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Influence of Testing Position on the Reliability of Hip Extensor Strength Measured by a Handheld Dynamometer
INFLUENCE OF TESTING POSITION ON THE RELIABILITY OF HIP EXTENSOR STRENGTH MEASURED BY A HANDHELD DYNAMOMETER Yi-Jing Lue,1,2 Ching-Lin Hsieh,3 Mei-Fang Liu,1,2 Shih-Fen Hsiao,1,2 Shu-Mei Chen,1,2 Jau-Hong Lin,1,2 and Yen-Mou Lu4 1 Department of Physical Therapy, College of Health Science, Kaohsiung Medical University, and Departments of 2 Physical Medicine and Rehabilitation and 4Orthopedics, Kaohsiung Medical University Hospital, Kaohsiung, and 3School of Occupational Therapy, College of Medicine, National Taiwan University, Taipei, Taiwan.
Hip extensors belong to an important muscle group that controls standing, walking and other functional activities. The prone position (PP) is commonly used to measure the strength of the hip extensors; however, the reliability of such measurements is poor. The aim of this study was to determine the effect of different testing positions, that is, the PP and the prone standing position (PSP), on the reliability of measurements of hip extensor strength. Intrasession reliability and interrater reliability studies were performed on 47 and 16 normal subjects, respectively. The muscle strength of the hip extensors was tested in both the PP and PSP. A handheld dynamometer and break test were used to measure the strength. Relative reliability and absolute reliability were assessed in both PP and PSP. For relative reliability, the intraclass correlation coefficient (ICC) was used to examine the level of reproducibility among measurements. Absolute reliability, the smallest real difference (SRD), was used to provide information on measurement error. The results showed that the reliability was better in PSP than in PP. For relative reliability, the values of ICCs were excellent in the intrasession reliability study, in both PP (ICC1,3 = 0.92) and PSP (ICC1,3 = 0.94). However, the interrater reliability was only excellent in PSP; the ICC2,3 were 0.92 in PSP and 0.65 in PP. For absolute reliability, the values of the SRD were much lower in PSP (29.8) than in PP (71.8), indicating that the measurement of muscle strength in PSP was more stable and had smaller measurement error than in PP. Changing the testing position from the traditional “prone” to “pronestanding” effectively improved both the relative reliability and the absolute reliability.
Key Words: hip extension, positions, reliability (Kaohsiung J Med Sci 2009;25:126–32)
Hip extensors play an important role in standing, walking and other functional activities. Studies have shown that the hip extensor strength is the most significant predictor of walking performance compared with the hip flexors, knee flexors/extensors and ankle
Received: Dec 1, 2008 Accepted: Mar 5, 2009 Address correspondence and reprint requests to: Dr Yen-Mou Lu, Department of Orthopedics, Kaohsiung Medical University Hospital, 100 Tzyou 1st Road, Kaohsiung 807, Taiwan. E-mail: [email protected]
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plantar flexors/dorsiflexors [1,2]. The elderly exhibit a reduced hip extension range during walking, which is exaggerated in patients suffering from a fall, and the coexistence of weak hip extensor strength is commonly found in many diseases [3–8]. Many methods are used to measure strength, including manual muscle testing, handheld dynamometer (HHD), and isokinetic machines [1,4,8–10]. Manual muscle testing procedures detect the magnitude of strength by grading muscle strength from 0 to 5. Complicated procedures such as using isokinetic machines or other fixation methods are often not practicable Kaohsiung J Med Sci March 2009 • Vol 25 • No 3 © 2009 Elsevier. All rights reserved.
Reliability of hip extensor strength measurement
due to the expense and time required to prepare the instruments. The HHD is a convenient, portable and inexpensive device for assessing isometric strength in a clinical setting. The use of HHD could aid the detection of strength deficits and in planning of clinical interventions [10,11]. The prone position is the standard position for measuring hip extensor strength [12]. However, the test–retest reliability in this position is poor. Studies have found that reliability measurements in the prone position, in normal healthy subjects and in cases of cerebral palsy, are unreliable [13–15]. Scott et al compared a handheld versus a dynamometer anchoring station to measure the reliability in normal healthy subjects and both intra- and interrater reliability were not good (intraclass correlation coefficient [ICC], 0.56–0.74) [14]. Reliability was also found to be unsatisfactory in patients with cerebral palsy; the intrasession and test–retest reliability were both poor and were unreliable for group means or individual scores [13,15]. When the strength measurement is not reliable, the perceived change in muscle strength after an intervention may only be the result of measurement error rather than actual change. Therefore, finding a more reliable method for hip extensor strength measurement is essential. Several statistical methods and indices have been proposed to examine the reliability of outcome measures, and both the relative and absolute reliability should be assessed [16–18]. For relative reliability, the ICC is the most commonly used statistical method because it shows the level of agreement among tests. For absolute reliability, the smallest real difference (SRD) can determine the extent of measurement error with a 95% confidence level [19,20], which provides useful information pertaining to the actual value for real change. A reliability test should show high ICC and low SRD values. Many studies have found poor reliability for hip extensor strength measured in the prone position, while in the study by Wang et al, the test–retest reliability was good for community-dwelling elderly patients who had suffered falls [21]. The major difference from previous studies was in the use of a different starting position; they used the prone standing position for measuring hip extensor strength. The reason for the poor reliability of hip extensor strength measurement may be because the initial prone body position does not allow the patient to exert maximum muscle force, Kaohsiung J Med Sci March 2009 • Vol 25 • No 3
whereas in the standing and leaning-forward-on-asupport positions, the maximum muscle force can be exerted. Few studies have compared the reliability of the two starting testing positions—the prone position (PP) and the prone standing position (PSP)—for the measurement of hip extensor strength. Therefore, the aim of this study was to assess the effect of changing the initial body position on the reliability of measurements of hip extensor strength. Furthermore, there are few comprehensive reliability studies for the measurement of hip extensor strength. In this study, we assessed both the relative and the absolute reliability.
MATERIALS AND METHODS Subjects Healthy volunteers were recruited from the local community. Subjects were excluded if they were pregnant, had cardiovascular symptoms, previous leg injury with symptoms present within 1 month, or had other systemic diseases. Subjects were asked to avoid intense exercise within the 24 hours prior to the test, but light activities were permitted. The research protocol was approved by the hospital’s institutional review board, and written informed consent was obtained from all subjects. The two parts of the study were designed accordingly; one was the intrasession reliability study, and the other was the interrater reliability study. All subjects (47 subjects; 23 males) participated in the intrasession reliability study, and some (16 subjects) also participated in the interrater reliability study.
Procedures A calibrated Micro FET2 handheld dynamometer (Hoggan Health Technologies Inc., West Jordan, UT, USA) was used to measure the strength of bilateral hip extensors. To compare the effects of position on the reliability of hip extensor measurement, hip extension was tested in two positions: the PP and the PSP (Figure) [21]. In PP, subjects were asked to lie prone on a height-adjustable table; the therapist adjusted the height of the table prior to testing to allow the therapist to produce the best exertion. The initial testing position of the leg was with the hip in hyperextension at 20° and the knee in full extension. In PSP, subjects stood and leaned forward on the height 127
Y.J. Lue, C.L. Hsieh, M.F. Liu, et al A
B
Figure. Testing positions for measuring hip extensor strength: (A) the prone position; (B) the prone standing position.
adjustable table. During the test, the subject’s trunk was supported by the table and the standing leg while the other leg was tested. The standing leg was in a comfortable and stable position, and the initial testing position of the leg tested was with the hip in flexion at 45° and the knee in full extension. The break test was used in this study. Physical therapists stabilized the subject with one hand and held the applicator of the dynamometer in the other. The applicator was held on the posterior surface of the thigh proximal to the knee during the test in both PP and PSP. The testing sequence of PP or PSP and the leg chosen (left/right) were randomized. The subject was asked to exert maximum force against the applicator, and the physical therapist rapidly applied a countering force to overcome the subject’s exertion; after that, the applicator was immediately removed from the limb and the force was recorded. Before the test, the physical therapists explained and demonstrated the break test and asked the subjects to practice once to familiarize themselves with the test method. The intrasession reliability was evaluated by one physical therapist using three trials for each position. Each trial lasted approximately 3 seconds, with 5-second rests between trials. A 1-minute rest was taken during the changeover between testing positions. The interrater reliability was tested in the same way in a 1-week time span by another physical therapist. Each position was tested in three trials, and the average strength was calculated. The testing order of the testing position (PP and PSP) of the second measurement session was the same as the first. Both physical therapists had over 10 years of clinical experience in orthopedic rehabilitation settings at the time of testing. Both participated in a 1-day training session 128
led by the principal investigator to standardize the evaluation procedures. Each physical therapist was blinded to the strength values obtained by the other.
Statistical analyses Relative reliability Intraclass correlation coefficients (ICCs) were calculated for intrasession ICC1,3 and interrater ICC2,3 analyses. The ICC was calculated as the ratio of the variance between subjects and the total variance. Absolute reliability The absolute reliability was examined with two repeated measurements from different raters. The standard error of measurement (SEM) and the SRD were used to estimate absolute reliability. The SEM was calculated using the formula (standard deviation of all interrater strength) × √(1 – ICCtwo raters). SRD was calculated as 1.96 × SEM × √2. A good measurement tool should have low SEM and low SRD values to detect changes in clinical trials [18,22,23]. The SRD% was calculated as the SRD divided by the mean of all measurements from tests 1 and 2 and multiplied by 100% to give a percentage value [24]. All statistical analyses were done with SPSS version 11.0 (SPSS Inc., Chicago, IL, USA) for Windows.
RESULTS The demographic data of subjects participating in this study are shown in Table 1. The age range was 22–31 years, and the body mass index range was 16.0–25.4 kg/m2. All participants confirmed that they had continued with their normal activities during the Kaohsiung J Med Sci March 2009 • Vol 25 • No 3
Reliability of hip extensor strength measurement Table 1. Demographic data of subjects participating in intrasession reliability and interrater reliability studies* Intrasession reliability study
Age (yr) Height (cm) Body weight (kg)
Interrater reliability study
All subjects (n = 47)
Men (n = 23)
Women (n = 24)
All subjects (n = 16)
Men (n = 8)
Women (n = 8)
22.6 ± 1.8 166.8 ± 7.6 56.1 ± 8.2
23.2 ± 2.2 172.9 ± 4.1 60.7 ± 5.8
22.1 ± 1.2 160.9 ± 5.1 51.4 ± 7.2
22.7 ± 2.1 167.2 ± 8.1 55.9 ± 8.0
23.4 ± 2.6 173.5 ± 4.8 60.6 ± 6.3
22.1 ± 1.4 160.9 ± 5.3 51.6 ± 7.4
*Data presented as mean ± standard deviation.
Table 2. Relative reliability and absolute reliability of two positions for measuring muscle strength of the hip extensor Relative reliability Position
PP PSP
Intrasession reliability
Absolute reliability
Interrater reliability
Interrater reliability
ICC
95% CI
ICC
95% CI
SEM
SRD
SRD%
0.92 0.94
0.87–0.95 0.91–0.97
0.65 0.92
−0.01 to 0.88 0.76–0.97
18.3 7.6
71.8 29.8
20.8% 8.4%
ICC = intraclass correlation coefficient; CI = confidence interval; SEM = standard error of measurement; SRD = smallest real difference; SRD% = SRD/mean strength × 100; PP = prone position; PSP = prone standing position.
interval between the two tests and had not done any intensive exercise within 24 hours of the test. The strengths of the left and the right limbs showed no significant differences; therefore, the ICC values of the right and the left side were averaged to represent that of each testing position. The results of the relative and absolute reliability analyses are shown in Table 2. According to a report by Fleiss, ICC values greater than 0.75 are regarded as having excellent reliability [25]. In the results of intrasession reliability, the ICC1,3 values for both PP and PSP were very high (0.92 and 0.94), demonstrating that both positions had excellent intrasession reliability. However, in the interrater reliability, the ICC2,3 values for PSP (0.92) were greater than that for PP (0.65), demonstrating that only PSP had excellent interrater reliability. We found that the values of SRD in PP (71.8 N) were more than twice those of PSP (29.8 N). Similarly, the SRD% was much smaller in PSP (8.4%) than in PP (20.8%). Both indicate that the strength measurement by different raters in the PSP was more stable than in the PP.
DISCUSSION This study investigated the reliability of two testing positions to measure the strength of hip extensors. Kaohsiung J Med Sci March 2009 • Vol 25 • No 3
We found that the reliability of measurement in PSP was better than that in PP. Therefore, PSP is a more stable measurement method among raters, which means it is able to detect real changes in strength after training or intervention. The interrater reliability for measuring hip extension strength is greatly affected by the test position. In this study, we found that the ICC2,3 value of the PSP was 0.92, which was much higher than that for the PP (0.65). To the best of our knowledge, no studies have compared the reliability of measuring the strength of hip extensors in both the PP and PSP. The possible reason for the poor reliability in PP may not simply be inadequate stability. We suggest that the insufficient interrater reliability measurement in PP is because the hip extensors are unable to perform a stable maximum contraction. In the PP position, the hip joint is in a hyperextended position and the subject must expend effort to lift his or her limb to keep the hip in hyperextension before testing [26,27]. Accordingly, subjects may be unable to sustain the maximum force in PP because the muscles are in a disadvantaged mechanical force position. However, in PSP, the hip joint is in a flexed position, so the hip extensor muscles are in a lengthened state, which creates a better length-tension relationship for muscles to produce greater force [28]. Therefore, subjects may be able to produce a more stable maximum force in PSP than in 129
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PP. In PSP, the subjects stood in a comfortable and stable position without control of the joint angle, with their weight borne by the standing leg. The difference in hip joint angle and the weight held by the standing leg might increase the measurement variance. Furthermore, the table should be adjusted so that it is low enough to allow a straight elbow and to maintain the elbow in a straight position for effective use of the therapist’s body weight to resist the patient’s effort and thus decrease the measurement variance. Absolute reliability indices can be used to determine the extent of measurement error caused by chance variation in measurements [29]. SEM, an index of absolute reliability, is a useful value for researchers to determine whether the change in a group is real and not due to measurement error [18–20,30]. The SEM data shown in Table 2 could provide information on measurement error in group comparisons for clinical research. The SRD value provides useful information on whether the change observed in the individual is real at the 95% confidence level. The large SRD% value indicates that it is uncertain whether the strength measurement method is reliable for clinicians to evaluate the changes in strength in individual cases. According to our results, changes in initial strength of less than 20.8% in PP and 8.4% in PSP could be due to natural variability over time, being within measurement error. The SRD% of PP was larger than the acceptable 10% of the mean score [24,31]. Three limitations of our study are worth noting. First, our sample size was small. Although small sample sizes (about 10–25) have been used in several reliability studies in measuring limb strength [13,32–35], a larger sample size would allow for more robust analyses in reliability studies [36]. Second, the two raters did not test each subject on the same day. This study investigated the interrater reliability with a 7-day interval rather than testing it on the same day. Some other studies have investigated interrater reliability of muscle strength with a 7-day interval [14,37]. The reason in this study for not testing each subject on the same day was to avoid muscle fatigue in the subjects and the raters. To avoid possible bias caused by the length of the testing interval, the interrater reliability could be investigated in the same session, better temporal arrangement is necessary to avoid fatigue of the subjects and raters. Third, the generalizability of our study is limited. Because this study was performed in normal adults, the results should not be 130
extrapolated to patients with diseases. Hip flexor contracture or insufficient flexibility of hip flexion are common symptoms in many diseases; in these cases, testing in PP cannot be performed [4,15,32]. We suggest that the PSP may be more suitable and reliable than other positions for measuring the strength of hip extensors in some diseases, but this hypothesis needs further study for confirmation. This study demonstrated that using the PSP as the initial position for the measurement of muscle strength of hip extensors effectively improved interrater reliability. The SRD% value of the hip extensor strength in PSP can provide a useful guide to ascertain whether the difference between two measurements is real or due to measurement error. We recommend the use of PSP for measuring the strength of hip extensors because the relative and absolute reliabilities were satisfactory in interrater reliability.
ACKNOWLEDGMENTS This study was supported by grants from the Department of Health (project DOH91-TD1063). We are grateful to the study participants for their contribution to this study.
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不同測量姿勢對髖伸肌肌力測量信度的影響 以手握式肌力測試計測量 呂怡靜
1,2
謝清麟
陳淑媚 1
1,2
3
劉玫舫
林昭宏
1,2
1,2
蕭世芬
呂衍謀
1,2
4
高雄醫學大學健康科學院 物理治療學系 2
4
高雄醫學大學附設醫院 復健科 骨科 3
台灣大學醫學院 職能治療學系
功能性活動表現與髖關節伸展肌 (extensor) 肌力的表現相關密切，傳統以手握式肌力 測試計 (handheld dynamometer) 在俯臥 (prone) 姿勢下測量髖關節伸展肌力，但 研究發現測量結果信度不佳。因此本研究目的嘗試以不同擺位姿勢，比較髖關節伸展 肌之測量信度。47 位及 16 位正常受試者分別接受髖關節伸展肌力測試之施測者內信 度 (intrasession reliability) 與施測者間信度 (interrater reliability) 分析，測試 方法採壓倒性測試 (break test)，受試者接受俯臥姿勢 (prone position、PP) 及俯臥 站 立 姿 勢 (prone standing position、PSP) 測 試。 結 果 方 面 以 組 內 相 關 係 數 (intraclass correlation coefficient、ICC) 數值表示相對信度，測試內信度在 PP (ICC 1,3 = 0.92) 和在 PSP (ICC 1,3 = 0.94) 下測試信度皆高，但施測者間信度只有在 PSP (ICC 2,3 = 0.92) 下測試信度高，而在 PP (ICC2,3 = 0.65) 下測試信度偏低。絕對 信度方面，最小真正改變值 (smallest real difference) 數值代表單一個案之測量誤 差，在 PSP (29.8) 下測試均小於在 PP (71.8) 下測試。臨床上建議髖關節伸展肌力的 測量應使用 PSP 測量，以 PSP 測試可有效改善以 PP 測量時施測者間信度不佳的問 題。 關鍵詞：髖關節伸展，擺位姿勢，信度 ( 高雄醫誌 2009;25:126–32)
收文日期：97 年 12 月 1 日 接受刊載：98 年 3 月 5 日 通訊作者：呂衍謀醫師 高雄醫學大學附設醫院骨科 高雄市 807 三民區自由一路 100 號 132
Kaohsiung J Med Sci March 2009 • Vol 25 • No 3
Hip extensors belong to an important muscle group that controls standing, walking and other functional activities. The prone position (PP) is commonly used to measure the strength of the hip extensors; however, the reliability of such measurements is poor. The aim of this study was to determine the effect of different testing positions, that is, the PP and the prone standing position (PSP), on the reliability of measurements of hip extensor strength. Intrasession reliability and interrater reliability studies were performed on 47 and 16 normal subjects, respectively. The muscle strength of the hip extensors was tested in both the PP and PSP. A handheld dynamometer and break test were used to measure the strength. Relative reliability and absolute reliability were assessed in both PP and PSP. For relative reliability, the intraclass correlation coefficient (ICC) was used to examine the level of reproducibility among measurements. Absolute reliability, the smallest real difference (SRD), was used to provide information on measurement error. The results showed that the reliability was better in PSP than in PP. For relative reliability, the values of ICCs were excellent in the intrasession reliability study, in both PP (ICC1,3 = 0.92) and PSP (ICC1,3 = 0.94). However, the interrater reliability was only excellent in PSP; the ICC2,3 were 0.92 in PSP and 0.65 in PP. For absolute reliability, the values of the SRD were much lower in PSP (29.8) than in PP (71.8), indicating that the measurement of muscle strength in PSP was more stable and had smaller measurement error than in PP. Changing the testing position from the traditional “prone” to “pronestanding” effectively improved both the relative reliability and the absolute reliability.
Key Words: hip extension, positions, reliability (Kaohsiung J Med Sci 2009;25:126–32)
Hip extensors play an important role in standing, walking and other functional activities. Studies have shown that the hip extensor strength is the most significant predictor of walking performance compared with the hip flexors, knee flexors/extensors and ankle
Received: Dec 1, 2008 Accepted: Mar 5, 2009 Address correspondence and reprint requests to: Dr Yen-Mou Lu, Department of Orthopedics, Kaohsiung Medical University Hospital, 100 Tzyou 1st Road, Kaohsiung 807, Taiwan. E-mail: [email protected]
126
plantar flexors/dorsiflexors [1,2]. The elderly exhibit a reduced hip extension range during walking, which is exaggerated in patients suffering from a fall, and the coexistence of weak hip extensor strength is commonly found in many diseases [3–8]. Many methods are used to measure strength, including manual muscle testing, handheld dynamometer (HHD), and isokinetic machines [1,4,8–10]. Manual muscle testing procedures detect the magnitude of strength by grading muscle strength from 0 to 5. Complicated procedures such as using isokinetic machines or other fixation methods are often not practicable Kaohsiung J Med Sci March 2009 • Vol 25 • No 3 © 2009 Elsevier. All rights reserved.
Reliability of hip extensor strength measurement
due to the expense and time required to prepare the instruments. The HHD is a convenient, portable and inexpensive device for assessing isometric strength in a clinical setting. The use of HHD could aid the detection of strength deficits and in planning of clinical interventions [10,11]. The prone position is the standard position for measuring hip extensor strength [12]. However, the test–retest reliability in this position is poor. Studies have found that reliability measurements in the prone position, in normal healthy subjects and in cases of cerebral palsy, are unreliable [13–15]. Scott et al compared a handheld versus a dynamometer anchoring station to measure the reliability in normal healthy subjects and both intra- and interrater reliability were not good (intraclass correlation coefficient [ICC], 0.56–0.74) [14]. Reliability was also found to be unsatisfactory in patients with cerebral palsy; the intrasession and test–retest reliability were both poor and were unreliable for group means or individual scores [13,15]. When the strength measurement is not reliable, the perceived change in muscle strength after an intervention may only be the result of measurement error rather than actual change. Therefore, finding a more reliable method for hip extensor strength measurement is essential. Several statistical methods and indices have been proposed to examine the reliability of outcome measures, and both the relative and absolute reliability should be assessed [16–18]. For relative reliability, the ICC is the most commonly used statistical method because it shows the level of agreement among tests. For absolute reliability, the smallest real difference (SRD) can determine the extent of measurement error with a 95% confidence level [19,20], which provides useful information pertaining to the actual value for real change. A reliability test should show high ICC and low SRD values. Many studies have found poor reliability for hip extensor strength measured in the prone position, while in the study by Wang et al, the test–retest reliability was good for community-dwelling elderly patients who had suffered falls [21]. The major difference from previous studies was in the use of a different starting position; they used the prone standing position for measuring hip extensor strength. The reason for the poor reliability of hip extensor strength measurement may be because the initial prone body position does not allow the patient to exert maximum muscle force, Kaohsiung J Med Sci March 2009 • Vol 25 • No 3
whereas in the standing and leaning-forward-on-asupport positions, the maximum muscle force can be exerted. Few studies have compared the reliability of the two starting testing positions—the prone position (PP) and the prone standing position (PSP)—for the measurement of hip extensor strength. Therefore, the aim of this study was to assess the effect of changing the initial body position on the reliability of measurements of hip extensor strength. Furthermore, there are few comprehensive reliability studies for the measurement of hip extensor strength. In this study, we assessed both the relative and the absolute reliability.
MATERIALS AND METHODS Subjects Healthy volunteers were recruited from the local community. Subjects were excluded if they were pregnant, had cardiovascular symptoms, previous leg injury with symptoms present within 1 month, or had other systemic diseases. Subjects were asked to avoid intense exercise within the 24 hours prior to the test, but light activities were permitted. The research protocol was approved by the hospital’s institutional review board, and written informed consent was obtained from all subjects. The two parts of the study were designed accordingly; one was the intrasession reliability study, and the other was the interrater reliability study. All subjects (47 subjects; 23 males) participated in the intrasession reliability study, and some (16 subjects) also participated in the interrater reliability study.
Procedures A calibrated Micro FET2 handheld dynamometer (Hoggan Health Technologies Inc., West Jordan, UT, USA) was used to measure the strength of bilateral hip extensors. To compare the effects of position on the reliability of hip extensor measurement, hip extension was tested in two positions: the PP and the PSP (Figure) [21]. In PP, subjects were asked to lie prone on a height-adjustable table; the therapist adjusted the height of the table prior to testing to allow the therapist to produce the best exertion. The initial testing position of the leg was with the hip in hyperextension at 20° and the knee in full extension. In PSP, subjects stood and leaned forward on the height 127
Y.J. Lue, C.L. Hsieh, M.F. Liu, et al A
B
Figure. Testing positions for measuring hip extensor strength: (A) the prone position; (B) the prone standing position.
adjustable table. During the test, the subject’s trunk was supported by the table and the standing leg while the other leg was tested. The standing leg was in a comfortable and stable position, and the initial testing position of the leg tested was with the hip in flexion at 45° and the knee in full extension. The break test was used in this study. Physical therapists stabilized the subject with one hand and held the applicator of the dynamometer in the other. The applicator was held on the posterior surface of the thigh proximal to the knee during the test in both PP and PSP. The testing sequence of PP or PSP and the leg chosen (left/right) were randomized. The subject was asked to exert maximum force against the applicator, and the physical therapist rapidly applied a countering force to overcome the subject’s exertion; after that, the applicator was immediately removed from the limb and the force was recorded. Before the test, the physical therapists explained and demonstrated the break test and asked the subjects to practice once to familiarize themselves with the test method. The intrasession reliability was evaluated by one physical therapist using three trials for each position. Each trial lasted approximately 3 seconds, with 5-second rests between trials. A 1-minute rest was taken during the changeover between testing positions. The interrater reliability was tested in the same way in a 1-week time span by another physical therapist. Each position was tested in three trials, and the average strength was calculated. The testing order of the testing position (PP and PSP) of the second measurement session was the same as the first. Both physical therapists had over 10 years of clinical experience in orthopedic rehabilitation settings at the time of testing. Both participated in a 1-day training session 128
led by the principal investigator to standardize the evaluation procedures. Each physical therapist was blinded to the strength values obtained by the other.
Statistical analyses Relative reliability Intraclass correlation coefficients (ICCs) were calculated for intrasession ICC1,3 and interrater ICC2,3 analyses. The ICC was calculated as the ratio of the variance between subjects and the total variance. Absolute reliability The absolute reliability was examined with two repeated measurements from different raters. The standard error of measurement (SEM) and the SRD were used to estimate absolute reliability. The SEM was calculated using the formula (standard deviation of all interrater strength) × √(1 – ICCtwo raters). SRD was calculated as 1.96 × SEM × √2. A good measurement tool should have low SEM and low SRD values to detect changes in clinical trials [18,22,23]. The SRD% was calculated as the SRD divided by the mean of all measurements from tests 1 and 2 and multiplied by 100% to give a percentage value [24]. All statistical analyses were done with SPSS version 11.0 (SPSS Inc., Chicago, IL, USA) for Windows.
RESULTS The demographic data of subjects participating in this study are shown in Table 1. The age range was 22–31 years, and the body mass index range was 16.0–25.4 kg/m2. All participants confirmed that they had continued with their normal activities during the Kaohsiung J Med Sci March 2009 • Vol 25 • No 3
Reliability of hip extensor strength measurement Table 1. Demographic data of subjects participating in intrasession reliability and interrater reliability studies* Intrasession reliability study
Age (yr) Height (cm) Body weight (kg)
Interrater reliability study
All subjects (n = 47)
Men (n = 23)
Women (n = 24)
All subjects (n = 16)
Men (n = 8)
Women (n = 8)
22.6 ± 1.8 166.8 ± 7.6 56.1 ± 8.2
23.2 ± 2.2 172.9 ± 4.1 60.7 ± 5.8
22.1 ± 1.2 160.9 ± 5.1 51.4 ± 7.2
22.7 ± 2.1 167.2 ± 8.1 55.9 ± 8.0
23.4 ± 2.6 173.5 ± 4.8 60.6 ± 6.3
22.1 ± 1.4 160.9 ± 5.3 51.6 ± 7.4
*Data presented as mean ± standard deviation.
Table 2. Relative reliability and absolute reliability of two positions for measuring muscle strength of the hip extensor Relative reliability Position
PP PSP
Intrasession reliability
Absolute reliability
Interrater reliability
Interrater reliability
ICC
95% CI
ICC
95% CI
SEM
SRD
SRD%
0.92 0.94
0.87–0.95 0.91–0.97
0.65 0.92
−0.01 to 0.88 0.76–0.97
18.3 7.6
71.8 29.8
20.8% 8.4%
ICC = intraclass correlation coefficient; CI = confidence interval; SEM = standard error of measurement; SRD = smallest real difference; SRD% = SRD/mean strength × 100; PP = prone position; PSP = prone standing position.
interval between the two tests and had not done any intensive exercise within 24 hours of the test. The strengths of the left and the right limbs showed no significant differences; therefore, the ICC values of the right and the left side were averaged to represent that of each testing position. The results of the relative and absolute reliability analyses are shown in Table 2. According to a report by Fleiss, ICC values greater than 0.75 are regarded as having excellent reliability [25]. In the results of intrasession reliability, the ICC1,3 values for both PP and PSP were very high (0.92 and 0.94), demonstrating that both positions had excellent intrasession reliability. However, in the interrater reliability, the ICC2,3 values for PSP (0.92) were greater than that for PP (0.65), demonstrating that only PSP had excellent interrater reliability. We found that the values of SRD in PP (71.8 N) were more than twice those of PSP (29.8 N). Similarly, the SRD% was much smaller in PSP (8.4%) than in PP (20.8%). Both indicate that the strength measurement by different raters in the PSP was more stable than in the PP.
DISCUSSION This study investigated the reliability of two testing positions to measure the strength of hip extensors. Kaohsiung J Med Sci March 2009 • Vol 25 • No 3
We found that the reliability of measurement in PSP was better than that in PP. Therefore, PSP is a more stable measurement method among raters, which means it is able to detect real changes in strength after training or intervention. The interrater reliability for measuring hip extension strength is greatly affected by the test position. In this study, we found that the ICC2,3 value of the PSP was 0.92, which was much higher than that for the PP (0.65). To the best of our knowledge, no studies have compared the reliability of measuring the strength of hip extensors in both the PP and PSP. The possible reason for the poor reliability in PP may not simply be inadequate stability. We suggest that the insufficient interrater reliability measurement in PP is because the hip extensors are unable to perform a stable maximum contraction. In the PP position, the hip joint is in a hyperextended position and the subject must expend effort to lift his or her limb to keep the hip in hyperextension before testing [26,27]. Accordingly, subjects may be unable to sustain the maximum force in PP because the muscles are in a disadvantaged mechanical force position. However, in PSP, the hip joint is in a flexed position, so the hip extensor muscles are in a lengthened state, which creates a better length-tension relationship for muscles to produce greater force [28]. Therefore, subjects may be able to produce a more stable maximum force in PSP than in 129
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PP. In PSP, the subjects stood in a comfortable and stable position without control of the joint angle, with their weight borne by the standing leg. The difference in hip joint angle and the weight held by the standing leg might increase the measurement variance. Furthermore, the table should be adjusted so that it is low enough to allow a straight elbow and to maintain the elbow in a straight position for effective use of the therapist’s body weight to resist the patient’s effort and thus decrease the measurement variance. Absolute reliability indices can be used to determine the extent of measurement error caused by chance variation in measurements [29]. SEM, an index of absolute reliability, is a useful value for researchers to determine whether the change in a group is real and not due to measurement error [18–20,30]. The SEM data shown in Table 2 could provide information on measurement error in group comparisons for clinical research. The SRD value provides useful information on whether the change observed in the individual is real at the 95% confidence level. The large SRD% value indicates that it is uncertain whether the strength measurement method is reliable for clinicians to evaluate the changes in strength in individual cases. According to our results, changes in initial strength of less than 20.8% in PP and 8.4% in PSP could be due to natural variability over time, being within measurement error. The SRD% of PP was larger than the acceptable 10% of the mean score [24,31]. Three limitations of our study are worth noting. First, our sample size was small. Although small sample sizes (about 10–25) have been used in several reliability studies in measuring limb strength [13,32–35], a larger sample size would allow for more robust analyses in reliability studies [36]. Second, the two raters did not test each subject on the same day. This study investigated the interrater reliability with a 7-day interval rather than testing it on the same day. Some other studies have investigated interrater reliability of muscle strength with a 7-day interval [14,37]. The reason in this study for not testing each subject on the same day was to avoid muscle fatigue in the subjects and the raters. To avoid possible bias caused by the length of the testing interval, the interrater reliability could be investigated in the same session, better temporal arrangement is necessary to avoid fatigue of the subjects and raters. Third, the generalizability of our study is limited. Because this study was performed in normal adults, the results should not be 130
extrapolated to patients with diseases. Hip flexor contracture or insufficient flexibility of hip flexion are common symptoms in many diseases; in these cases, testing in PP cannot be performed [4,15,32]. We suggest that the PSP may be more suitable and reliable than other positions for measuring the strength of hip extensors in some diseases, but this hypothesis needs further study for confirmation. This study demonstrated that using the PSP as the initial position for the measurement of muscle strength of hip extensors effectively improved interrater reliability. The SRD% value of the hip extensor strength in PSP can provide a useful guide to ascertain whether the difference between two measurements is real or due to measurement error. We recommend the use of PSP for measuring the strength of hip extensors because the relative and absolute reliabilities were satisfactory in interrater reliability.
ACKNOWLEDGMENTS This study was supported by grants from the Department of Health (project DOH91-TD1063). We are grateful to the study participants for their contribution to this study.
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不同測量姿勢對髖伸肌肌力測量信度的影響 以手握式肌力測試計測量 呂怡靜
1,2
謝清麟
陳淑媚 1
1,2
3
劉玫舫
林昭宏
1,2
1,2
蕭世芬
呂衍謀
1,2
4
高雄醫學大學健康科學院 物理治療學系 2
4
高雄醫學大學附設醫院 復健科 骨科 3
台灣大學醫學院 職能治療學系
功能性活動表現與髖關節伸展肌 (extensor) 肌力的表現相關密切，傳統以手握式肌力 測試計 (handheld dynamometer) 在俯臥 (prone) 姿勢下測量髖關節伸展肌力，但 研究發現測量結果信度不佳。因此本研究目的嘗試以不同擺位姿勢，比較髖關節伸展 肌之測量信度。47 位及 16 位正常受試者分別接受髖關節伸展肌力測試之施測者內信 度 (intrasession reliability) 與施測者間信度 (interrater reliability) 分析，測試 方法採壓倒性測試 (break test)，受試者接受俯臥姿勢 (prone position、PP) 及俯臥 站 立 姿 勢 (prone standing position、PSP) 測 試。 結 果 方 面 以 組 內 相 關 係 數 (intraclass correlation coefficient、ICC) 數值表示相對信度，測試內信度在 PP (ICC 1,3 = 0.92) 和在 PSP (ICC 1,3 = 0.94) 下測試信度皆高，但施測者間信度只有在 PSP (ICC 2,3 = 0.92) 下測試信度高，而在 PP (ICC2,3 = 0.65) 下測試信度偏低。絕對 信度方面，最小真正改變值 (smallest real difference) 數值代表單一個案之測量誤 差，在 PSP (29.8) 下測試均小於在 PP (71.8) 下測試。臨床上建議髖關節伸展肌力的 測量應使用 PSP 測量，以 PSP 測試可有效改善以 PP 測量時施測者間信度不佳的問 題。 關鍵詞：髖關節伸展，擺位姿勢，信度 ( 高雄醫誌 2009;25:126–32)
收文日期：97 年 12 月 1 日 接受刊載：98 年 3 月 5 日 通訊作者：呂衍謀醫師 高雄醫學大學附設醫院骨科 高雄市 807 三民區自由一路 100 號 132
Kaohsiung J Med Sci March 2009 • Vol 25 • No 3