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00005768-200705000-0001800005768_2007_39_872_bishop_thermoregulatory_5miscellaneous-article< 124_0_29_10 >Medicine & Science in Sports & Exercise©2007The American College of Sports MedicineVolume 39(5)May 2007pp 872-879Effects of Active versus Passive Recovery on Thermoregulatory Strain and Performance in Intermittent-Sprint Exercise[APPLIED SCIENCES: Biodynamics]BISHOP, DAVID1,2; RUCH, NICOLE1; PAUN, VANESSA11School of Human Movement and Exercise Science, UWA, AUSTRALIA; 2Team Sport Research Group, Facota' di Scienze Motorie, Universita' degli Studi di Verona, ITALYAddress for correspondence: David Bishop, Ph.D., Team Sport Research Group Facoltà di Scienze Motorie, Università di Verona, Via Casorati 43, 37131 Verona, Italy; E-mail: bishop@motorie.univr.it.Submitted for publication June 2006.Accepted for publication November 2006.ABSTRACTPurpose: Team sports contain high-intensity sprints separated by active recovery (AR) and passive recovery (PR). The beneficial effects of AR on repeated-sprint performance, for short exercise duration, in thermoneutral environments, are well known. However, team sports are often performed in hot environments for prolonged periods. Therefore, the aim was to investigate the thermal strain of AR versus PR during prolonged, intermittent-sprint exercise.Methods: Eight men performed two intermittent-sprint tests in the heat (35°C, 44% relative humidity (RH)), with either AR or PR.Results: No differences were found between conditions for mean work (AR: 3739.5 ± 204.7 J; PR: 3814.0 ± 161.3 J) or power per sprint (AR: 1257 ± 64 W; PR: 1245 ± 47 W). AR was associated with a significantly higher heart rate (HR), muscle (Tmu), rectal temperature (Tre), body temperature (Tb), and skin temperature (Tsk) after 7, 10, and 25 min, respectively. Body heat storage, and physiological and cumulative heat-strain indices, were significantly higher in AR compared with PR. The differences in Tmu and thermoregulatory strain between AR and PR were greater than the differences in Tre and Tb.Conclusions: These results likely can be attributed to a greater rate of whole-body heat loss during the AR protocol. Because AR has previously been associated with a greater muscle pump, a greater blood flow to surface veins and inactive musculature may have been maintained, allowing greater heat dissipation than during PR, when blood was likely to be pooling in the legs. Despite the greater increase in body temperature and heat strain in AR than in PR, there was no difference in performance, possibly because critical temperature levels were not reached in this study.High-intensity sprints separated by active recovery (AR) and passive recovery (PR) represent the typical motion pattern of various team sports (28). The ability to perform repeated sprints is likely influenced by the nature of the recovery between sprints. Although most of the recovery between repeated sprints in a men's field hockey game was reported to be spent in AR of varying speeds (28), the remaining time consists of PR; PR is also used in various team sport training drills. It is well documented that compared with PR, AR has a beneficial effect on the power output during subsequent high-intensity performance during maximal exercise lasting several minutes, when followed by 10 or more minutes of AR (30% V˙O2max) (15). The same findings have been reported regarding high-intensity sprint performance (four or five 6-s sprints separated by 5 min of AR or PR) (1). Supporting these findings, studies focusing particularly on power output have shown both mean and peak power to be higher in AR than in PR during 6-s (27) and 15-s (4) sprints that are separated by 30-s and 3-min recovery periods, respectively. In contrast, in a study using 15-s sprints separated by 15 s of AR (50% V˙O2peak) or PR, time to exhaustion was found to be greater in PR (745 s) than in AR (445 s) (7). Thus, AR seems to improve subsequent sprint performance, except when time to exhaustion is measured and/or recovery time is very short.To date, no study has investigated the effects of active versus passive recovery on high-intensity, intermittent-sprint performance in hot conditions. When environmental conditions are hot, compared with moderate or cool, humans have a decreased capacity to perform high-intensity sprint exercise (23). Although some authors have suggested elevated rectal temperature to be the key factor in the decreased exercise performance seen in the heat (23), others have primarily associated the reduction in exercise performance with an increased heat strain (14). Therefore, differences in rectal temperature and heat strain between AR and PR conditions may affect high-intensity intermittent-sprint performance.The type of recovery between sprints is likely to influence both heat production and dissipation. As the muscles continue to work during AR, they will continue to produce heat. The effects of AR versus PR on heat dissipation are less clear. Stroke volume during PR has been reported to decline to below preexercise levels and is proposed to be caused by the pooling of venous blood in motionless muscles (29). These authors suggest that continuous limb movement after exercise was important in maintaining stroke volume and cardiac output at higher levels. Therefore, one possible benefit of AR is that it will maintain blood flow to the active muscle and promote the transfer of heat from the core to the periphery. Despite this, it has been reported that AR results in a significantly elevated Tre, compared with passive recovery, after six 1-min bouts of exercise (8). Given the lack of information regarding the effects of recovery between repeated sprints on thermoregulatory responses, the aim of this study was to investigate the physiological and performance responses to high-intensity intermittent-sprint exercise with either short AR or PR in the heat. It was hypothesized that active recovery between sprints would increase body heat storage and thermal strain, resulting in a decrease in intermittent-sprint performance.METHODSSubjects.Eight male recreational team sport athletes (age: 21.2 ± 2.1 yr; height: 177.1 ± 5.1 cm; mass: 74.2 ± 8.3kg; V˙O2peak: 50.70 ± 4.5 mL·kg−1·min−1; lactate threshold: 175 ± 20 W) volunteered to participated in this study. All subjects were informed about the purpose and risks of the procedures adopted in the study and gave written consent before commencing. The participants were required to abstain from caffeine-containing foods and beverages and to avoid alcohol consumption for 24 h before each test. They were asked to complete a food diary for the 24 h before each test and to avoid vigorous exercise for 48 h before each test. Two hours before each test, the subjects were required not to consume food or beverages other than water. Subjects were asked to prepare for each testing session as they would for an important match and to prepare for each test in an identical manner (including replicating the meals recorded in the food diary). All procedures described in this project were approved by the institutional ethics committee.Experimental overview.Each participant completed a preliminary session that contained the familiarization session for the graded-exercise test (GXT). In this session, anthropometric data (standing height and body mass) were also collected. At least 48 h later, subjects performed the GXT test to obtain their V˙O2peak and to determine their required power output for AR. Two days after the GXT, subjects completed a familiarization of the intermittent-sprint test (IST). This session also included familiarization to the perceptual scale. The two experimental sessions were then performed in a random, counterbalanced, crossover design, with each trial being no fewer than 4 d apart. Each experimental trial required subjects to exercise in a hot environment (34.9 ± 0.1°C, 45.5 ± 1% relative humidity (RH)), with all trials conducted at the same time of day for each subject, to control for diurnal effects.GXT.The GXT was performed on an air-braked, front-access cycle ergometer (Repco, Sydney, Australia) and consisted of graded 4-min exercise steps using an intermittent protocol (1 min of rest between stages). The test commenced at 70 W; thereafter, intensity increased by 30 W every 4 min until volitional exhaustion. Subjects were required to maintain the set power output, which was displayed on a computer screen in front of them. The criteria for reaching V˙O2peak were the attainment of a plateau in oxygen consumption (an increase of less than 0.15 L·min−1) and/or a respiratory exchange ratio greater than 1.15. Strong verbal encouragement was provided to each subject throughout and especially towards the end of the test. The lactate threshold, V˙O2peak, and the power output for the AR exercise protocol were determined from the GXT. The lactate threshold was calculated using the modified Dmax method. This is determined by the point on the polynomial regression curve that yields the maximal perpendicular distance to the straight line connecting the first increase in lactate concentration above resting level and the final lactate point.Intermittent-sprint test.The IST was performed on an air-braked, front-access cycle ergometer (Repco, Sydney, Australia). The IST was based on a motion analysis of international men's field hockey (28) and consisted of 37 min of intermittent-sprint exercise (Fig. 1). The AR protocol was divided into blocks of approximately 2 min, consisting of a 4-s sprint, AR of 100 s, and PR of 20 s. One PR block contained a 4-s sprint and 120 s of PR until the next sprint. In the present study, AR was chosen at 35% V˙O2peak (approximately 87 ± 3 W), consistent with various other studies that have used workloads corresponding to 30-40% V˙O2peak (1,3,9).FIGURE 1- Schematic overview of the exercise protocol containing 18 × 2-min blocks of either active or passive recovery. B, blood sample; HR, heart rate; Tre, rectal temperature; Tsk, skin temperature; Tmu, muscle temperature; RPE, rating of perceived exertion.Gas analysis.During the GXT, expired air was continuously monitored. Oxygen uptake and carbon dioxide production were calculated every 15 s using a computerized online gas-analysis system comprising a ventilation monitor (Morgan, Reinham, Kent, UK), an Ametek S3A oxygen analyzer, and an Ametek DC3A carbon dioxide analyzer (Ametek, Paoli, PA), all of which were calibrated before testing. The gas analyzer was calibrated immediately before and verified after each test using one (α-certified) β-grade gas mixture (BOC Gases, Chatswood, Australia). The ventilometer was calibrated before exercise using a 1-L syringe in accordance with the manufacturer's instructions. The ventilometer and gas analyzers were connected to an IBM PC that measured and displayed the variables.Blood sampling and analysis.A hyperemic ointment (Finalgon, Boehringer Ingelheim, Germany) was applied to the earlobe 5-7 min before initial blood sampling. Capillary blood samples (35 or 100 L) were taken in heparinized glass capillary tubes (Radiometer, Copenhagen, Denmark) for the GXT and the intermittent-sprint exercise trials, respectively. During the GXT, blood samples were taken at the end of every 4-min stage. During the intermittent-sprint exercise trials, blood samples were taken immediately before the start of either protocol and 17 and 35 min into each protocol (Fig. 1). Plasma lactate concentration was determined using a blood-gas analyzer (ABL 625, Radiometer, Copenhagen, Denmark). The blood-gas analyzer was calibrated before each test using precision standards and was routinely assessed by external quality controls.Temperature measurements.Tre was measured using a thermistor (Model RET-1, Physitemp Instruments, Clifton, NJ) from a site 10 cm beyond the anal sphincter. Skin thermistors (Model SST-2, Physitemp Instruments, Clifton, NJ) were attached to four sites: the upper chest, lower forearm, upper thigh, and medial side of the calf. All readings were recorded at 5-s intervals. Mean skin temperature (Tsk) was computed via the formula:Equation (Uncited)where Tch is the chest, Tfa is the forearm, Tth is the thigh, and Tca is the calf temperature (17). Mean body temperature (Tb) was calculated asEquation (Uncited)Sixty minutes before the IST, subjects were asked to empty the contents of a topical anaesthetic (Emla, Atra Zeneca, North Ride, Australia) onto a 2-cm2 area of the thigh that covered the site where muscle temperature was to be recoded, with a dressing affixed directly over the cream. Tmu was monitored via a needle thermistor probe (Model T-204A, Physitemp Instruments, Clifton, NJ) inserted 4 cm into the anaesthetized area of the vastus lateralis muscle. This technique involved the use of an 18-gauge, 50-mm polyethylene needle (Cathlon and Critikon, Markham, Ontario, Canada) piercing the skin to place a catheter in the muscle. The needle was then removed, leaving the catheter in the muscle. After this, the temperature probe was inserted through the catheter into the muscle and the catheter was removed, leaving only the probe in the muscle. The probe was covered and securely fixed to subject's thigh, to allow movement and continual measurement of Tmu. Temperature measurements were taken to nearest 0.01°C every 10 s during the IST from all thermistor sites. For the results, the last 30 s of every 5 min were averaged. The thermal core-to-skin-gradient was calculated as Tre − Tsk, the core-to-muscle gradient as Tre− Tmu, and the muscle-to-skin-gradient as Tmu − Tsk.Heat-strain/storage calculations.The physiological strain index (PSI) was calculated according to Moran et al. (19) using the following equation:Equation (Uncited)where Trefin and HRfin were the rectal temperature and heart rate (HR) values simultaneously measured at the end of the intermittent exercise protocol. Tre0 and HR0 were the seated initial rectal and HR resting values in the chamber before the beginning of the exercise protocol.The cumulative heat-strain index (CHSI) was also used to assess the total physiological strain experienced by subjects exposed to an exercise heat stress (10). Mathematically, the index was calculated as follows:Equation (Uncited)where HR0 = initial lowest HR (bpm), and t = time (min) from the onset of measurements.The body heat storage rate (S) was determined by the rate of change in Tb, body mass (m), DuBois surface area (AD) (6), and specific heat of the body (0.97 W·kg−1) by use of the following equation:Equation (Uncited)Perceptual scale.Ratings of perceived exertion (RPE) were obtained immediately before blood sampling throughout both experimental trials using a perceptual scale, where 6 was very, very light and 19 was very, very hard.HR.HR was monitored telemetrically throughout all exercise protocols using a polar HR monitor (Accurex Plus, Polar Electro, Kempele, Finland).Statistical analysis.The results of the study were analyzed using two-way (condition vs time) analysis of variance with repeated measures for time. When significant main or interaction effects were found, post hoc comparisons were employed (Student-Newman-Keuls test) to locate the significant differences. Pre and final values (Tre, Tmu, Tsk, HR, and plasma lactate concentrations) were also analyzed by paired-samples t-tests. Values are expressed as means ± SD, and significance was accepted at P < 0.05.RESULTSPerformance.Work produced in individual sprints was not significantly different in the AR or the PR exercise protocols (Fig. 2A). There also were no significant differences between AR and PR for peak power (Fig. 2B). No significant differences were found between conditions for mean work per sprint (AR: 3739 ± 205 J; PR: 3814 ± 161 J) or mean peak power per sprint (AR: 1257 ± 64 W; PR: 1245 ± 47 W) during the IST.FIGURE 2- Work (A) and peak power (B) per sprint in the AR and PR exercise protocols performed in hot conditions. Values are mean ± SD (N = 8).Thermoregulatory responses.There were significant increases in both Tre and Tb from the start to the end of the exercise protocol during both AR and PR, and these increases were significantly greater in the AR than in the PR condition (Table 1). As a consequence, the final Tre and Tb values were also significantly greater in AR than in PR (Fig. 3A,B). Whereas Tmu exhibited a similar pattern of increase in both exercise protocols, after 10 min, Tmu was significantly higher in AR compared with PR. The largest increase in Tmu occurred within the first 5 min of exercise, with Tmu then plateauing after approximately 15-20 min of exercise (Fig. 3A). Significant differences were also present between pre- and postexercise muscle temperatures for both protocols (Table 1). Tmu exceeded Tre after 5-10 min in the AR mode and reached similar values as Tre in the PR mode after 20 min (Fig. 3A). There was a significant increase in Tsk in AR only (Table 1), resulting in significant differences between AR and PR after 25 min of exercise (Fig. 3A).TABLE 1. Differences between pre- and postexercise values and the final values in the active recovery and passive recovery exercise protocols performed in hot conditions.FIGURE 3- Tre, Tmu, and Tsk (A) and Tb (B) during the AR and PR exercise protocols performed in hot conditions. Values are mean ± SD (N = 8). *Significantly different between recovery conditions (P < 0.05).The core-to-skin thermal gradient (Fig. 4) was significantly higher during the PR protocol after 15, 25, 35, and 37 min, with differences also approaching significance at 20 min (P = 0.089) and 30 min (P = 0.056). Pre- and postexercise values for the thermal core-to-skin gradient were significantly different for the PR but not for the AR exercise protocol (Table 1). A different pattern was seen for the thermal muscle-to-skin gradient (Fig. 4), with the gradient significantly greater in AR, compared with PR, only in the first 10 min of exercise. Pre- and postexercise values were also significantly different in both conditions. The muscle-to-core thermal gradient (Fig. 4) was significantly higher in AR than in PR from minutes 5 to 20, followed by an apparent plateau in both conditions (after 15 min in AR and after 20 min in PR). Pre- and postexercise values were significantly different in both recovery conditions (Table 1). In the PR condition, Tmu did not exceed Tre, resulting in a muscle-to-core thermal gradient that was never greater than zero.FIGURE 4- Thermal core-to-skin gradient (Tre - Tsk), muscle-to-skin gradient (Tmu - Tsk), and muscle-to-core gradient (Tmu - Tre) in the AR and PR exercise protocols performed in hot conditions. Values are mean ± SD (N = 8). * Significantly different from alternate recovery condition (P < 0.05); b Significantly different from zero (P < 0. 05).Body heat storage was significantly higher for the AR exercise protocol compared with the PR exercise protocol (AR: 61.1 ± 5.1 W·m−2; PR: 38.7 ± 4.6 W·m−2). Significantly higher PSI (AR: 6.3 ± 0.5 vs PR: 4.0 ± 0.5) and CHSI values (AR: 43.1 ± 16.3 vs PR: 11.3 ± 4.3) were also achieved during the AR exercise protocol compared with the PR exercise protocol.HR.Throughout the exercise protocols, HR was significantly higher during AR than PR (AR final: 156 ± 7 bpm; PR final: 123 ± 6 bpm). Although there was a significant increase in HR during both protocols, there was a significantly greater difference between pre- and postexercise values during the AR protocol (pre-post AR difference: 75 ± 6 bpm; pre-post PR difference: 42 ± 3 bpm).Plasma lactate concentrations.There were no significant differences in plasma lactate concentration between protocols at any time (AR final: 6.0 ± 0.5 mM; PR final: 6.3 ± 0.8 mM), although there were significant differences between the pre and final values for both exercise protocols (pre-post difference: 4.9 ± 0.6 mM; pre-post difference: 4.7 ± 0.8 mM).RPE.No significant differences in RPE between recovery conditions were found for any time period (final values: AR: 14.5 ± 0.5; PR: 13.5 ± 1.0).DISCUSSIONThe major findings of this study are that the differences in Tmu, Tsk, and thermoregulatory strain between AR and PR were greater than the differences in Tre and Tb. This suggests that although greater heat was produced by the muscles during the AR protocol, there was also greater transfer of heat from the muscles to the skin. Despite differences in the thermoregulatory responses, intermittent-sprint performance in the heat did not differ between the two recovery conditions. This contrasts with the results of previous research conducted in thermoneutral conditions (4,27).Thermoregulatory responses.During exercise, the rate of heat liberation is increased as a result of a higher metabolic energy turnover in the exercising muscles. Because the efficiency to convert chemical energy to external work is less than 20-25%, a large proportion of the liberated energy appears as heat in the active muscle (24). This local metabolic heat production (plus vascular heat delivery from the viscera) results in a rapid increase in Tmu, especially during the first 5 min of exercise (26). Thus, the greater Tmu after AR compared with after PR can be attributed largely to the greater work (and hence heat production) during AR. Consistent with previous research (26), the rate of increase in Tmu in the present study tended to plateau after approximately 20 min.It has been reported previously that heat produced in the muscle is conducted to the blood flowing through the muscles and is distributed in this way to the rest of the body core, promoted by the increased circulation to the exercising muscle (24). Furthermore, a relatively constant difference between Tre and Tmu has been reported through a range of exercise intensities (26). However, Gisolfi and Robinson (11) have suggested that the rate of heat accumulation within the core region is attenuated to a large degree by an increase in the rate of whole-body heat loss. This was supported by their observation that muscle heat seemed to be lost quickly across the leg surface, with the temperature of the blood in a leg vein dropping progressively as it traveled from the ankle to the knee in a subject walking on a treadmill (11). Thus, the smaller differences in Tre than in Tmu between the AR and PR conditions may be attributable to greater transfer of heat from the muscle to the skin during AR. Because the temperature of the skin is a function of the quantity of heat that flows into it, this is supported by the greater Tsk in AR. This suggests that in the present study, there was a greater attenuation of heat accumulation in the core during AR than during PR, and this was mediated by a greater increase in the rate of whole-body heat loss (nonevaporative and evaporative heat loss).To dissipate the heat transferred to the skin, several mechanisms are available (i.e., conduction, convection, radiation, and evaporation). Evaporation is the predominant heat-loss mechanism when ambient temperatures exceed Tsk (as was the case in the present study). Furthermore, it has been estimated that when the ambient temperature is 35°C, evaporation contributes approximately 90% to whole-body heat loss (21). An increase in Tre is the main determinant for sweating (30), stimulating the cutaneous vasodilatation that enables the high blood flow needed for sweating (24).It has been suggested that continuous limb movement during AR is also important in maintaining skin blood flow by activating the skeletal muscle pump at higher levels compared with PR (29). This is supported by research showing that cutaneous vascular conductance was elevated in AR compared with PR conditions (16). This is likely to have contributed to the greater Tsk in AR compared with PR. Skin blood-flow response during exercise has previously been reported to be higher in the upper inactive limbs than in the lower active limbs (25). Therefore, it is proposed that compared with PR, AR was able to maintain a greater blood flow to the skin of the upper limbs in the present study, with Tch dropping significantly in PR (but not in AR) and with final Tch being 1.0°C higher in AR than in PR. In contrast, at the end of exercise (start of PR), venous pressure in the legs has been reported to decrease, resulting in venous blood pooling in the previously active musculature, and decreased cardiac filling (29). These changes, along with decreased input from the mechanoreceptors, have been reported to decrease skin blood flow (11), potentially reducing the heat lost across the skin. Because the mechanisms for active vasodilation, along with central command, have been associated with the sweat response (16), heat loss through evaporation is also likely to be decreased during PR. Thus, it is possible that PR was not able to maintain as great a blood flow to the upper limbs, because of the venous pooling in the lower limbs, resulting in less heat transfer to the chest and the inactive limbs.Despite the likely increase in heat loss during AR, body heat storage remained significantly higher in the AR condition compared with the PR condition. The greater heat storage in the AR condition, despite no differences in Tre between recovery conditions, can largely be attributed to the greater heat transfer to the skin in AR than in PR. Because the heat-storage calculations were originally developed to compare the same exercise in different environments, they may be limited in comparing different exercises in the same environment. The greater heat storage in the AR condition most likely contributed to the greater heat strain in the AR than in the PR condition. Whereas the PSI values are comparable with, or higher than, previously reported values (13,20), the more recent CHSI has not been reported for a similar exercise task. Nonetheless, the greater heat storage supports the finding that the AR condition resulted in greater heat strain compared with the PR condition but that the increase in Tre was limited by the greater heat transfer to the skin and the greater whole-body heat loss during the AR condition.Performance.Although a greater increase in rectal temperature (12,23) and an increased heat strain (12,14) have been suggested to impair exercise performance in the heat, mean and peak power were not significantly different between recovery conditions. This can probably be attributed to the modest increases in temperature recorded and the length of the recovery between sprint efforts. It has been reported previously that 40-m sprint times (˜6 s) were not significantly different between the first and last of 15 sprints when 2 min of recovery were allowed between sprints (2). Thus, the 2-min recovery time in the present study was likely to have been sufficient for recovery from the 4-s all-out sprints, regardless of the type of recovery performed, even in hot, humid conditions. This may explain why our results differ from previous research that has reported smaller decrements in repeated-sprint performance with, AR compared with PR, when short sprints (6 s) were separated by 30 s of recovery (27). Thus, type of recovery may only affect intermittent-sprint performance when there is a decline in sprint performance from the first to the last sprint.The absence of a significant difference in performance between conditions also may be related to the modest increases in temperature recorded. Although the IST was based on the physiological demands of one half of a team sport competition, the final Tre was "only" 38.15-38.42°C. This is below what has been previously proposed to be a critical Tre for both prolonged intermittent (39.4-40.0°C (5,22)) and prolonged continuous (˜40°C (12)) exercise in the heat. Further research is required to investigate the effects of recovery on intermittent-sprint performance in the heat for a longer time period (˜80-90 min, which would be more indicative of the typical game duration for a team sport).Physiological responses.HR was found to be significantly higher in the AR than in the PR protocol after 3 min. Final HR in the present study was 156 ± 7 and 123 ± 6 bpm for the AR and PR conditions, respectively. Although these values are lower than those reported for other intermittent exercise in the heat, these previous studies have involved longer sprint durations (20-s sprints; 180 and 186 bpm (18)) and/or have been performed until exhaustion (180 and 186 bpm (23)). In a previous study investigating AR and PR in the heat, HR remained at 128 ± 11 bpm in AR, compared with decreasing values during PR (8). The higher HR in the AR recovery condition is also consistent with previous recovery studies in thermoneutral conditions incorporating four to five 6-s sprints and an AR at 35% V˙O2peak (1). The higher HR in the AR condition can most likely be attributed to the increase in competition for blood supply between the heart and the exercising muscles, and the increased skin blood flow necessary to carry heat to the skin when there is a concomitant decrease in the Tre - Tsk thermal gradient (Qsk = heatskin/c[Tre − Tsk] (24)). 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