Parasympathetic reactivation after repeated sprint exercise

Page 1Articles of 39 in PresS. Am J Physiol Heart Circ Physiol (March 2, 2007). doi:10.1152/ajpheart.00062.2007 Parasympathetic reactivation after sprinting Parasympathetic reactivation after repeated sprint exercise Running title: Parasympathetic reactivation after sprinting Martin Buchheit1, Paul B. Laursen2 and Saïd 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, F-80025 France 2 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 Copyright Information Copyright © 2007 by the American Physiological Society. 1 Page 2 of 39 Parasympathetic reactivation after sprinting Abstract The purpose of this study was to examine the effects of muscular power engagement, anaerobic participation, aerobic power level and energy expenditure on post-exercise parasympathetic reactivation. We compared the response of heart rate (HR) following repeated sprinting with that of exercise sessions of comparable net energy expenditure and anaerobic energy contribution. Fifteen moderately-trained athletes performed: 1) 18 maximal all-out 15-m sprints interspersed with 17 s of passive recovery (RS), 2) a moderate isocaloric continuous exercise session (MC) at a similar level of mean oxygen uptake ( V& O2) to that of the RS trial, and 3) a high-intensity intermittent exercise session (HI) conducted at a similar level of anaerobic energy expenditure to that of the RS trial. Subjects were immediately seated following the exercise trials and beat-to-beat HR was recorded for 10 min. Parasympathetic reactivation was evaluated through 1) immediate post-exercise HR recovery, 2) the time course of the root-mean-square for the successive R-R interval difference between successive 30-s segments (RMSSD30s) and 3) heart rate variability vagal-related indexes calculated on the last 5-min stationary period of recovery. RMSSD30s increased during the 10min period following the MC trial, whereas it remained depressed following both the RS and HI trials. Parasympathetic reactivation indexes were similar for the RS and HI trials, but lower than for the MC trial (P < 0.001). When considering data of the three exercise trials together, only anaerobic contribution was related to HR-derived indexes. Parasympathetic reactivation is highly impaired following RS exercise and appears to be mainly related to anaerobic process participation. Keywords: heart rate recovery, vagal-related indexes, autonomic activity, sprint interval training. Copyright Information 2 Page 3 of 39 Parasympathetic reactivation after sprinting INTRODUCTION Repeated sprint (RS) training, characterized by recurring sessions of brief repeated bouts of supramaximal exercise, may be a time-efficient strategy for inducing metabolic adaptations in human skeletal muscle (15). Adaptations shown following a short time-course of RS training include increases in resting glycogen content (20), increases in the maximal activities of various enzymes involved in glycolytic (31) and oxidative energy provision (10, 20), an increase in H+ buffer capacity (17, 20) as well as decreases in cycling time-trial performance (10, 15, 20). As a result, RS training has been proposed as a viable alternative to classically prescribed submaximal endurance training (10, 17, 20). Today, there are growing social and psychological reasons to encourage RS training within clinical populations. First, RS training is remarkably time-efficient and more compatible with the Western world’s time-poor modern lifestyle. Second, the concept of RS training may also be more attractive compared with continuous exercise for sedentary individuals who have difficulty handling exercise sessions that are perceived to be of a long duration and of a monotonous nature. Third, the high level of muscular power needed to perform RS training stresses more Type II muscle fibers, which comprise approximately onehalf of the fibers within the thigh (vastus) and calf (gastrocnemius) muscle of most people and of which are not recruited during low-intensity exercise. Not surprisingly, RS training has been shown to result in a maintenance and even an improvement in muscular strength (17). While the effectiveness of RS training for maintaining and improving muscular performance is established (47), its influence on post-exercise autonomic function is unknown. Knowledge of this effect however is critical for clinicians dealing with patients in certain disease states that may leave them more prone to adverse cardiovascular events. Indeed, sympathetic hyperactivity (4) or reduced cardiac vagal tone (3) after exercise may Copyright Information 3 Page 4 of 39 Parasympathetic reactivation after sprinting confer a poor cardioprotective background and underlie ischemic heart disease and the pathogenesis of malignant ventricular arrhythmias and sudden cardiac death. To quantify parasympathetic reactivation after exercise, the time-course of HR recovery (HRR) and HR variability (HRV) indexes have been used (7, 13, 22, 42, 45). The validity of these markers has been examined with the use of drugs which cause a parasympathetic blockade (i.e. atropine) (22, 45). The simplest and most used HRR index is the number of heart beats recovered within 60 s after the cessation of exercise (13). Fitting post-exercise HR recovery to a first-order exponential decay curve has also been used (7, 42). Regarding HRV, it is the vagal-related indexes, such as the root-mean-square of successive differences of R-R intervals (RMSSD) or the power density in the high-frequency (HF) range obtained by spectral analysis that are the most widely used methods (50). Finally, a new and simple temporal time-varying parasympathetic index has recently been proposed by Goldberger et al. (22). This index is derived from the time course of the RMSSD measured on successive 30-s segments over the recovery period. The acute effects of a single exercise bout on HRV have been placed into long- and short-term categories. Twenty-four to 48 hours after exercise, a rebound of parasympathetic activity, which seems independent of the exercise type undertaken (38), has often been described (18, 26). Concerning the short-term evolution of autonomic activity, an initial decrease in HRV and vagal-related indexes has been observed within minutes to hours following exercise (38, 39, 51). Moreover, vagal restoration has been shown to be more delayed following intense (80% V& O2 peak) compared with moderate intensity exercise (50% V& O2 peak (38, 39)). Parasympathetic activity has also been shown to be more impaired following resistance exercises compared with a moderate cycling effort (27). Nevertheless, in all of these previous studies, the absence of anaerobic metabolite measurements has made less-clear the influence that muscular power engagement, anaerobic participation and aerobic Copyright Information 4 Page 5 of 39 Parasympathetic reactivation after sprinting power level have on post-exercise parasympathetic reactivation. For RS exercise, although V& O2 peak is sometimes reached (16), this is not always the case (21, 47); so that, from a metabolic point of view, the aerobic exercise intensity could be considered to be ‘submaximal’. Nevertheless, sprinting requires the development of very high muscular power, so that the exercise might also be considered to be ‘supramaximal’ from a muscular and anaerobic point-of-view. As a result, the effects of RS on parasympathetic reactivation are difficult to predict. The primary purpose of the present study was to quantify the time-course of the parasympathetic reactivation following RS exercise and to observe the respective effects of muscular power engagement, anaerobic participation, aerobic power level and energy expenditure on post-exercise autonomic control. We used multiple HR-derived parasympathetic indexes measured following RS exercise and compared these to indexes obtained after exercises of 1) comparable levels of net energy expenditure to distinguish the specific effect of anaerobic process participation and muscular power engagement and 2) comparable anaerobic contribution to observe the explicit incidence of net energy expenditure. Finally, we used multiple linear regressions to show the respective consequence of each metabolic and mechanical exercise characteristic on post-exercise autonomic regulation. METHODS Participants Based on the assumption that a 8 ± 5 beats.min-1 difference in HHR60 is meaningful (7, 13), we used Minitab 14.1 Software (Minitab Inc, Paris, France) to determine that a sample size of nine subjects was needed to provide a power of 80% with an alpha of 0.05. Although Copyright Information 5 Page 6 of 39 Parasympathetic reactivation after sprinting evaluating the incidence of RS exercise on autonomic function is especially crucial in sedentary individuals and patients, we preferred here to recruit moderately trained subjects, because we expected that they would be more able to cope with this innovative experimentation (n = 15, age, 21.3 ± 3.1 yr; height, 178.1 ± 7.5 cm; body mass, 73.8 ± 10.4 kg; muscle mass, 30.7 ± 0.3 kg; body surface area, 1.91 ± 0.05 m²). All participants were routinely involved (6.6 ± 2.7 h·wk-1) in various intermittent activities (soccer, handball, basketball or tennis), and without history or clinical sign of cardiovascular or pulmonary diseases. Muscle mass was estimated based on forearm and thigh calf girths and skinfolds (33), while body surface area (BSA) was calculated according to the formula provided by Mosteler et al (37). Subjects were not currently taking prescribed medications and presented with normal levels of blood pressure and electrocardiographic patterns. The study conformed to the recommendations of the Declaration of Helsinki and participants gave voluntary written consent to participate in this experiment, which was approved by the local ethics committee. Experimental design Subjects performed at the same time of day (±1 h) over a 2-week period one graded aerobic test and three sessions of short exercise bouts. Each test was separated by at least 48 h. All tests were performed on an indoor synthetic track where ambient temperature ranged from 18 to 22°C. The graded maximal aerobic test was performed first, followed by the three other tests in a random and balanced order for each subject. The exercise bouts consisted of a repeated sprinting bout, a submaximal and continuous bout completed at comparable net energy expenditure to that of the RS bout, as well as a high-intensity intermittent bout completed at a comparable level of anaerobic energy expenditure to that of the RS bout. Subjects were familiarized with the exercise procedure prior to commencement of each test. Subjects were asked not to perform exercise on the day prior to a test, and to consume their usual last meal at least 3 h before the scheduled test time. All three of the experimental Copyright Information 6 Page 7 of 39 Parasympathetic reactivation after sprinting exercise bouts were preceded by a supervised and standardized warm-up consisting of 5 min running at 45% of VIFT (corresponding to a ‘maximal intermittent aerobic reference velocity’, see below) along with a few athletic drills (i.e. skipping) and short bursts of progressive accelerations on the track. Exercise bouts began 2 min after this warm-up. Maximal graded aerobic test. Maximal aerobic performance of each subject was assesses using a 30-15 Intermittent Fitness Test (30-15IFT). This intermittent shuttle field test elicits peak oxygen uptake ( V& O2 peak) and has been shown to be accurate for individualizing intermittent shuttle running exercise (6). Moreover, this test has been shown to be reliable (intraclass correlation coefficient = 0.96) for the final running speed (VIFT). The 30-15IFT consists of 30-s shuttle runs interspersed with 15s passive recovery periods. For this test, 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. Subjects were required to run back and forth between two lines set 40 m apart at a pace which was governed by a prerecorded beep. The prerecorded beep allowed the subject to adjust their running speed within a 3-m zone placed in the middle and at each extremity of the field. During the 15-s recovery period, subjects walked in the forward direction towards the closest line (at either the middle or end of the running area, depending on where their previous run had stopped); this line is where they would start the next run stage from. Subjects were instructed to complete as many stages as possible, and the test ended when the subject could no longer maintain the required running speed or when they were unable to reach a 3-m zone in time with the audio signal on three consecutive occasions. The velocity attained during the last completed stage was determined as the subject’s VIFT. Respiratory gas exchanges were measured using an automated portable metabolic system (VO2000, Medgraphics; St. Paul, MN, USA)(34). Before each test, the O2 and CO2 analysis systems were calibrated. The V& O2 peak was defined as the highest V& O2 attained in a 20-s period. Subjects were considered to have Copyright Information 7 Page 8 of 39 Parasympathetic reactivation after sprinting reached V& O2 peak if at least two of the following criteria were met: 1) a respiratory exchange ratio greater than 1.1, 2) a maximal HR attained within 10 beats.min-1 of the age-predicted maximum, and 3) volitional fatigue (28). Repeated sprint exercise. RS exercise consisted of repeated 15-m sprints with 17 s of passive recovery (52). Subjects ran in the opposite direction following the 17-s rest. Total duration of the test was 6 min. Respiratory gas exchanges were measured during the test as previously described. Equivalent RS net energy expenditure exercise. This trial was used to evaluate the influence of the muscular power engagement and/or anaerobic participation on post-exercise parasympathetic reactivation. The ‘aerobic’ intensity (% V& O2peak) of this moderate and continuous (MC) exercise was similar to that measured during RS and corresponded to 65% of VIFT (6). The exercise duration was determined a-priori in order to achieve similar levels of net energy expenditure compared with the RS trial. Total calculated MC trial duration ranged from 368 to 501 s. As for other exercises, respiratory gas exchanges were measured during the test. Equivalent RS anaerobic energy exercise. In order to examine the influence of net energy expenditure on parasympathetic reactivation, we attempted to compare the RS trial to that which occurred during a 12-min high-intensity intermittent (HI) exercise trial. Our choice of such an exercise trial of that duration was based on prior work showing that, at appropriate duration and intensity, HI exercise 1) is associated with similar lactate accumulation to that observed after RS exercise, and 2) that HI exercise induces twice the net energy expenditure of a short RS exercise (>200 Kcal) (2). The exercise consisted of 30-s runs at 90% of VIFT (corresponding to 100 to 105 % of maximal aerobic velocity(6)) with intermittent 30-s passive recovery periods. During the 30-s exercise period, athletes were required to run back and forth over the 40 m area so that they covered the distance determined according to their Copyright Information 8 Page 9 of 39 Parasympathetic reactivation after sprinting VIFT. Run interval pace was provided by a digital timer that produced a sound every 30 s from the start to the end of the exercise interval. After a 30-s rest, the subject ran again in the opposite direction. Respiratory gas exchanges were also measured during the test as previously described. Blood lactate measurement. Three minutes after the end of each exercise set, a fingertip blood sample (5 µL) was collected and blood lactate concentration [La]b was determined (Lactate Pro, Arkray Inc, Japan) (44). The accuracy of the analyzer was checked before each test using standards. Beat-to-beat HR analyses Materials. An electrode transmitter belt (T61, Polar Electro, Kempele, Finland) was fitted to the chest of each subject as instructed by the manufacturer, after application of conductive gel. A Polar S810 HR monitor (Polar Electro, Kempele, Finland) was used to continuously record beat-to-beat HR during each exercise and each subsequent recovery phase (19). Data treatment. All R-R series recorded by the S810 were extracted on an IBM compatible PC with the processing program (Polar Precision Performance SW 4.03, Polar Electro, Kempele, Finland). Occasional ectopic beats (irregularity of the heart rhythm involving extra or skipped heartbeats - i.e. extrasystole and consecutive compensatory pause) were visually identified and manually replaced with interpolated adjacent R-R interval values. Post-exercise HR recovery assessment. Beat-to-beat HR was analyzed during the 10-min recovery period immediately following each exercise. As soon as exercise was stopped, all subjects immediately sat passively on a chair placed adjacent to the track. Time duration between the end of exercise and sitting was less than 5 s. Particular attention to this detail was made because differences in body posture have been shown to result in different absolute HR recovery values (49). HRR was calculated by three ways. The first HRR index was defined as the absolute difference between the final HR observed at the end of exercise (mean of 5 s) and Copyright Information 9 Page 10 of 39 Parasympathetic reactivation after sprinting the HR recorded 60 s following (mean of 5 s, HRR60s) (7, 13). The second HHR index was calculated as proposed by Imai et al. (29), using a semi-logarithmic regression technique. The natural logarithm of heart beats during the initial rapid HR decrease (from the 10th to the 40th s) was plotted against the elapsed time of exercise, and a linear regression analysis was applied. The time constant of the short-time post exercise HR decay (T30) was thus determined as the negative reciprocal of the slope of the regression line. HRR was further analyzed by fitting the 10-min post-exercise HR recovery into a first-order exponential decay curve (7). A HR time constant (HRR ) was then produced by modelling the resultant 10-min of HR data using an iterative technique (Sigmaplot 10, SPSS Science; Chicago, IL, USA) by the following equation: HR = HR0 + HRamp e(-T/ HRR ) , where HR0 = resting (final) heart rate; HRamp = maximal HR (HRmax) – HR0 and T = time (s). A single exponential model was applied for all subjects, as previously used models (i.e. biexponential) (41) did not significantly improve the gain of variance for the fit between modelled and measured HR data. HRR and HRamp were retained for statistical analysis. Time-varying vagal-related HRV index. While a progressive increase in the R-R interval is generally observed over the initial 5 min of recovery , on shorter scales (i.e. 15-60 s), the curve is piecewise linear with superimposed oscillations. Thus, a time-varying vagal-related index, the root mean square of successive differences in the R-R intervals (RMSSD), was calculated for each of the subsequent 30-s segments of recovery (RMSSD30s) (22). To smooth out transient outliers in the heart rate variability plots (heart rate variability versus time in recovery), a median filter operation was performed in which each value was replaced with the median of the value as well as the preceding and following values. The first and last values were not median filtered (22). Short-term resting HRV analysis. HRV analyses were performed on the last 5 min of the 10-min recovery period in the sitting-resting position which assures stableness of the data. Copyright Information 10 Page 11 of 39 Parasympathetic reactivation after sprinting While respiratory rate is often controlled in HRV studies, we did not control respiratory rate in our participants as we did not want to perturb the natural return of HR to baseline. Nevertheless, respiratory rate was always in HF range (> 0.15-0.50 Hz) and did not differ significantly during the last 5 min of recovery in the three conditions. Therefore respiratory rate would have influenced the HRV indexes of each subject in the same way. The mean HR (HR5-10min), the standard deviation of normal R-R intervals (SDNN5-10min), the percentage of successive R-R differences greater than 50 ms (pNN505-10min) and the root-mean-square difference of successive normal R-R intervals (RMSSD5-10min) were calculated for the 5-min period. Power frequency analysis was performed sequentially with a fast Fourier transform based on a nonparametric algorithm with a Welsh window after the ectopic-free data were detrended and resampled. A fixed linear resampling frequency of 1024 equally-spaced points per 5-min period was used. The power densities in the LF band (0.04-0.15 Hz) and the high HF band (> 0.15-0.50 Hz) were calculated from each 5-min spectrum by integrating the spectral power density in the respective frequency bands. The SDNN5-10min, pNN505-10min, RMSSD5-10min, LnHF5-10min, the normalized HF power (HFnu5-10min, calculated as the HF/(LF+HF) ratios) and the HR5-10min were retained for statistical analysis. Energy contribution to each exercise bouts Total aerobic energy contribution. Total aerobic energy contribution for the trials was calculated as the sum of the oxygen uptake measured during the exercise and the following 10 min plus the available oxygen stores assumed to be 2.3 ml O2.kg body mass-1 (1). Total anaerobic energy contribution. Total anaerobic energy contribution during the trials, expressed as oxygen equivalents, was calculated based on the estimated phosphocreatine and lactate contribution (La-PCr methods). The anaerobic energy contribution of phosphocreatine was estimated to be 37.0 ml O2.kg-1 muscle mass (11), while the anaerobic glycolytic energy Copyright Information 11 Page 12 of 39 Parasympathetic reactivation after sprinting contribution was calculated at 3.0 ml O2 equivilents.kg-1 body mass for each 1 mmol.l-1 increase in [La-]b above pre-exercise levels (11), assumed to be 1 mmol.l-1. Net energy expenditure during exercise. Net energy expenditure during exercise was obtained while converting total net oxygen cost of each exercise into Kcal, assuming that the consumption of 1L O2 in the human body yields 4.99 Kcal, a value applicable only if the respiratory quotient (RQ) is 0.96. However, since the energy equivalent of 1L O2 only varies from 4.68 to 5.05 Kcal throughout the range of RQ values from 0.71 to 1.00, the minor effects of RQ would have been negligible (11). The total net oxygen cost of each exercise trial was calculated as the sum of the total aerobic and total anaerobic energy contributions minus resting V& O2, which was defined as the mean V& O2 during the 10-min period preceding the maximal aerobic test (24). Estimated mechanical power output. Mechanical power output during each of the exercise bouts was estimated from the formula given by di Prampero et al. (11), which takes into account the time of the exercise, the distance covered as well as the BSA of the individual. We also assumed a muscular efficiency of 25%. Statistical analyses Data are presented as means and standard errors (±SE). The distribution of each variable was examined with the Kolmogorov-Smirnov and Shapiro-Wilk normality tests. Since absolute HF power values were skewed, HF power density was transformed by taking its natural logarithm to allow parametric statistical comparisons that assume a normal distribution. A one-way analysis of variance (ANOVA) with Tukey’s post hoc test was used to compare the variables of HRR60s, T30, HRR , HRamp, SDNN5-10min, pNN505-10min, RMSSD5-10min, LnHF510min, HFnu5-10min as well as the HR5-10min distributions between the three exercise bouts. For time-varying RMSSD30s, a 3 (exercise trial) x 20 (time) repeated measures ANOVA was used to examine for main effects and/or interactions of intensity and time. When statistical Copyright Information 12 Page 13 of 39 Parasympathetic reactivation after sprinting significance was identified, a Tukey‘s post hoc test was used to further delineate differences between exercise trial or time. Multiple linear regressions were used to establish the respective relationships between parasympathetic indexes and exercise characteristics. Others polynomial regressions were rejected on the basis of importantly higher residuals. Adjusting calculations based on age and body mass index did not significantly change the outcomes. Moreover, as data were homogenous, we did not need to make adjustments to avoid overfitting. All statistical analyses were carried out using Minitab 14.1 Software (Minitab Inc, Paris, France) with the level of significance set at P<0.05. RESULTS Maximal aerobic test. Mean V& O2 peak, maximal HR and [La]b were 51.6 ± 1.4 ml.min. -1kg1 O2, 200 ± 2 beats.min-1 and 10.9 ± 0.9 mmol.l-1. Mean VIFT was 18.9 ± 1.1 km.h-1. Cardiorespiratory measures during the three exercise trials. All subjects successfully completed the 360-s RS and the 720-s HI exercise trials. The mean duration of MC was 422 ± 21 s. As expected, mean V& O2 was similar between RS and MC trials (78.4 ± 8.3 vs. 76.0 ± 9.5 % V& O2peak, P = 0.18) and significantly higher during the HI trial (91.1 ± 5.2 % V& O2peak, P < 0.001). Mean HR for the RS trial (87 ± 2 %HRmax) was higher than for the MC trial (80 ± 3 %HRmax, P = 0.01), while mean HR for the HI trial (93 ± 4 %HRmax) was greater than both RS and MC trials (P < 0.001). Energy system contribution and energy expenditure during the three exercise trials. Mean (±SE) values of cumulated net V& O2 during exercise, estimated total oxygen stores, measured [La]b, [La]b O2 equivalent, PCr O2 equivalent, % aerobic energy contribution and % anaerobic energy contribution to exercise during the trials are presented in Table 1. Mean (±SE) values for the net energy expenditure and total aerobic and anaerobic contribution to Copyright Information 13 Page 14 of 39 Parasympathetic reactivation after sprinting the RS, MC and HI trials are summarized in Figure 1. The net O2 cost was marginally higher for the MC trial compared with the RS trial (P = 0.04), but more than twice as high during the HI trial compared with RS and MC trials (P < 0.001). The difference in net exercise O2 cost between the RS and MC trials was compensated for by the higher anaerobic energy contribution observed during the RS trial. As a result, the net energy expenditure was equivalent. Mean anaerobic energy contribution was comparable between the RS and HI trials, and was significantly higher than the MC trial (P < 0.001). As well, the net energy expenditure for the HI trial was more than double that of the RS and MC trials. Mechanical power during the three exercise trials. Figure 1 (upper right plot) illustrates the estimated mean power developed during RS, MC and HI trials. Mean estimated power during RS was almost four-times higher compared with the power developed during the MC trial, and nearly three-times higher than the power developed during the HI trial (11.4 ± 0.2 vs. 3.1 ± 0.1 and 4.6 ± 0.1 W.kg-1, P < 0.001). Power during the HI trial was also marginally higher than during the MC trial (P < 0.001). Parasympathetic reactivation following the three exercise trials. The time course of the RR intervals following each exercise trial is illustrated in Figure 2, while the HR recovery and HRV indexes are presented in Table 2. HRR60s, T30, SDNN5-10min, pNN505-10min, RMSSD510min, lnHF5-10min, HFnu5-10min and HR5-10min were comparable across the RS and HI trials, but were significantly lower compared to the MC trial (P < 0.001); no difference was observed across trials for HRamp. HRR was shorter after the MC exercise compared to after the RS trial (P<0.001), and both were shorter compared to after the HI trial (P < 0.001). Correlation coefficients of the regression line for T30 were 0.99 ± 0.01 for the three exercise trials. A significant time by exercise trial interaction was observed for RMSSD30s during the 10-min period after MC; however post-hoc analysis revealed that RMSSD30s remained constant and similar for the RS and HI trials (Figure 3). Concerning HR recovery indexes, simple linear Copyright Information 14 Page 15 of 39 Parasympathetic reactivation after sprinting regression analyses demonstrated that the relationship was stronger between HRR60s and T30 (r = 0.84, P < 0.001) than between HRR60s and HRR (r = 0.68, P < 0.001) or between T30 and HRR (r = 0.66, P < 0.001). Relationships between HR recovery indexes and resting HRV indexes were all significant and moderate to strong (0.53 < r < 0.81). However correlation coefficient values were higher for short-term compared to long-term indexes (T30 vs. LnHF5-10min: r = 0.81, P < 0.001 and T30 vs. LnHF5-10min: r = 0.75, P < 0.001; whereas HRR vs. LnHF5-10min: r = 0.53, P < 0.001). Relationship between parasympathetic reactivation indexes and exercise characteristics. Results from multiple linear regressions between HR-derived parasympathetic indexes and exercises characteristics are presented in Table 3. Only anaerobic process contribution was significantly related to vagal-restoration indexes. Figure 4 presents the simple linear relationships between the time constant of short-term HR recovery (T30) and oxygen equivalent for the anaerobic energy contribution (r = 0.83, P < 0.001). DISCUSSION This aims of this study were to examine the time-course of the parasympathetic reactivation using indexes of heart rate variability following repeated sprint running, and to quantify the post-exercise autonomic regulation response in relation to exercise bouts of equal net energy expenditure, mean (aerobic) exercise intensity, muscular power and anaerobic process participation. Results revealed that, compared with moderate continuous exercise of a similar energy expenditure, parasympathetic reactivation during the first 10-min of recovery was significantly more delayed after the repeated sprint exercise. Second, most of the vagal HR-derived indexes were similar following repeated sprint running compared with a highintensity intermittent running bout of similar anaerobic energy release and double the energy expenditure. Thus, we present novel data to suggest that anaerobic contribution and other Copyright Information 15 Page 16 of 39 Parasympathetic reactivation after sprinting factors associated with a high level of fast-twitch fiber recruitment (i.e. central stress command, catecholamine and sympathetic cotransmitter release, lactate and H+ accumulation) rather than mean aerobic power or net energy expenditure, are of primary importance in determining the level of parasympathetic reactivation after repeated sprint running. Post-exercise parasympathetic reactivation after RS exercise. Parasympathetic reactivation, i.e. the time course of RMSSD30s, was highly impaired after the RS trial, so that the index did not change during the 10-min recovery period (Figure 2). Although short-term autonomic regulation of HR following RS running has not been directly documented, the delayed parasympathetic reactivation we have shown following RS exercise is consistent with what has been presented in the literature. Limited research in this area has revealed that the convalescence of the autonomic control of HR appears dependent upon the exercise intensity. Indeed, HR recovery following moderate to high-intensity exercise (80% V& O2peak) has revealed slower short-term restoration of vagal modulation compared with low intensity exercise (50% V& O2peak) (39, 51), even for isocaloric exercise trials (39). Moreover, in 2004, Mourot et al. compared the effect of a hard interval training session (9 x (4min 79% V& O2peak, 1min 100% V& O2peak)) to that of a constant load isocaloric one (48 min at 79% V& O2peak), and found that the return of parasympathetic activity during the first hour of recovery also appeared to be slower following the high-intensity interval training session (38). While the exercise sessions in this study were completed at similar levels of total work, mean aerobic power was higher during the high-intensity interval training session compared with the continuous exercise session, and anaerobic metabolites were not measured (38). Thus, prior studies have not been definitive with respect to the influence that muscular power engagement, anaerobic participation and aerobic power level have on post-exercise parasympathetic reactivation. Copyright Information 16 Page 17 of 39 Parasympathetic reactivation after sprinting Effect of muscular power engagement and anaerobic process participation on parasympathetic reactivation. Parasympathetic reactivation was significantly lowered following the RS trial compared with MC exercise. Mean aerobic power and net energy expenditure were comparable between the RS and MC trials, while muscular power and anaerobic energy contribution were different. The present study design and the mathematical adjustments made enabled us to make a clear distinction between the factors that are of primary importance in determining parasympathetic reactivation level. Multiple linear regression analyses revealed, when considering data of the three exercise trials together, that anaerobic contribution but not muscular power engagement is significantly related to all of post-exercise parasympathetic reactivation indexes (Table 3 and Figure 4). Therefore we suggest that the impaired parasympathetic activity after RS exercise may be primarily related to the high anaerobic process participation and associated plasma metabolites. Although the incidence of lactate accumulation on post–exercise autonomic regulation has still not been directly evaluated, the effect of high-intensity repeated muscular contraction has been recently examined. Heffernan et al. (27) found that a resistance workout (10-repetition maximum test) perturbed the parasympathetic reactivation of heart rate significantly more than did an aerobic exercise session (30min cycling at 65% V& O2peak). Muscular power production during the RS trial in the present study was high enough to be compared with that commonly performed during resistance training and the anaerobic contribution might have been even greater. This slowing of vagal restoration following resistance training or RS training may thus be related to the heightened sympathetic activity that occurs during exercise (43) and the persistent elevation of adrenergic factors (25) and local metabolites during recovery (epinephrine, norepinephrine, H+, lactate, inorganic phosphate, etc) (21, 47). For example, Perini et al. (40, 41) have reported strong and significant linear relationships between HR recovery kinetics and norepinephrine plasma levels. Nevertheless, post-exercise HR recovery is a multifaceted Copyright Information 17 Page 18 of 39 Parasympathetic reactivation after sprinting phenomenon. Various investigations have demonstrated complex interactions between the sympathetic and vagal systems with respect to heart rate regulation (30, 32, 35, 36), resulting in reduced (35) or amplified vagal stimulation (30, 32). As recently demonstrated by Sunagawa and co-workers (30, 35, 36), in the presence of sympathetic activation, HR response to vagal stimulation essentially depends on the type and site of adrenergic receptors most selectively activated under a given condition. For example, Sunagawa et al. have shown that the dynamic HR response to vagal stimulation can be attenuated via activation of adrenergic receptors on the preganglionic and/or postganglionic cardiac vagal nerve terminals by high plasma norepinephrine levels (35), but not by cardiac postganglionic sympathetic nerve stimulation (36). In contrast, elevated cardiac sympathetic nerve activity can augment the dynamic HR response to vagal nerve stimulation via activation of the postjunctional adrenergic cascade (30, 32). Unfortunately in the present study we did not measure any objective indexes of muscular power engagement (i.e. EMG measurement) or system stress (i.e. sympathetic muscle nerve activity, hormones and muscle metabolite sampling) to assess objectively the level of contribution this may have had on HRV markers. Nevertheless, since high plasma norepinephrine levels have been repeatedly observed after high -intensity or sprint exercise (5, 40, 41), we put forth that the poor parasympathetic reactivation level observed after RS and HI exercise in the present study might be partly attributed to high sympathetic activity and associated metabolites persistence. Additionally, the potential presence of postexercise hypotension after acute exercise could have stimulated the arterial baroreflex, elevating sympathetic outflow and delaying the restoration of vagal tone. Changes in plasma volume have been reported to alter cardiac autonomic balance (48) and the occurrence of a plasma volume shift during the exercise trials in the present study could have altered the parasympathetic reactivation indexes. Copyright Information 18 Page 19 of 39 Parasympathetic reactivation after sprinting Effect of net energy expenditure on parasympathetic reactivation. All HR vagalrelated indexes were different between the RS and MC trials, whereas these two exercises were equivalent with respect to their net energy expenditure. Moreover, a doubling of the net energy expenditure while maintaining a similar level of anaerobic energy contribution (RS vs. HI), was not associated with any further lowering of most indexes of parasympathetic reactivation (9 of the 10 indexes considered; Table 2). We also cannot exclude the possibility thatour 10 min post-exercise recording duration may have been too short to reveal differences between RS and HI in stationary HRV indexes. Nevertheless, these findings conform with the studies of Parekh and Lee (39) and Mourot et al. (38) which have shown that the total work or energy expenditure is not the main factor influencing parasympathetic reactivation after exercise. It should be noted however that HRR was significantly different between the RS and the HI trials, whereas HRR60s and T30 were not. These differences, together with the moderate correlation coefficients we found between short-term recovery indexes (HRR60s and T30) and HRR , confirm previous investigations that have shown that HRR , in contrast to T30 or HRR60s, is influenced by additional factor(s) and not only vagal modulation (29, 4042). While comparing the effect of parasympathetic versus complete autonomic blockade on T30 and HRR , Imai et al. (29) found that T30 was primary mediated by vagal activity, whereas HRR explained not only the prompt parasympathetic reactivation (rapid initial decrease in HR) but also the level of sympathetic activity (slow second decrease). Perini et al. (41) found that, in the absence of autonomic control in heart-transplanted recipients, HR decrease began only after 50s (which corresponds to the slow second phase) and was related to plasma norepinephrine clearance. Thus, the present differences shown between our RS and HI trials suggest that HI exercise might be associated with a higher persistence of postexercise sympathetic activity compared to RS exercise, despite a similar time course of vagal Copyright Information 19 Page 20 of 39 Parasympathetic reactivation after sprinting activity restoration (similar HRR60s and T30). This possible higher norepinephrine plasma concentration after the HI versus the RS trial could in part be attributed to differences in exercise bout duration between the two exercises, as sprint times during the RS trial were almost ten-fold shorter than the intermittent runs during the HI trial. Exercise bout duration has been shown to be essential in determining the level of muscle desoxygenation and consequently the level of anaerobic participation and plasma cathecholamine accumulation (12). Implications for the clinical use of RS training. Although evaluation of the influence that RS exercise has on autonomic function is crucial in sedentary individuals and patients, we intentionally recruited moderately-trained subjects for the present study because we expected that they would likely be able to cope better with this innovative experimentation. Trained subjects often display high HR recovery indexes (7), so the present results should be viewed with caution when inference is made as to what might occur in a sedentary subject. Whether individuals with low activity levels show a similar response to RS exercise needs to be confirmed. In summary, the present study has shown that short-term parasympathetic reactivation is impaired following repeated sprint running, and that anaerobic contribution rather than muscular engagement and net (aerobic) energy expenditure appears to be of primary importance in determining the level of parasympathetic reactivation. This finding may be of particular importance for clinicians wishing to prescribe sprint training to clinical populations. Longitudinal studies that examine the effect of long-term repeated sprint training on autonomic control of the heart, parasympathetic reactivation and cardiovascular risk prognostic are warranted. Copyright Information 20 Page 21 of 39 Parasympathetic reactivation after sprinting Acknowledgements The authors would like to thank the subjects for their participation in the study, as well as Daniel Mercier and Irmant Cadjjiov for their helpful comments during the preparation of this manuscript. Copyright Information 21 Page 22 of 39 Parasympathetic reactivation after sprinting References 1. Barstow TJ, Lamarra N, and Whipp BJ. Modulation of muscle and pulmonary O2 uptakes by circulatory dynamics during exercise. J Appl Physiol 68: 979-989, 1990. 2. Billat LV. Interval training for performance: a scientific and empirical practice. Special recommendations for middle- and long-distance running. Part II: anaerobic interval training. Sports Med 31: 75-90, 2001. 3. Billman GE. Aerobic exercise conditioning: a nonpharmacological antiarrhythmic intervention. J Appl Physiol 92: 446-454, 2002. 4. Billman GE. 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The relationship between repeated sprint ability and the aerobic and anaerobic energy systems. J Sci Med Sport1: 100 -110, 1998. Copyright Information 27 Page 28 of 39 Parasympathetic reactivation after sprinting Figure legends Fig. 1. Mean ± SE net exercise energy expenditure (NEE), mean power, net oxygen cost, oxygen equivalent of net anaerobic energy cost, and parasympathetic reactivation indexes (number of heart beats recovered in 60 s after exercise cessation (HRR60s) and high frequency power (HF) or R-R intervals for each of the three exercise trials: repeated sprinting (RS), moderate continuous (MC) and high-intensity exercise (HI) running. *: significant difference vs. RS (P <0.001). †: significant difference between HI and MC (P <0.001). Fig. 2. Mean ± SE R-R intervals during recovery from three different exercise trials: repeated sprint (RS), moderate continuous (MC) and high-intensity exercise (HI) running. The R-R interval was calculated as the mean R-R intervals over the 5-s segment of interest. For the sake of clarity, symbols indicating significant differences vs. RS have not been added. Fig. 3. Mean ± SE root mean square of successive difference of the R-R intervals measured on successive 30-s segments (RMSSD30s) during the 10-min recovery period, as calculated for each of the three exercise trials: repeated sprint (RS), moderate continuous (MC) and highintensity exercise (HI) running. *: significant difference vs. RMSSD30s at the end of exercise (P < 0.001). †: significant difference vs. RS (P < 0.001). Fig. 4. Relationship between time constant of short-time heart rate recovery (T30, see methods) and oxygen equivalent of the net anaerobic energy cost of each of three exercise trials. Dotted lines represent 95% confidence intervals. Copyright Information 28 Page 29 of 39 Parasympathetic reactivation after sprinting Table 1. Energy system contributions of the three exercise trials. RS MC HI 254.7 ± 8.8 276.7 ± 9.1* 563.8 ± 14.8*† O2 stores (ml.kg-1) 2.3 ± 0.0 2.3 ± 0.0 2.3 ± 0.0 [La]b (mmol.l-1) 10.9 ± 0.6 3.5 ± 0.2* 11.6 ± 0.5† [La]b O2 equivalent (ml.kg-1) 29.6 ± 1.7 7.4 ± 0.7* 31.9 ± 1.6† PCr O2 equivalent (ml.kg-1) 15.2 ± 0.2 15.2 ± 0.2 15.2 ± 0.2 % Aerobic energy contribution 85.2 ± 0.7 92.4 ± 0.3* 97.3 ± 0.4*† % Anaerobic energy contribution 14.8 ± 0.7 7.6 ± 0.3* 2.7 ± 0.4*† Cumulated V& O2 (ml.kg-1) Mean ± SE mean and cumulated O2 consumption, estimated oxygen stores (myoglobin content), blood lactate concentration ([La]b), estimated blood lactate concentration O2 equivalent, estimated PCr O2 equivalent and relative contribution of aerobic and anaerobic energy processes for each of the three exercise trials: repeated sprint (RS), moderate continuous (MC) and high-intensity exercise (HI) running. *: significant difference vs. RS (P <0.001). †: significant difference between HI and MC (P <0.001). Copyright Information 29 Page 30 of 39 Parasympathetic reactivation after sprinting Table 2. Heart rate (HR) indexes of parasympathetic reactivation following each exercise trial. Parasympathetic reactivation indexes RS MC HI 37.45 ± 1.92 54.34 ± 3.11* 35.94 ± 1.70† 288.30 ± 19.90 144.11 ± 12.21* 295.20 ± 30.46† HRR (s) 74.47 ± 3.71 54.41 ± 6.39* 100.99 ± 7.02*† HRamp (bpm) 72.27 ± 2.76 76.54 ± 2.99 78.19 ± 1.94 HR5-10min (bpm) 112.28 ± 2.60 91.67 ± 2.45* 119.51 ± 2.84† SDNN5-10min (ms) 13.50 ± 1.13 37.28 ± 3.77* 12.03 ± 1.19† pNN505-10min (%) 0.02 ± 0.01 1.63 ± 0.82* 0.00 ± 0.00† RMSSD5-10min (ms) 4.96 ± 0.85 16.19 ± 2.93* 3.38 ± 0.35† Ln HF5-10min (ms2) 1.52 ± 0.35 4.45 ± 0.32* 1.16 ± 0.28† HFnu5-10min 0.19 ± 0.03 0.15 ± 0.01 0.22 ± 0.03 HHR60s (bpm) T30 (s) Mean ± SE parasympathetic reactivation indexes (number of heart beat recovered in 60 s after exercise cessation (HRR60s), time constant of short-time heart rate recovery (T30), time constant (HRR ) and amplitude (HRamp) of HR during the 10-min recovery, mean HR (HR510min), standard deviation of normal R-R intervals (SDNN5-10min), percentage of successive R- R differences greater than 50 msec (pNN505-10min), root mean square of successive difference of the R-R intervals (RMSSD5-10min), high frequency power (LnHF5-10min) of R-R intervals and the normalized HF power (HFnu5-10min) calculated on the last 5 min of the recovery periods after each of the three exercise trials: repeated sprint (RS), moderate continuous (MC) and high-intensity exercise (HI) running. *: significant difference vs. RS (P <0.001). †: significant difference between HI and MC (P <0.001). Copyright Information 30 Page 31 of 39 Parasympathetic reactivation after sprinting Table 3. Relationships between post-exercise parasympathetic reactivation indexes and mechanical and metabolic exercise characteristics. Anaerobic Constant Overall Aerobic Contribution NEE Muscular Power Contribution P HHR60s T30 HRR 74.45 ± 4.92 -22.48 ± 31.16 8.59 ± 11.97 <0.001 0.48 0.48 HR5-10min 68.43 ± 6.87 <0.001 RMSSD5-10min 29.64 ± 4.21 <0.001 Ln HF5-10min 7.88 ± 0.74 <0.001 P 0.01 ± 0.01 0.03 ± 0.15 0.08 ± 0.06 0.04 ± 0.03 -0.01 ± 0.02 -0.01 ± 0.01 0.97 0.86 0.19 0.26 0.73 0.49 relationship P -0.43 ± 0.22 2.99 ± 1.33 1.16 ± 0.52 0.63 ± 0.29 -0.12 ± 0.18 -0.04 ± 0.03 0.05 0.03 0.03 0.03 0.49 0.24 P -0.08 ± 0.07 0.62 ± 0.44 0.1 ± 0.17 -0.01 ± 0.09 -0.06 ± 0.06 -0.01 ± 0.01 0.27 0.17 0.96 0.89 0.2 0.22 P -0.83 0.61 9.12 ± 3.78 -0.85 1.47 0.58 0.83 -0.89 0.50 -0.23 0.09 0.19 0.02 0.57 0.49 0.09 0.06 r² P 0.60 <0.001 0.73 <0.001 0.57 <0.001 0.55 <0.001 0.47 <0.001 0.65 <0.001 Mean -coefficient (± SE) and P values from multiple linear regressions between post-exercise parasympathetic reactivation indexes [number of heart beats recovered in 60 s after exercise cessation (HRR60s), time constant of short-time heart rate recovery (T30), time constant (HRR ) of HR during the 10-min recovery, mean HR (HR5-10min), standard deviation of normal R-R intervals (SDNN5-10min), root mean square of successive difference of the R-R intervals (RMSSD5-10min) and the high frequency power (LnHF5-10min) of R-R intervals calculated on the last 5 min of the recovery periods after each exercise trial] and four exercise characteristics: aerobic contribution (net oxygen cost), anaerobic contribution (oxygen equivalent of anaerobic exercise), net energy expenditure and muscular power developed during each exercise trails. Copyright Information 31 Page 32 of 39 Parasympathetic reactivation after sprinting *† 200 NEE (Kcal) 14 Mean power (W.kg-1) 250 150 100 50 700 *† 600 500 400 300 200 100 70 8 *† 6 * 4 2 60 † 50 40 30 * 20 10 0 6 * 50 † 40 * 5 30 Ln HF (msec2) 60 HRR60s (bpm) 10 0 O2 equivalent of net exercise anaerobic cost (ml.kg-1) Net exercise O 2 cost (ml.kg-1) 0 12 20 4 3 † 2 1 10 RS MC 0 HI RS MC HI Fig. 1. Copyright Information 32 Page 33 of 39 Parasympathetic reactivation after sprinting 800 R-R intervals (ms) 700 600 500 400 MC RS HI 300 200 0 100 200 300 400 500 600 Recovery time (s) Fig. 2. Copyright Information 33 Page 34 of 39 Parasympathetic reactivation after sprinting 16 † † 14 RMSSD30s (ms) † 12 * * 10 * † † † † † † † * * * * † †* † † * * * * * * * * * * MC RS HI 8 6 4 2 0 0 60 120 180 240 300 360 420 480 540 600 Recovery time (s) Fig. 3. Copyright Information 34 Page 35 of 39 Parasympathetic reactivation after sprinting 500 r = 0.83 P < 0.001 n = 45 T30 (s) 400 300 200 MC RS HI 100 0 0 10 20 30 40 50 60 70 Oxygen equivalent of net anaerobic energy cost (ml.kg-1) Fig. 4. Copyright Information 35 Net exercise O2 cost (ml.kg-1) HRR60s (bpm) 50 0 700 600 500 400 300 200 * MC *† † HI 4 2 0 60 50 40 30 20 10 0 6 5 4 3 2 1 0 Copyright Information Ln HF (msec ) 100 70 60 50 40 30 20 10 RS Mean O2 equivalent of net exercise anaerobic cost (ml.kg-1) 2 RS * * MC † † HI Page 36 of 39 Page 37 of 39 800 R-R intervals (ms) 700 600 500 400 MC RS HI 300 200 0 100 200 300 400 Recovery time (s) Copyright Information 500 600 Page 38 of 39 16 † † 14 RMSSD30s (ms) † 12 * * 10 * † † † † * * * * * * * †* * * * * * † † † * * † † † MC RS HI 8 6 4 2 0 0 60 120 180 240 300 360 420 480 540 600 Recovery time (s) Copyright Information Page 39 of 39 500 r = 0.83 P < 0.001 n = 45 T30 (s) 400 300 200 MC RS HI 100 0 0 10 20 30 40 50 60 Oxygen equivalent of net anaerobic energy cost (ml.kg-1) Copyright Information 70