Reliability of quadriceps muscle power and explosive force, and relationship to physical function in people with chronic obstructive pulmonary disease an observational prospective multicenter study (Physiotherapy

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ISSN: 0959-3985 (Print) 1532-5040 (Online) Journal homepage: https://www.tandfonline.com/loi/iptp20

Reliability of quadriceps muscle power and explosive force, and relationship to physical function in people with chronic obstructive

pulmonary disease: an observational prospective multicenter study

Kim-Ly Bui, Nathalia Maia, Didier Saey, Gail Dechman, François Maltais, Pat G Camp & Sunita Mathur

To cite this article: Kim-Ly Bui, Nathalia Maia, Didier Saey, Gail Dechman, François Maltais, Pat G Camp & Sunita Mathur (2019): Reliability of quadriceps muscle power and explosive force, and relationship to physical function in people with chronic obstructive pulmonary disease:

an observational prospective multicenter study, Physiotherapy Theory and Practice, DOI:

10.1080/09593985.2019.1669233

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Published online: 19 Sep 2019.

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Reliability of quadriceps muscle power and explosive force, and relationship to physical function in people with chronic obstructive pulmonary disease: an observational prospective multicenter study

Kim-Ly Bui, PT, MSc â, Nathalia Maia, BScPT, MSc^b, Didier Saey, PT, PhDâ, Gail Dechman, PT, PhD^c, François Maltais, MDâ, Pat G Camp, PT, PhD^d,e, and Sunita Mathur, PT, PhD ^b

aCentre de Recherche, Institut universitaire de cardiologie et pneumologie de Québec, Université Laval, Québec, Canada;^bDepartment of Physical Therapy, University of Toronto, Toronto, Canada;^cSchool of Physiotherapy, Dalhousie University, Halifax, Canada;^dCentre for Heart and Lung Innovation, University of British Columbia and St. Paul’s Hospital, Vancouver, Canada;^eDepartment of Physical Therapy, University of British Columbia, Vancouver, Canada

ABSTRACT

Background: Muscle power declines with age and is a stronger determinant of physical function than strength. Muscle power using computerized dynamometry has not been investigated in COPD.

Objectives: To determine: 1) test-retest reliability of quadriceps power using a standardized protocol with computerized dynamometry; and 2) associations between quadriceps strength and power, and functional capacity.

Design/Setting: Prospective observational study in four Canadian research labs.

Participants: People with mild to very severe COPD.

Methods: Tests were conducted on two days. Quadriceps muscle maximal strength was evaluated during a static maneuver using maximal voluntary isometric contractions (MVIC). Rate of torque development (RTD) during MVIC was used to assess explosive force. Muscle power was measured using a dynamic, isotonic protocol from which peak and average power and peak velocity were derived. Functional capacity was assessed with the Short Physical Performance Battery (SPPB).

Reliability was assessed using intraclass correlation coefficients (ICC), standard error of measurements (SEM), and Bland Altman plots. Spearman and Pearson correlation coefficients were used for associations.

Results: 65 patients (age 69 ± 8 years; FEV₁48 ± 21% of predicted) were included. ICC was 0.77 for RTD and 0.87–0.98 for isotonic power measures (95%CI 0.63–0.99, p < .001); SEM < 10% for average/peak power and peak velocity, and > 30% for RTD. SPPB had moderate correlation with average power, but not with MVIC or RTD.

Conclusion: The standardized isotonic protocol with computerized dynamometry was reliable in assessing quadriceps power in COPD. Our data highlights that average power correlates best with functional capacity, indicating higher relevance than static measures when investigating determinants of function.

ARTICLE HISTORY Received 17 December 2018 Revised 24 May 2019 Accepted 15 August 2019 KEYWORDS

Chronic obstructive pulmonary disease; muscle power; reproducibility of results; isotonic contractions;

physical functional performance

Background

Limb muscle dysfunction is an important systemic con- sequence of chronic obstructive pulmonary disease (COPD) because it is closely related to important clinical outcomes such as exercise intolerance, reduced quality of life and increased mortality risk (Maltais et al., 2014;

Swallow et al., 2007). Muscle strength, defined as the ability of a muscle to generate force (Robles et al.,2011), has been well described in people with COPD (Mathur et al.,2018; Swallow et al.,2007). Functional limitations as well as low levels of physical activity are associated with low muscle strength (Bernabeu-Mora et al.,2017; Rausch Osthoff et al.,2013). Muscle power, defined as the ability

to produce energy (in Joules) in a short period of time, can be expressed by the product of force and velocity (Reid and Fielding,2012), and is another important determinant of muscle function. Interestingly, muscle power shows a steeper decline with aging than muscle strength (Izquierdo et al.,1999). This decline in muscle power may be linked to the neuromuscular changes that occur with aging such as the loss of size of type II (fast-twitch) muscle fibers, a reduction in the number of spinal motor neurons and excitable motor units, and a decrease in contraction time (Aagaard et al.,2010; Lexell, Taylor, and Sjöström, 1988). In addition, muscle power has been shown to be more strongly associated with functional capacity and

CONTACTSunita Mathur [email protected] Department of Physical Therapy, University of Toronto, 160-500 University Ave, Toronto, ON M5G 1V7, Canada.

https://doi.org/10.1080/09593985.2019.1669233

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mobility, such as gait speed and balance, than muscle strength in older adults (Accettura, Brenneman, Stratford, and Maly,2015; Bean et al.,2002b,2007,2003;

Cadore et al., 2014; Reid and Fielding,2012; Van Roie et al., 2011). Physical function and balance are often impaired in people with COPD to a greater extent than age-matched controls (Beauchamp et al.,2009,2012; Bui, Nyberg, Maltais, and Saey,2017; Eisner et al.,2008; Gulart et al., 2015; Karpman and Benzo, 2014; Oliveira et al., 2013) and may contribute to some restrictions in per- forming activities of daily living (Butcher, Meshke, and Sheppard, 2004). Indeed patients with COPD present deficits in systems underlying balance control (i.e. postural stability, anticipatory and reactive balance control) and show a delayed reaction time for balance recovery, which put them at higher risk of falls than older adults (Beauchamp et al.,2012). Non fallers with COPD perform the Timed-Up and Go test in a mean time of 14s compared to 17s in fallers with COPD, which is longer than the usual cutoff predicting falls in the older population (Beauchamp et al.,2009). COPD was also shown to be associated with 9% reduction in mean lower extremity function assessed with the Short Physical Performance Battery (SPPB), and 20% reduction in the 6 minute walk distance when compared with a matched reference group after controlling for sex, race, height, smoking history and education (Eisner et al.,2008). Thus assessing the association of muscle power with a composite functional test which incorporates various aspects of physical function is relevant in COPD. To this extent, the SPPB is a valid and reliable test used to characterize functional limitations in older adults, assessing balance, gait speed and the ability to rise from a chair (Bernabeu-Mora et al.,2015; Guralnik et al.,1994; Medina-Mirapeix et al.,2016; Trombetti et al., 2016).

Various protocols can be used to evaluate how rapidly the neuromuscular system produces force.

Rate of torque development (RTD) from isometric contractions is a measure that indicates explosive force, defined by Maffiuletti and colleagues as“the ability to increase torque as quickly as possible during a rapid voluntary contraction realized from a low or resting level”. More specific measures of rapid force development includes measures of muscle power, such as peak or average power during isokinetic and isotonic contractions (Reid and Fielding,2012), and surrogate measures such as peak velocity during isotonic contractions (Maffiuletti et al., 2016; Sleivert and Wenger, 1994;

Stauber, Barill, Stauber, and Miller,2000; Webber and Porter,2010).

Muscle power can also be estimated using functional tests such as the Stair Climb Power Test (SCPT), which was initially developed in community dwelling older

adults with mobility limitations (Bean et al., 2002a).

In people with COPD, there have been limited studies evaluating muscle power or explosive force. One study investigated the relationships between physical activity and muscle power using a bilateral leg press exercise at 50% of one-repetition maximum (Hernandez et al., 2017), and two studies used the SCPT to investigate its associations with muscle strength and functional performance (Butcher et al., 2012; Roig et al., 2010).

To our knowledge, no studies to date have utilized computerized dynamometry, considered as the gold standard tool to measure muscle performance, to evaluate muscle power nor explosive force in COPD. The objectives of this study were to determine: 1) test-retest reliability of quadriceps muscle power (i.e. peak and average power, peak velocity) and explosive force (rate of torque development) using a computerized dynamometer; and 2) the association between quadriceps muscle maximal strength (isometric), muscle power and explosive force, and physical function using the SPPB in people with COPD.

Methods Design

This is a sub-study from a prospective, observational, multicenter trial conducted in four sites across Canada between May 2016 and January 2018 (Bui et al., 2019).

Ethical approval from the Research Ethics Boards of each participating site were obtained and all participants signed an informed consent form before study participa- tion. Participants attended two sessions held two to seven days apart. Demographic and clinical characteristics were obtained at the first visit, as previously described: age, smoking index, spirometry, anthropometric measures using bioelectrical impedance analysis, and the modified Medical Research Council Dyspnea Scale score. At each session, SPPB, muscle maximal strength, and explosive force, and muscle power were assessed consistently in this order (Bui et al.,2019).

Sample population

Patients between 40 and 89 years of age, with a medical diagnosis of mild to very severe COPD were included.

Participants with any condition that could alter maximal strength performance (i.e. recent immobilization, severe neurologic, psychiatric/cognitive or musculoskeletal conditions, involvement in a muscle strengthening program in the previous 3 months, and corticosteroid injections or oral corticosteroids use in the previous three months) or with a body mass index above 35 kg/m²were excluded

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from the study (Bui et al.,2019). Participant recruitment was performed through each center’s patient database, recruitment posters, and/or support groups/maintenance pulmonary rehabilitation programs.

Assessments

A detailed manual of standard operating procedures was provided to all sites in order to ensure consistency in the instructions and assessments among sites. Data collectors at three sites were trained by an experienced investigator (SM) for computerized dynamometry assessments; evaluators at the coordinating site were already experienced with all the procedures. Printout and raw data from pilot tests performed independently at each site were then sent to the training investigator (SM) for quality review.

Evaluation of quadriceps muscle maximal strength, explosive force, and power

Quadriceps muscle maximal strength, explosive force and power were assessed on the right leg, using the Biodex System 4 or System Pro 4 computerized dynamometer (Shirley, New York, US). Raw data signals for torque, position and velocity were sampled at 100 Hz and saved as text files using the Biodex software.

Muscle maximal strength was evaluated using an static standardized protocol (Mathur, Makrides, and Hernandez,2004), with five trials of maximal voluntary isometric contractions (MVIC) performed at 90 degrees knee flexion, separated by one-minute rest breaks, as previously described (Bui et al.,2019). No gravity correction was needed in this protocol as the weight of the limb at this angle did not influence the torque.

A dynamic, isotonic knee extensor protocol was administered at least ten minutes following the MVIC assessment, to minimize fatigue. The isotonic load was set at 20% of the highest MVIC. The range of motion was set from approximately 90–95 degrees of knee flexion to full extension. The participants performed a warm-up set of three isotonic contractions, followed by two sets of ten maximal isotonic contractions at the pre-set load (Accettura, Brenneman, Stratford, and Maly, 2015). A two-minute rest was given between sets. Participants were instructed to push as“hard and fast” as possible during the isotonic quadriceps contractions and strong verbal encouragement was given throughout the trials. Knee flexion back to the starting position was passively ensured by the computerized dynamometer between quadriceps contractions.

Gravity correction was applied using the computerized

dynamometer software (i.e. limb weight correction in the Biodex software).

The muscle explosive force measure, RTD, expressed in Newton-meters per second (Nm/s), was calculated from the raw data signals of torque and time from the MVIC trials. Using MatLab™ software, the torque at 40% and 80% of peak were identified and divided by the time (in seconds) between these two points:

RTD40-80% (Nm/s) = (torque80% – torque40%)/

(time_80%-time_40%) (Kiriella et al., 2018; Webber and Porter,2010).

Power measures obtained from the isotonic protocol were: 1) average power (i.e. average over a full knee extension range of motion, in Watts); 2) peak velocity (i.e. the highest velocity achieved during the knee extension, in degrees/s); and 3) peak power (i.e. the highest power achieved during a single isotonic contraction, in Watts). The first two measures were obtained directly using the Biodex “reports” function, and the average for the two sets was used for analysis.

Peak power was calculated from the raw data signals for torque, position and velocity, using AcqKnowledge software (version 3.1, Biopac Systems Inc), where power (Watts) = torque (Newton-meters) x angular velocity (radians/s). Peak power was calculated as the highest power achieved for each of the middle five contractions in each set of ten contractions. The middle five contraction were chosen since peak power tends to plateau during this period (Sheppard et al.,2019). Then the mean of the three highest peak power values for each set was calculated. The mean of the peak power values from the two sets was used for further analysis (Sheppard et al., 2019).

Functional capacity

Functional capacity was assessed with the SPPB at least fifteen minutes before strength and power testing.

Evaluators followed the National Institute on Aging protocol (http://hdcs.fullerton.edu/csa/Research/docu ments/SPPBInstructions_ScoreSheet.pdf) as described by Guralnik et al. (1994). The SPPB includes three separate standardized tasks: 1) static balance (i.e. standing with feet together, semi-tandem and tandem positions); 2) usual gait speed (meters/second); and 3) five-time sit-to- stand (time, in seconds). According to the tasks performance, an ordinal score between 0 to 4 is given to each of them, and then summed to provide a total score out of twelve, with higher scores representing better performance (Guralnik et al.,1994). Balance ordinal score, the fastest of two gait speed trials and sit-to-stand time were also individually reported and used for the correlation analyses with muscle function.

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Statistical analysis

Sample size was based on Hopkins (2000) estimates for reliability studies, with an aim to recruit and test a minimum of 50 participants. All statistical analyses were performed on SPSS software (IBM^® Corporation, SPSS statistics, version 22). The variables were described using means and standard deviations. Homogeneity of variance for muscle maximal strength, explosive force and power measures was confirmed with Levene’s test.

Level significance of ? = 0.05 was used for all analysis.

Reliability analysis was conducted using intraclass correlation coefficients (ICC, two-way mixed model, absolute agreement), standard error of the measurement (SEM) and Bland-Altman plots. The SEM is expressed in the same unit than the measure for which it is calculated. It represents a measure of absolute reliability indicating how much a score varies from its “true” score in repeated measurements, with higher values representing poorer reliability (Domholdt, 2000). SEM was calculated using the following equation: SEM = [√((X – Y)²/(Z-1))

*√(1-ICC)]; where: √ = square root; X = measurement;

Y = average of all measurements; Z = number of participants; ICC = intraclass correlation coefficient

SEM expressed in percentage (SEM/total mean x 100) was also used for comparisons between measurements.

Bland-Altman plot was used to show agreement between measurements and helps investigate whether there is a relationship between the measurement errors and the true value (Bland and Altman,1986).

Spearman correlation coefficients were used to analyze the associations between muscle maximal strength, explosive force, and power variables with SPPB total and balance scores (ordinal data), and Pearson correlation coefficients were used for associations between muscle function and SPPB other subcomponents (i.e. usual gait speed and sit-to-stand time).

Results

Sixty-five participants were tested in total. One subject was excluded only from the RTD analysis, and another from the peak power analyses due to techni- cal errors with raw data files. Another subject was

excluded specifically from the average power and peak velocity analyses due to loss of data. Thus, a total of 64 participants’ data were analyzed for each power variable.

Sample characteristics

Subject characteristics are presented in Table 1 (Bui et al.,2019).

Test-retest reliability

Isotonic quadriceps power is reported inTable 2 and demonstrated high reliability for all variables (average power ICC = 0.98, peak velocity ICC = 0.87, peak power ICC = 0.98). Low SEM (< 10% of the mean) demonstrated high absolute reliability for the same three variables (Table 2). However, only moderate reliability was found for RTD derived from the isometric contraction (ICC = 0.77; Table 2, and with a SEM of 30% of total mean). Bland-Altman plots demonstrated less agreement between days and greater bias for the RTD explosive force variable than for isotonic power measures (Figure 1a-d).

Average power showed the highest agreement between visits (Figure 1b).

Table 1.Subject characteristics.

People with COPD (n = 65)

Age (years) 69 ± 8

Sex (M/W) 31/34

Body mass index (kg*m⁻²) 26 ± 5

Smoking history (pack-years) 41.0 ± 25.5

Percent body fat (%; n = 46) 35.3 ± 8.5

FVC (L) 2.7 ± 0.9

FVC (% predicted) 81 ± 24

FEV1(L) 1.2 ± 0.5

FEV1(% predicted) 48 ± 21

FEV1/FVC (%) 46 ± 15

GOLD classification of airflow obstruction (1/

2/3/4)

5/24/22/14 mMRC dyspnea scale (0/1/2/3/4) 0/16/21/17/6 Data are presented in mean ± SD or number of observations.

Abbreviations: M: men; FEV1: forced expiratory volume in one second;

FVC: forced vital capacity; GOLD: Global initiative for chronic obstructive lung disease; mMRC: modified Medical Research Council; W: women

Table 2.Test-retest reliability of quadriceps muscle explosive force and power measures (n = 64).

Variable

Day 1

(mean ± SD) Day 2 (mean ± SD)

Mean

(mean ± SD) ICC* SEM SEM (% of grand mean)

RTD40-80%(Nm/s) 176 ± 111 158 ± 102 167 ± 96 0.77 [0.63–0.86] 50.3 30.2

Average power (Watts) 84 ± 52 90 ± 53 87 ± 52 0.98 [0.95–0.99] 7.4 8.5

Peak power (Watts) 255 ± 137 266 ± 143 260 ± 139 0.98 [0.97–0.99] 17.1 6.6

Peak velocity (deg/s) 321 ± 55 329 ± 50 325 ± 49 0.87 [0.78–0.92] 18.9 5.8

RTD40-80%= rate of torque development between 40 and 80% of peak torque; ICC = intraclass correlation coefficient; SEM = standard error of measurement;

*all ICC values were significant atp< 0.001.

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Correlations between muscle function measures and functional capacity

All correlations of maximal strength, explosive force, and power measures with SPPB total score and its components are presented in Table 3. SPPB total score, balance score, gait speed and sit-to-stand time (mean ± SD) were: 10.7 ± 1.6 points, 3.6 ± 0.8 points, 1.0 ± 0.2 m/s, and 11.5 ± 3.5 s, respectively. 44% of patients attained the maximum score of 12 points on the SPPB. Isometric maximal strength did not correlate significantly with gait speed and the sit-to-stand time, and showed only a low significant correlation with balance score (r = 0.278, p = .025). Explosive force measures from isometric testing did not correlate significantly with any functional measures. Peak velocity

and average power demonstrated weak to moderate significant correlations with all functional measures, with average power demonstrating the highest correlations (Table 3).

Discussion

To our knowledge, this is the first study investigating test- retest reliability of isotonic quadriceps power measures using computerized dynamometry, and their relationships with physical function using a multi-component functional test in people with COPD. Our data showed that isotonic muscle power measures were highly reliable in people with COPD, with peak velocity, and peak and average power showing SEM below 10%. Dynamic power, represented by average power and peak velocity, were significantly related to function while static muscle maximal strength (MVIC) and explosive force measures (RTD) were not. These results suggest the use of dynamic power measures in future research investigating determinants of physical function in people with COPD.

Test-retest reliability

Our results demonstrate high reliability of dynamic power variables obtained using a standardized isotonic protocol in people with COPD. These findings are similar to those shown in older adults, with ICCs vary- ing between 0.81 and 0.98 for dynamic quadriceps power measures using dynamometry (Callahan et al., 2007; Hartmann, Knols, Murer, and De Bruin, 2009;

Maffiuletti et al., 2007; Van Driessche, Van Roie, Van Wanseele, and Delecluse, 2018). A recent validation Figure 1.Bland-Altman plots for (a) Rate of Torque Development 40-80% (b) Average Power (c) Peak Velocity (d) Peak Power.

Table 3.Pearson and Spearman correlation coefficients between isometric quadriceps strength explosive force, and power measures, and functional capacity.

SPPB total score

Balance score

Gait speed (m/s)

5STS time (s)

MVIC (Nm) r-value .243 .278* .227 −.159

p-value .051 .025 .069 .205

RTD r-value .150 .231 .193 −.129

p-value .237 .067 .126 .308

Average power (watts)

r-value .457** .403** .321** −.322**

p-value .000 .001 .009 .009

Peak Power (watts)

r-value .370** .349** .321** −.237

p-value .003 .005 .010 .060

Peak velocity (deg/s)

r-value .308* .262* .293* −.334**

p-value .013 .035 .018 .007

In gray: Spearman correlation coefficients

**Correlation is significant at the 0.01 level (2-tailed); *Correlation is significant at the 0.05 level (2-tailed). Abbreviations: 5STS: Five-repetition sit- to-stand; s:second; SPPB: Short Physical Performance Battery; m/s: meter/

second; Nm: Newton-meter; deg/s: degree/second; MVIC: isometric maximal voluntary contraction; RTD: rate of torque development between 40 and 80% of peak MVIC in isometric assessment

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study examined the test-retest reliability of quadriceps power during isotonic contractions at 25% of MVIC, in 63 older adults with a similar age range as our sample (Van Driessche, Van Roie, Van Wanseele, and Delecluse, 2018). Similar to our findings, they also reported ICCs of greater than 0.90 and SEMs less than 10% of the mean for peak velocity and peak power. Moreover another study reported similar findings for isotonic dorsi- and plantar flexion power assessments in older women (Webber and Porter, 2010).

Our findings for the reliability of RTD resemble findings from previous studies in healthy adults (ICCs of 0.61 to 0.85; SEM of 31.5% of mean) (Mentiplay et al.,2015; Sleivert and Wenger,1994). Its lower reliability may be related with limitations of the measure such as the ability of participants to focus on pushing both “fast” and “as hard as possible” during an isometric test. Some participants may have even started to contract before the dynamometer recording which could result in a steeper ramp initially and contribute to variability in the data. These limitations may be overcome by providing more practice trials.

Furthermore, Mentiplay et al. (2015) reported that the reliability of RTD measurements may be improved when it is calculated across successive time intervals (e.g. change of force between contractions at 100 ms and 50 ms) instead of between percentages of peak torque. The variation in our RTD measures may have been reduced if we had had a higher sampling rate and used this approach in our analysis. Despite its known lower reliability (Maffiuletti et al.,2016), this marker of muscle explosive force could be used to estimate the ability of the neuromuscular system to rapidly produce force when only isometric testing is available.

Associations between muscle function and physical function

We assessed dynamic power using average and peak power, and peak velocity during isotonic testing. These power measures showed low to moderate correlations with functional capacity, whereas no significant correlations were found with static explosive force using RTD. It could be argued that the lower reliability of RTD could have had an impact on the strength of associations with functional capacity. The variable showing the highest correlations with SPPB total score and its components was average power (r = 0.32 to 0.41), a dynamic measure of muscle function which presents similar physiological requirements than cer- tain functional activities. Indeed, activities such as walking require muscles to work over a dynamic

range of motion at the joint while resisting a constant load (limb’s weight and the gravity) (Cairns, Knicker, Thompson, and Sjogaard,2005; Cheng and Rice,2005).

Power calculated using the Stair Climb Power Test (SCPT) has also been associated with isokinetic con- centric and eccentric quadriceps strength (r = 0.41 to 0.79 and r = 0.718, respectively) (Butcher et al., 2012;

Roig et al., 2010) and with the six-minute walk and Timed Up and Go tests (r = 0.68 and r =−0.46, respectively). In older men with COPD, six-minute walk distance was moderately correlated with power assessed using a bilateral one-repetition maximum (1RM) leg- extension test at 50% of 1RM, but not at 70% of 1RM (Hernandez et al.,2017). These stronger correlations at lower percent of maximal strength relate to our results since we assessed power at 20% of MVIC. Moreover, our results are also consistent with those found in frail older adults where there was a moderately strong, negative correlation between quadriceps power at 30 and 60% of 1RM, and usual gait speed and the ability to rise from a chair (Casas-Herrero et al.,2013).

A low but significant correlation was found between MVIC and balance score from the SPPB, but not with other functional tests. These results are consistent with our previous study in which quadriceps strength obtained with handheld dynamometry was also weakly associated with balance (r = 0.32) but not with SPPB total score (Bui et al.,2019). These findings suggest that static maximal strength assessment regardless of the instrument used does not reflect the functional limitations of people with COPD. Also, despite the relevance of assessing balance in COPD, the balance sub-score of the SPPB has not been validated as a stand-alone component, and only allows for scoring between 0 to 4, which limits interpretation of its associations with muscle function variables.

Limitations

The participants in our study were functioning at a high level. Indeed the mean SPPB score was 11/12 and 44% of the participants scored 12/12 on the test.

Therefore a ceiling effect could explain the absence of a correlation between most functional variables and quadriceps strength, as well as the low to moderate strength of other correlations. The quadriceps muscle is not the sole contributor to the performance of functional activities, possibly explaining the low to moderate correlations between quadriceps power and function. Assessing other muscle groups such as hip extensors and ankle plantar- and dorsi-flexors should be also relevant when investigating associations

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between lower limb muscle function and physical function.

Despite the high reliability of our quadriceps power measures and the associations with physical function, this assessment method is not easily translated into the clinical setting as computerized dynamometry is typically most of the time used in laboratory and research settings.

Functional power tests such as the SCPT may be more applicable in the clinical setting, and should be validated against gold-standard dynamometry measures for the COPD population.

Finally, another limitation is the analysis method we used for the RTD calculations at 40-80% of peak torque instead of at specific time windows, such as 0-50 ms or 0-100 ms. As we had a low sampling rate of 100 Hz, it was not possible to reliably analyze these early time windows. This might have had an impact on our associations with functional capacity since Maffiuletti et al.

(2016) had previously reported in a narrative review studies which used time windows for their analysis of explosive force found associations between RTD and 5-meter sprint time in strength-trained athletes (Tillin, Pain, and Folland, 2013), and self-reported functional limitations assessed with the Knee Outcome Survey Activities of Daily Life Scale questionnaire in people with total knee arthroplasty (Maffiuletti, Bizzini, Widler, and Munzinger,2010).

Conclusion

Dynamic power measures, assessed with a standardized isotonic protocol on a computerized dynamometer, were reliable and significantly correlated with physical function in COPD. Among the different measures, average power was the power measure showing the best reliability and the stronger association with mobility and function. Assessments targeting specific muscle type of contraction (e.g. isotonic vs isometric) when investigating determinants of physical function are encouraged in future studies. Functional tests should be preferred to muscle function assessments when specific documentation of functional status is wanted.

Declaration of Interest

The authors declare no conflict of interest.

Funding

This work was supported by the Canadian Lung Association.

ORCID

Kim-Ly Bui http://orcid.org/0000-0001-7458-6524 Sunita Mathur http://orcid.org/0000-0001-5564-9796

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