I need a simple summary article

Associations of the Stair Climb Power Test With Muscle Strength and Functional Performance in People With Chronic Obstructive Pulmonary Disease: A Cross-Sectional Study Marc Roig, Janice J. Eng, Donna L. MacIntyre, Jeremy D. Road, W. Darlene Reid

Background. The Stair Climb Power Test (SCPT) is a functional test associated with leg muscle power in older people.

Objective. The purposes of this study were to compare the results of the SCPT in people with chronic obstructive pulmonary disease (COPD) and people who were healthy and to explore associations of the SCPT with muscle strength (force- generating capacity) and functional performance.

Design. The study was a cross-sectional investigation.

Methods. Twenty-one people with COPD and a predicted mean (SD) percentage of forced expiratory volume in 1 second of 47.2 (12.9) and 21 people who were healthy and matched for age, sex, and body mass were tested with the SCPT. Knee extensor and flexor muscle torque was assessed with an isokinetic dynamometer. Functional performance was assessed with the Timed “Up & Go” Test (TUG) and the Six-Minute Walk Test (6MWT).

Results. People with COPD showed lower values on the SCPT (28%) and all torque measures (�32%), except for eccentric knee flexor muscle torque. In people with COPD, performance on the TUG and 6MWT was lower by 23% and 28%, respectively. In people with COPD, the SCPT was moderately associated with knee extensor muscle isometric and eccentric torque (r�.46) and strongly associated (r�.68) with the 6MWT. In people who were healthy, the association of the SCPT with knee extensor muscle torque tended to be stronger (r�.66); however, no significant relationship between the SCPT and measures of functional performance was found.

Limitations. The observational design of the study and the use of a relatively small convenience sample limit the generalizability of the findings.

Conclusions. The SCPT is a simple and safe test associated with measures of functional performance in people with COPD. People with COPD show deficits on the SCPT. However, the SCPT is only moderately associated with muscle torque and thus cannot be used as a simple surrogate for muscle strength in people with COPD.

M. Roig, PT, PhD, is a postdoctoral fellow at the University of Copenha- gen, Copenhagen, Denmark. He was a doctoral student in Rehabili- tation Sciences at the University of British Columbia, Vancouver, British Columbia, Canada, at the time of the study. Dr Roig’s institutional mailing address is: Department of Exercise and Sport Sciences and Department of Neuroscience and Pharmacology, Panum Institute, University of Copenhagen, Copen- hagen, Denmark. Address all correspondence to Dr Roig at: [email protected].

J.J. Eng, PT/OT, PhD, is Professor, Department of Physical Therapy, University of British Columbia, and Scientist, Rehab Research Lab, GF Strong Rehabilitation Centre, Van- couver, British Columbia, Canada.

D.L. MacIntyre, PT, PhD, is Asso- ciate Professor, Department of Physical Therapy, University of Brit- ish Columbia, and Scientist, Rehab Research Lab, GF Strong Rehabilita- tion Centre.

J.D. Road, MD, is Professor, De- partment of Medicine, University of British Columbia.

W.D. Reid, PT, PhD, is Professor, Department of Physical Therapy, University of British Columbia, and Director, Muscle Biophysics Labora- tory, Vancouver Coastal Health Re- search Institute.

[Roig M, Eng JJ, MacIntyre DL, et al. Associations of the Stair Climb Power Test with muscle strength and functional perfor- mance in people with chronic ob- structive pulmonary disease: a cross-sectional study. Phys Ther. 2010;90:1774 –1782.]

© 2010 American Physical Therapy Association

Research Report

Post a Rapid Response to this article at: ptjournal.apta.org

1774 f Physical Therapy Volume 90 Number 12 December 2010

D ow

nloaded from https://academ

ic.oup.com /ptj/article-abstract/90/12/1774/2737794 by guest on 13 February 2020

Chronic obstructive pulmonarydisease (COPD) is a clinicalcondition with numerous sys- temic manifestations1 that com- pound the loss of function com- monly observed in people with this respiratory disease.2 Skeletal muscle dysfunction has recently gained rec- ognition as an important systemic ef- fect of COPD that contributes to func- tional limitations in people with COPD (for a review, see Mador and Bozka- nat3). In COPD, skeletal muscle dys- function is characterized by deficits in muscle strength (force-generating ca- pacity) and endurance,4 muscle atro- phy (ie, reductions in muscle mass),5

and alterations in muscle composition (eg, increased intramuscular fat).6 The relevance of assessing muscle strength in people with COPD has been em- phasized in recent studies. For exam- ple, Decramer et al7 found that knee extensor muscle weakness was a use- ful measure of health care resources in people with COPD. More recently, knee extensor muscle strength was identified as a simple and powerful predictor of mortality in people with COPD.8 Other studies have confirmed the clinical importance of assessing muscle strength as an important factor associated with exercise capacity in people with COPD (for a review, see Debigare and Maltais9).

Muscle strength in people with COPD traditionally has been assessed with either isokinetic or isotonic dyna- mometers. However, most of these de- vices are large, expensive, and not ac- cessible to clinicians and often require procedures involving complex and time-consuming protocols. Recently, more-sophisticated methods for as- sessing muscle activation, such as fem- oral magnetic stimulation, have been made available.10 These techniques are appealing to researchers because they permit potential confounders re- lated to the volitional capacity of an individual to be minimized and thus reduce testing variability. However, these methods also require specialized

equipment that can be impractical in clinical settings. Therefore, alternative means of assessing functional muscle performance in people with COPD are needed.

The Stair Climb Power Test (SCPT) recently was proposed as a simple and safe measure associated with mea- sures of lower-limb muscle strength, power, and functional performance in older adults.11 Because the SCPT does not require additional equipment, it could be a reasonable alternative to more-sophisticated tests for measuring lower-limb muscle impairments in people with COPD. We conducted a cross-sectional study with the primary aim of comparing the results of the SCPT in people with moderate to se- vere COPD and people who were healthy (control group) and matched for age, sex, and body mass. The sec- ondary aim was to determine associa- tions between the SCPT and measures of muscle strength and functional per- formance. We hypothesized that peo- ple with COPD would show lower val- ues on the SCPT than people who were healthy and that the SCPT would correlate with knee extensor muscle torque. We also hypothesized that the SCPT would show moderate correla- tions with measures of functional performance.

Method Participants People with COPD and people who were healthy (control group) were recruited on a voluntary basis in the local community from October 2008 to June 2009. Information about the study was distributed in waiting rooms of hospitals, pulmonary func- tion laboratories, and community and senior centers in the area of Vancouver, British Columbia, Canada.

The inclusion criterion for people with COPD was moderate to severe disease (stage II or III), as deter- mined with the Global Initiative for Obstructive Lung Disease.12 Exclusion

criteria were an acute exacerbation13

within the 3 months before the study; regular participation in a for- mal exercise rehabilitation program during the 1-year period before the study; current smoking; comorbid cardiovascular or neurological dis- ease or lower-extremity musculo- skeletal problems that could inter- fere with or could cause undue risk during the performance of a study test; use of portable supplemental oxygen on a continuous basis; and �1-antitrypsin deficiency without a significant smoking history.

To be included in the study, people who were healthy had to be seden- tary. “Sedentary” was defined as “only performs activities with low meta- bolic cost, including light activities such as slow walking or cooking.”14(p174)

Exclusion criteria were regular par- ticipation in a formal exercise pro- gram during the 1-year period before the study; current smoking; and re- spiratory, cardiovascular, or neuro- logical disease or lower-extremity musculoskeletal problems that could interfere with or could cause undue risk during the performance of a study test.

Thirty-eight people with COPD and 26 people who were healthy (con- trol group) were recruited. Twenty- two people were excluded because they did not meet the criteria. The details of the recruitment process and the reasons for exclusion are shown in Figure 1. After exclusion, 21 non– oxygen-dependent, clinically sta- ble people (11 men and 10 women)

Available With This Article at ptjournal.apta.org

• Audio Abstracts Podcast

This article was published ahead of print on October 28, 2010, at ptjournal.apta.org.

Muscle Strength and Functional Performance in People With COPD

December 2010 Volume 90 Number 12 Physical Therapy f 1775

D ow

nloaded from https://academ

ic.oup.com /ptj/article-abstract/90/12/1774/2737794 by guest on 13 February 2020

with moderate to severe COPD12 and 21 people who were healthy but sed- entary (control group) and matched for age, sex, and body mass were in- cluded in the study. The characteris- tics of the participants are shown in Table 1. People with COPD had mod- erate to severe levels of airflow limita- tion, which were confirmed by their lower values for spirometric measures in comparison with those of people who were healthy. All participants gave written informed consent before participation.

To estimate an appropriate sample size, standardized effect sizes antici- pated from previous studies assess- ing strength deficits in people with COPD6 were calculated by dividing the difference between the mean scores for the COPD and control groups by the pooled standard devi- ation.15 Standardized effect sizes ranged from 0.8 to 1.2. Assuming a standardized effect size of 0.8, 80% power, and an �2 level of .05, we estimated that at least 20 participants

per group were needed to detect dif- ferences in muscle strength.15

Initial Screening Before testing was begun, partici- pants were asked to complete a risk screening questionnaire to guarantee their suitability for participation and to minimize any potential risk.16 To confirm the severity of COPD and to rule out pulmonary disease, people with COPD and people who were healthy, respectively, underwent spi- rometry with a portable spirometer (CPFS/D spirometer*). Spirometry was performed in accordance with the guidelines of the American Tho- racic Society and the European Re- spiratory Society.17

Measurements SCPT. In the SCPT, the time re- quired to climb a staircase of 10 stairs as quickly and safely as possi- ble is recorded. In brief, participants were instructed to ascend the 10 stairs as quickly as they could. They were allowed to use the handrail only to increase their own safety; they were specifically instructed not to use the handrail to ascend the stairs faster. A more detailed descrip- tion of the procedures of the SCPT is available elsewhere.11 In brief, veloc- ity is calculated by dividing the dis- tance, which in this case is the total vertical height of the stairs, by the time required to climb the 10 stairs (v�d/t). Force is calculated by mul- tiplying body weight by acceleration (F�m � a); the latter is the constant for the effect of gravity (9.81 N). Thus, power (P�F � v) for the SCPT is the product of (total vertical height of the stairs/time) � (body weight � 9.81).11 Participants per- formed 2 trials of the SCPT, with at least 2 minutes of recovery between the trials. The same tester (M.R.) timed the SCPT manually with a stopwatch, and the trial with the

* Medical Graphics Corp, 350 Oak Grove Pkwy, St Paul, MN 55127-8507.

Included people who were healthy

(n=21)

Included people with COPD

(n=21)

Total people screened

(n=64)

People with COPD (n=38)

People who were healthy (n=26)

Excluded (n=5)

Not sedentary=1 Smoking=1 Lung dysfunction=1 Age not matched with other group=2

Excluded (n=17)

Comorbidities=2 Smoking=3 Disease severity=8 Oxygen dependent=3 α1-antitrypsin deficiency=1

Figure 1. CONSORT diagram showing the recruitment process and the reasons for exclusion. COPD�chronic obstructive pulmonary disease.

Muscle Strength and Functional Performance in People With COPD

1776 f Physical Therapy Volume 90 Number 12 December 2010

D ow

nloaded from https://academ

ic.oup.com /ptj/article-abstract/90/12/1774/2737794 by guest on 13 February 2020

faster time was saved for further analysis.

Knee extensor and flexor muscle strength. The concentric and ec- centric average peak torque of the knee extensor and flexor muscles of the dominant leg (determined by preference for kicking a ball) was assessed with a KinCom dynamome- ter (version 5.30).† In brief, partici- pants were seated on the KinCom dynamometer with the knee joint aligned with the center of rotation of the dynamometer. The resistance pad was positioned distally at 75% of the distance from the lateral epi- condyle to the lateral malleolus. Sta- bilizing belts were used to secure the pelvis and the upper body and to minimize extraneous movements. An angular velocity of 30°/s was chosen because it showed high re- producibility (intraclass correlation coefficient [ICC (2,1)]�.99) in a pre- vious study of people with COPD.18

The testing range of motion was set at 10 to 90 degrees and at 20 to 80 degrees of knee flexion for the knee extensor and flexor muscles, respec- tively. This range of motion was se- lected to diminish joint discomfort and force variability produced by the action of the hamstring muscles at

the end of knee extension. After a warm-up consisting of 5 submaximal contractions, participants were asked to extend their knee as fast and as hard as they could. Knee flexor mus- cles were tested with the same pro- cedures except that participants were asked to bend their knee. The same apparatus was used to assess the isometric knee extensor muscle average peak torque at 90 degrees of knee flexion. For this test, partici- pants were asked to extend their knee as fast and as hard as they could and to maintain force production for 3 seconds. To ensure maximal effort, participants were vigorously encour- aged. Three maximal trials for each muscle contraction and muscle group were recorded, with 2 minutes of recovery between the trials. Values from the 3 trials were averaged to obtain the peak torque.

Functional performance. Func- tional performance was assessed with the Timed “Up & Go” Test (TUG) and the Six-Minute Walk Test (6MWT). In brief, the TUG was per- formed by timing the ability to stand up from a chair, walk 3 m, cross a line marked on the floor, turn around, walk back to the chair, and sit down.19

Participants performed 2 trials of the TUG, with 2 minutes of recovery be- tween the trials. The same tester

(M.R.) timed the TUG manually, and the trial with the faster time was saved for further analysis. The 6MWT was performed once according to the guidelines of the American Thoracic Society.20 In brief, participants were instructed to walk as far as they could in 6 minutes in a 25-m hallway, and the distance walked was recorded. Mean peak oxygen uptake (peak V̇O2) val- ues for each group were estimated from the 6MWT with the following equation21:meanpeakV̇O2�4.948 � 0.023 � mean 6MWT distance in meters.

Data Analysis Assumptions of the normality of the data distribution for all variables were explored through histograms and normality plots.22 Pearson or Spearman (for data not normally dis- tributed) correlations were used to explore associations and multicol- linearity among dependent variables and potential covariates. Collinear covariates showing weaker associa- tions (r�.5) were eliminated. A re- gression model with age and body weight as covariates was used to de- termine differences between groups in all measures. The regression model was adjusted for age and body weight because these variables were inde- pendently correlated with measures

† Chattanooga Group Inc, 1430 Decision St, Vista, CA 92081.

Table 1. Characteristics of Study Participantsa

Characteristic

Control Group (n�21) COPD Group (n�21)

X (SD) Range X (SD) Range

Age (y) 67.4 (8.6) 50–85 71.2 (8.1) 50–85

Height (m) 1.7 (0.1) 1.4–1.8 1.6 (0.1) 1.5–1.8

Weight (kg) 70.8 (14.6) 47–104 70.5 (14.2) 48.3–103.4

BMI (kg/m2) 25.3 (4) 20–36 25.7 (4.2) 20.1–38.4

FEV1 (% predicted) 98.1 (11.9) 76–114 47.2 (12.9) b 32–72

FVC (% predicted) 108 (20.6) 67–141 80.4 (15.9)b 42–102

FEV1/FVC (% predicted) 69.5 (7.7) 57–85 52.4 (17.2) b 27–71

a COPD�chronic obstructive pulmonary disease, BMI�body mass index, FEV1�forced expiratory volume in 1 second, FVC�forced vital capacity. b Significant difference between groups at P�.05.

Muscle Strength and Functional Performance in People With COPD

December 2010 Volume 90 Number 12 Physical Therapy f 1777

D ow

nloaded from https://academ

ic.oup.com /ptj/article-abstract/90/12/1774/2737794 by guest on 13 February 2020

of functional performance and mus- cle torque, respectively. Pearson product moment correlations were used to explore associations between the SCPT and measures of knee exten- sor muscle torque and functional per- formance. The strength of the cor- relations (r) was categorized as low (0 –.25), moderate (�.25–.50), strong (�.50 –.75), and very strong (�.75).15

Test-retest reliability for all measures was calculated with ICCs (2,1).15 Data are presented as means, standard devi- ations, and 95% confidence intervals. All analyses were performed using the Statistical Package for the Social Sci- ences‡ with 2-tailed probability tests with the level of significance set at P�.05.

Role of the Funding Source Dr Roig received support through a Strategic Training Fellowship in Re- habilitation Research from the CIHR Musculoskeletal and Arthritis Insti- tute, a Fellowship in Respiratory Rehabilitation from the BC Lung As- sociation, and a Graduate Fellow- ship from the University of British Columbia. This project was sup-

ported by the Canadian Lung Asso- ciation and the Canadian Physio- therapy Association.

Results SCPT The results of the regression model for the SCPT are shown in Table 2. All participants were able to perform the SCPT safely, and no adverse ef- fects or unexpected events were re- ported. Compared with the control group, the COPD group developed, on average, 28% lower power during the SCPT. Test-retest reliability for the SCPT within the same session was very high (ICC [2,1]�.90).

Knee Extensor and Flexor Muscle Strength and Functional Performance The results of the regression model for measures of muscle torque and func- tional performance are shown in Table 2. Compared with the control group, the COPD group showed significantly lower values for all torque measures (�32%), except for eccentric knee flexion, for which the difference was not statistically significant. In the COPD group, test-retest reliability for isometric knee extension torque (ICC

[2,1]�.91) and for isokinetic knee ex- tension and flexion torque (ICC [2,1]�.90 –.97) was high. The control group showed similar reliability co- efficients for isometric knee exten- sion torque (ICC [2,1]�.90) and isokinetic knee extension and flex- ion torque (ICC [2,1]�.92–.96).

In the COPD group, performance on the TUG was significantly lower (23%). Test-retest reliability for the TUG within the same session was very high (ICC [2,1]�.95). The dis- tance walked in the 6MWT was 28% lower in the COPD group than in the control group. The mean (SD) peak V̇O2 values were 13.9 (1.5) and 17.8 (1.5) mL/kg/min in the COPD and control groups, respectively. The mean difference between groups in peak V̇O2 values was 3.8 mL/kg/min (95% confidence interval�2.9 – 4.7) (P�.001).

Association Between the SCPT and Measures of Muscle Strength and Functional Performance The Pearson correlation analysis con- firmed that the SCPT was very strongly associated (r�.66, P�.001) with measures of knee extensor

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

Table 2. Regression Model for Differences Between People With Chronic Obstructive Pulmonary Disease (COPD Group) and People Who Were Healthy (Control Group) on Various Measuresa

Measure Control Group

(n�21)b COPD Group

(n�21)b

Mean Difference (95% Confidence

Interval) P

Stair Climb Power Test (W) 378.2 (121.3) 266.2 (80.3) 112.1 (74.2, 150) �.001

Torque (N�m)

Knee extensor muscle, concentric 80.3 (31.8) 55.7 (23.8) 24.6 (10.6, 38.6) .001

Knee extensor muscle, eccentric 144.4 (50.9) 98.5 (29.2) 45.9 (25.8, 66.1) �.001

Knee extensor muscle, isometric 74.8 (21.3) 100.2 (31.6) 25.3 (11, 39.7) .001

Knee flexor muscle, concentric 37.8 (15.4) 28.4 (10.9) 9.5 (1.3, 17.6) .024

Knee flexor muscle, eccentric 54.5 (19.7) 46.5 (17.1) 8 (�2, 18) .114

Functional performance

Timed “Up & Go” Test (s) 7.7 (1.1) 9.5 (2.3) 1.8 (0.7, 2.9) .002

Six-Minute Walk Test (m) 554.9 (65) 394.6 (64.1) 160.2 (117.4, 203) �.001

a The regression model was adjusted for age and body weight. b Data are means (standard deviations).

Muscle Strength and Functional Performance in People With COPD

1778 f Physical Therapy Volume 90 Number 12 December 2010

D ow

nloaded from https://academ

ic.oup.com /ptj/article-abstract/90/12/1774/2737794 by guest on 13 February 2020

muscle concentric, eccentric, and isometric torque in the control group. Knee flexor muscle concen- tric torque (r�.46, P�.03) and ec- centric torque (r�.61, P�.003) also showed moderate to strong associa- tions with the SCPT in the control group. In contrast, associations be- tween the SCPT and measures of torque in the COPD group tended to be more moderate and statistically significant only for eccentric (r�.53, P�.015) and isometric (r�.46, P�.043) knee extensor muscles and for eccentric knee flexor mus-

cles (r�.47, P�.043). The associa- tions between the SCPT and the var- ious torque measures are shown in Table 3.

When the relationships between the SCPT and measures of functional performance were explored (Fig. 2), the Pearson correlation analysis re- vealed notable differences between groups. In the COPD group, the SCPT was strongly associated with the 6MWT (r�.68, P�.001) and moderately associated with the TUG (r��.46, P�.04). In contrast, in the

control group, there was no signifi- cant relationship between the SCPT and either the 6MWT (r�.43, P�.052) or the TUG (r�.25, P�.28) (Fig. 2).

Discussion The main finding of this study was that compared with people who were healthy, people with moderate to severe COPD developed signifi- cantly lower power during the SCPT. Interestingly, although the SCPT was only moderately related to thigh muscle torque in the COPD group,

Table 3. Associations Between the Stair Climb Power Test and Various Measures of Muscle Torque in People Who Were Healthy (Control Group) and People With Chronic Obstructive Pulmonary Disease (COPD Group)a

Torque Measure

Pearson Correlation Coefficient, r (95% Confidence Interval), for:

Control Group (n�21)

COPD Group (n�21) Both Groups

Knee extensor muscle, concentric .81 (.58, .92)a .41 (�.02, .72) .74 (.56, .55)a

Knee extensor muscle, eccentric .86 (.68, .94)a .53 (.11, .79)b .84 (.72, .91)a

Knee extensor muscle, isometric .66 (.32, .85)a .46 (.02, .75)b .70 (.50, .83)a

Knee flexor muscle, concentric .46 (.04, .74)b .23 (�.24, .61) .48 (.20, .69)a

Knee flexor muscle, eccentric .61 (.24, .82)a .47 (.03, .75)b .57 (.32, .75)a

a Significantly associated at P�.01. b Significantly associated at P�.05.

Figure 2. Scatter plots showing associations of the Stair Climb Power Test with the Six-Minute Walk Test (left) and the Timed “Up & Go” Test (right) in people with chronic obstructive pulmonary disease (COPD group) (●) and people who were healthy (control group) (�). Regression lines with correlation coefficients for the COPD group (solid lines) and the control group (dashed lines) are shown.

Muscle Strength and Functional Performance in People With COPD

December 2010 Volume 90 Number 12 Physical Therapy f 1779

D ow

nloaded from https://academ

ic.oup.com /ptj/article-abstract/90/12/1774/2737794 by guest on 13 February 2020

this relationship was very strong in the control group. In contrast, al- though the SCPT was strongly to moderately related to the 6MWT and the TUG in the COPD group, weaker or nonsignificant relationships were found in the control group.

The lower power developed during the SCPT suggested that, like muscle strength,6 leg muscle power is possi- bly impaired in people with COPD. However, we did not formally assess leg muscle power and thus cannot confirm that the SCPT is associated with more-mechanistic measures of muscle power in people with COPD. Because muscle power and muscle strength are closely related,23 we hy- pothesized that the SCPT could be related to measures of knee extensor muscle torque in people with COPD. Of some surprise was the finding that knee extensor muscle torque in the COPD group was only moder- ately correlated with the SCPT, in contrast to the strong association in the control group. The high correla- tion of the SCPT with knee extensor muscle torque in the control group is consistent with the results of a pre- vious report11; in that study, the SCPT was found to be associated (r�.47) with leg muscle power in older adults, as assessed with iso- tonic pneumatic machines. In con- trast, the results of the present study indicated that the SCPT cannot be used as a surrogate for muscle strength in people with COPD.

The differences between groups in the relationships between muscle torque and the SCPT emphasized the importance of analyzing these unique relationships independently. A potential explanation for the dif- ferent degrees of association be- tween groups is that in people with COPD, factors other than muscle strength come into play during the SCPT. Indeed, our results indicated that during the performance of the SCPT, people with COPD were not

able to develop powerful muscle contractions in accordance with their torque levels. Because stair climbing is a complex functional task that requires the coordinated in- tegration of various body systems (ie, musculoskeletal, pulmonary, cardio- vascular, and vestibular systems),24 it is possible that other factors, such as ventilatory limitation, reduced pos- tural control, or even fear of falling,25

limited the performance of the SCPT in the COPD group. Perhaps the min- imal effects of these limiting factors on participants in the control group enhanced the capacity to develop powerful muscle contractions that were more closely dependent on muscle strength.

The COPD group also showed signif- icant deficits in functional perfor- mance, as assessed with the TUG and the 6MWT. These findings are con- sistent with the results of a previous study in which the investigators ex- amined whether the TUG and the 6MWT were sensitive enough to dis- criminate between fallers and non- fallers in a group of people with COPD.26 Compared with the results of the present study, however, the scores obtained in the TUG and the 6MWT in the fall risk study were approximately 25% lower. These re- sults may have been due to the higher level of impairments in the participants in the latter study, as reflected in their lower mean (SD) percentage of forced expiratory vol- ume in 1 second—predicted to be 41.5 (17)—and the use of supple- mental oxygen in 46% of their par- ticipants. In general, our results are in line with those of other studies in which investigators, using similar mobility tests, reported deficits in functional performance in people with COPD compared with people who were healthy.27–29

We explored the similarities and dif- ferences between the SCPT and other functional tests, including the

6MWT and the TUG. In view of a recent report that showed no associ- ation between the time required to climb 44 stairs and the distance covered in the 6MWT in people with severe COPD (mean forced expiratory volume in 1 second�33 [SD�13]),30 the strong association between the SCPT and the 6MWT in people with COPD was not ex- pected. Indeed, although the short and intense muscle contractions elic- ited during the SCPT indicate that this test requires mainly the activa- tion of anaerobic processes, the 6MWT has been traditionally used as the gold standard for the measure- ment of aerobic capacity.31 A plausi- ble explanation for the strong asso- ciation between the SCPT and the 6MWT is that, because of the func- tional impairments in people with COPD, the 6MWT imposes a high physiological demand on them.32 In- deed, a recent study confirmed that the same functional test (ie, the 6MWT) may provide different phys- iological information depending on the level of impairments in the participants.33

Although the scatter plot indicated a moderate relationship between the SCPT and the 6MWT (Fig. 2), no sig- nificant associations between the SCPT and measures of functional performance were found in the con- trol group. At face value, these data may appear to conflict with those of a previous study,33 which revealed a very strong inverse association (r��.83) between the time to com- plete the SCPT and the distance cov- ered in the 6MWT in older adults (mean age�72.7 years [SD�4.6]). An explanation for this disparity in findings is that power (which con- siders distance climbed and body weight), not the time to complete the SCPT, was calculated in the present study. When we correlated the time to complete the SCPT with the 6MWT in the control group, the association was similar to that

Muscle Strength and Functional Performance in People With COPD

1780 f Physical Therapy Volume 90 Number 12 December 2010

D ow

nloaded from https://academ

ic.oup.com /ptj/article-abstract/90/12/1774/2737794 by guest on 13 February 2020

reported in the previous study (r��.7, P�.001).33

Another potential explanation for the weak associations of the SCPT with the 6MWT and the TUG in the control group is that the high levels of power developed during the SCPT in this group contributed only mar- ginally toward the performance of these functional tests. For example, a recent study indicated that the ca- pacity to produce muscle power at a high velocity and a lower resistance is more important than simply the magnitude of muscle power or strength during the performance of a walking task.34 This finding is consis- tent with the analysis of outliers in our scatter plot (Fig. 2), which re- vealed that higher levels of power produced by 2 participants in the control group during the SCPT were not accompanied by greater dis- tances walked during the 6MWT. Thus, the relationship between the SCPT and other measures of func- tional performance, such as the 6MWT, may not follow a linear pat- tern.35 Instead, after a threshold of muscle power is achieved,36,37 fur- ther improvements in muscle power may not translate to better functional performance.38

Limitations The present study had some limita- tions. The most important limita- tions are the observational (cross- sectional) design of the study and the use of a convenience sample. Results from cross-sectional studies should be interpreted with caution because conclusions regarding cause and ef- fect based on clinical correlates can- not be drawn.15 Although our sam- ple size estimation was appropriate for detecting significant differences between groups in most measures, the low correlations between some measures might have been related to the relatively small number of partic- ipants. In addition, the use of a con- venience sample might have biased

motivation for participation (ie, se- lection bias).

Conclusions We explored the use of the SCPT, which is a functionally relevant test associated with leg muscle power in older adults.11 People with COPD showed deficits on the SCPT, as well as on measures of muscle strength and functional performance. Al- though our results indicated that this test cannot be used as a simple sur- rogate for muscle strength, the SCPT is a simple, safe, and yet informative alternative for assessing functional performance in people with COPD. The clinical relevance of this test is that, although most functional tests used in people with COPD tend to focus on measures of aerobic capac- ity (eg, walking tests), the SCPT em- phasizes the capacity to exert rapid contractions with the muscles of the lower limb. Detailed information re- garding muscle power deficits in people with COPD is not yet avail- able. Muscle power is particularly important in clinical evaluations be- cause it is more closely related to functional performance,39 postural control,40,41 and possibly fall preven- tion strategies (eg, fast anticipatory movements)42,43 than is muscle strength. Therefore, the SCPT may add some new clinical information to the characterization of functional impair- ments in people with COPD.

Larger studies assessing the nature of the relationships among measures of muscle strength, muscle power, and functional performance in people with COPD are needed. More stud- ies assessing cardiorespiratory re- sponses to the SCPT and the feasibil- ity of this test for people with more severe COPD also are needed. In ad- dition, the sensitivity and responsive- ness of the SCPT for detecting func- tional improvements after training interventions should be explored.

Dr Roig, Dr Eng, Dr Road, and Dr Reid pro- vided concept/idea/research design. Dr Roig, Dr Eng, and Dr Reid provided writing. Dr Roig and Dr MacIntyre provided data col- lection. Dr Roig and Dr Reid provided data analysis and project management. Dr Reid provided fund procurement and clerical sup- port. Dr Roig and Dr Road provided partici- pants. Dr MacIntyre and Dr Reid provided facilities/equipment. Dr Road and Dr Reid provided institutional liaisons. Dr MacIntyre, Dr Road, and Dr Reid provided consultation (including review of manuscript before sub- mission). The authors thank Jennifer Rurak for editing a preliminary version of the manuscript.

The Clinical Research Ethics Board of the University of British Columbia approved the study.

A poster with some data from this study was presented at the Annual Meeting of the American Thoracic Society; May 14 –19, 2010; New Orleans, Louisiana.

Dr Roig received support through a Strategic Training Fellowship in Rehabilitation Research from the CIHR Musculoskeletal and Arthritis Institute, a Fellowship in Respiratory Rehabili- tation from the BC Lung Association, and a Graduate Fellowship from the University of British Columbia. This project was supported by the Canadian Lung Association and the Canadian Physiotherapy Association.

This article was submitted March 10, 2010, and was accepted August 13, 2010.

DOI: 10.2522/ptj.20100091

References 1 Agusti AGN. Systemic effects of chronic

obstructive pulmonary disease. Proc Am Thorac Soc. 2005;2:367–370.

2 MacIntyre RN. Muscle dysfunction associ- ated with chronic obstructive pulmonary disease. Respir Care. 2006;51:840 – 852.

3 Mador MJ, Bozkanat E. Skeletal muscle dys- function in chronic obstructive pulmo- nary disease. Respir Res. 2001;2:216 –224.

4 Janaudis-Ferreira T, Wadell K, Sundelin G, Lindstrom B. Thigh muscle strength and endurance in patients with COPD com- pared with healthy controls. Respir Med. 2006;100:1451–1457.

5 Mathur S, Takai KP, MacIntyre DL, Reid WD. Estimation of thigh muscle mass with magnetic resonance imaging in older adults and people with chronic obstruc- tive pulmonary disease. Phys Ther. 2008; 88:219 –230.

6 Mathur S, MacIntyre DL, Forster BB, et al. Preservation of eccentric torque of the knee extensors and flexors in patients with COPD. J Cardiopulm Rehabil Prev. 2007;27:411– 416.

Muscle Strength and Functional Performance in People With COPD

December 2010 Volume 90 Number 12 Physical Therapy f 1781

D ow

nloaded from https://academ

ic.oup.com /ptj/article-abstract/90/12/1774/2737794 by guest on 13 February 2020

7 Decramer M, Gosselink R, Troosters T, et al. Muscle weakness is related to utiliza- tion of health care resources in COPD pa- tients. Eur Respir J. 1997;10:417– 423.

8 Swallow EB, Reyes D, Hopkinson NS, et al. Quadriceps strength predicts mortality in patients with moderate to severe chronic obstructive pulmonary disease. Thorax. 2007;62:115–120.

9 Debigare R, Maltais F. The major limitation to exercise performance in COPD is lower limb muscle dysfunction. J Appl Physiol. 2008;105:751–753.

10 Man WD, Moxham J, Polkey MI. Magnetic stimulation for the measurement of respi- ratory and skeletal muscle function. Eur Respir J. 2004;24:846 – 860.

11 Bean JF, Kiely DK, LaRose S, et al. Is stair climb power a clinically relevant measure of leg power impairments in at-risk older adults? Arch Phys Med Rehabil. 2007;88: 604 – 609.

12 Rabe KF, Hurd S, Anzueto A, et al. Global strategy for the diagnosis, management, and prevention of chronic obstructive pul- monary disease: GOLD executive summa- ry. Am J Respir Crit Care Med. 2007;176: 532–555.

13 Rodriguez-Roisin R. Toward a consensus definition for COPD exacerbations. Chest. 2000;117:398 – 401.

14 Pate RR, O’Neill JR, Lobelo F. The evolving definition of “sedentary.” Exerc Sport Sci Rev. 2008;36:173–178.

15 Portney LG, Watkins MP. Foundations of Clinical Research: Applications to Prac- tice. 2nd ed. Englewood Cliffs, NJ: Pren- tice Hall Inc; 2000.

16 American College of Sports Medicine. AC- SM’s Guidelines for Exercise Testing and Prescription. Philadelphia, PA: Lippincott, Williams & Wilkins; 2000.

17 Brusasco V, Crapo R, Viegi G. Coming to- gether: the ATS/ERS consensus on clinical pulmonary function testing. Eur Respir J. 2005;26:1–2.

18 Mathur S, Makrides L, Hernandez P. Test- retest reliability of isometric and isokinetic torque in patients with chronic obstruc- tive pulmonary disease. Physiother Can. 2004;56:94 –101.

19 Podsiadlo D, Richardson S. The Timed “Up & Go”: a test of basic functional mobility for frail elderly persons. J Am Geriatr Soc. 1991;39:142–148.

20 ATS Committee on Proficiency Standards for Clinical Pulmonary Function Laborato- ries. ATS statement: guidelines for the six- minute walk test. Am J Respir Crit Care Med. 2002;166:111–117.

21 Ross RM, Murthy JN, Wollak ID, Jackson AS. The six minute walk test accurately estimates mean peak oxygen uptake. BMC Pulm Med. 2010;10:31–39.

22 Ramsey FL, Schafer D. The Statistical Sleuth. 2nd ed. Pacific Grove, CA: Dux- bury; 2002.

23 Skelton DA, Greig CA, Davies JM, Young A. Strength, power and related functional ability of healthy people aged 65– 89 years. Age Ageing. 1994;23:371–372.

24 Startzell JK, Owens DA, Mulfinger LM, Ca- vanagh PR. Stair negotiation in older peo- ple: a review. J Am Geriatr Soc. 2000;48: 567–580.

25 Tiedemann AC, Sherrington C, Lord SR. Physical and psychological factors associ- ated with stair negotiation performance in older people. J Gerontol A Biol Sci Med Sci. 2007;62:1259 –1265.

26 Beauchamp MK, Hill K, Goldstein RS, et al. Impairments in balance discriminate fall- ers from non-fallers in COPD. Respir Med. 2009;103:1885–1891.

27 Butcher SJ, Meshke JM, Sheppard MS. Re- ductions in functional balance, coordina- tion, and mobility measures among pa- tients with stable chronic obstructive pulmonary disease. Cardiopulm Rehabil. 2004;24:274 –280.

28 Eisner MD, Blanc PD, Yelin EH, et al. COPD as a systemic disease: impact on physical functional limitations. Am J Med. 2008;121:789 –796.

29 Eisner MD, Iribarren C, Yelin EH, et al. Pulmonary function and the risk of func- tional limitation in chronic obstructive pulmonary disease. Am J Epidemiol. 2008; 167:1090 –1101.

30 Dreher M, Walterspacher S, Sonntag F, et al. Exercise in severe COPD: is walking different from stair-climbing? Respir Med. 2008;102:912–918.

31 Solway S, Brooks D, Lacasse Y, Thomas S. A qualitative systematic overview of the measurement properties of functional walk tests used in the cardiorespiratory domain. Chest. 2001;119:256 –270.

32 Marquis N, Debigare R, Bouyer L, et al. Physiology of walking in patients with moderate to severe chronic obstructive pulmonary disease. Med Sci Sports Exerc. 2009;41:1540 –1548.

33 Bean JF, Kiely DK, Leveille SG, et al. The 6-minute walk test in mobility-limited el- ders: what is being measured? J Gerontol A Biol Sci Med Sci. 2002;57:M751–M756.

34 Cuoco A, Callahan DM, Sayers S, et al. Im- pact of muscle power and force on gait speed in disabled older men and women. J Gerontol A Biol Sci Med Sci. 2004;59: 1200 –1206.

35 Ferrucci L, Guralnik JM, Buchner D, et al. Departures from linearity in the relation- ship between measures of muscular strength and physical performance of the lower extremities: the Women’s Health and Aging Study. J Gerontol A Biol Sci Med Sci. 1997;52:M275–M285.

36 Manini TM, Visser M, Won-Park S, et al. Knee extension strength cutpoints for maintaining mobility. J Am Geriatr Soc. 2007;55:451– 457.

37 Ploutz-Snyder LL, Manini T, Ploutz-Snyder RJ, Wolf DA. Functionally relevant thresh- olds of quadriceps femoris strength. J Ger- ontol A Biol Sci Med Sci. 2002;57:B144 – B152.

38 Buchner DM, Larson EB, Wagner EH, et al. Evidence for a non-linear relationship be- tween leg strength and gait speed. Age Ageing. 1996;25:386 –391.

39 Bean JF, Leveille SG, Kiely DK, et al. A comparison of leg power and leg strength within the InCHIANTI study: which influ- ences mobility more? J Gerontol A Biol Sci Med Sci. 2003;58:728 –733.

40 Orr R, de Vos NJ, Singh NA, et al. Power training improves balance in healthy older adults. J Gerontol A Biol Sci Med Sci. 2006; 61:78 – 85.

41 Horak FB, Nashner LM. Central program- ming of postural movements: adaptation to altered support-surface configurations. J Neurophysiol. 1986;55:1369 –1381.

42 Skelton DA, Kennedy J, Rutherford OM. Explosive power and asymmetry in leg muscle function in frequent fallers and non-fallers aged over 65. Age Ageing. 2002;31:119 –125.

43 Evans WJ. Exercise strategies should be designed to increase muscle power. J Gerontol A Biol Sci Med Sci. 2000;55: M309 –M310.

Muscle Strength and Functional Performance in People With COPD

1782 f Physical Therapy Volume 90 Number 12 December 2010

D ow

nloaded from https://academ

ic.oup.com /ptj/article-abstract/90/12/1774/2737794 by guest on 13 February 2020