findings is that power at 40% 1-RM represents power capability only at an arbitrary level. During walking and other activities, the largest amount of power that can be produced may be more important than the power production at a predetermined relative intensity.

Power at 90% 1-RM explained more of the variance in the SPPB total score and the SPPB sit-to-stand subscale score than peak power and power at a low relative intensity. Given that the demands on the musculoskeletal system to rise from a chair are more than double the requirements for normal walking, (21) the finding that power at a high relative intensity has a stronger relationship to the sit-to-stand subscale score than other measures of power is to be expected. The stronger relationship to the SPPB total score may be attributable to the sit-to-stand subscale having a greater influence on the total score than the other subscales. These findings are similar to those of Salem et al, (21) who found that peak torque values for the knee extensor muscles were related more to high-intensity activities than to low-intensity activities. This is the first study, to our knowledge, that has considered the importance of power production at an almost maximal load in older adults. Our findings are important because they support the hypothesis made earlier (14) that the importance of muscle power may vary on the basis of the intensities of the activities being performed.

The LLFDI functional limitation component provides an overall self-assessment of functional limitations by asking questions about activities such as stair climbing, getting dressed, running, pouring water, or opening a door. This outcome measure assesses a wider range of functional limitations than physical performance tests and better reflects the diverse challenges that older adults experience daily. Our findings indicated that peak power explained more of the variance in the LLFDI functional limitation component than the other measures of power. These findings further support the greater importance of overall peak power than of power at a particular relative intensity.

Although various investigators (16,48,50,65) have found curvilinear relationships of measures of impairments in strength and power to functional limitations, this was not the case in the present study. The lack of curvilinear relationships may have been attributable to the degrees of impairments and functional limitations demonstrated in our study population. Subjects in the present study tended to be older, to have lower SPBB scores, and to have lower gait speeds than subjects in studies in which curvilinear relationships were found. Perhaps the subjects in the present study were below the threshold values of strength and power needed to perform functional activities without difficulty, leading to linear rather than curvilinear relationships. The other possibility is that the relatively small number of subjects in the present study made it difficult to identify significant curvilinear relationships.

Relationships of Impairments in Muscle Strength and Power to Disability

One of the novel aspects of the present study was the examination of the relationships of impairments in muscle strength and power to disability by mediation analysis. Mediation analysis provided an approach to addressing the indirect relationships of impairments in strength and power to disability by taking into consideration the principles of the disablement model. The results of the present study support the finding of previous studies that strength impairments may lead to the development of disability in older adults. This is the first study, to our knowledge, that has examined how lower-extremity power is related to disability in a manner that is cognizant of the indirect relationships between impairments and disability.

Power measurements consistently had stronger indirect effects on disability than the strength measurement. When the SMWT was the mediator, peak power and power at 40% 1-RM had almost the same indirect effects on disability, whereas power at 90% 1-RM had a much smaller indirect effect. When the SPPB total score was the mediator, power at 90% 1-RM had a stronger indirect effect on disability than peak power and power at 40% 1-RM. These data support both strength and power being important factors in affecting disability. Whether peak power, power at a low relative intensity, or power at a high relative intensity was more important was based on the mediator used to link impairments and disability.

Recommendations

The results of the present study have some meaningful implications for clinicians who work with older adults. Exercise should focus on improving both strength and power. Strength is important, but power affects function in ways that strength does not. It is vital not only to have large amounts of force production but also to be able to produce force quickly. Although the overall results of the present study demonstrated that peak power explains a larger amount of the variance in most functional activities, there are certain situations in which power at different relative intensities is important. On the basis of the findings of the present study, older adults may benefit from improving power at both low and high intensities. Designing an exercise program to improve both strength and power does not add any additional time requirements. When a resistance training program is performed at a high velocity rather than a low velocity, power increases to a significantly greater degree without any detrimental effects on strength gains in older adults. (37,66) High-velocity resistance training has been performed safely in studies involving older adults and has been shown to have low rates of adverse effects. (60)

Limitations

Some limitations of the present study need to be addressed. First, the sample size of the study--30 total subjects--was small. Additionally, the study had a sex imbalance, with 25 of the 30 subjects being women. However, although the number of subjects in the study was small, power analysis determined that only 20 subjects would be needed for 80% power. Nonetheless, the application of our results to the general population of older adults with mild to moderate functional limitations should be done with caution.

This study involved a cross-sectional design, and none of the results proved causal relationships of impairments in muscle strength and power to functional limitations and disability in older adults. Interventional studies in which strength and power are improved through exercise and then functional limitations and disability levels are examined need to be done to support causal links. Longitudinal studies that involve tracking impairments in strength and power, functional limitations, and disability also would establish causal relationships.

Conclusion

The results of this study suggest that, in community-dwelling older adults with mild to moderate functional limitations, impairments in lower-extremity strength and power are related to functional limitations and disability. Power has a stronger relationship to functional limitations and disability than does strength. Peak power appears to have the strongest influence on functional limitations and disability, except in the performance of tasks that require high relative intensities.

Dr Puthoff provided concept/idea/research design, writing, data collection and analysis, and project management. Dr Nielsen provided consultation (including review of manuscript before submission). The authors acknowledge Genesis Outpatient Rehabilitation Services, Genesis Medical Center, Davenport, Iowa, for providing space to conduct the study.

A platform presentation of this work was given at the Combined Sections Meeting of the American Physical Therapy Association; February 14-17, 2007; Boston, Mass.

This study was approved by the University of Iowa Human Subjects Review Committee, Iowa City, Iowa, and the Genesis Health System Institutional Review Board, Davenport, Iowa.

Dr Puthoff received funding from the American Physical Therapy Association Section on Geriatrics Adopt-A-Doc Award during the academic years 2004-2005 and 2005-2006, when this study was completed.

This article was submitted June 19, 2006, and was accepted May 4, 2007.

DOI: 10.2522/ptj.20060176

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* Keiser Corp, 2470 S Cherry Ave, Fresno, CA 93706.

([dagger]) SPSS Inc, 233 S Wacker Dr, Chicago, IL 60606.

ML Puthoff, PT, PhD, is Assistant Professor, Physical Therapy Department, St Ambrose University, 518 W Locust St, Davenport, IA 52803 (USA). Address all correspondence to Dr Puthoff at: [e-mail].

DH Nielsen, PT, PhD, is Professor Emeritus, Graduate Program in Physical Therapy and Rehabilitation Science, University of Iowa, Iowa City, Iowa.

[Puthoff ML, Nielsen DH. Relationships among impairments in lower-extremity strength and power, functional limitations, and disability in older adults. Phys Ther. 2007;87:1334-1347.]

Table 1.
Subject Demographic Data

Characteristic (a)      [bar.X]   SD      Range

Age (y)                  77.3     7.0   67.0-91.0
BMI (kg/[m.sup.2])       29.8     8.1   18.0-62.8
MMSE score               28.7     1.4   25.0-30.0
No. of chronic health     2.7     1.4    0.0-6.0
  conditions
No. of prescription       4.7     3.4    0.0-14.0
  medications

(a) BMI=body mass index, MMSE=Mini-Mental State Examination.
Chronic health conditions were heart disease, pulmonary
disease, hypertension, diabetes, arthritis, osteoporosis,
cerebrovascular accident, and history of cancer.

Table 2.

Reliability of Strength and Power Measurements

                     Intraclass    Standard Error   No. of Subjects
Strength and Power   Correlation   of the           With Data from
Measurements (a)     Coefficient   Measurement      Both Sessions

Strength             .97           55.86 N          26
Peak power           .97           38.29 W          26
Power at 40% 1-RM    .95           39.41 W          20
Power at 50% 1-RM    .99           24.17 W          25
Power at 60% 1-RM    .97           37.02 W          26
Power at 70% 1-RM    .94           46.48 W          26
Power at 80% 1-RM    .89           59.53 W          26
Power at 90% 1-RM    .87           27.26 W          26

(a) 1-RM = 1 repetition maximum.

Table 3.

Strength and Power Measurements (a)

Strength and Power                                     No. of
Measurements (a)           [bar.X]   SD     Range     Subjects

Strength (N/kg)             15.5     4.0   7.9-26.3      30
Peak power (W/kg)            7.6     2.7   2.9-14.4      30
Power at 40% 1-RM (W/kg)     7.1     2.7   2.9-13.6      26
Power at 90% 1-RM (W/kg)     5.7     2.4   2.2-13.5       3

(a) 1-RM=1 repetition maximum.

Table 4.

Results of Regression Analyses of Relationships of Strength and Power
Measurements to Short Physical Performance Battery (SPPB) Scores

                                                    Partial
Measurements (a)              Coefficient   SE     [R.sup.2]      P

SPPB total score
  Strength                       0.32       0.10      .28      .003 (b)
  Peak power                     0.46       0.15      .26      .004 (b)
  Power at 40% 1-RM              0.31       0.16      .14      .063
  Power at 90% 1-RM              0.64       0.15      .39      .000 (b)
SPPB gait speed
  Strength (c]                   0.03       0.01      .31      .000 (b)
  Peak power (c]                 0.05       0.01      .35      .000 (b)
  Power at 40% 1-RM (c]          0.05       0.01      .31      .001 (b)
  Power at 90% 1-RM (c]          0.05       0.01      .24      .002 (b)
SPPB sit-to-stand times
    (rank-transformed data)
  Strength                       -1.18      0.35      .29      .002 (b)
  Peak power                     -1.75      0.51      .29      .002 (b)
  Power at 40% 1-RM              -1.40      0.56      .20      .020 (b)
  Power at 90% 1-RM              -2.21      0.56      .34      .001 (b)

(a) 1-RM = 1 repetition maximum.

(b) Statistically significant.

(c] Sex was significantly related to the dependent variable and was
entered into  the regression model prior to strength or power data.

Table 5.

Results of Regression Analyses of Relationships of Strength and Power
Measurements to Six-Minute Walk Test (SMWT) Distance and Late Life
Function and Disability Index (LLFDI) Functional Limitation Component
Score

                                         Partial
Measurements (a)     Coefficient  SE    [R.sup.2]      P

SMWT distance
  Strength              12.84     3.08     .38      .000 (b)
  Peak power            21.39     4.20     .48      .000 (b)
  Power at 40% 1-RM     19.68     4.65     .43      .000 (b)
  Power at 90% 1-RM     23.78     4.81     .47      .000 (b)
LLFDI functional
    limitation
    component score
  Strength              0.91      0.25     .32      .001 (b)
  Peak power (c]        1.27      0.33     .35      .001 (b)
  Power at 40% 1-RM     1.17      0.37     .31      .004 (b)
  Power at 90% 1-RM     1.15      0.41     .22      .010 (b)

(a) 1-RM = 1 repetition maximum.

(b) Statistically significant.

(c] Data for one subject were excluded from analysis because the score
was an outlier.

Table 6.
Mediation Analysis Results

Mediator   Independent          Dependent    Indirect   95% Confidence
(a)        Variable (b)         Variable     Effect     Interval (c)

SMWT       Strength             Disability   0.479      0.087-1.014 (d)
           Peak power           Disability   0.968      0.126-1.959 (d)
           Power at 40% 1-RM    Disability   1.038      0.290-2.141 (d)
           Power at 90% 1-RM    Disability   0.887      0.041-1.861 (d)
SPPB       Strength             Disability   0.395      0.069-0.896 (d)
           Peak power           Disability   0.564      0.074-1.263 (d)
           Power at 40% 1-RM    Disability   0.394      0.014-1.059 (d)
           Power at 90% 1-RM    Disability   0.817      0.167-1.819 (d)

(a) SPPB = Short Physical Performance Battery, SMWT=Six-Minute
Walk Test.

(b) l-RM=1 repetition maximum.

(c) As determined by bootstrapping with bias correction.

(d) Values indicate a significant indirect effect.