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Phys Ther. Jan 2010; 90(1): 67–74.
PMCID: PMC2802823

Decreased Muscle Strength Relates to Self-Reported Stooping, Crouching, or Kneeling Difficulty in Older Adults

Abstract

Background

Bending down and kneeling are fundamental tasks of daily living, yet nearly a quarter of older adults report having difficulty performing or being unable to perform these movements. Older adults with stooping, crouching, or kneeling (SCK) difficulty have demonstrated an increased fall risk. Strength (force-generating capacity) measures may be useful for determining both SCK difficulty and fall risk.

Objective

The purposes of this study were: (1) to examine muscle strength differences in older adults with and without SCK difficulty and (2) to examine the relative contributions of trunk and leg muscle strength to SCK difficulty.

Design

This was a cross-sectional observational study.

Methods

Community-dwelling older adults (age [X±SD]=75.5±6.0 years) with SCK difficulty (n=27) or without SCK difficulty (n=21) were tested for leg and trunk strength and functional mobility. Isometric strength at the trunk, hip, knee, and ankle also was normalized by body weight and height.

Results

Compared with older adults with no SCK difficulty, those with SCK difficulty had significant decreases in normalized trunk extensor, knee extensor, and ankle dorsiflexor and plantar-flexor strength. In 2 separate multivariate analyses, raw ankle plantar-flexor strength (odds ratio [OR]=0.97, 95% confidence interval [CI]=0.95–0.99) and normalized knee extensor strength (OR=0.61, 95% CI=0.44–0.82) were significantly associated with SCK difficulty. Stooping, crouching, and kneeling difficulty also correlated with measures of functional balance and falls.

Limitations

Although muscle groups that were key to rising from SCK were examined, there are other muscle groups that may contribute to safe SCK performance.

Conclusions

Decreased muscle strength, particularly when normalized for body size, predicts SCK difficulty. These data emphasize the importance of strength measurement at multiple levels in predicting self-reported functional impairment.

Stooping, crouching (ie, bending down), and kneeling movements are an integral component of many common activities, including gardening, shopping, and cleaning. Limitations in stooping, crouching, or kneeling (SCK) are associated with an increased likelihood of limitations in other lower-body functional tasks such as lifting and prolonged standing1 and also are associated with fall risk.2 As with standing up from a chair, SCK movements require coordination of the whole-body center of mass over a wide range of postures in order to prevent a loss of balance or fall. Moving from stance into crouching and kneeling involves significant ankle, knee, and hip range of motion, whereas stooping movements are characterized by a reduction of knee movement.3 In addition to mobility requirements, SCK movements can constitute a significant challenge to the balance and strength (force-generating capacity) capacities of older adults, which may explain why “bending down” tasks are included in clinical assessments such as the Physical Performance Test4 and Berg Balance Scale5 that are thought to predict fall risk. However, even though SCK difficulty may be a significant indicator of mobility and independence in older adults, few data exist regarding the determinants of SCK difficulty and its association with fall risk.

The prevention of falls in older adults is the focus of much research effort. A key component of avoiding falls is the motor system's ability to produce joint torques to counteract perturbations that would lead to losses of balance.6 The neuromuscular system invokes a series of response strategies involving extremity and trunk musculature710 to produce joint torques at various body segments in order to maintain the center of mass over the base of support or return the center of mass rapidly to within the base of support when external perturbations have altered its position. Muscle weakness contributes to both falls and self-reported SCK difficulty in older adults, but the relative contribution of different muscle groups, such as proximal (trunk and hip) versus distal (ankle) muscle strength, is not entirely clear.11,12

The association of reduced lower-extremity muscle forces, particularly ankle strength, with falls is well established.1315 Leg strength also has been associated with functional performance such as gait speed and chair rises.16,17 Ankle dorsiflexor and ankle plantar-flexor strength measures predict performance on some (Timed “Up & Go” Test [TUG] and Berg Balance Scale)15 but not all (unipedal stance time [UST])18 common clinical balance tests that are thought to predict falls in older adults. Although knee extensor strength is associated with static and dynamic capabilities as well as functional ability in older adults,19,20 the role of more proximal trunk and hip muscles in determining functional performance has not been well characterized. Given that control of the flexing trunk is critical to avoiding falls when losses of balance occur,9 it is important to determine the relative contributions of both trunk and lower-extremity strength to functional tasks such as SCK.

The purposes of this study were: (1) to examine trunk and lower-extremity muscle strength differences in older adults with and without SCK difficulty and (2) to examine the relative contributions of trunk and leg muscle strength to SCK difficulty that may predict falls in older adults with a range of balance impairments. Of particular interest are the relative contributions of muscle strengths that have not been well studied in older adults, including strength at the trunk and hip. These data will advance the understanding of the contribution of strength measures, particularly of the proximal lower extremity and trunk, to self-reported functional performance and may allow a more thoughtful approach to the use of strength training in improving balance and thereby reducing falls. Given the previous data on the importance of ankle function in balance performance and fall risk, we hypothesized that lower-extremity strength would be significantly decreased in older adults with SCK difficulty and that distal strength measures would be the main predictors of SCK difficulty.

Method

Participants

Functionally independent, community-dwelling older adults were recruited largely from a database maintained by the University of Michigan Older Americans Independence Center Human Subjects and Assessment Core. Inclusion criteria for the study were age over 65 years, ability to speak and understand English, and ability to stand for 5 minutes without an assistive device. A nurse practitioner performed a screening medical history and physical examination and excluded those whose medical conditions precluded the ability to complete the test battery, such as those individuals who: (1) were medically unstable (eg, chest pain upon exertion, dyspnea, acute infection), (2) reported severe and frequent back or lower-extremity pain, or (3) reported severe neurologically induced impairments that might affect balance (eg, history of cerebrovascular accident, Parkinson disease). Out of 50 recruited individuals, 2 were excluded due to severe osteoporosis and severe and frequent back pain. During screening, participants rated their ability to stoop, crouch, or kneel, according to a 5-point difficulty scale, based on a single question on the Established Populations for Epidemiologic Studies of the Elderly (EPESE) questionnaire21: no difficulty (n=21), a little (n=13), some (n=9), a lot (n=2), or unable to do (n=3). Participants were categorized into 1 of 2 groups: a no SCK difficulty group (n=21), if they reported no difficulty, or an SCK difficulty group (n=27) if they reported any SCK difficulty. Participants signed a written informed consent form approved by the University of Michigan Medical School Institutional Review Board. Based on previous work,12 power analysis22 revealed that a sample size of 21 for each group was required to achieve a power of 0.80 with an alpha level of .05 in detecting strength differences between older adults with and without SCK difficulty.

Instrumentation and Measures

Self-report health measures.

The nurse practitioner used self-report (interview-based) health measures to obtain patient data at the medical screening. The total number of chronic medical conditions was ascertained by asking participants if they had a previous history of osteoarthritis, rheumatoid arthritis, osteoporosis, myocardial infarction, stroke, joint replacement, Parkinson disease, or peripheral neuropathy. Dizziness was determined by asking participants if they had a current episode of light-headedness or vertigo. Self-reported leg joint limitations were determined by the report of joint range-of-motion limitations due to pain or stiffness in the hip, knee, or ankle.

Clinical balance measures.

The 3 clinical balance measures that were examined were the UST, the TUG, and the maximum step length (MSL). The UST is a commonly used clinical balance measure. Deficits in UST, defined as the inability to stand unsupported on one leg for more than 5 seconds, is a strong predictor of injurious falls.23 Participants stood on their preferred leg while their arms were folded. The foot of the remaining leg was lifted and held approximately 5.08 cm (2 in) from the medial malleolus of the stance leg. Participants performed a practice trial followed by 2 experimental trials. The best time (maximum 30 seconds) was recorded as the UST. The UST has excellent interrater reliability (intraclass correlation coefficient [ICC]=.99).24

The TUG is a reliable measure of functional mobility and dynamic balance in older adults (intrarater ICC=.99, interrater ICC=.99).25 Participants sat in a chair and, on command, stood up and walked 9.84 ft (3 m) at their “comfortable and safe pace” before turning around and returning to the seated position. The time to complete this task is the person's TUG score. Scores exceeding 14 seconds have been associated with increased fall risk in older adults.26 Participants performed a practice trial, followed by 3 experimental trials. The TUG was recorded as the mean time (in seconds) of the 3 trials.

The MSL test is a reliable test of stepping ability, which correlates with standard balance measures (eg, unipedal stance, tandem stance, tandem walk), functional mobility measures, (eg, TUG, Performance-Oriented Mobility Assessment, Six-Minute Walk Test) and fall history (intrarater ICC=.91).2729 Standing with arms folded and feet together, participants stepped forward maximally with their dominant leg and returned to the original starting position. Leg dominance was ascertained using a simple screening question, namely the preferred foot used to kick a soccer ball. Participants performed a practice trial followed by 5 experimental trials, and MSL was recorded as the mean distance stepped over the 5 trials. To account for individual anthropometric differences, the MSL was normalized as a percentage of body height (% height).

Strength measures.

Isometric peak torque of the trunk, hip, and knee extensors, as well as of the ankle plantar flexors and dorsiflexors of the dominant lower extremity, was evaluated using the Biodex multijoint isokinetic dynamometer.* After a practice trial, participants exerted a maximal contraction for 2 trials of 3 seconds each. Participants received approximately 45 seconds of rest between trials, the best of which was recorded as the peak torque. If the 2 torque measurements differed by more than 15%, an additional trial was performed to achieve consistency.

Trunk extensor isometric strength was evaluated using a back attachment* affixed to the multijoint isokinetic dynamometer. Participants were supported and stabilized in the seated position via chest, pelvic, and thigh straps, with the arms folded and the trunk and lower extremities configured at approximately 90 degrees to each other. Participants exerted a maximal trunk extensor contraction against a pad located at the interscapular region.

Hip extensor isometric strength was evaluated in the functional standing position as previously described.30 Briefly, this measure required the participants to stand supported and stabilized upright in a standing frame with the lower extremity straight and the hip flexed 15 degrees in the sagittal plane. Participants exerted a maximal hip extensor contraction by pushing against a pad located immediately distal to the popliteal fossa.

Knee extensor isometric strength was evaluated in the seated position with the knee flexed to 90 degrees and chest and thigh straps providing support and stabilization. With the arms folded, participants exerted a maximal knee extension contraction against a pad located at the anterior distal tibia.

Isometric strength of the ankle plantar flexors and dorsiflexors was evaluated with the participants in a semireclined position with the tibia parallel to the floor, the foot and tibia at 90 degrees to each other, and the knee and hip at approximately 30 and 60 degrees of flexion, respectively. The lower leg was supported with a pad and straps, and with arms folded, participants either pushed maximally into (plantar flexors) or pulled away from (dorsiflexors) a footplate in which the foot was tightly secured with straps. Peak torque values are expressed as raw strength (in N · m) or as normalized strength, as a percentage of the product of body weight (BW) (newtons) and body height (BH) (meters). as done in previous studies (% BW × BH).12,29

Reliability of isometric strength testing using the Biodex multijoint dynamometer system has been demonstrated previously in measurements of knee strength for older men (intrarater ICC=.90).31

Fall-related measures.

Data for fall-related measures were obtained during the medical screening by asking participants whether they had fallen within the past year. In the event that a fall was reported, the number of falls incurred and whether medical treatment was sought for their injuries (ie, injurious falls) were ascertained. For analysis, the number of falls and number of injurious falls within the past year were dichotomized (ie, falls ≥2 and injurious falls ≥1).

Data Analysis

All statistical analyses were carried out in SPSS 16.0 for Windows. To test for normality in continuous variables, the Kolmogorov-Smirnov test was used. Group comparisons for continuous participant description, self-reported health, clinical balance, and strength data in older adults with and without SCK difficulty were evaluated using an independent-samples t test when data were normally distributed and a Mann-Whitney U test when data were not normally distributed (eg, number of chronic medical conditions, UST, trunk extensor strength). To determine group differences in dichotomous variables, either the Pearson chi-square test (eg, sex, dizziness, self-reported leg joint limitations) or the Fisher exact test (eg, fall-related measures) was performed. Relationships among strength, clinical balance measures, and SCK difficulty were evaluated using Spearman correlation coefficients, using the full 5-point scale of SCK difficulty. A forward stepwise binary logistic regression analysis included all the strength variables with a significant correlation (P<.05) to determine which measurements were important predictors of self-reported SCK difficulty. A log transformation resulted in a normal distribution for UST and trunk strength measures. Statistical analyses were carried out with and without the log transformation to the UST and trunk strength measurements, but yielded similar results. Thus, for ease of interpretation, the untransformed variables are presented in this article. Two separate logistic regression models, one for normalized strength and one for raw strength, are presented. We performed an analysis of multicollinearity among the variables. Even with all strength measures entered into a linear regression model, we found no evidence of collinearity, as the variance inflation factor was not greater than 10 for any raw or normalized strength measure.32 A significance level of .05 was used for all correlational and regression analyses, and a Bonferroni correction was used for multiple SCK group comparisons of strength (.05/5).

Role of the Funding Source

This work was funded, in part, by the National Institute on Aging; the Department of Veterans Affairs Office of Research and Development, Clinical Science and Rehabilitation Research and Development Services; and the Dorothy and Herman Miller Fund for Mobility Research in Older Adults.

Results

Participant Characteristics and Strength Measures

Forty-eight older adults (62.5% women, 37.5% men) participated in the study. Participant characteristics (demographic, self-reported health, clinical balance, strength, and fall-related data) are presented for the entire sample in Table 1. Compared with older adults without SCK difficulty, older adults with SCK difficulty were older and had increased self-reported leg joint limitations, decreased UST, increased TUG scores, and decreased MSL. The incidence of 2 or more falls within the previous year (P=.096) tended to be higher in older adults with SCK difficulty.

Table 1.
Characteristics of Older Adults With and Without Stooping, Crouching, or Kneeling (SCK) Difficulty

Compared with older adults without SCK difficulty, mean (SD) raw ankle plantar-flexor strength was significantly lower in those with SCK difficulty (76.4±25.6 N·m versus 53.9±29.8 N·m, respectively) (Tab. 1). Raw ankle dorsiflexor and knee extensor strength were decreased in older adults with SCK difficulty versus those without SCK difficulty, but when the Bonferroni correction was applied, the difference was not statistically significant.

After normalization of strength data to account for body size (eg, body height and weight), mean normalized strength was higher in older adults without SCK difficulty compared with those with SCK difficulty in ankle dorsiflexors (2.4±0.9 % BW × BH versus 1.6±0.6 % BW × BH, respectively), ankle plantar flexors (6.8±2.4 % BW × BH versus 4.3±2.3 % BW × BH, respectively), knee extensors (11.1±2.2 % BW × BH versus 7.6±3.0 % BW × BH, respectively), and trunk extensors (6.3±3.7 % BW × BH versus 4.3±2.3 % BW × BH, respectively) (P<.01) (Figure). There were tendencies toward greater normalized hip extensor strength in participants without SCK difficulty compared with those with SCK difficulty (7.1±2.0 % BW × BH versus 5.8±2.0 % BW × BH, respectively), but this observation was not significant after correcting for multiple comparisons.

Figure.
Mean normalized isometric peak torque (% body weight × body height) for all muscle groups tested in older adults with and without stooping, crouching, or kneeling (SCK) difficulty. Error bars show 1 standard deviation (* indicates P<.01). ...

Correlations With SCK Difficulty

Considering the full 5-point ordinal scale of SCK difficulty, SCK difficulty was associated with raw knee extensor (r=−.43) and ankle plantar-flexor (r=−.39) strength (P<.01), and to a lesser degree with ankle dorsiflexor strength (r=−.31, P<.05). However, no significant correlations between SCK difficulty and raw hip or trunk extensor strength were observed. Normalization of strength variables led to strong associations between all strength measures and SCK difficulty, such that decreased strength was associated with SCK difficulty (r=−.37 to −.58, P<.01) (Tab. 2). Stooping, crouching, or kneeling difficulty also was strongly correlated with functional mobility in all 3 clinical balance tests (P<.005), particularly with UST (r=−.62).

Table 2.
Correlations Between Stooping, Crouching, or Kneeling (SCK) Difficulty and Both Normalized Strength and Clinical Balance Measuresa

Strength as Predictor of SCK Difficulty

Based on findings from the primary analyses, raw ankle and knee strength values were entered in a forward stepwise binary logistic regression. The model showed that ankle plantar-flexor strength (odds ratio [OR]=0.97, 95% confidence interval [CI]=0.95–0.99) was significantly associated with SCK difficulty (P=.014), but explained only 11% of the variance. Considering the primary analyses of normalized strength, all 5 strength measures were entered into a forward stepwise binary logistic regression model. The model showed that the most significant predictor for SCK difficulty was normalized knee extensor strength (OR=0.61, 95% CI=0.44–0.82, P=.001), which explained 26% of the variance in SCK difficulty (Tab. 3).

Table 3.
Results of Logistic Regression Analyses of Muscle Strength Measurements as Predictors of Stooping, Crouching, or Kneeling Difficultya

Discussion

The results of this study highlight the significant association between strength and self-reported SCK difficulty. We hypothesized that lower-extremity strength would be significantly decreased in older adults with SCK difficulty and that distal strength measures would be the main predictors in SCK difficulty. Results from the primary analyses of raw and normalized strength are mostly in agreement with our hypothesis, as leg strength, particularly after normalization, was significantly decreased in older adults with SCK difficulty. Two novel findings of this study were: (1) the significant correlation between SCK difficulty and both normalized strength and functional balance measures, and (2) that among a broad sample of leg and trunk muscle strengths, those having the greatest association with SCK difficulty were raw ankle plantar-flexor and normalized knee extensor strengths. Overall, these findings suggest that in older adults, the major strength determinant of SCK difficulty is strength of the distal leg musculature, thereby providing a common link with functional tests of balance.

This is the first study to examine the differences in trunk and lower-extremity muscle strength in older adults with SCK difficulty. Although ankle plantarflexor strength was found to be a significant predictor of SCK difficulty, normalization of strength measures to account for differences in body size demonstrated that normalized knee extensor strength is a more significant predictor of SCK difficulty as evaluated by the percentage of variance explained (26% for the knee versus 11% for the ankle). As in the present study, decreased lower-extremity strength has been found to be associated with SCK limitations12,33 and limited functional performance in daily activities such as rising from a chair, walking, or stair ascent or descent.3436 Similarly, frequent fallers have reported decreased lower-extremity strength,37 and older adults with a high fall risk have been best identified by maximum isometric push-off force in a leg press apparatus.38

The significant correlations found between SCK difficulty and functional balance tests such as UST, TUG, and MSL extend the results of other investigators who found an increased risk of falls in older adults who reported trouble bending down to the floor.2 Distal strength consistently had stronger correlations to SCK difficulty than proximal strength measures and may provide a common link between SCK difficulty and functional balance tests. Adequate ankle dorsiflexor and plantar-flexor strength may be required to generate corrective torques about the ankle to maintain equilibrium by moving the center of mass forward or backward during stooping movements, due to the limited range of motion at the knee and hip. Knee extensor strength would be expected to play a significant role in the recovery to an upright stance after crouching or kneeling. Similarly to SCK movements, lower-extremity strength is of primary importance to the performance of functional tests such as the UST, TUG, and MSL.1618,39 Even though fall-related measures such as the incidence of more than 2 falls or an injurious fall were not found to be significantly different between older adults with and without SCK difficulty, this might have been due to the overall health of this study cohort or limitations in sample size.

After normalization of strength measures, hip extensor and trunk extensor isometric strength were found to correlate with SCK difficulty. These muscle groups would be expected to play a role in performing stooping tasks and recovery of the trunk after bending down to the ground, but appear to be of less significance to distal musculature as seen in the multivariate analysis. A possible explanation for the lack of significance of hip and trunk extensors is that additional muscle groups, such as lateral hip muscles (eg, hip abductors), may be playing a role in the stabilization of the torso during the large motions undertaken while stooping, crouching, or kneeling.

The results of this study have important implications for clinicians working to reduce fall risk in older adults. Rehabilitation or intervention programs aimed at addressing deficits in self-reported SCK performance should focus on improving distal strength. Although reduced strength is a significant contributor to SCK difficulty, older adults with SCK difficulty also may benefit from more comprehensive programs that address balance confidence, coordination, leg joint limitations such as stiffness and pain, and sensory capacities.

A limitation of this study is that although we included participants with a wide range of SCK difficulty and balance capabilities, some of whom may be at risk for a fall, the participants were all community-dwelling volunteers and generally active. Therefore, it is undetermined whether similar results would be seen in a frailer cohort of older adults. Future studies should include older adults who are regular fallers to understand the strength determinants of SCK difficulty and clinical measures of balance and trunk control in those having the highest risk for falls. Finally, a further limitation of this study is that although it focused on muscle groups that were key to rising from SCK, there are other muscle groups, such as hip abductors, that may contribute to safe SCK performance, particularly as the older adult lowers himself or herself.

Conclusions

Decreased muscle strength, particularly when normalized for body size, predicts SCK difficulty. These data emphasize the importance of strength measurement at multiple levels in predicting self-reported functional impairment. Further studies are needed to determine whether rehabilitation programs with a focus on training specific muscle groups are effective in improving self-reported functional performance and whether improvements in self-reported functional performance are associated with fewer falls in older adults.

Footnotes

All authors provided concept/idea/research design, writing, and data collection and analysis. Dr Goldberg and Dr Alexander provided project management, participants, and consultation (including review of manuscript before submission). Dr Alexander provided fund procurement, facilities/equipment, institutional liaisons, and clerical support. The authors thank Diane Scarpace, Brad Grincewicz, Ravi Goswami, and Eric Pear for assistance with participant screening and data collection.

This study was approved by the University of Michigan Medical School Institutional Review Board.

This work was funded, in part, by grant F31AG02468 from the National Institute on Aging (NIA) (National Research Service Award to Mr Hernandez); grant AG08808 from the Claude D. Pepper Older Americans Independence Center, University of Michigan; NIA grant AG10542; and NIA Institutional Training Grant T32 AG00114 (Multidisciplinary Research Training in Aging). This work also was supported by the Department of Veterans Affairs Office of Research and Development, Clinical Science and Rehabilitation Research and Development Services, and the Dorothy and Herman Miller Fund for Mobility Research in Older Adults. Dr Alexander is a recipient of the K24 Mid-Career Investigator Award in Patient-Oriented Research (grant AG109675) from NIA. Dr. Goldberg is the recipient of the 2004 Fellowship for Geriatric Research Award from the Section on Geriatrics of the American Physical Therapy Association.

*Biodex Medical Systems Inc, 20 Ramsey Rd, Shirley, NY 11967.

SPSS Inc, 233 S Wacker Dr, Chicago, IL 60606.

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