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CLINICAL SCIENCES: Clinical Investigations

Knee strength and lower- and higher-intensity functional performance in older adults

SALEM, GEORGE J.; WANG, MAN-YING; YOUNG, JEAN T.; MARION, MICHAEL; GREENDALE, GAIL A.

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Medicine & Science in Sports & Exercise: October 2000 - Volume 32 - Issue 10 - p 1679-1684
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Abstract

The knee extensor and flexor muscles are important contributors to human locomotor efficiency and stability. Their use during slow and fast walking (17,29), stair climbing (2,20), and chair standing (21,22) activities has been well documented using electromyographic, kinematic, and kinetic analyses. Clinically, loss of strength in these muscle groups is associated with gait and movement dysfunction, including knee hyperextension and toe drag during gait, an inability to stand from low chair heights, progressive knee joint pathology, and pain (1,15,25).

Despite strong evidence supporting the importance of knee extensors and flexors in the performance of functional activities, observed relations between measurements of knee strength and assessments of functional performance in older men and women have not been consistent. For example, statistically significant correlations have been reported between peak knee strength and relatively higher-intensity functional measures, such as chair-stand time, in both “frail institutionalized” (r = −0.63) (9) and “independent healthy” (r = −0.35 to −0.5) volunteers (27). However, nonsignificant relations between measures of peak knee strength and relatively lower-intensity functional measures, such as walking speed, have been reported for both “clinically healthy” (8) and “mildly to moderately frail” volunteers (5). These conflicting results suggest that the relations between functional measures of performance and measures of knee strength in older adults are complex and that the strength of these relations may be modified by the relative intensity of the functional performance tests performed.

The intensity of exercise is classically defined as the power output associated with the performance of an activity, expressed as the work performed per unit time (28). Data from biomechanical studies can be used to assess power output and, thus, the relative intensity of various tests of functional performance (20,29,30). These data suggest that functional performance measures frequently used in the assessment of older adults (e.g., walking speed, standing reach, timed chair stands, and timed stair climbs) (7,9,10,19,26,27) can be separated into relatively lower-intensity and higher-intensity measures. The purpose of this investigation was to explore the relations among isokinetic knee flexor and extensor peak strength, work capacity, and relatively lower- and higher-intensity measures of physical functional performance.

METHODS

Subjects.

The present analysis uses baseline data from a randomized controlled trial of a physical activity intervention study (12). Sixty-two men and women volunteers, aged 60 yr or greater, were recruited from community sites such as seniors centers and retirement programs in the greater Los Angeles area. All subjects reviewed and signed a written informed consent previously approved by the UCLA and University of Southern California Institutional Review Boards. Study exclusion criteria included: 1) coronary heart disease, angina, or congestive heart failure; 2) systolic BP greater than 160 or diastolic BP greater than 95; 3) use of antiseizure medications, anticoagulants, or more than two antihypertensive medications; 4) metabolic bone disease or use of any osteoinductive drugs; 5) long-term musculoskeletal pain that limited physical function; 6) history of injurious falls; 7) use of an ambulation assistive device; and 8) previous hip fracture. All physical, strength, and functional performance measures were collected during a single laboratory visit while the subjects wore a tee shirt and gym shorts supplied by the investigators.

Functional measures.

Functional performance was measured in the morning after the subjects completed self-report demographic, behavioral, and medical history questionnaires. Specific tests were selected because they had previously been used to assess functional performance in older subjects (7,9,13,14,26) and because we believed that they represented a range of performance intensity.

During performance of all functional and strength tests, research associates gave participants standardized directions and uniform encouragement prompts. Timed walking tests included an 8-foot normal walk, a 50-foot normal walk, and a 50-foot brisk walk. All walking tests were conducted on a tiled walkway while participants wore their own shoes. During the “normal” walking tests, participants were instructed to walk at their “normal” pace. During the 50-ft “brisk” walking task, participants were told to select a pace they would use to “cross a street at a traffic signal.” A timed sit-to-stand test was performed using a firm, armless chair (45 cm high). Participants were asked to fold their arms across their chest and perform five repetitions of standing and sitting at a “normal” pace. A timed stair climb was performed on a flight of 13 stairs. Participants were instructed to ascend the stairs at their “normal” pace and were encouraged to use the handrail if needed. A standing reach test was performed as follows: Participants stood erect with their right shoulder against a wall and were instructed to make a fist, and reach out with their right arm parallel to the floor. Laboratory assistants made sure the participants did not flex or rotate their trunk during this initial measurement. A mark was recorded on the wall to identify the subject’s “erect-reach” distance. Participants were then instructed to “reach as far as possible” while maintaining their foot position. Again, a mark was recorded on the wall to identify the subject’s “bending-reach” distance. Participants performed three trials; the maximum difference between the erect- and bending-reach distance was the subject’s standing reach.

Isokinetic measures.

Bilateral knee flexion and extension strength, and work capacity were measured with a Kin-Com 500H isokinetic dynamometer (Chattecx, Chattanooga, TN). Isokinetic testing was performed using standard isokinetic testing protocols (3,4,8,11,16,19) and was sequenced after the functional testing to provide the participants with an adequate lower-extremity “warm-up” before collection of the maximum-effort isokinetic data. The participants were positioned against a back support providing a hip flexion angle of approximately 90°. Stabilizing belts were secured over the chest, lap, and distal one-third of the thigh; arms were crossed during testing. The resistance pad was positioned at a two-finger-width distance above the malleoli on the lower leg. Knee flexion/extension range of motion was 100–160° (measured posteriorly), and all tests were performed at 60°·s1. Participants practiced five submaximal repetitions of knee flexion/extension before the start of recording. During recording, consistent motivational verbal prompts were employed; no visual feedback was provided. Maximum strength was recorded as the peak flexion and peak extension torque produced during five continuous, concentric, maximum-effort, flexion/extension repetitions (Fig. 1A). Peak torque is routinely referenced as the absolute strength of a subject (26) and has been used extensively in studies assessing knee strength in elders (3,4,8,11,16,19). Repeated measures reliability of peak extension and flexion torques measured using the same protocol was tested in our laboratory in a separate group of 19 older adults. These measures were highly reliable with Cronbach’s alpha values of between 0.96 and 0.99. Work capacity was recorded as the total flexion and total extension work (sum of the product of torque and angular displacement or area underneath the torque/angle curve) performed during 20 continuous, concentric, flexion/extension repetitions (Fig. 1B). The total work performed over many repetitions is an indicator of a subject’s muscular endurance (23). Researchers have recommended using work capacity over other indices of endurance because of this measure’s high repeatability (18,24). Collectively, the peak-torque and work-capacity measures are referred to as “isokinetic measures.”

Figure 1
Figure 1:
Concentric, multiple-repetition, isokinetic knee extension and flexion curves performed at 60°·s−1, through a range of 100° to 160° measured posteriorly. A, Representative torque/time curve illustrating peak flexion and extension torques produced during five continuous repetitions. B, Representative torque/time curve illustrating flexion and extension torques produced during twenty continuous repetitions. Work capacity was calculated as the total work performed during 20 continuous repetitions.

Statistical methods.

Bilateral isokinetic measures were collected; because correlations between right and left limbs were strong (r > 0.80), analyses presented here are limited to the right side only. Simple linear regression models were used to fit each functional performance outcome measure to each isokinetic predictor measure. Adjusted regression models were controlled for age, body mass, and height. Using backward-selection, models for which an isokinetic predictor measure was significant (at P ≤ 0.05) but a covariate was not significant (at P > 0.10) were refitted dropping the insignificant covariate terms. The change due to model reduction in the estimated coefficients of the isokinetic terms was then assessed. None of the reduced regression models were appreciably different from the full models (statistically significant estimated regression coefficients for isokinetic measures changed by no more than 11.1%); thus, models adjusted for age, body mass, and height are reported throughout.

RESULTS

Relevant participant characteristics are provided in Table 1. All participants tolerated the testing procedures without difficulty, and none reported having experienced musculoskeletal pain during a 7-d follow-up phone call.

Table 1
Table 1:
Demographic and anthropometric characteristics of participants.

Table 2 presents descriptive statistics for isokinetic and functional performance measures. Average peak extension and flexion torques were 60% and 40% greater in the male subjects, respectively; whereas, average total extension and flexion work was 67% and 43% greater in the male subjects, respectively.

Table 2
Table 2:
Average values of peak isokinetic and functional tests.

Results of linear models that examined the associations between each functional measure (outcome) as a function of peak knee extensor torque are summarized in Table 3. Considering the timed tests, greater strength was consistently and statistically significantly related to faster performance times. Compared with the remainder of the timed tests, stair climbing and chair standing showed the strongest relations to peak knee extension torque, as evidenced by their higher beta coefficient values.

Table 3
Table 3:
Linear relations between selected functional tests and right knee peak extension torque (N = 60).a

Illustrated in Table 4, the magnitude and pattern of associations between peak knee flexion torque and the timed performance measures were similar to those seen for peak knee extension torque: greater strength predicted faster performance, all associations were statistically significant (except the 50-foot normal walk which was borderline significant), and the relative magnitude of the associations was greatest for stair climbing and chair standing.

Table 4
Table 4:
Linear relations between selected functional tests and right knee peak flexion torque (N = 62).a

Greater total knee extension work was also related to faster timed performance (Table 5). Similar to findings in peak extensor and flexor torque, the strength of the relations was greatest for stair climbing and chair standing. Time to perform all walking tasks was also lower in relation to greater work, but these associations were of borderline significance for the normal-speed walking tests.

Table 5
Table 5:
Linear relations between selected functional tests and right knee total extension work (N = 60).a

Table 6 presents results of linear models that examined relations between total knee flexion work and each of the timed functional tasks. Greater work was again related to faster functional performance, with the strongest associations evident between the chair stand test and total knee flexion work.

Table 6
Table 6:
Linear relations between selected functional tests and right knee total flexion work (N = 60).a

DISCUSSION

This study demonstrates that for timed measures of functional performance, greater knee strength and work capacity are linearly associated with better performance, and the relations between knee strength and functional performance are greatest for those measures believed to represent higher-intensity functional tasks (e.g., brisk walking, stair climbing, and chair standing). Isokinetic strength and work capacity measures explained between 41% and 54% of the variance in the higher-intensity functional models but only between 31% and 33% of the variance in the lower-intensity functional models. The strength of the associations, approximated by the beta coefficients of the strength and work terms, was also greater for the higher-intensity functional tasks.

The physical performance measures used in this study have predictive validity in older women and men. In large, population-based samples, poorer performance on the lower extremity battery, which includes the 8-foot walk test and timed chair stands, predicts nursing home admission and death (13). The capability of these functional tests to predict future disability (inability to perform tasks of daily life) and death persists even in large samples of subjects showing no disability at baseline (14). Further, the predictive validity of the lower extremity battery is independent of comorbid medical conditions (14). Inability to execute the functional reach test is a strong predictor of recurrent falls (9). For example, those who are unable to reach are 8 times more likely to fall, and those with a reach less than 6 inches are 4 times more likely to fall than those with a reach greater than 10 inches.

Although the relative intensity of the six functional measures used here has not been formally assessed, published data can be used to support their separation into higher and lower intensity categories. Biomechanical studies indicate that the power output of the knee musculature during stair ascent (20) and chair standing (30) is greater than double the power output during normal gait (29). Researchers, to date, have not reported the comparative power output between normal and “brisk” walking; however, knee extensor moments associated with “fast walking” are approximately double those associated with normal walking (29). Thus, relatively lower-intensity functional measures included the standing reach, 8-foot normal walk, and 50-foot normal walk tasks, whereas the relatively higher-intensity functional measures were likely the 50-foot brisk walk, chair stand, and stair climb tasks.

Our findings that the relations among knee strength, work capacity, and functional performance are greater for the higher-intensity functional tasks is consistent with previously published hypotheses suggesting a strength “threshold” for functional activities in older adults (6,31). In these models, strength below a minimum threshold results in functional impairment, whereas strength above a minimum threshold is in excess of what is required for normal function. Figure 2 illustrates the effects of functional activity intensity on these relations. For lower-intensity functional tasks, the hypothetical relation between strength and function is curvilinear, a minimum strength threshold is relatively easy to identify, and there is an extended flat region (i.e., the area where strength is in excess of what is required for normal function). Conversely, for the higher-intensity functional tasks, a minimum strength threshold is relatively difficult to identify and there is a relatively short or nonexistent flat region. We believe our subject pool included a majority of participants with knee strength and work capacity in excess of what is required for normal walking and the standing reach task (i.e., the lower-intensity measures). Consequently, greater knee strength and work capacity were only moderately associated with improved performance (i.e., knee strength and work capacity explained between 31% and 33% of the variance in these models). Conversely, we believe that few of our subjects possessed a knee strength and work capacity in excess of what is required for brisk walking, chair-standing, and stair-climbing activities. Consequently, greater knee strength and work capacity were more strongly associated with improved performance, and these isokinetic measures explained between 41% and 54% of the variance in these models.

Figure 2
Figure 2:
Hypothetical relations between lower- and higher-intensity timed measures of functional performance and strength. For lower-intensity functional tasks, the hypothetical relation is curvilinear, a minimum strength threshold is relatively easy to identify, and there is an extended flat region. Conversely, for the higher-intensity functional tasks, a minimum strength threshold is relatively difficult to identify and there is a relatively short or nonexistent flat region.

Interpretation of the results of this study should be made with an appreciation of the study’s limitations. The study participants can be described as a relatively homogeneous group of ambulatory, high-functioning seniors. Consequently, the extrapolation of these results to physically frail elders or master athletes is not recommended. This study only examined the relations between knee musculature and functional performance and did not consider addition muscle groups (e.g., ankle and hip) that are known contributors to ambulation. Additionally, this study relied exclusively upon isokinetic testing and linear regression analyses to identify the associations between muscular and functional performance. Additional biomechanical analyses, including kinematic, kinetic, and electromyographic investigations, are needed to increase our understanding of the underlying neuromuscular and skeletoarticular contributions to the performance of important functional tasks of daily life.

The preservation of functional independence through physical activity intervention is a complex problem requiring 1) an examination of the underlying neuromuscular and skeletoarticular contributions to normal function, 2) the identification of specific exercises and activities that target those underlying contributions, 3) an assessment of the feasibility-of-performance and prescription-adherence associated with these exercises and activities, and 4) testing of whether interventions that increase muscular strength also improve functional performance. By characterizing the associations among knee strength and work capacity and the performance of lower- and higher-intensity functional tasks, this study increases our understanding of the underlying neuromuscular and skeletoarticular contributions to functional performance. These observations provide an important first step and can be used to inform the research design of studies that aim to preserve or improve function through the enhancement of physical capability.

This research was supported by the Claude D. Pepper Older Americans Independence Center, grant no. NIA P60 AG10415.

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Keywords:

AGING; ISOKINETIC; PEAK TORQUE; TOTAL WORK; FUNCTIONAL TASKS

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