Cardiorespiratory and autonomic responses to 30-15

Intermittent Fitness Test in team sport players

Running title: Cardiorespiratory and autonomic responses to 30-15IFT

M. Buchheit1, H. Al Haddad1, G. P. Millet2, P.M. Lepretre3, M. Newton3, and S. Ahmaidi1

1 Laboratoire de Recherche, EA 3300 «APS et conduites motrices : « Adaptations

Réadaptations », Faculté des Sciences du Sport d’Amiens, Université de Picardie Jules Verne,

80025 Amiens – France.

2Aspire, Academy for Sports Excellence, PO Box 2287, Doha, Qatar.

3 Laboratoire de Biomécanique et Physiologie, Institut National du Sport et de l'Education

Physique, 11, Av. du Tremblay, 75012 Paris – France.

4 School of Exercise, Biomedical and Health Sciences, Edith Cowan University, Joondalup,

WA, Australia

Address for correspondence: Martin Buchheit, PhD Laboratoire de Recherche Adaptations Réadaptations (APS et conduites motrices) Faculté des Sciences du Sport Allée P. GROUSSET 80025 AMIENS CEDEX 1 France Tel : +333.22.82.89.36 Fax : +333.22.82.79.10 Email :martin.buchheit@u-picardie.fr

Abstract

Objective: The 30-15 Intermittent Fitness Test (30-15IFT) is as an attractive alternative to

classic continuous incremental field tests for defining a reference velocity for interval training

prescription in team sport athletes. The aim of the present study was to compare

cardiorespiratory and autonomic responses to 30-15IFT with those observed during a standard

continuous test (CT).

Methods: In 20 team-sport players (20.9 ± 2.2 y), cardiopulmonary parameters were

measured during exercise and for 10 min after both tests. Final running velocity, peak lactate

([La]peak) and rating of perceived exertion (RPE) were also measured. Parasympathetic

function was assessed during the post-exercise recovery phase via heart rate (HR) recovery

time constant (HRRτ) and HR variability (HRV) vagal-related indices.

Results: At exhaustion, no difference was observed in mean oxygen uptake (V O2),

respiratory exchange ratio, HR or RPE between 30-15IFT and CT. In contrast, 30-15IFT led to

significantly higher minute ventilation, [La]peak, and final velocity than CT (p<0.05 for all

parameters). Regarding ventilatory thresholds (VThs), all cardiorespiratory measurements

were similar and well correlated between the two tests. Parasympathetic function was lower

after 30-15IFT than after CT, as indicated by significantly longer HHRτ (81.9 ± 18.2 vs. 60.5 ±

19.5, for 30-15IFT and CT, respectively, p<0.001) and lower HRV vagal-related indices (i.e.,

the root mean square of successive R-R intervals differences (rMSSD): 4.1 ± 2.4 and 7.0 ± 4.9

ms, p<0.05).

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Conclusions: The 30-15IFT is accurate for assessing VThs and V O2 peak, but alters post-

exercise parasympathetic function more than a continuous incremental protocol.

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Keywords: intermittent exercise, aerobic capacity, field test, heart rate variability,

parasympathetic reactivation.

Introduction

High intensity aerobic training performed in team sports often include intermittent and

shuttle runs in order to reproduce running patterns specific to competition.[1, 2] The 30-15

Intermittent Fitness Test (30-15IFT),[3] a graded intermittent and shuttle field test, leads to a

maximal running velocity (VIFT) that is determined through an effort involving physiological

variables similar to those solicited during shuttle interval training sessions (i.e. ability to

change direction and both aerobic and anaerobic energy systems). The use of VIFT has been

shown to be more accurate than individual vV O2max (defined as the lowest running velocity

that elicits V O2max during a continuous graded test [4]) for getting players with different

physiological profiles to a similar level of cardiorespiratory demand, and thus for

standardizing training content at a team level.[3] Nevertheless, the physiological responses

induced by the 30-15IFT have not been compared to a gold-standard continuous incremental

test (CT) [5] for identifying submaximal (i.e.; ventilatory thresholds) and maximal

cardiorespiratory variables. To date, it has only been shown that VIFT is very well related to

maximal oxygen uptake determined in the laboratory and that maximal heart rate (HR) [3] is

achieved.

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Assessment of post-exercise autonomic regulation has recently emerged as an original

way to estimate the short-term physiological strain of exercise, since it can indirectly provide

insight into perturbation of body regulatory processes and homeostasis.[6, 7] Post-exercise

autonomic assessment can also transiently highlight increased cardiovascular risk [8-10] in

specific populations (i.e.; sedentary individuals engaging in rehabilitation programs). Post-

exercise parasympathetic activity has never been described after any graded intermittent

exercise. Nevertheless, since supramaximal and intermittent exercise has been shown to

strongly impair post-exercise autonomic function [6] and body’s homeostasis,[11] it could be

expected that the 30-15IFT, leading to higher running velocities than vV O2max,[3] would

have a more depressive effect on post-exercise autonomic function that CT.

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The aim of the present study was thus to compare cardiorespiratory and autonomic

responses between the 30-15IFT and a graded continuous field test.

METHODS

Subjects. Twenty moderately-trained healthy male team-sport players (20.9 ± 2.2 yr, 179.8 ±

5.8 cm and 76.4 ± 9.4 kg, playing soccer, handball or basketball and training 4.9 ± 1.9

hrs.week-1) participated in the study, which conformed to the recommendations of the

Declaration of Helsinki and was approved by the local institutional research ethics committee.

Incremental procedures

All subjects were familiar with field exercise evaluation and were tested on two separate days

on a synthetic indoor track (temperature ~22 °C). The 30-15IFT consists of 30-s shuttle runs

interspersed with 15-s passive recovery periods. Velocity was set at 8 km.h-1 for the first 30-s

run and speed was increased by 0.5 km.h-1 every 30-s stage thereafter.[3] Subjects were

required to run back and forth between two lines set 40 m apart at a pace which was governed

by a pre-recorded beep. During the 15-s recovery period, subjects walked in a forward

direction towards the closest line (at either the middle or end of the running area, depending

on where their previous run had stopped) and commenced the next run stage from this line.

The graded continuous field test was adapted from the extensively used test of Léger at al.

:[12] initial speed of 10 km.h-1 increased by 0.5 km.h-1 per 1-min stage with subjects running

at a pace which was governed by a pre-recorded beep.

The velocity (km.h-1) attained during the last completed stages of each test was determined as

the subject’s VIFT and VCT for 30-15IFT and CT, respectively.

Physiological measurements.

Respiratory gas exchange and HR were measured using an automated breath-by-breath

system (K4b2, Cosmed, Rome, Italy [13]), which was calibrated as recommended by the

manufacturer before each test. Cardiorespiratory data (oxygen uptake (V O2), carbon dioxide

production (V CO2), breathing frequency (BF), tidal volume (VT), minute ventilation (V E),

respiratory exchange ratio (RER), O2 pulse, end tidal volume PO2 (PETO2 %), PCO2 (PETCO2

%)) and HR were filtered and averaged on a 5-s basis. For both tests, peak oxygen uptake

(V O2peak) was arbitrarily defined as the highest V O2 values attained in one 20-sec period.

CO2, BF, VT, V E, RER at V O2peak were retained as V CO2peak, BFpeak, VTpeak,

Epeak, RERpeak and O2pulse peak. A HR peak attained coincident with the maximal

predicted value, [La]peak > 8 mmol.l-l , respiratory exchange ratio (RER) > 1.1 and volitional

fatigue were required to confirm the maximal nature of the test. The velocity associated with

O2 peak (vV O2peak30-15IFT and vV O2peakCT) were the lowest running velocities which

elicited a V O2 value equal to V O2peak during the 30-15IFT and CT, respectively [4]. Three

minutes after the end of each exercise set a fingertip blood sample (5 μL) was collected in the

20 subjects ([La]bpeak, Lactate Pro, Arkray Inc, Kyoto, Japan). Additionally, every four

stages during the 30-15ITF (i.e., every 2 km.h-1, during the 15-s recovery periods), lactate

profile was determined in 11 of the 20 subjects. The accuracy of the analyzer was checked

before each test using standards.

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Ventilatory thresholds assessment.

Cardiorespiratory data were averaged at each stage level. For the 30-15IFT, data from the 30-s

exercise and the successive 15-s recovery periods were averaged to produce a single 45-s

stage value. As described by Whipp et al.,[14] the first ventilatory threshold (VTh1) was

established as the velocity at which V E /V O2 and PETO2 began to increase without a

simultaneous increase in PETCO2. The second ventilatory threshold (VTh2 [also called RCP,

respiratory compensation point]), was determined as the velocity corresponding to the first

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decrease in PETCO2, with a corresponding increase in V E /V CO2 following the steady-state

phase after the VTh1. All thresholds measurements were made by visual inspection of graphs

of time plotted against each relevant respiratory variable measured in the testing. The visual

inspections were made by two experienced exercise physiologists and the results were

compared and then averaged. The difference in the values determined by the two assessors

was < 2%.

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Beat-to-beat HR recordings

Beat-to-beat HR data were additionally recorded with a Polar S810 HR monitor (Polar

Electro, Kempele, Finland).[15] All R-R interval series were extracted (Polar Protrainer 5.1,

Polar Electro, Kempele, Finland) and occasional ectopic beats were visually identified and

manually replaced with interpolated adjacent R-R interval values. As previously described,[7]

parasympathetic function was assessed during the 10 min period following both CT and 30-

15IFT, by 1) analysing the first 30 s (from the 10th to the 40th s) of HR recovery (HRR) via

semi-logarithmic regression analysis (T30, short-term time constant), 2) by taking the

complete time constant of the HR decay obtained by fitting the 10-min post-exercise HR

recovery into a first-order exponential decay curve (HRRτ), 3) by calculating a time-varying

vagal-related index (the root-mean-square difference of successive normal R-R intervals on

subsequent 30-s non overlapped segments, rMSSD30s), or 4) by analysing usual stationary

vagal-related HRV indices,[7] calculated as the last 5 min of the 10-min recovery period in

the seated position. Mean HR (HR5-10min), standard deviation of normal R-R intervals

(SDNN5-10min), RMSSD5-10min, power densities in the low (LF, 0.04-0.15 Hz) and the high

frequency band (HF, > 0.15-0.50 Hz), LnHF5-10min (natural-log of HF) and the normalized LF

power (LFnu5-10min, calculated as the LF/(LF+HF) ratios) were computed.[16] All indices

were considered to provide insight into parasympathetic activity, except LFnu5-10min, which

was used as a relative index of the sympathetic component of sympathovagal balance.[16]

Statistical analyses

Descriptive data are presented as mean and standard deviation (±SD). Normality of the

distribution was verified via the Shapiro Wilk’s test. Student T-test for paired data, Pearson’s

coefficient correlation (r) calculated from linear regressions and Bland & Altman limits of

agreement [17] were used to compare ventilatory measures, [La]bpeak and RPE measures

between 30-15IFT and CT at the three intensities (VTh1, VTh2 and O2peak). Bland & Altman

plots were presented to illustrate the intra-individual agreement in V O2peak and HRpeak for

the 30-15IFT and CT [17]. HR(V)-derived indices and running velocity were compared with

paired-student T-test and Pearson’s coefficient correlation. For time-varying rMSSD30s, a 2-

factor repeated measures ANOVA with one between factor (test type; 30-15IFT versus CT)

and a within factors (time; 19 repeated measures during the recovery) was used to examine

the main effects and/or interactions of test type and time during recovery. For each ANOVA,

if a significant interaction was identified, a Tukey‘s post hoc test was used to further delineate

differences between training type, period and time. The level of significance was set at P <

0.05. Statistical analyses were carried out using Minitab 14 Software (Minitab Inc, Paris,

France).

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Results

Velocities at exhaustion, and first and second ventilatory thresholds were all higher

during 30-15IFT compared to CT : 19.4 ± 0.9 vs. 15.4 ± 1.1 km.h-1 (p<0.001) for VIFT and VCT,

respectively; 18.3 ± 1.3 vs. 14.1 ± 1.4 km.h-1 (p<0.001) for VIFTV O2peak and VCTV O2peak; 16.9

± 1.3 vs. 13.6 ± 1.3 km.h-1 (p<0.001) for 30-15IFT and CT at VTh1; and 13.1 ± 1.2 vs. 10.1 ±

0.6km.h-1 (p<0.001) for 30-15IFT and CT at VTh2, respectively.

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Mean maximal cardiorespiratory responses of both tests are presented in Table 1.

Figure 1 illustrates the Bland & Altman plot for V O2peak and HR. All values remained in the

95% confidence interval. V O2 peak, V CO2 peak, E peak and [La]b peak measured for 30-

15IFT as a percentage of CT are shown in Figure 2. It appears that whereas V O2 peak was

similar, V CO2 peak, V E peak and [La]b were significantly higher in 30-15IFT compared to CT

(all p<0.05). There were no differences in absolute values for V O2 peak, VT peak, RER peak,

O2pulse peak, HR peak and RPE between 30-15IFT and CT. In contrast, 30-15IFT led to

significantly higher BF peak, V E peak and [La]b peak than CT. For the 11 subjects in whom

blood samples were collected during the 30-15IFT, mean blood lactate concentration was 1.1 ±

0.1 mmol.l-1 at rest and increased with running velocity to reach 12.3 ± 2.8 mmol.l-1 3 min

after the end of exercise (Figure 3).

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Table 1. Maximal values of cardiovascular variables of interest for the 30-15 Intermittent

Fitness Test (30-15IFT) and the continuous graded test (CT).

30-15IFT CT 95% Limits of

agreement r² (p)

VO2peak (ml.min-1.kg-1) 54.5 ± 6.6 53.5 ± 7.3 -9.8; 11.8 0.58 (0.001)

BFpeak (cycles.min-1) 54.0 ± 10.8* 48.1 ± 8.9 -14.4; 26.2 0.28 (0.04)

VTpeak (l) 2.7 ± 0.3 2.7 ± 0.4 -0.6; 0.6 0.34 (0.01)

VEpeak (l.min-1) 141.4 ± 20.1* 128.7 ± 22.1 -25.3; 50.8 0.36 (0.01)

VCO2peak (ml.min-1.kg-1) 60.3 ± 9.7* 56.4 ± 6.4 -8.5; 15.5 0.60 (0.001)

RERpeak 1.11 ± 1.1 1.06 ± 0.1 -0.2; 0.2 0.31 (0.05)

O2pulsepeak (ml.beat-1) 22.1 ± 2.9 21.3 ± 3.1 -4.2; 5.9 0.41 (0.004)

HRpeak (beats.min-1) 195.0 ± 8.4 192.5 ± 10.1 -9.2; 14.1 0.71 (0.000)

[La]peak (mmol.l-1) 11.8 ± 2.3** 8.9 ± 2.1 -2.8; 8.5 0.02 (0.54)

RPE 7.1 ± 1.1 6.6 ± 1.2 -1.5; 2.1 0.30 (0.08)

Mean (±SD) values of peak oxygen uptake (VO2peak), breathing frequency (BFpeak), tidal

volume (VTpeak), minute ventilation (VEpeak), carbone dioxide production (VCO2peak),

respiratory exchange ratio (RERpeak) oxygen pulse (O2pulsepeak) at VO2peak, peak heart rate

(HRpeak), rate of perceived exertion (RPE), peak blood lactate concentration ([La]peak), and

rate of perceived exertion (RPE) measured during and after the continuous graded test (CT)

and the 30-15 Intermittent Fitness Test (30-15IFT). * : significant difference compared to CT

(p<0.05). ** : significant difference compared to CT (p<0.01).

With regard to ventilatory thresholds, all cardiorespiratory measurements were similar

between the two tests (Table 2). Person’s correlation coefficients show (Table 1 and 2) that all

cardiorespiratory variables observed during both tests, at the three intensities, were

moderately to well correlated (i.e.; r² = 0.58, p=0.001 for V O2 peak). &

Table 2. Cardiovascular variables of interest at the first (VTh1) and second ventilatory threshold (VTh2) for the 30-15 Intermittent Fitness Test

(30-15IFT) and the continuous graded test (CT).

V VTh1 Th2

30-15IFT CT 95% Limits

of agreementr (p) 30-15IFT CT

95% Limits of

agreement r (p)

VO2 (ml.min-1.kg-1) 38.6 ± 4.7 39.8 ± 4.8 -10.2; 7.6 0.45 (0.01) 49.8 ± 6.6 50.3 ± 6.9 -10.5; 9.5 0.52 (0.001)

BF (cycles.min-1) 32.8 ± 5.2 33.6 ± 6.1 -10.1; 6.6 0.53 (0.001) 44.2 ± 6.9 43.0 ± 6.3 -11.7; 14.1 0.29 (0.02)

VT (l) 2.2 ± 0.4 2.2 ± 0.5 -0.5; 0.4 0.70 (0.000) 2.6 ± 0.3 2.7 ± 0.4 -0.4; 0.5 0.51 (0.002)

VE (l.min-1) 70.3 ± 11.6 73.44 ± 9.4 -22.2; 16.0 0.36 (0.03) 114.2 ± 13.2 113.4 ± 14.4 -32.2; 33.9 0.58 (0.01)

VCO2 (ml.min-1.kg-1) 35.47 ± 5.3 37.2 ± 4.1 -10.8; 7.7 0.31 (0.04) 52.5 ± 9.7 53.1 ± 6.6 -11.8; 10.6 0.64 (0.000)

RER 0.91 ± 0.1 0.93 ± 0.1 -0.1; 0.1 43 (0.004) 1.05 ± 0.1 1.06 ± 0.1 -0.2; 0.2 0.34 (0.01)

O2pulse (ml.beat-1) 18.5 ± 2.5 18.5 ± 3.2 -6.0; 5.9 0.28 (0.05) 20.8 ± 3.0 20.5 ± 3.3 -4.6; 5.1 0.48 (0.001)

HR (beats.min-1) 159.6 ± 13.5 161.0 ± 14.4 -15.0; 12.1 0.68 (0.000) 182.9 ± 9.7 183.6 ± 10.3 -12.7; 11.3 0.62 (0.000)

Mean (±SD) values of oxygen uptake (VO2), breathing frequency (BF), tidal volume (VT), minute ventilation (VE), carbon dioxide production

(VCO2), respiratory exchange ratio (RER) oxygen pulse (O2pulse) and heart rate (HB) at the fist (VTh1) and second (VTh2) ventilatory threshold

during the continuous graded test (CT) and the 30-15 Intermittent Fitness Test (30-15IFT). * : significant difference compared to CT (p<0.05).

Parasympathetic function was appreciably more impaired following 30-15IFT than after

CT, as indicated by significantly lower T30 (p<0.05), longer HHRτ (p<0.001), lower

rMSSD5-10min (p<0.01) and decreased LnHF5-10min, (p<0.01) (Table 3). Conversely, LFnu5-10min

values indicated a higher shift toward sympathetic predominance after 30-15IFT than CT

(p<0.05). HHRτ and rMSSD5-10min observed following the 30-15IFT, expressed as a percentage

of that observed after CT are presented in Figure 2. rMSSD30s, did not display any significant

change during the 10-min recovery period following 30-15IFT whereas it increased

significantly after CT (Figure 3).

Table 3. Post-exercise parasympathetic indices calculated for the 30-15 Intermittent Fitness

Test (30-15IFT) and the continuous graded test (CT).

30-15IFT CT

T30 (s) 261.4 ± 65.9* 221.8 ± 65.9

HRRτ (s) 81.9 ± 18.2† 60.5 ± 19.5

SDNN5-10min (ms) 13.8 ± 6.3* 18.6 ± 7.6

RMSSD5-10min (ms) 4.1 ± 2.4** 7.0 ± 4.9

LnHF5-10min 1.6 ± 1.0* 2.3 ± 1.5

LFnu5-10min 0.86 ± 0.1* 0.77 ± 0.1

HR5-10min (bpm) 110.6 ± 10.2* 105.5 ± 14.1

Mean (±SD) values for short-term HR time constant (T30), long-term time constant of the HR

decay (HRRτ), standard deviation of normal R-R intervals (SDNN5-10min), root-mean-square

difference of successive normal R-R intervals (RMSSD5-10min), natural-log of high-frequency

power density (LnHF5-10min), normalized low-frequency power (LFnu) and mean HR (HR5-

10min) calculated for the 5-min period. *: significant difference versus CT (P < 0.05); **:

significant difference versus CT (P < 0.01). †: significant difference versus CT (P < 0.001).

DISCUSSION

The present study is the first to describe cardiorespiratory and autonomic responses to

a shuttle and intermittent field test, the 30-15IFT. Peak oxygen uptake and most

cardiorespiratory parameters at the two ventilatory thresholds were similar for the 30-15IFT

and classic graded continuous field test. In contrast, the 30-15IFT elicited higher peak minute

ventilation and maximum carbon dioxide production, which could be related to the final

higher running velocity reached and the increased anaerobic contribution in 30-15IFT

compared to CT. Finally, post-exercise autonomic function was characterized by a decreased

parasympathetic activity and a greater sympathetic predominance following the 30-15IFT than

after CT.

Continuous and linear exercise testing is not the optimal method for determining a

reference velocity for scheduling shuttle interval training sessions in team sport players.[3]

Therefore, there is a need for designing protocols such as the 30-15IFT that reproduce the

characteristics of specific intermittent exercises practiced in those sports (i.e.; change of

direction, acceleration, deceleration and rest periods).

Our results demonstrate the validity of the 30-15IFT for assessing maximal aerobic

power. We observed that the 30-15IFT elicits similar peak O2 uptake, VT, RER, O2pulse and

HR to a classic graded continuous field test, which we used as a standard reference of aerobic

power assessment (Table 1).[5, 18] The parameters obtained for the two tests were all

moderately-to-well correlated and 95% intervals of agreement (i.e., Figure 1 for V O2peak

and HR peak) were similar to those usually reported for field tests comparison.[19, 20] Mean

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O2 bias (-1.60; 3.01 ml.-1min.kg-1) was within the range of V O2peak interdian variations (2.4

to 5.9%) [21] or that of technical error measurement (≈3% [13]). The present results can be

compared to previous studies of young soccer players [19, 20] that have shown incremental

intermittent testing (i.e.; the Yo-Yo intermittent endurance test, which employs a protocol

similar to the 30-15IFT) to elicit similar V O2 peak or HR peak values to those observed during

standard treadmill CT. Our results contrast however with those reported by Girard et al. [22,

23] using specific intermittent tennis and squash tests, since V O2 peak was higher during

intermittent racket tests than those on a treadmill. The higher oxygen consumption observed

in the racket sport tests [16, 17] may be due to the involvement of upper body muscles

required for the simulated ball hitting action. We, however, observed significantly higher BF

peak, E peak and V CO2 peak during the 30-15IFT than the CT. One may assume that the

discontinuous or shuttle [24] nature of the 30-15IFT explains in part the higher ventilatory

parameters observed, since the coupling of locomotion-ventilation is altered by the changes in

direction, stride patterns and velocity. Signals from neural afferents in contracting muscles as

a result of increased VIFT compared to VCT may have also led to an excitatory effect on central

respiratory drive.

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Although blood lactate may not be the best measure to demonstrate the relative

contribution of anaerobic energy supply in sports with discontinuous exercise of long

duration,[25] its significantly higher concentration observed after the 30-15IFT could also

account for the increase in ventilation parameters. Metabolic acidosis may have contributed to

an enhancement of peripheral chemoreceptor activity resulting in increased ventilation.[26]

The significant higher CO2 production observed for 30-15IFT may also be directly related to

higher blood buffering of protons (H+) compared to CT.

In terms of assessment of ventilatory thresholds, the present results show that the 30-

15IFT produced comparable values to CT. All cardiorespiratory parameters were similar and

well correlated at the two ventilatory thresholds, and the confidence intervals were acceptable

(Table 2). With the exception of the two specific racket test studies,[22, 23] that reported

similar VThs values for field intermittent and treadmill tests, previous running studies paid

exclusive attention to maximal cardiorespiratory values.[19, 20] As the intermittent nature of

exercise is known to increase anaerobic participation,[27, 28] the accuracy of the 30-15IFT to

predict VThs was difficult to gauge. However, data suggest that anaerobic participation

during intermittent work might only affect ventilatory measures at high intensities (higher

than VTh2). Thus, the present results may also encourage the use of the 30-15IFT for VThs

assessments.

This is also the first study to describe HR recovery and post-exercise autonomic

function following a graded intermittent test. The time-varying vagal-related rMSSD30s did

not display any significant increase during the 10-min recovery period after 30-15IFT, whereas

it showed a time-effect increase after CT, indicative of effective parasympathetic reactivation

(Figure 3). This apparently reduced parasympathetic modulation (and conversely increased

sympathetic activity) after the 30-15 IFT compared to CT was corroborated by the significantly

slower HRR time constants (T30 and HRRτ), the lower vagal-related HRV indices and higher

sympathetic-related data (Table 3). Nevertheless, present inference to parasympathetic effect

on HR should be considered with caution. Various investigations have demonstrated,

especially at high workloads, complex interactions between the sympathetic and vagal

systems with respect to heart rate regulation resulting in reduced [29] or amplified vagal

stimulation.[30, 31]

The reduced parasympathetic modulation after 30-15IFT is in agreement with previous

data showing that anaerobic participation and muscle power engagement, rather than aerobic

intensity per se, influence the post-exercise parasympathetic reactivation.[6] Although the

direct link between HRV and homeostasis has not been objectively determined, we can

suggest that the particular autonomic status observed after the 30-15IFT might be related to a

more disturbed post-exercise homeostasis (i.e., increased system stress metabolite

accumulation [32]) as confirmed by the higher blood lactate concentration after the 30-15IFT.

This is in line with results reported earlier, where frequent changes in exercise intensity

during 40 min cycling were shown to disturb homeostasis and increase post-exercise

metabolism to a greater extent than constant exercise conducted at a similar mean

intensity.[28] All vagal HR-derived indices observed here were higher than those reported

after 12-min of high-intensity exercise [6] (HIT, 30 s running at 90% of VIFT, interspersed

with 30 s of passive recovery) in comparable subjects using extremely similar methods (i.e.,

T30 = 81.9 ± 18.2 and 100.99 ± 7.02 s after 30-15IFT and HIT [6]). This indicates that the 30-

15IFT has an important impact on homeostasis and autonomic nervous system activity but that

it is of lower magnitude than traditional interval-training exercises.[33, 34] Since it is believed

that a background low in parasympathetic activity following exercise is related to a transiently

increased risk for adverse cardiovascular invents,[8-10, 16] the present data suggest limiting

the use of the 30-15IFT to team sports athletes and to employ the traditional CT in sedentary or

unfit subjects for aerobic function assessment. However, as per exercise intensity

prescription,[33, 34] the use of the 30-15IFT could be indicated for the latter stages of a

(rehabilitation) training program in sedentary individuals or patients.

To conclude, by using the 30-15IFT protocol, the assessment of peak oxygen uptake

and ventilatory thresholds appear to be as accurate as a classic graded continuous test. These

data support the use of the 30-15IFT, not only as an accurate training instrument but also as

valid tool to explore aerobic function. Nevertheless, the slower HR recovery and increased

sympathetic predominance after the 30-15IFT suggests that its physiological strain is higher

than that of CT. The appreciably depressed post-exercise parasympathetic activity encourages

reserving its use to already trained athletes.

Acknowledgements

The authors would like to thank Anne-Lyse Behaegel for her help in data collection, the

subjects for their participation in the study, as well as Irmant Cadjjiov for his helpful

comments during the preparation of this manuscript.

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Figure legends

Figure 1. Bland & Altman plot of peak oxygen uptake (V O2peak) and peak heart rate (HR

peak) measured for the 30-15IFT vs. continuous test (CT).

&

Figure 2. Mean (±) value of peak oxygen uptake (V O2 peak), carbon dioxide uptake (V CO2

peak) and minute ventilation (V E peak) at V O2peak, peak blood lactate concentration

([La]b), long-term heart rate recovery time constant (HRRτ) and root mean square of

successive R-R interval differences (rMSSD) for 30-15IFT (white bars), expressed as a

percentage of values observed for CT (black bars). *: significant difference versus CT (P <

0.05).

& &

& &

Figure 3. Evolution of blood lactate concentration ([La]b) during the 30-15IFT and 3 min after

test termination. Means ± SD and individual values for 11 subjects are reported.

Figure 4. Average (±SD) root mean square of successive differences in the R-R intervals

measured on successive 30-s segments (rMSSD30s) during the 10-min recovery period after

the 30-15 Intermittent fitness test (30-15IFT) or the graded continuous one (CT)*: significant

difference vs. rMSSD30s at 30s after the end of exercise during the same test session (P <

0.05).

Mean 30-15IFT and CT VO2peak (ml.min-1.kg-1)

40 45 50 55 60 65 70VO2p

eak

30-1

5 IFT

- VO

2pea

k C

T (m

l.min

-1.k

g-1)

-15

-10

-5

0

5

10

15Mean + 2 SD

Mean - 2 SD

Mean diff.

Mean 30-15IFT and CT HR peak (bpm)

175 180 185 190 195 200 205 210 215

HR

pea

k 30

-15 IF

T - H

R p

eak

CT

(bpm

)

-15

-10

-5

0

5

10

15

20

Mean + 2 SD

Mean - 2 SD

Mean diff.

Figure 1

0

50

100

150

200CT 30-15IFT

VO

2pea

k

VC

O2p

eak

V Epe

ak

[La]

bpea

k

rMS

SD30

s

HR

R

% o

f CT *

*

**

*

Figure 2

Velocity (km.h-1)

[La]

b (m

mol

.L-1

)

0

2

4

6

8

10

12

14

16

18

10 12 14 16 18 20 3' rec

Figure 3

Time (s)0 100 200 300 400 500 600

rMSS

D30

s (m

s)

2

4

6

8

10CT 30-15IFT

***

Figure 4