Measuring Muscle Strength for People With Chronic Obstructive Pulmonary Disease: Retest Reliability of Hand-Held Dynamometry

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1 32 ORIGINAL ARTICLE Measuring Muscle Strength for People With Chronic Obstructive Pulmonary Disease: Retest Reliability of Hand-Held Dynamometry Simone D. O Shea, PT, Nicholas F. Taylor, PhD, PT, Jennifer D. Paratz, PhD, PT ABSTRACT. O Shea SD, Taylor NF, Paratz JD. Measuring muscle strength for people with chronic obstructive pulmonary disease: retest reliability of hand-held dynamometry. Arch Phys Med Rehabil 2007;88:32-6. Objective: To evaluate the retest reliability and quantify the degree of measurement error when measuring isometric muscle strength with a hand-held dynamometer for people with chronic obstructive pulmonary disease (COPD). Design: Retest reliability of hand-held dynamometry for 4 muscle groups was assessed on 2 occasions separated by a 2-week interval. Setting: Community rehabilitation center. Participants: Eight men and 4 women (mean age standard deviation, y) with moderately severe COPD (percentage of predicted forced expiratory volume in 1 second, 41.5% 17.7%). Interventions: Not applicable. Main Outcome Measures: Muscle strength (in kilograms). Statistical analysis was conducted by calculating intraclass correlation coefficients and 95% confidence intervals for both group and individual scores. Results: All reliability coefficients were greater than.79. Muscle strength would need to increase by between 4% and 18% in groups of people with COPD and between 34% and 58% in a person with COPD to be 95% confident of detecting real changes. Conclusions: Hand-held dynamometry is suitable for monitoring change in muscle strength and testing hypotheses for groups of people with COPD. However, hand-held dynamometry is not likely to detect changes in muscle strength for a person with COPD. Key Words: Muscles; Pulmonary disease, chronic obstructive; Rehabilitation; Reliability and validity by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation SKELETAL MUSCLE DYSFUNCTION, in particular strength deficits, has been linked with exercise intolerance and higher utilization of health care resources for people with chronic obstructive pulmonary disease (COPD). 1 Progressive From the Physiotherapy Department, Wodonga Regional Health Service, Wodonga, Australia (O Shea); Musculoskeletal Research Centre, School of Physiotherapy, La Trobe University, Victoria, Australia (O Shea, Taylor); and School of Health and Rehabilitation Sciences, University of Queensland, Brisbane, Australia (Paratz). No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the author(s) or upon any organization with which the author(s) is/are associated. Reprint requests to Nicholas F. Taylor, PhD, PT, School of Physiotherapy, La Trobe University, Victoria 3086, Australia, N.Taylor@latrobe.edu.au /07/ $32.00/0 doi: /j.apmr resistance exercise programs have been shown to improve the ability of muscle to generate force in this population 2 ; therefore, assessment of muscle strength before and during pulmonary rehabilitation may aid identification of muscle weakness, allow prescription of targeted training programs, and provide a mechanism for monitoring progress. However, the reliability of methods used for measuring muscle strength in COPD have been examined in only 1 study. 3 Mathur et al 3 examined retest reliability of a Cybex II dynamometer for measuring isokinetic and isometric biceps and quadriceps strength in 20 people with moderately severe COPD. For both isokinetic and isometric strength measures, Mathur 3 reported reliability coefficients (ICCs) greater than.94 within a testing session (3 trials) and greater than.88 between testing sessions (1wk apart). The standard error of measurement also was small between trials and testing sessions, indicating good reliability of the testing procedures. No changes were detected in cardiovascular responses during performance of the maximal strength maneuvers, suggesting that the protocol used was safe for people with COPD. 3 One of the advantages of using isokinetic dynamometers to assess muscle strength for people with COPD is the ability to assess strength dynamically through a range of movements at various velocities, which may more accurately reflect functional performance. 3 However, isokinetic strength testing protocols can be time consuming to perform in the clinical setting, and the size and the expense of equipment can also be prohibitive. To our knowledge, methods for reliably assessing muscle strength in the clinical setting for people with COPD have not previously been examined. Clinically, hand-held dynamometers represent a simple, portable, and relatively inexpensive alternative to isokinetic machines for assessing muscle strength. Although using hand-held dynamometers is more expensive than performing simple manual muscle tests, hand-held dynamometers provide quantification of strength. Manual muscle tests are typically graded on an ordinal scale (usually 0 5) in relation to evidence of muscle contraction, gravity, and the amount of resistance that can be applied by the examiner. 4 Therefore, manual muscle tests provide an indication of strength status but may be relatively insensitive to changes in muscle strength, particularly at the higher end of the scale. 5,6 In a comparison measuring knee extensor strength with both manual muscle tests and hand-held dynamometry, Bohannon 5 found great variability in the range of strength scores ( N) obtained by participants achieving the maximal manual muscle test grade. These findings highlight the need for clinical measures that can discriminate changes occurring in muscle strength. The reliability of hand-held dynamometers has been examined in numerous populations including healthy young and elderly adults, 7-12 community-dwelling elderly fallers, 13 people with acquired brain injury, 14 older adults after hip fracture, 15 and adolescents with cerebral palsy. 16 Despite the use of varied testing procedures and muscle groups across studies, retest reliability coefficients within a testing session have generally

2 MEASURING MUSCLE STRENGTH IN COPD, O Shea 33 Table 1: Muscle Strength Testing Procedure Muscle Group Body Position Starting Position Hip abductors Standing Hip abducted 20. Resistance applied to the lateral aspect of the distal thigh (12cm above the knee joint line). Tester stabilizes the pelvis contralateral to the side being tested. Participant is positioned in front of a raised plinth or bench to assist with balance. Knee extensors Sitting Participant seated on a raised plinth. Knee flexed 70. Resistance applied to the anterior tibia, 5cm above the lateral malleolus. For comfort a towel is placed between dynamometer and tibia. Pectorals Sitting Shoulder abducted 30 and flexed 75 ; elbow flexed 45 (stop sign position). Resistance applied through the palm of the hand; therefore, a force strut needs to be placed over the curved force plate to ensure an even distribution of force during testing. Shoulder flexors Sitting Shoulder flexed 90 and elbow flexed 15. Resistance applied to the anterior arm over the biceps, 4cm above the elbow crease. A towel can be used for comfort as required. been greater than and between sessions (range, 2d to 6wk) greater than ,15,16 Coefficients of variation between trials or testing sessions have also generally been reported as less than 15%. 13,15,17 Therefore, hand-held dynamometers would appear suitable for measuring and detecting changes in muscle force generation for people with COPD. However, because measures of reliability are specific to the populations and testing procedures used, the findings of previous studies may not be applicable to people with COPD, in whom deconditioning, muscle weakness, osteoporosis, and fluctuating disease stability may affect not only the reliability but the safety of performing hand-held dynamometry. The purpose of this study was to examine the retest reliability of measuring isometric muscle strength for people with COPD and to quantify the degree of change required to overcome measurement error. Establishing the degree of measurement error was important for allowing clinicians to make judgments about and detect changes in muscle strength for groups and individuals with COPD. METHODS Participants Nine men and 4 women with COPD were recruited from a database of former pulmonary rehabilitation participants at a regional health service. Sample size estimates undertaken according to the method described by Walter et al 18 indicated that 12 people would be appropriate if reliability coefficients were anticipated to be between 0.6 and 0.9, which is well within the range reported in previous investigations ,15,16 People were excluded from the investigation if they had respiratory conditions other than COPD, were unable to provide written informed consent, or were actively participating in a rehabilitation program. Procedure Ethics committee approval was obtained to undertake this investigation, and all participants completed a written consent form before the initial testing session. Retest reliability of isometric muscle strength was assessed over a 2-week interval, which was considered appropriate because stability could be assumed over this time, yet the measures were sufficiently far apart that any changes were not due to learning effects. Participants were instructed to carry on with their usual activities and were advised not to commence any new exercises between testing sessions. A physiotherapist (body mass, 70kg) experienced in assessing people with COPD performed measurement procedures at both sessions. The muscle strengths of 4 muscle groups (hip abductors, knee extensors, pectorals, shoulder flexors) were assessed with a hand-held dynamometer. a The testing positions used for assessing muscle strength were based on previous descriptions 16,19 and guidelines provided by the dynamometer manufacturer s manual. During all tests, the dynamometer was placed perpendicular to the limb and force was applied gradually over a 4-second period to allow maximal muscle fiber recruitment. 20 The assessor placed the limb to be examined in the starting position (table 1) and instructed each participant to hold the limb in that position against the increasing resistance applied to it by the examiner. During each test the assessor counted out loud for 4 seconds and at the end of the test period advised the participant to relax the limb. Participants were not provided with encouragement during testing or given knowledge of their results between trials. Participants performed 3 trials for each muscle group, with the final 2 trials averaged to provide a strength score (in kilograms). 16 A bilateral strength score was derived by adding the averaged strength scores from the left and right sides for each muscle tested. This was relevant because bilateral strength is necessary for functional activities both with the lower limbs (eg, sit to stand) and upper limbs (eg, lifting activities). To avoid fatigue, trials were alternated between the left and right limbs for the muscle being tested, and a recovery period of 30 seconds to 1 minute also was provided between trials. Before undertaking testing at the second session, participants were asked to report any adverse events experienced from the initial testing. The same strength testing procedure was completed at the second testing session. Statistical Methods Retest reliability was expressed as ratios in the form of ICCs and in the unit of measurement through 95% confidence intervals (CIs) for both the group and individual mean scores. ICCs were calculated according to model 2,1 21 : BMS EMS ICC 2,1 EMS (k 1)EMS k(jms EMS) N where k is the number of observers, N is the number of participants examined, BMS is the between-subjects mean squares, EMS is the residual mean square, and JMS is the intervention mean square. The ICC takes account of systematic and random error, and it was used to summarize the strength of the retest reliability. ICC 2,1 also takes account of variance between test retest scores and was applicable because it assumes that the rater was a part of a larger population of raters. 21 ICCs were interpreted with the following guidelines: good reliability is greater than.75, moderate is between.50 and.75, and poor less than

3 34 MEASURING MUSCLE STRENGTH IN COPD, O Shea Table 2: Characteristics of Participants Characteristic Age (y) Height (m) Weight (kg) Body mass index (kg/m 2 ) FEV 1 (% predicted) Comorbidities (n) Medications (n) Mean SD Abbreviations: FEV 1, forced expiratory volume in 1 second; SD, standard deviation. The following equation was used to calculate the 95% CIs for interpreting changes in mean scores for a group of people with COPD: 95% CI (group) Mdiff t SDdiff N where Mdiff is the mean difference of retest minus test scores and SDdiff is the standard deviation of the difference between retest and test scores. To determine the degree of change required in a person with COPD, the 95% CI was calculated by substituting N equal to 1 in the above equation. 23 RESULTS One man withdrew from the study because he was unable to complete the second testing session because of an unrelated illness. The characteristics of the remaining 12 participants are given in table 2. All but 1 participant had an extensive smoking history, and long-term oxygen therapy had been prescribed for 33% of participants. Cardiovascular problems were the most commonly reported comorbidities (n 7), followed by musculoskeletal conditions such as osteoarthritis (n 4) and osteoporosis (n 4). All participants confirmed they had continued with their normal activities during the interval between test and retest. Reliability coefficients and 95% CIs are given in table 3. For all measures, the group 95% CI traversed zero, indicating no systematic change between testing sessions. Good reliability was shown for all muscle groups with reliability coefficients greater than or equal to.79. For groups of people with COPD, muscle force generation for the 4 muscle groups tested would need to increase by 4% for the pectorals to 18% for the hip abductors to give 95% confidence that real changes in strength had occurred. A person with COPD would need to improve the force-generating capacity in the 4 muscle groups tested by 33% for the pectorals to 58% for the hip abductors to be confident of overcoming measurement error. Adverse Events Several minor effects, including transient muscle soreness of less than 3 days in duration (6 participants), minor shin bruising (3 participants), shin soreness (3 participants), and muscle soreness greater than 3 days (1 participant), were reported after the first occasion of strength testing. However, all participants returned to complete the repeat testing session. DISCUSSION The protocol used for assessing isometric muscle strength in this study had good retest reliability, as evidenced by ICCs greater than or equal to.79. The correlation coefficients of the current study are similar to retest reliability coefficients reported in assessing the isometric strength of using hand-held dynamometers in other populations 12,15,16 and not inconsistent with results reported with people with COPD using a Cybex dynamometer (ICC.88). 3 However, sole use of reliability coefficients for interpreting reliability may be misleading, because ICCs represent a ratio of error variance to overall variance and as such are influenced by the data distribution. 22,24 Reliability coefficients are also unable to provide clinicians with information regarding what represents a change in their clients. 25 Therefore, a reliable measure should also be able to interpret changes occurring between groups, allowing clinicians or researchers to examine the effectiveness of a group intervention. Of 6 located studies (reporting sufficient data) examining progressive resistance training for people with COPD, the weighted mean increase in muscle strength was 31% (range, 12.4% 63.5%). According to the current study, a group of people with COPD would need to increase the hip abductor force by an average of 3.6kg (18%), quadriceps force by 7.4kg (17%), pectoral force by 1.2kg (4%), or shoulder flexor force by 3.7kg (15%) to be 95% confident that changes were not just related to measurement variability. Because these values fall well within the degree of change expected after progressive resistance training, use of a hand-held dynamometer may be suitable for detecting mean changes in muscle strength for groups of people with COPD. Clinicians also need to assess changes in muscle strength occurring in individual patients to aid treatment choices, to determine the effectiveness of chosen interventions, and to monitor progress over time. The dynamometric testing protocol used in this study may not be suitable for detecting changes in muscle strength on an individual basis, because to overcome the degree of measurement error, force generating capacity would need to increase by at least 10 to 19kg (34% 58%) in the muscle groups tested, which is generally above that expected with progressive resistance training. One of the main problems regarding the reliability of handheld dynamometers for measuring muscle strength is the reliance on the assessor to perform the test. Of particular importance is the ability of the assessor to generate sufficient force to counteract the force produced by the muscle being tested. The Table 3: Reliability Coefficients and 95% CI Muscle Group Mean Test SD* Mean Retest SD* ICC 2,1 Mdiff SDdiff Group 95% CI Individual 95% CI Hip abductors* to to Quadriceps* to to Pectorals* to to Shoulder flexors* to to *In kilograms (combined left and right). Mean difference (mean retest mean test).

4 MEASURING MUSCLE STRENGTH IN COPD, O Shea 35 potential for strength to be underestimated or for a ceiling effect to occur has greater potential in larger, stronger muscle groups, such as the quadriceps. 7,8,32 An investigation 33 examining the effect of assessor strength on hand-held dynamometric measures showed that the forces elicited for most muscle groups were relative to the strength of the assessor, with the strongest assessor always eliciting the greatest measures. Participants in the current investigation, when compared with mean knee extensor strength values for people 70 to 79 years of age, tended to be weaker by approximately 40% (74kg vs 42kg). 8 This weakness along with the strength of the assessor (body mass, 70kg) may have reduced the potential for assessor strength to have affected measures and accounted for the good reliability we observed for the quadriceps (ICC.87). Hand-held dynamometry used to measure muscle strength did cause some mild, transient adverse effects for people with COPD. However, previous studies investigating isokinetic and 1-repetition maximum (1-RM) strength testing in older adults 34 and 1-RM strength testing in people with COPD 35 have also reported residual muscle soreness posttesting, indicating that people with COPD and older adults may be more susceptible to muscle soreness with maximal strength testing. Therefore, it is recommended that participants are warned about the possibility of muscle soreness for several days after testing and that cushioning is used as required between the limb and the dynamometer forceplate. Study Limitations The small sample size may represent a limitation of the current investigation, even though the number of participants is equal to the sample size estimated for a study of this nature. 18 A larger sample size may not change the estimate of reliability but may serve to narrow the confidence intervals about the reliability coefficient. Future work could investigate interrater reliability of the dynamometer for assessing muscle strength in people with COPD. Because assessor strength has been highlighted as an important factor, it has previously been recommended that when different examiners may be conducting hand-held dynamometry, interrater reliability should be determined. 36 CONCLUSIONS Hand-held dynamometry is a simple, portable, and inexpensive option for clinicians wanting to obtain measures of isometric muscle strength for people with COPD. 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