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DAY-TO-DAY VARIABILITY IN CARDIORESPIRATORY RESPONSES TO HYPOXIC CYCLE EXERCISE Meaghan J. MacNutt1, Carli M. Peters2, Catherine Chan2, Jason Moore2, Serena Shum2 and A. William Sheel2 1 2

Life Sciences, Quest University, Squamish, BC, Canada School of Kinesiology, University of British Columbia, Vancouver, BC, Canada

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Correspondence William Sheel, Ph.D. 6108 Thunderbird Blvd. Vancouver, BC, Canada V6T 1Z3 Phone: (604) 822-4459 Fax: (604) 822-9451 E-mail: [email protected] MacNutt et al.

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ABSTRACT (word count = 248)

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Repeatedly performing exercise in hypoxia could elicit an independent training response and

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become an unintended co-intervention. The primary purposes of this study were to determine if

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hypoxic exercise responses changed across repeated testing and assess the day-to-day variability

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of commonly used measures of cardiorespiratory and metabolic responses to hypoxic exercise. . Healthy young males (23±2 yrs) with a maximal O2 consumption (Vo2max) of 50.7±4.7 mL·kg-

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Participants completed 3-min stages at 20%, 40%, 60%, and 10 % of individual peak power. . With increasing exercise intensity there were increases in ventilation (VI), O2 consumption . . (VCO2), CO2 production (VCO2), respiratory exchange ratio (RER), heart rate (HR), blood lactate

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([LA]), and ratings of perceived exertion for legs (RPElegs) and respiratory system (RPEresp)

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along with a reduction in oxyhaemoglobin saturation (%SpO2) (all p0.05). Most measures were highly repeatable across

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testing sessions with the coefficient of variation (CV) averaging ≤ 10% of the mean value in all . . variables except VO2 (17%), VCO2 (11%) and [La] (17%). For HR and %Spo2, the CV was
0.05) had recovered towards baseline after 3 min at 10% Pmax. These

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results indicate that our exercise challenge was of insufficient duration and/or intensity to elicit

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even the earliest signs of an aerobic training response after five non-consecutive sessions.

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Repeatability of exercise responses

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Within subject repeatability of cardiorespiratory responses to hypoxic rest and exercise

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was generally very good, with average CVs across workloads as low as 3 and 4% for SpO2 and

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HR, respectively. Measurement of respiratory gas exchange was less repeatable and is reflected . . . . in CVs > 10% for Vo2, Vco2 and VI/Vo2. However, as with most other cardiorespiratory

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responses measured here, variability was markedly higher during rest and recovery than exercise

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at 20, 40 and 60% of Pmax, likely reflecting behavioural, rather than physiological differences

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from day to day. CVs for most variables are substantially reduced when only these exercise . . stages are considered and the result is most notable for Vo2 (17  12%), Vco2 (11  8%), and

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RER (10  7%). Overall, day to day variability of cardiorespiratory responses to hypoxic

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exercise were similar to those previously reported from a normoxic exercise test – re-test study

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on a much larger sample population (Wilmore et al. 1998). Also in agreement with our findings, . . Wilmore and colleagues reported that variability in HR, Vo2, Vco2 and RER decreased in heavier

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MacNutt et al.

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exercise. Conversely, we demonstrate that RPE CVs are lowest at rest and tend to increase

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throughout exercise and recovery. Decreasing repeatability of RPE scores with increasing

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intensity has also been previously reported in normoxic exercise testing (Lamb et al. 1999).

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Conclusion and utility for other researchers We developed an exercise challenge that was appropriate to use for repeated testing of

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the cardiorespiratory responses to hypoxic exercise. The hypoxic stimulus (FIO2=0.13, ~3800 m)

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was sufficient to significantly alter cardiorespiratory responses in rest and exercise, but the

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frequency and duration of hypoxic exposure was insufficient to elicit measurable IH-induced

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changes in ventilation. The cycle protocol duration (3-min stages at 20, 40, 60 and 10% of

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normoxic peak power) was not of sufficient intensity, duration or frequency to elicit a training

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response when repeated five times during a two-week period. With most cardiorespiratory

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responses measured in our study the variability was greater under resting conditions than during

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exercise. The lower day-to-day variability of exercise responses indicates that the exercise

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challenge elicited repeatable responses. Therefore, this protocol allows detection of changes in

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cardiorespiratory responses to hypoxic exercise that might occur during exposure to chronic or

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intermittent hypoxia, or in response to therapeutic interventions for pathological hypoxaemia or

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high altitude illness. Information is provided so the protocol can be modified to address other

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research questions. For example, it might be desirable to customize the exercise challenge to

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maximize statistical power. Using our collective findings, investigators can select an exercise

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intensity that will maximize the hypoxic effect size and/or minimize the within-subject

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variability for a cardiorespiratory variable of interest.

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Acknowledgements This study was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada. MJ MacNutt was supported by a NSERC Canada Graduate Scholarship award. We are indebted to our research participants for their patience, commitment, and enthusiastic participation in this study.

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Table 1. Physical and physiological characteristics of study participants. Age

Height

Mass

. VO2max

Peak Power

(yrs)

(m)

(kg)

(mL·kg-1·min-1)

(W)

1

22.1

1.82

86.2

44.0

314

2

21.7

1.78

86.2

50.0

329

3

24.0

1.63

64.4

47.6

263

4

20.5

1.83

74.4

50.1

340

5

27.0

1.88

84.6

57.6

398

6

23.9

1.67

65.8

55.9

280

7

19.1

1.70

67.4

46.6

267

8

22.0

1.67

51.6

53.4

238

50.7±4.7

304±52

Subject

Mean ± SD 22.5±2.4 1.75±0.09 72.6±12.5 2 3

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Table 2. Effect variables. Percent .change in minute . of hypoxia (∆HYPOXIA) on cardiorespiratory . ventilation (VI), heart rate (HR), O2 consumption (VO2), CO2 production (VCO2), respiratory exchange ratio (RER), oxyhaemoglobin saturation (Spo2) and rate of perceived exertion for legs (RPElegs) and breathing (RPEresp) from normoxic to hypoxic exercise. H1 to H5 were collapsed into a single hypoxic condition (H) and mean change (± SD) is reported for each workload and as a global mean across all workloads. P values are given for the effect of hypoxia in a 2 (condition) x 5 (work load) RM ANOVA on raw cardiorespiratory responses and for the effect of workload on percent change from normoxia to hypoxia, determined by one-way RM ANOVA. ∆HYPOXIA (% change) rest

p values

20%

40%

60%

10%

mean

Pmax

Pmax

Pmax

Pmax

change

hypoxia

work load

. VI

na

↑ 34±15

↑ 51±11

↑ 68±18

↑ 121±52

↑ 66±43

Day-to-day variability in cardiorespiratory responses to hypoxic cycle exercise.

Repeatedly performing exercise in hypoxia could elicit an independent training response and become an unintended co-intervention. The primary purposes...
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