Muscular fitness as a mediator of quality cardiopulmonary resuscitation

    Muscular fitness as a mediator of quality cardiopulmonary resuscitation A. L´opez-Gonz´alez PhD, M. S´anchez-L´opez PhD, A. Garcia-Hermoso PhD, J. L´opez-Tendero MSci, J. Rabanales-Sotos PhD, V. Mart´ınez-Vizca´ıno MD, PhD PII: DOI: Reference:

S0735-6757(16)30274-1 doi: 10.1016/j.ajem.2016.06.058 YAJEM 55917

To appear in:

American Journal of Emergency Medicine

Received date: Revised date: Accepted date:

7 April 2016 13 June 2016 13 June 2016

Please cite this article as: L´ opez-Gonz´ alez A, S´anchez-L´opez M, Garcia-Hermoso A, L´ opez-Tendero J, Rabanales-Sotos J, Mart´ınez-Vizca´ıno V, Muscular fitness as a mediator of quality cardiopulmonary resuscitation, American Journal of Emergency Medicine (2016), doi: 10.1016/j.ajem.2016.06.058

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ACCEPTED MANUSCRIPT Title: Muscular fitness as a mediator of quality cardiopulmonary resuscitation Short running title: Muscular fitness and cardiopulmonary resuscitation

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Author list: López-González, A. PhD School of Nursing, University of Castilla-La Mancha, Albacete, Spain E-mail: [email protected]

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Sánchez-López, M. PhD University of Castilla-La Mancha, Health and Social Research Centre, Cuenca, Spain. E-mail: [email protected]

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Garcia-Hermoso, A. PhD Laboratorio de Ciencias de la Actividad Física, el Deporte y la Salud, Facultad de Ciencias Médicas, Universidad de Santiago de Chile, USACH, Chile E-mail: [email protected]

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López-Tendero, J. MSci University of Castilla-La Mancha, Health and Social Research Centre, Cuenca, Spain. E-mail: [email protected]

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Rabanales-Sotos, J. PhD University of Castilla-La Mancha, Health and Social Research Centre, Cuenca, Spain E-mail: [email protected]

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Martínez-Vizcaíno, V. MD, PhD University of Castilla-La Mancha, Health and Social Research Centre, Cuenca, Spain. Universidad Autónoma de Chile, Facultad de Ciencias de la Salud, Talca, Chile.

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E-mail: [email protected] Corresponding author information: Ángel López González E-mail: [email protected] School of Nursing, University of Castilla-La Mancha, Albacete, Spain Edificio Benjamín Palencia, 02071 Albacete Teléfono: + (34) 969 599200 Extensión: 2717. Fax: + (34) 967 599267 Keywords: Cardiac arrest, first responders, resuscitation Word count: 2597

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Keywords: cardiac arrest, first responders, resuscitation

Author’s contribution: All authors have made substantial contributions to all of the

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following: (1) the conception and design of the study, or acquisition of data, or analysis and interpretation of data; (2) drafting the article or revising it critically for important intellectual content; (3) final approval of the version to be submitted.

ACCEPTED MANUSCRIPT Introduction The out-of-hospital cardiac arrest poses a challenge to the health system. Survival rates after these events mainly depend only on effective chest compressions, early defibrillation, and

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advanced life support [1]. There is consensus that high quality cardiopulmonary resuscitation

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(CPR) is associated with a higher survival rate and remains essential to improving outcomes [2]. The technique for effective external chest compression (ECC) and positive pressure ventilation

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is periodically reviewed by international organizations [1,2].

Authors indicate that the rescuer’s physical fatigue decreases the quality of ECC a few

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minutes after the start of the CPR [3-8]. Weight status has been also associated with the quality of ECC, such that those who are underweight perform CPR worse than those with normal weight or excess weight [4,5,9,10]. High levels of muscle strength are positively correlated with the number of adequate ECCs performed [5,10,11]. On the other hand, weight status has been

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related to muscle strength. Increases in body mass index (BMI) are associated with improved performance in physical fitness tests that do not involve lifting the body, such as dynamometry

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[12-14]. In this way, muscle strength might be a potential mediator or play a confounding role

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in the analysis of the relationship between weight status and adequate quality of the ECC. Mediation analyses are the statistical procedures usually employed in order to clarify the

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relationship between two variables and how this relationship can be modified, mediated or confounded by a third variable. When the third variable (the mediator) carries the influence of a given independent variable on a given dependent variable, the effect of mediation occurs [15]. To our knowledge, no study has examined the mediator role of muscle strength on the relationship between BMI and adequate ECC parameters by mediation analysis. The objectives of this study were twofold: 1) to analyse, in college students, the relationship between BMI and muscle strength with adequate ECC parameters; and 2) to examine whether the association between BMI and adequate ECC parameters is mediated by muscle strength. Methods Study design and participants

ACCEPTED MANUSCRIPT A cross-sectional analysis of the evaluation of a CPR performance test was conducted from September 2011 to April 2012 which included sixty-three university students (19 men, 44 women), aged 19 to 43 years, from the Nursing Faculty of the Albacete Campus in the

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University of Castilla-La Mancha, Spain (Table 1). All of them had been previously trained in CPR based on European Resuscitation Council Guidelines [16]. All the participants were

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required to be able to achieve a maximal voluntary cardiopulmonary exercise test. All the participants were informed in detail about the nature and risks of this study, and

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were provided with written informed consent. The study protocol was approved according to the Helsinki declaration by the Clinical Research Ethics Committee of the University Hospital from Albacete, Spain. Participants suffering from any cardiovascular and/or orthopaedic injury/dysfunction were excluded.

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Training program

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The participants (in groups of 15) received, 48 hours before the measurement of the anthropometric variables and physical fitness, a 30´ session of standardized training in basic

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CPR. This training, supervised by an instructor who followed the CPR Personal Anytime Learning Programs method, allows each student to practice with a mannequin following the

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instructions of a DVD, and the amendments of the instructor [17]. Measurements

Socio-demographic variables (age, sex, education and residence) were collected and the following were also measured in all subjects.

Anthropometry Weight was measured to the nearest 100g with a calibrated digital scale (SECA model 861; Vogel & Halke, Hamburg, Germany), with the participant barefoot and in light clothes. Height was measured to the nearest millimetre with a wall-mounted stadiometer (SECA model

ACCEPTED MANUSCRIPT 220; Vogel & Halke, Hamburg, Germany), with the students standing straight against the wall without shoes to align the spine with the stadiometer. The head was positioned with the chin parallel to the floor. Finally, the mean of the two measurements of weight and height was used

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to calculate BMI (kg/m2). BMI was categorized according to the age and gender cut-off points

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defined by the World Health Organization in underweight (<18.5 kg/m2), normal weight (18.5-

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24.9 kg/m2) and overweight/obesity (≥25 kg/m2) [18]. Physical fitness tests

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Muscle strength: the maximum strength of the upper body (capacity to produce the maximum muscular tension with a muscle contraction) was evaluated using digital handgrip dynamometer Takei TKK 5101 (rank, 5–100kg, accuracy, 0.1 kg), which measured the force of maximum grip strength in both hands alternately (with previous grip adjustment of the

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dynamometer based on the hand size), making two attempts with each hand, with the subject standing up and leaving the arms relaxed and parallel to the body [19]. The final score was the

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mean of the four measures (kg). Various factors may confound strength performance tests. In

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addition to sex, age, level of physical activity, and skill, body size is a well-recognized factor affecting muscle strength. Thus, to avoid the potential biasing effect of body weight on the

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estimation of muscular fitness, handgrip was adjusted for body weight (in kg)0.67 in line with standard assumptions about morphologic effects as some authors have suggested [20,21]. Cardiorespiratory fitness (CRF) was estimated through peak or maximum oxygen uptake determined by the maximum effort test according to the Bruce ramp protocol on a treadmill ergometer (HP-Cosmos, model Pulsar 3P). The treadmill stress electrocardiogramtesting was performed with the electrocardiograph Cardinal Health model Ergoline ER800. Exhaled breath was registered by an effort basal spirometer and an automatic system of analysis of exhaled gas (model Oxycom Alpha; Jaeger). The instruments were calibrated before each session according to protocol and barometric pressure, temperature and humidity corrections. CRF was dichotomized as higher and lower using the Cooper Institute cut-off points for 20–30

ACCEPTED MANUSCRIPT year old adults (VO2max 38 ml/kg/min for women and 43 ml/kg/min for men). Evaluation of participants’ cardiopulmonary resuscitation ability

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External chest compressions: after the training session, each participant performed CPR

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on a manikin Laerdal Resusci-Anne-SkillReporter (Medical Laerdal; Stavanger, Norway)

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without interruption (30:2 compression-ventilation ratio) for 20 minutes or until exhaustion. Auditory feedback was provided and measured by the internal metronome of the

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manikin, and visual feedback was shown on a monitor that allowed visualisation of the depth of compressions, incomplete release, released pressure, increased duration, faster compression and slower compression.

During the test, ECCs were considered adequate items, according to the

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recommendations of the European Resuscitation Council [16], when the following conditions were achieved: (a) a rate of 100–120 min-1; (b) 100 per cent of compressions in the centre of the

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patient’s chest; (c) full chest recoil after each compression 100 per cent of the time; (d) 100 per

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cent of compressions with a depth ranging between 50 and 60 mm; and (e) taking approximately equal amounts of time for the compression and relaxation phases 100 per cent of the time.

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The measurements were obtained minute by minute during the test, which ended once the participants reached the objective (20 minutes of ECC) or could not continue because of physical limitations such as physical exhaustion or pain in their extremities. All measurements were taken under standard conditions by the same researchers. Statistical analysis The variables were expressed as the mean ± standard deviation. Statistical normality of the variables was tested using both graphical (normal probability plot) and statistical procedures (Kolmogorov-Smirnov test). All variables fit acceptably to a normal distribution.

ACCEPTED MANUSCRIPT Muscle strength was categorized in three levels according to quartiles (low = Q1; medium = Q2–Q3; high = Q4). ANCOVA models were estimated to test the differences in correct ECC parameters by weight status and muscle strength categories, controlling for age,

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sex and CRF (model 1); in a second step, muscle strength or BMI were added depending on the fixed factor variable (model 2). Pairwise post-hoc comparisons were examined using the

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Bonferroni test.

To examine whether the association between BMI and correct ECC parameters was

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mediated by muscle strength, linear regression models were estimated based on the procedures outlined by Baron and Kenny [15] by using the PROCESS macro for SPSS recommended by Preacher and Hayes [22]. The first equation regressed to the mediator (muscle strength) on the independent variable (BMI). The second equation regressed to the dependent variables (correct

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ECC parameters and adequate ECC) on the independent variable. The third equation regressed to the dependent variable on both the independent and the mediator variables.

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The following criteria were used to establish mediation: 1) the mediator will be

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significantly related to the independent variable; 2) the dependent variable will be significantly related to the independent variable; 3) the dependent variable will be significantly related to the

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mediator; and 4) when the mediator is included in the regression model, the association between the independent and dependent variables must be attenuated. In addition, following the steps described by Sobel, we also assessed mediation [23]: first, we estimated the attenuation or indirect effect (i.e. the effect of the independent variable on the mediator from the first regression model multiplied by the effect of the mediator on the dependent variable obtained from the third regression model); and second, we divided the indirect effect by its standard error and performed a Z-test under the null hypothesis in which the indirect effect is equal to zero. This analysis was adjusted by age, sex and CRF. A bilateral criterion for statistical significance of p ≤ 0.05 was used. All statistical analyses were performed using the software IBM SPSS 22.0.

ACCEPTED MANUSCRIPT Results All participants except one completed the study, the exception because he suffered from

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arm pain. Table 1 displays the anthropometric and physical fitness characteristics by gender.

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Mean differences in adequate ECC parameters according to both weight status and muscle strength categories are shown in Table 2. Underweight subjects achieved lower results

compression

rate

(p=0.033),

correct

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than those with normal weight and overweight/obese in several dependent variables including: compression

depth

(p<0.001),

relation

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compression/decompression (p=0.001) and adequate ECC (p<0.001) (model 1). After adjusting for muscle strength (model 2), the differences remained, except for the compression rate. Moreover, participants in the low muscle strength quartile were lower in both

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correct compression depth (p=0.001) and adequate ECC (p<0.001) than participants in the

Mediation analysis

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medium/high quartile after adjusting for confounding variables.

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When we tested the mediator role of muscle strength in the relationship between BMI and adequate ECC (Figure 1), in the first regression equation BMI was positively

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associated with muscle strength (p ≤ 0.001). In the second equation, BMI was also positively associated with adequate ECC (p ≤ 0.001). Finally, in the third equation, when BMI and muscle strength were simultaneously included in the model, both muscular strength (p ≤ 0.001) and BMI (p ≤ 0.05) were positively associated with adequate ECC. Both of them indicated that the effect of BMI on adequate ECC was partially mediated by muscle strength. The percentage of total effect mediated by muscle strength was 55 per cent (z = 2.396; p = 0.008) when adequate ECC was the dependent variable. Similar results were obtained in the analysis of the mediator role of muscle strength in the relationship between BMI and correct compression depth; therefore

ACCEPTED MANUSCRIPT muscle strength may be considered as a partial mediator. Conversely, the relationship between BMI and the other correct chest compression parameters was not mediated by

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muscle strength, since the above mentioned criteria for the mediation analysis were not

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observed (data not shown).

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Discussion

The main finding of this study is that there is a significant association between weight

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status and muscle strength with adequate ECC parameters in college students in an extreme fatigue situation or after 20 minutes of CPR. Students with normal weight and overweight/obesity and a high level of muscle strength are capable of performing better ECC manoeuvres than those with underweight or those included in other muscle strength categories.

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The present study is the first to investigate the involvement of muscle strength in the

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relationship between BMI and the ability to perform adequate ECC in college students using mediation analysis. Taken together, these results suggest that muscle strength acts as a partial

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mediator on the relationship between BMI and adequate ECC in the population described. The influence that a greater BMI has on adequate ECC has been demonstrated [4,9,23-

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27], and like other authors [6,9,10,26,28], our data indicate that adequate ECCs decrease in number as a consequence of the decrease of compression depth. Furthermore, in our study a lower BMI was also associated with decreased compression rate, and although it seems to contradict what previously reported by us and others [9,10], it confirms that it is a weak association that may be due to the feedback system, as others suggest [30]. Our data show that, in accordance with Hansen et al. and Ock et al. [5,11], high levels of handgrip strength were associated with a greater number of adequate ECCs; thus we claim that for prolonged CPR (> 5min), the quality of ECC, and particularly, the depth of ECC, are positively related to muscle strength.

ACCEPTED MANUSCRIPT Our mediation analysis revealed that the effect of muscular fitness on adequate ECC was partially mediated by BMI. We confirm the proven bivariate relationships between BMI and adequate ECC, and clarify the mediating role of muscle strength in the relationship between

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BMI and adequate ECC. Thus, there seems to be a direct and positive relationship between BMI and adequate ECC where the muscle strength is a mediator factor.

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Our results confirm that the heaviest people perform CPR better than those who are underweight, but also suggest that for low weight people, a high level of strength in arms might

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offset the negative influence of their weight status.

Limitations of this study include that it is a laboratory study and generalization of the results to real situations should consider that many environmental variables, which can influence the quality of ECC (location of the patient, meteorological factors, individual factors

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of the patient, stress, etc.), have not been included. A second limitation is that the muscle’s ability has only been measured as a single peak force against a physical object (maximal muscle

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strength). Repeated measurements of a muscle’s ability to continue generating peak forces for a

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prolonged time period or during repeated contractions against a physical object (muscle strength endurance) could have provided greater power to the study’s conclusions. A third limitation is

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that the hand grip strength measures primarily forearm flexor strength. But the muscles of the upper arm and shoulders do most of the work during ECC. Lastly, the influence on the performance of the study participants of the real-time audiovisual feedback during the CPR manoeuvres, might limit the generalizability of our findings in situations where this feedback procedure is not used. Future research would elucidate this concern. Conclusions We have shown that the ability to provide adequate ECC is influenced by the muscular strength of rescuers, which should be advised that exercises focused on improving upper body muscle strength might improve the ability to perform correct resuscitation manoeuvres.

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Conflict of interest statement

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The authors report no conflicts of interest. The authors alone are responsible for the

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content and writing of the paper.

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Acknowledgements

The authors wish to thank the participants of this study for their enthusiastic collaboration, and also want to acknowledge their gratitude to Margarita Carrión and Maria Jose

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García, nurses of the Sports Medicine Centre from Albacete, Spain.

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Table 1. Demographic, anthropometric and physical fitness variables of study population, by sex. Men

(n=63)

(n=19)

Age (years)

22.7 (5.2)

23.3 (5.5)

22.4 (5.0)

0.516

Weight (kg)

63.68 (10.32)

73.68 (7.20)

59.36 (8.28)

<0.001

Height (m)

1.67 (0.80)

1.74 (0.05)

1.63 (0.06)

<0.001

Body mass index (kg/m2)

19.02 (2.63)

21.11 (2.11)

18.11 (2.31)

<0.001

Underweight Normal weight

(VO2max; ml/kg/min)

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Cardiorespiratory fitness

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(n=44)

0

62.8

<0.001

51.6

89.5

34.9

<0.001

4.8

10.5

2.3

<0.001

1.85 (0.50)

2.41 (0.46)

1.60 (0.28)

<0.001

40.7 (7.4)

49.1 (6.0)

37.1 (4.3)

<0.001

Values are means ± standard deviation except when indicated.

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P

43.5

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Overweight/obesity Handgrip/weight0.67

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Weight status (%)

Women

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Total

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Crude data Underweight

Normal weight

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Table 2. Mean differences in correct chest compression parameters according to weight status (BMI) and muscular fitness (handgrip/weight0.67) categories.

Overweight/Obese

BMI Model 1

Model 2

p

Post-hoc†

p

Post-hoc†

p

Post-hoc†

(N)

(O)

Compression rate (min-1)

99.1±4.46

99.2±4.55

106.9±9.57

0.063

-

0.033

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0.053

-

Compressions with correct location (%)

90.7±14.9

91.8±16.6

98.9±1.5

0893

-

0.747

-

0.788

-

Compressions with adequate release pressure on the chest (%)

99.7±0.9

96.1±15.5

97.9±3.3

0.781

-

0.492

-

0.554

-

Correct compression depth (%)

36.4±30.6

70.7±28.3

87.7±11.4

0.018

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<0.001

U
0.006

U
Relation compression/decompression

37.5±4.7

40.9±4.3

46.1±5.4

<0.001

U
0.001

U
0.005

U
Adequate compression (%)

34.1±29.9

70.2±25.4

84.2±11.7

0.012

U
<0.001

U
0.002

U
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TE

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Low

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(U)

Handgrip/weight0.67 Crude data

Medium

High

(L)

(M)

(H)

Compression rate (min-1)

99.3±7.23

99.5±4.9

Compressions with correct location (%)

86.2±2.6

Compressions with adequate release pressure on the chest (%)

99.0±2.0

Model 1

Model 2

p

Post-hoc†

p

Post-hoc†

p

Post-hoc†

100.1±2.0

0.567

-

0.907

-

0.935

-

96.2±5.6

98.0±3.3

0.062

-

0.055

-

0.094

-

96.8±15.8

98.1±3.8

0.904

-

0.815

-

0.777

-

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35.6±27.2

52.7±32.4

87.6±15.5

0.010

Relation compression/decompression

39.8±5.1

38.6±4.6

41.7±5.3

0.068

Adequate compression (%)

32.3±24.7

52.5±31.1

85.3±14.9

0.004

CR I

Values are means ± SD.

L
<0.001

L
0.001

L
-

0.133

-

0.888

-

L
<0.001

L
<0.001

L
PT

Correct compression depth (%)

NU S

Model 1: adjusted for age, sex and cardiorespiratory fitness. Model 2: model 1 covariates plus handgrip/weight0.67 or body mass index (BMI) depending on the fixed factor variable.

MA

Categories of BMI are underweight, normal weight and overweight/obese according to cut-offs defined by the World Health Organization. Categories of muscular fitness (an index was measured by the sum of the standardized Z-score of handgrip dynamometry/weight and standing long jump) are low, medium and high, representing the first, second, third and fourth quartiles. †

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CE P

TE

D

All the pairwise mean comparisons using the Bonferroni post-hoc test were statistically significant (p<0.001).

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CE

PT

ED

MA NU

SC

RI P

T

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Figure 1. Muscle strength mediation models of the relationship between body mass index (BMI) and adequate external chest compressions (ECC) controlling for age, sex and cardiorespiratory fitness (CRF). *p ≤ 0.05; **p ≤ 0.001.