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The Reliability and Validity of Handheld Dynamometry for the Measurement of Lower-Extremity Muscle Strength in Older Adults

Arnold, Cathy M1,3; Warkentin, Kathryn D2; Chilibeck, Philip D1; Magnus, Charlene R A1

Journal of Strength and Conditioning Research: March 2010 - Volume 24 - Issue 3 - p 815-824
doi: 10.1519/JSC.0b013e3181aa36b8
Original Research
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Arnold, CM, Warkentin, KD, Chilibeck, PD, and Magnus, CRA. The Reliability and Validity of Handheld Dynamometry for the Measurement of Lower-Extremity Muscle Strength in Older Adults. J Strength Cond Res 24(3): 815-824, 2010-Lower-extremity muscle strength is important in predicting fall risk in older adults. Handheld dynamometry (HHD) is a tool used to measure isometric muscle strength in the older adult, but few studies have evaluated the utility of HHD for muscle groups beyond knee extension. The purpose of this study was to evaluate the reliability of HHD at the hip, knee, and ankle and to compare HHD strength values to other isometric dynamometry (ID) and to balance and recovery in older adults. This was a repeated measures study design of 18 men and women, age 65 to 92 years of age, who had HHD strength testing 3 to 7 days apart by the same examiner and repeat testing on the same day using 2 independent examiners. ID strength, balance, step length, and reaction time were measured once. HHD demonstrated good intrarater and interrater reliability for isometric strength at the hip and knee but was not a reliable measure for ankle strength. The HHD was a valid measure of isometric strength at the hip and knee, demonstrating moderate to high correlation values when compared to ID strength measures (r = 0.57-0.86; p < 0.05). Hip and knee strength was positively associated to step length and reaction time but not to balance (r = 0.40-0.71; p < 0.05). In conclusion, HHD is a reliable and valid assessment tool for measuring strength at the hip and knee in older adults, and greater strength in these muscles is associated with longer step length and decreased reaction time, which are important components of balance recovery in older adults. HHD can be used as an effective strength measurement tool for the older adult population.

1College of Kinesiology, University of Saskatchewan, Saskatoon, Saskatchewan, Canada; 2Bourassa and Associate Rehabilitation Center, Saskatoon, Saskatchewan, Canada; and 3School of Physical Therapy, University of Saskatchewan, Saskatoon, Saskatchewan, Canada

Address correspondence to Cathy M. Arnold, cathy.arnold@usask.ca.

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Introduction

Physiologic changes associated with aging result in diminished muscle strength (4,15). Although there is individual variation, muscle strength significantly declines, on average 1 to 2 % per year after the age of 50 years, with steeper declines observed after age 65 years (29). There are several methods of measuring muscle strength such as isokinetic dynamometers, manual muscle testing, and handheld dynamometry (HHD). Testing with an isometric action may be more clinically applicable for older adults because limited joint range of motion or the presence of joint pain may make concentric actions difficult (3). HHD using an isometric action is a viable option because it is a simple, inexpensive tool.

Previous research supports reliability of HHD testing knee extension strength in both healthy older adults and those with chronic disease or functional limitations, with 2 of these studies also confirming reliability of the instrument for hip abduction and hip flexion (20,21,26,32); however, only 1 study has comprehensively evaluated reliability for muscle groups at the hip, knee, and ankle in the older adult population (32). Difficulties in comparing these studies include a variety of types of HHD used and differences in protocol and positioning. There has been good agreement between measurements of quadriceps strength by HHD and a standard isometric Biodex System 3 dynamometer (ID) in a supine position; however, HHD was found to underestimate strength compared to the ID (17). HHD requires little training to use, is easy to administer, is portable, and is less costly than ID, but it is not clear if HHD is a reliable, valid tool to measure strength in other lower-extremity muscles and in alternative positions for knee extension.

Evaluating validity of HHD should also incorporate comparison to measures of function and balance because ultimately it is strength related to the ability to perform daily tasks that is relevant to older adult health and wellness. Decreased lower-extremity muscle strength is one of the stronger predictors of fall risk in older adults(30), but it is not clear if this is because of a relationship of strength to general functional mobility, specifically to balance, or because of the impact it has on activities such as walking. Although lower-extremity strength, primarily knee extension strength, correlates moderately with general mobility measures (26) and some measures of balance such as single-leg stance, it is not consistently related to other balance measures such as postural sway (balance in quiet stance) or functional reach (dynamic balance) (5,25). Muscle strength is more strongly related to measures of gait performance such as stride length and gait speed (5,25). Some evidence suggests that muscle strength is associated with reaction time, but this is inconsistent, with few studies evaluating the direct relationship of lower-extremity strength to a stepping reaction time or step length (9). Most studies have evaluated quadriceps strength, with very little research on the relationship of other muscles at the hip to balance and reaction time (19).

Our first purpose was to confirm intrarater and interrater reliability of HHD in an older adult population for muscle groups at the hip, knee, and ankle, incorporating different start positions than what has been reported in the literature previously. Secondary purposes were to evaluate validity of HHD by comparing measurements to the gold standard measure of ID and to evaluate the association of strength measures to other indicators of fall risk, functional tests of balance, and lower-extremity mobility related to step length and recovery.

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Methods

Experimental Approach to the Problem

The reliability and validity of HHD were tested in a sample of 18 older men and women as a pilot of the tool to be used in a larger clinical trial. A repeated measures design with repeat testing 3 to 7 days apart was used. This time frame allowed for adequate recovery following strength testing with not enough time to observe changes in muscle strength as a result of other factors. ID was chosen as the gold standard measure of isometric strength to compare with HHD because it is a reliable and valid measure of hip and knee strength in older adults (1). The fall risk parameters included both gait parameters of step length, reaction time, and standing balance under different environmental challenges. All of these measures have confirmed reliability and validity (reported later).

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Subjects

Posters were distributed to recreational centers, senior residences, and older adult exercise programs to recruit participants. Volunteers were telephone screened to confirm eligibility. Inclusion criteria were (a) aged 65 years or older and (b) living independently in the community. Exclusion criteria included (a) hip or knee replacement surgery, (b) a severe balance deficit, or (c) any medical or neurologic condition that contraindicated maximal muscle action. Thirteen subjects were excluded from the study because of time constraints (5), presence of a medical or neurologic condition jeopardizing safety of testing (5), or history of hip or knee surgery (3). All participants were informed of the experimental risks and signed an informed consent document prior to testing. Ethical approval for this study was granted by the University of Saskatchewan Biomedical Ethics Review Board for use in human subjects.

Participants (14 women and 4 men) had a mean age of 74 years (65-92 years), height of 165 cm (153-181 cm), and weight of 73.3 kg (56.4-119.9 kg). Eleven of the 18 participants reported arthritis in either their knees or hips and 10 out of 11 reported associated pain. Usual pain was graded as mild or moderate by all but 2 of the 10. One reported severe pain, and the other did not grade pain. No participants discontinued testing because of pain and there were no reports of increased pain during or between test sessions. Of the 18 participants, 6 reported at least 1 fall within the past year (defined as any body part coming to rest on the ground, floor, or other lower surface, not as a result of fainting) and 12 reported no history of falls. The mean number of comorbidities reported was 2.2 (range 1-4). All participants lived independently in the community. When asked to rate their level of mobility on a scale from 1 to 10, with 1 being extremely limited mobility (i.e., confined to a wheelchair) and 10 being no mobility restriction, the range of scores was 4 to 10 with a mean of 8.7. Two participants used a roller walker for outdoor use only and 1 participant used a cane. All testing occurred in summer months (June to July).

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Procedure

A questionnaire was used to assess demographic information including use of medication, presence of medical conditions, location and duration of any lower-extremity pain, and history of falls. Subjects were tested on 2 occasions with repeat testing by 2 testers on Day 2. Tester 1 was a physical therapist with 20 years of clinical experience, and Tester 2 was a physical therapy student in the second year of her studies. These 2 testers were chosen because their differing levels of clinical experience are realistic for a clinical setting and therefore increase external validity. The HHD protocol was reviewed and practiced by both testers prior to the study. On Day 1, Tester 1 administered the following tests twice: HHD for hip flexion (HF), hip extension (HE), hip abduction (HA), knee extension (KE), and ankle dorsiflexion (DF); maximal step length (step length) forward, backward, and sideways with left and right extremities; the modified clinical test of balance and sensory interaction (balance); and lower-extremity reaction time (reaction time). Muscle strength was also assessed by ID on Day 1. On Day 2, both Tester 1 and Tester 2 repeated HHD and step length, balance, and reaction time. Testers 1 and 2 were blinded to HHD testing by the other by being out of the room during the test. Both testers were also blinded to previous measurements on Day 1. Balance and stepping tests were also repeated twice to confirm intrarater and interrater reliability of these measurements for this sample. Reliability values for these measures are reported in the methods section. ID was not repeated because we confirmed reliability of the ID protocol in previous studies(4,27). Although the order of balance and step tests was the same for all participants, the order of HHD and ID was randomized during Test 1.

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Handheld Dynamometry (HHD)

The Lafayette Manual Muscle Tester, Model # 01163, was used (Lafayette Instrument Inc., Lafayette, Indiana; Figure 1). The muscle actions and starting positions tested were KE (sitting), DF (sitting), HA (in lying and standing), HF (in sitting and standing), and HE (standing). HA and HF were performed in 2 positions to standardize the same position used for ID and to evaluate lying positions often used clinically and in other studies. A standard protocol was used based on a similar study (32) with some minor modifications as described later. Instructions for each position and test were given to the participant prior to testing. If a trial did not appear to demonstrate maximal effort or the participant was not following instructions, the trial was repeated. If the HHD registered an error message because the dynamometer was not positioned perpendicular to the test surface, the test was repeated. Almost half of the sample reported familiarity with this tool.

Figure 1

Figure 1

For each muscle group tested the limb was first moved to the set position. Instructions for the participant were to hold the position, trying to exert as much force as possible against the pad. The tester met the effort of the participant and did not break his or her hold (make test) (23). Cuing for the action was “push … hold, hold, hold, relax.” This resulted in a hold of approximately 5 seconds, which is an adequate time period to generate maximal force (32) and was consistent with the ID protocol. Each participant performed 2 trials for each leg. The mean peak force (recorded in kilograms) achieved between 2 trials was used as the strength measure. The order of testing was HF (sitting), KE, DF, HA (lying), HE, HF (standing), and HA (standing). The HHD pad was placed perpendicular to the limb for all test positions except for DF, where the pad needed to be angled approximately 45 degrees for some participants as a result of irregularities in the contour of the foot. HF (sitting), KE, and DF were tested in the same position sitting on the edge of a plinth with the hips and knees bent at 90 degrees and a stool used to place the feet at 90 degrees of DF (Figure 2). The participants were allowed to place their hands on the plinth for support but were asked not to lean their trunk backward. For HF (sitting) the dynamometer pad was placed 3 finger widths proximal to the top of the patella with the thigh approximately 10 degrees off the plinth during the isometric action. For KE the dynamometer pad was placed just proximal to the lateral malleolus, with the knee at a 45-degree angle. For DF the pad was placed on the dorsum of the foot at the base of the metatarsals. The participant was asked to lift the forefoot just off the stool at approximately 10 degrees of DF or the available range. For HA (lying) the participant laid supine on the plinth with both hips extended and a small pillow under the head. The dynamometer pad was placed 3 finger widths above the lateral joint line of the knee with the hip at approximately 10 degrees of abduction. HE was tested with the participant standing facing the plinth with the hip in 0 degrees extension or as close as possible given any limitations in range (thigh in relation to vertical). The plinth was raised to the participant's hip level so he or she could place hands on it for support. The participant was told not to lean onto the plinth during the action. The command was to push the leg back keeping the foot off the ground by slightly bending the knee. HF (standing) was tested with the participant standing with his or her right side to the plinth using it for support, lifting the hip into approximately 80 degrees of HF (thigh in relation to vertical). HA (standing) was tested facing the plinth to use it for support. The participant was asked to push out to the side keeping the toes pointing forward and perform the action from 10 degrees of abduction. External stabilization of the spine and pelvis was not used in this study because we were attempting to mimic a clinical situation. The tester instructed and monitored the participant to maintain the trunk in a stable position. The test was repeated and instructions were reinforced if there was any change in trunk position.

Figure 2

Figure 2

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Isometric Dynamometry

ID was tested using the Biodex System 3 (Biodex Medical Systems Inc., Shirley, NewYork). Isometric actions were assessed in KE, HF, HE, and HA. DF was not tested because a previous study found poor reliability for this measure (1). Two research assistants (different than the testers performing HHD) with training in ID protocol performed all ID testing following a standard protocol (27). Each test was performed in the same order for each subject: Right KE, left KE, right HA, right HE, left HA, left HE, right HF, and left HF. This order was chosen for time efficiency of changing attachments and moving the equipment. Instructions for positioning and testing were explained to the participant and verification of understanding was confirmed prior to the test. Participants had the opportunity to become familiar with exerting force on the pad by moving through the full range of motion prior to the isometric test. If there was any discomfort from the pad, this was adjusted. Three isometric actions of 5 seconds were performed for each movement with a 30-second rest period between actions. The instructions were the same as for HHD: “Push, hold, hold, hold, and relax.” The highest peak torque obtained was recorded in newton meters (Nm). KE was tested with the participant in a seated position with the hips at 90 degrees and the knee at 45 degrees. Stabilizing straps were applied diagonally across the chest, waist, and just above the knee on the leg not being tested. The dynamometer attachment was adjusted so the pad was placed just proximal to the lateral malleolus of the leg being tested and the knee joint was in line with the axis of rotation of the dynamometer. All hip movements were performed from a standing position, with the hip joint in line with the dynamometer axis of rotation. Participants placed their hands on the machine at waist level for balance and to stabilize the standing position. If the participant lost balance, the test was repeated. The dynamometer attachment was adjusted so the pad was placed 3 finger widths above the lateral joint line of the knee for HA, HF, and HE. Participants were asked to keep their foot just off the ground with knee slightly flexed for abduction and extension. HA was performed with the leg at an angle of 10 degrees of abduction. For HE the action was performed from 0 degrees or as close as their hip range would allow. HF was performed with the leg set at 80 degrees of flexion. All hip joint angles were referenced from thigh to vertical. All measurements on the ID were corrected for the effects of gravity on the leg and the dynamometer's resistance pad.

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Measurement of Leg Length

To convert HHD to Nm and compare absolute values for ID and HHD, leg length was measured for 17 of the 18 participants. One participant refused to take part in this measurement. The femur lengths were measured from the greater trochanter to the lateral aspect of the knee joint. Because the HHD was positioned 3 finger widths above the lateral knee joint for comfort, 5 cm was subtracted from this measure. The lower leg measures were from the lateral knee joint to the lateral malleolus. Femur lengths were used to convert HHD to torque values for HF, HE, and HA. Lower leg length was used for conversion for KE.

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Maximal Step Length

Maximal step length is a clinical test of functional stepping that correlates moderately to highly with other balance measures and self-reported physical function (6). Our intrarater and interrater reliability of this test was ICC = 0.83 to 0.96. Participants started with both feet in a 30.5-cm square box with tape measures secured to an adhesive mat on the floor in 4 directions (forward, left, right, and back). Participants were instructed to take a maximal but comfortable step, without losing balance or lifting the back foot, with arms at side, not holding on to anything. If the heel of the opposite foot lifted or the participant lost balance, the trial was repeated. The mean of 3 trials for each leg in each direction was used as the final score.

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Lower-Extremity Reaction Time

A Lafayette digital timer was used with 2 photo cells taped to the floor that automatically stopped the timer once the participant's foot stepped between the cells. Participants stepped forward to a line marked at the average of their left and right 60% maximal step length in the forward direction. One practice trial was given. The Lafayette timer was initiated with a manual touch and a simultaneous auditory cue. Participants stepped as quickly as possible with the bulk of the forefoot landing between the photo cells. Five trials were repeated consecutively for the right then left leg. The mean of 10 trials was calculated. Intrarater and interrater reliability for combined means for 5 trials of left and right step was confirmed (ICC = 0.86, 0.81).

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Modified Clinical Test of Sensory Interaction and Balance

This modified clinical test is a reliable and valid clinical tool, measuring total time to balance in 4 conditions: standing, feet together with eyes open and closed, standing feet together on foam with eyes open and closed (7,28) We confirmed reliability with ICC values for intrarater and interrater reliability ranging from 0.85 to 0.92. A high-density foam block was used (height = 6.4 cm). Participants were given 3 trials to achieve 30 seconds in each position. If they did not achieve 30 seconds on the first trial, the mean of 2 or 3 trials was used for a total possible score of 120 seconds. The trial was stopped if participants opened their eyes, uncrossed their arms, lost balance, or if any part of the foot lifted off the floor or foam.

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Data Analysis

SPSS version 14.0 was used for all data analysis. To evaluate the intrarater and interrater reliability of HHD, the intraclass correlation coefficient (ICC) or R coefficient was calculated using a 2-way mixed model with absolute agreement. The strength of reliability correlation coefficients was interpreted based on ranges of poor (<0.69), fair (0.70-0.79), good (0.80-0.89), and high (0.90-1.00) (8). Standard error of measurement (SEM = [SD√(1-ICC)]) (10), was also calculated to provide an estimate of measurement error over repeated tests, or the variability expected for repeat testing in the units of measurement used. HHD measures for HF (standing), HA (standing), HE, and KE were converted to newton meter by converting kilograms to newtons and multiplying by the limb length. The differences between HHD measurements for participants with and without lower-extremity pain were compared using 1-way analysis of variances (ANOVA). Pearson product moment correlation coefficients and ICCs were used to compare HDD and ID for concurrent validity and to evaluate the absolute agreement between the 2 measurement tools. Paired t-tests were run to determine if there were any significant differences between ID and HHD. Pearson product moment correlations (Pearson r) and coefficients of determination were used to measure validity of HHD by the association to balance, step length, and reaction time. For verification of validity, high correlations were defined as >0.70, moderate 0.50-0.69, low 0.26-0.49, and little or no correlation 0.00-0.25 (10). All values are presented as group means ± SD. Significance was set at α ≤ 0.05.

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Results

HHD Reliability

Means and intrarater and interrater reliability data for HHD are presented in Tables 1 and 2. Same-day intrarater reliability was also calculated from 2 repeat tests done on Day 1 by Tester 1 (data not shown) and all measurements were highly reliable (ICC = 0.90 to 0.98). Test-retest reliability (3 to 7 days later) was fair to high with DF having the lowest ICC value. Interrater reliability on the same day was good-high for hip and knee measures but poor for DF. This study was not designed to compare subgroups with (n = 10) and without (n = 8) lower-extremity pain, but of interest, the mean values were significantly higher (p < 0.05) for 7 of the 12 measures (right and left HF sitting, right HF standing, right and left HA standing and lying) for those who did not report pain. The data did not demonstrate any differences in ICC values for those who reported lower-extremity pain compared to those who did not (ICC values ranging from 0.72-0.88 interrater and 0.83-0.94 intrarater for those with pain and ICC values ranging from 0.74-0.92 interrater and 0.71-0.89 intrarater for those without pain). Both HA and HF were repeated in 2 different positions. Although values were slightly higher for HF in sitting as opposed to standing, all positions demonstrated similar reliability values.

Table 1

Table 1

Table 2

Table 2

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HHD Validity

The absolute values for ID and HHD converted to Nm and the relationship of the 2 measurement tools are reported in Table 3. There were significant differences in ID versus HHD measurements for right and left KE and right HE, with ID measures consistently higher than HHD. Pearson r and ICC correlations of HHD to ID were moderate to high supporting concurrent validity. Table 4 reports Pearson r correlations of the HHD muscle strength measurements with step length, balance, and reaction time. HE and HA (standing) were used because it was assumed these tests would be more reflective of functional strength related to balance and gait. DF was not included because the reliability values were poor. For step length, correlations for strength in both the stabilizing leg and the stepping leg were reported. There was little or no correlation of hip or knee muscle strength to balance in quiet stance. Hip and knee strength were moderately correlated to reaction time except for left KE, right HE, and HF, which were slightly less than moderate. Both hip and knee strength were moderately to highly correlated with step length on the stepping leg. The muscle group that corresponded with the step direction had higher correlations-for example, left HA with stepping to the left side, right HE for stepping backward with the right leg. However, HF did not demonstrate as high correlations for stepping forward compared to the other directions. For hip and knee strength of the stabilizing limb, there was moderate to high correlations for all muscle groups. Of note were consistently higher correlations of the opposite limb muscle strength when stepping to the side. A second way of evaluating the relationship of lower-limb strength to these functional parameters is the percent of variance shared by the 2 variables or the coefficient of determination. The coefficient of determination for strength (using a mean of left and right strength values) to reaction time was 31% for HA, 23% for HE, 27% for HF, and 26% for KE. Coefficients of determination were higher when determining the relationship of muscle strength to stepping, particularly for the hip musculature as opposed to KE. For example, the coefficient of determination for the stabilizing hip abductors when stepping was 49%, 40% for HE, and 42% for HF. Similar values were found for hip strength in the stepping limb.

Table 3

Table 3

Table 4

Table 4

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Discussion

Evaluating the reliability of HHD in a broad spectrum of lower-extremity muscle actions in different starting positions adds to the research evaluating the use of this device. Further, the data from this study of the relationship of lower-extremity strength to balance, stepping, and recovery help to direct future research in evaluating factors associated with fall risk.

This study confirmed the reliability of HHD found in other studies for measuring muscle strength at the hip and knee in older adults (20,21,26,32) and provides new data of a protocol that is reliable in different starting positions for testing at both the hip and knee. There have been reports of lower reliability when measuring hip muscle strength in positions where stabilization of the pelvis may be more difficult such as prone (11) and standing (24). We found acceptable reliability for measuring hip strength in lying, sitting, and standing positions. This is important for the clinician or coach because it provides them with different position options in using this test. It was noted that subjects could exert more force in positions when stabilized in sitting with the muscle in a slightly more shortened position (HF in sitting at 90 degrees) compared to standing positions (HF in standing at 80 degrees). For clients for whom stabilization in standing is difficult, the sitting position may be preferable. HHD was easy to administer and did not require significant training or set-up time. Although the sample size was not large enough to make definitive conclusions about the reliability of HHD comparing participants with lower-extremity pain versus those that did not have pain, the reliability coefficients were similar for both groups.

We found HHD to be reliable for assessing strength at the hip and knee but not for ankle DF. The placement of the dynamometer pad for DF often caused discomfort, which may have affected the motivation of the participant to produce a maximal, consistent action. Because of individual differences in contours of the dorsal aspect of the foot, the pad placement may have differed even for those not experiencing pain. The lever arm for this test is very short, and small differences in placement of the HHD from test to test along this short lever arm will make large differences in measured force. Other studies have also found the measurement of DF strength to be less reliable than other strength measurements (4). Wang et al. (32) achieved higher DF reliability values than in the present study using HHD but still found that the ankle measurements had the highest variation when compared to measurements at the knee and hip.

Validity of HHD was supported with moderate to high correlations with ID, stepping, and reaction time but not with balance. Torque measurements for ID were significantly higher than HHD, for KE and right HE. KE for ID was stabilized using a chair with a back and straps around the chest, waist, and femur, whereas there was no stabilization for HHD. Another study supports this finding as HHD under measured quadriceps strength tested in supine as compared to Biodex ID (17). Cahalan et al. (3) evaluated isometric and isokinetic muscle strength for all hip musculature in both younger and older adults using a Cybex II isokinetic dynamometer with a body stabilizing frame. The torque values they found for older women were higher than ours (i.e., mean 51 Nm ± 18 for HF compared to 33 Nm ± 22 and 69 Nm ± 20 compared to 37.3 Nm ± 15 for HA, using average of left and right). Differences are likely a result of a younger age range for the Cahalan et al. sample (40 to 81 years) compared to ours (65 to 92 years) and the use of the body stabilization frame. Although they did not compare results to other methods of torque measurement, others have found high correlations of HHD to Biodex ID for KE in an elderly population (17). The Biodex (System 3) is a standardized device used to measure both isometric and isokinetic strength; however, the equipment takes up much space and is costly to utilize in a clinical environment. A longer set-up time was noted as a limitation for the ID in standing positions for participants with lower-extremity pain or weakness. For these individuals, HHD is a viable alternative.

HHD strength measures were also associated with functional tests of stepping and reaction time. The stabilizing limb showed consistently high associations to the length of step with the stepping limb. About half-40 to 50%-of the variance in the distance the participant could step was explained by hip strength in the stabilizing limb. Both HA and HE are important in the ability to stabilize the stance limb to allow for maximal swing of the opposite extremity. Gluteus medius controls lateral balance during the mid-stance phase of gait (33). Weakness results in a decreased ability to stabilize the limb, resulting in shortened stride and decreased ability to weight shift laterally on the stance limb (2). This may decrease the ability to take a maximal step to protect one from falling. Weakness in HE can result in decreased ability to accelerate the stance hip as it moves from HF at heel strike to HE at toe off. The gluteus maximus is an important contributor to HE and KE during stance (1). Other studies have also provided evidence that loss of HE range and strength may be biomechanical contributors to fall risk (13,14).

Although KE strength has been associated with stepping ability and gait speed (16,18), little is known about the relationship of the stepping limb's hip strength to step length. We found that hip strength, and to a lesser degree KE strength, in the stepping limb was associated with stepping distance. Being able to take a maximal step is an important protective reaction for balance. Older adult fallers tend to use protective responses more often than nonfallers, particularly at low levels of postural disturbance (22). The first step response to a slip or trip may require as much as a full maximal step length to recover balance (31). Hip strength appears to be an important component in stepping and thus crucial in the ability to react to prevent a fall.

There was no relationship of hip or knee strength to balance in quiet stance with feet together under 4 sensory conditions. This is a demanding balance test with a variety of systems involved such as vision, proprioception, sensation, and neuromuscular and vestibular control. If we were able to control for other factors such as visual loss or vestibular dysfunction, we may have seen stronger associations. Other studies also support a stronger relationship of lower-extremity strength to components of gait and dynamic balance as opposed to standing balance. Ringsberg et al. (25) found no relationship of KE strength to balance as measured on a platform with eyes open and closed and lower correlations to standing on 1 leg, as opposed to the association of KE strength to gait performance. Another study reports knee extensor strength associated with dynamic balance measures such as figure-of-8 running (12). Lower-extremity strength may contribute more to dynamic balance activities such as stepping, gait, and reaction time than the ability to maintain balance in quiet stance. Our results are consistent with these studies and add further to the literature by supporting the association of hip strength and quadriceps strength to lower-extremity reaction time and step length. Hip strength may be an important component in preventing falls by being able to elicit an effective protective response. This study found that hip strength plays an important role in predicting how far someone can step in all directions and how quickly they can step. Strength of ankle muscles may also be important, but we did not have a reliable measure of ankle strength in this study to evaluate this association. Future research should evaluate the different contributions of hip, knee, and ankle strength to balance, gait, falls, and fall risk.

The limitations of this study include the following: (a) a small sample size, with both genders, and presence of lower-extremity pain for some, making it difficult to compare outcomes for these subgroups, and (b) several tests run concurrently, which may have caused fatigue for some individuals.

In conclusion, we found that HHD was a reliable and valid tool for measuring muscle strength for hip and knee musculature but not at the ankle in an older adult population. Hip and knee muscle strength is associated with step length and reaction time-important intrinsic factors in preventing falls.

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Practical Applications

HHD is a simple evaluation tool that can be used in exercise and clinical settings to measure hip and knee isometric muscle strength in both male and female older adults with or without lower-extremity pain. Weakness of hip and knee muscle strength is associated with fall risk, and simple screening measures such as HHD may be valuable in identifying older adults at risk. The findings in this study of the relationship of hip abductor, flexor, and extensor strength to ability to stabilize the limb, step farther, and have quicker reaction times also suggest that these may be important muscle groups to train in fall prevention exercise programs.

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Acknowledgments

The authors would like to acknowledge Lisa Skarpinsky for her assistance with testing, Nathan Jantz for his role in developing the Biodex protocol, Dr. Bob Faulkner for his advice and help with protocol development, and Zoe Arnold and Justin Fisher for their assistance with photographs.

Financial Acknowledgement: The authors acknowledge funding support from the University of Saskatchewan College of Medicine Summer Student Project (author KW) and University of Saskatchewan Student Employment Program (Author CM).

The authors disclose that there are no professional relationships with companies or manufacturers who will benefit from the results of the present study and that the results of the present study do not constitute endorsement of the strength assessment devices evaluated.

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Keywords:

fall risk; pain; isometric strength; gait; balance

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