Heart Rate Does Not Reflect the %VO2max in Recreational Runners during the Marathon

ise physiologists and coaches prescribe heart rate zones (between 65 and 80% of maximal heart rate, HRmax) during a marathon because it supposedly represents speci c metabolic zones and the percentage of . V O 2maxbelow the lactate threshold. The present study tested the hypothesis that the heart rate does not re ect the oxygen uptake of recreational runners during a marathon and that this dissociation would be more pronounced in the lower performers' group (>4 h). While wearing a portable gas exchange system, ten male endurance runners performed an incremental test on the road to determine . V O 2max, HRmax, and anaerobic threshold. Two weeks later, the same subjects ran a marathon with the same device for measuring the gas exchanges and HR continuously. The %HRmaxremained stable after the 5th km (between 88% and 91%,p= 0.27), which was not signi cantly different from the %HRmaxat the ventilatory threshold (89 4% vs. 93 6%, p= 0.12).

. V O 2max, HRmax, and anaerobic threshold. Two weeks later, the same subjects ran a marathon with the same device for measuring the gas exchanges and HR continuously. The %HRmaxremained stable after the 5th km (between 88% and 91%,p= 0.27), which was not signi cantly different from the %HRmaxat the ventilatory threshold (89 4% vs. 93 6%, p= 0.12). However, the % . V O 2maxand percentage of the speed associated with . V O 2maxdecreased during the marathon (81 5 to 74 5 % . V O 2maxand 72 9 to 58 14 %v . V O 2max,p< 0.0001). Hence, the ratio between %HRmaxand % . V O 2maxincreased signi cantly between the 5th and the 42nd km (from 1.01 to 1.19,p= < 0.001). In conclusion, pacing during a marathon according to heart rate zones is not recommended. Rather, learning about the relationship between running sensations during training and racing using RPE is optimal. Keywords: self-pace run; energy cost of running; exercise physiology; endurance; running performance; pacing 1. Introduction Marathon running has increased in popularity since humans rst set foot on the moon and with this popularization emerged a new category of runners, namely: “recre- ational marathon runners” [1–3]. These runners are eager [4], less well trained, and experimenters [5,6]. Hal Higdon, an experienced coach, and marathon runner (the rst American 1964 Boston marathon nisher; 2:21:55) [7], purports that the key to performance in the marathon is proper pacing. However, ill-advised pacing remains the biggest prob- lem for recreational marathoners as many runners continue to “hit the wall” late in the marathon due to their pacing strategies [8]. More than 80% of runners who “hit the wall” during a marathon report cardio-respiratory distress—increases and decreases in heart rate (HR) and feeling a general malaise and burnout after the 30th km [9]. The marathon is the ultimate exercise in both intensity and duration. It has been shown that elite [10] and recreational runners can reach 100% of their . V O2maxduring the marathon [11,12]. The ability to sustain a high fraction of . V O2maxis a good indicator of

(HR) and feeling a general malaise and burnout after the 30th km [9]. The marathon is the ultimate exercise in both intensity and duration. It has been shown that elite [10] and recreational runners can reach 100% of their . V O2maxduring the marathon [11,12]. The ability to sustain a high fraction of . V O2maxis a good indicator of marathon perfor- mance [5,13–16]. Indeed, recreational marathon runners often take over twice the time (of the winner) to nish a marathon, and thus sparing muscle glycogen becomes even more Int. J. Environ. Res. Public Health2022,19, 12451.

Int. J. Environ. Res. Public Health2022,19, 12451 2 of 13 critical in order not to suffer from severe fatigue and “hit the wall” [17]. The most widely accepted theory for the association between low muscle glycogen and impaired muscle contractile function is glycogen depletion resulting in a reduction in ATP regeneration. Consequently, the muscles cannot maintain an adequate energy supply to the processes involved in excitation-contraction coupling, leading to an inability to translate muscle drive into an expected force; when this occurs, cramping and fatigue lead to “hitting the wall”. This is supported by observations of decreased phosphocreatine in addition to an increase in free adenosine diphosphate and inositol monophosphate following prolonged glycogen-depleting exercise [18,19]. Runners often pre-plan their pacing efforts using a pacing pro le according to past marathon performances [20], feedback from the perceived exertion rate [12], or heart rate and speed. A small pacing error can result in feelings of a subpar performance or severe fatigue and “hitting the wall”. Studies show that previous marathon experience in uences a runner's pace and that there is an interaction between feedback (heart or respiratory rates) and the rate of perceived exertion (RPE) [12,21]. Exercise physiologists and coaches prescribe heart rate zones (between 65 and 80% of maximal heart rate, HRmax) during a marathon because it supposedly represents speci c metabolic zones and the percentage of . V O2maxbelow the lactate threshold, allowing the sparing of muscle glycogen [17]. Heart rate does not serve as an indicator of environmental factors but rather provides an indication of the body's exercise response when environmental factors change. A prior study [22] that measured the cardiac output (CO) and the stroke volume (SV), of 14 recreational runners during a marathon which they completed in an average time of 3 h 30 min 45 min, showed that they elicited a higher fraction of HRmaxthan the one of their SV and CO (87.0 1.6% vs. 77.2 2.6%, and 68.7 2.8%, for HR, SV and CO, respectively,p< 0.05) [23]. Furthermore, data collected during an of cial marathon race showed that HR was elevated throughout the marathon and

an average time of 3 h 30 min 45 min, showed that they elicited a higher fraction of HRmaxthan the one of their SV and CO (87.0 1.6% vs. 77.2 2.6%, and 68.7 2.8%, for HR, SV and CO, respectively,p< 0.05) [23]. Furthermore, data collected during an of cial marathon race showed that HR was elevated throughout the marathon and increased over time, but without knowing an individual's baseline . V O2kinetics and HRmax, there is no suf cient information concerning relative intensity [23]. Indeed, HR is ineffective for estimating the metabolic zone due to cardiovascular drift. A study of 280 recreational marathoners (2 h 30 min–3 h 40 min) showed that the relationship between heart rate increases for each meter run (cardiac cost) [24], and performance speed (m/s) was highly dependent on pacing strategy [25]. A higher increase in cardiac cost was associated with lower performance, resulting in a probable dissociation with the . V O2and HR over time. Consequently, a wrong pacing strategy may lead to an erroneous estimation of an athlete's metabolic zones, i.e., A non-corresponding increase between HR and . V O2. In the same way, performance speed (v, m/s) in running depends on the maximal metabolic power available to the athlete throughout the effort and on the economy of running (1): v=Emax/C=F . VO2max/Cr (1) where Cr (mlO2 m 1 kg 1 ) is the energy cost of running the fraction (F, a dimensionless number) is the percentage of . V O2maxthat can be sustained over a race. Any increase in Cr would inevitably lead to a decrease in v [17]. In this regard, Cr increase with distance completed during simulated competitions that are shorter or identical in duration to the marathon or half marathon. For longer distances, results are equivocal: Cr did not increase after a 6-h ultramarathon, but it increased after 5 h and after 8 h of running at a pace corresponding to 55% and 40% of . V O2max, respectively [26]. Despite a drop in running speed that the . V O2level will stay relatively high. We already know that runners

For longer distances, results are equivocal: Cr did not increase after a 6-h ultramarathon, but it increased after 5 h and after 8 h of running at a pace corresponding to 55% and 40% of . V O2max, respectively [26]. Despite a drop in running speed that the . V O2level will stay relatively high. We already know that runners will never attain the same percentage of . V O2maxas the percentage of HRmaxand here, the aim of the present study is to test the hypothesis of a possible dissociation between the increases in HR and . V O2of recreational runners during the completion of an actual marathon. Therefore, this disassociation between HR and . V O2, could be that most recreational runners will

Int. J. Environ. Res. Public Health2022,19, 12451 3 of 13 probably be maintaining a marathon pace above their metabolic zone if they use HR as a pacing determinant. This will especially be true in cases where their marathon time are longer than 4 hours. The estimation of the metabolic zone as a percentage of the maximal oxygen uptake (% . V O2max) using HR data during the marathon is not reliable due to the different time courses of HR and . V O2, which is even more pronounced in longer runs (4 h). 2. Materials and Methods 2.1. Subjects Our subjects were ten male, recreational endurance runners (mean standard de- viation (SD) age: 41.7 7.7 years; weight: 73.2 4.7 kg; and height: 180.5 7.0 cm) (Table). Table 1.Anthropometric characteristics of the subjects and their personal best in the marathon. Subjects Level Age (years) Weight (kg) Height (cm) BMI 1 High 47 71 175 23.1 2 High 44 82 183 24.5 3 High 33 71 177 22.7 4 High 34 68 181 20.8 5 High 37 74 193 19.9 6 Low 50 71 170 24.6 7 Low 37 66 173 22.1 8 Low 33 77 180 23.8 9 Low 53 75 186 21.7 10 Low 49 77 187 22.0 Mean 41.7 73.2 180.5 22.5 SD 7.7 4.8 7.0 1.5 All study subjects were volunteers and were asked not to modify their habitual training. They were selected for having homogenous physiological and endurance char- acteristics[27–29] , and half of the runners had previously run at least one marathon. All subjects declared to have habitually trained 3 to 4 times weekly (50–80 km/week) over more than ve years. All subjects performed a high-intensity interval training session once per week of 6 1000 m at 90–100% of HRmaxand a 15–25 km tempo session at 90–100% of their average marathon speed. The study was approved by the Institutional Review Board (IRB Sud-Est V, Grenoble, France; reference: 2018-A01496-49), and all participants were provided with study information and provided written consent. 2.2. The Incremental Maximal Test and the Marathon Race All subjects performed an incremental test (the

at 90–100% of HRmaxand a 15–25 km tempo session at 90–100% of their average marathon speed. The study was approved by the Institutional Review Board (IRB Sud-Est V, Grenoble, France; reference: 2018-A01496-49), and all participants were provided with study information and provided written consent. 2.2. The Incremental Maximal Test and the Marathon Race All subjects performed an incremental test (the Universit²de Montr²al track test, L²ger and Boucher, 1980) on the road using a portable gas exchange system for determining . V O2max, HRmax, and anaerobic threshold. The UM-TT has been validated as a valid eld test of maximal and functional aerobic capacity and suggests that it can be additionally used for exercise prescription [30,31]. The UM-TT was conducted on a 400 m track with cones placed every 20 m. Pre- recorded sound beeps indicated when the subject needed to be near a cone to maintain the imposed speed. A longer sound marked speed increments. The rst step was set to 8.5 km h 1 , with a subsequent increase of 0.5 km h 1 every minute. When the runner was unable to maintain the imposed pace and thus failed to reach the cone in time for the beep on two consecutive occasions, the test was terminated. The speed corresponding to the last completed step was recorded as the v . V O2max(km h 1 ). During the UM-TT, . V O2max was con rmed by a visible plateau in . V O2(O2mL kg 1 min 1 ) with a standard increase in exercise intensity, and any indicative secondary criteria (visible signs of exhaustion; HRmax 10 beats min 1 ) around the point of volitional exhaustion and an RPE of 19–20.

Int. J. Environ. Res. Public Health2022,19, 12451 4 of 13 2.3. The Experimental Measurements The following gas exchange variables were measured: . V O2, ventilation (VE), ventila- tory equivalents for oxygen (VE/ . V O2) and carbon dioxide (VE/ . V CO2). Data from the last 30 seconds of each exercise stage were considered representative measurements of each stage. Maximal . V O2and HR were recorded as the highest values obtained for the last 30 secons period before exhaustion. The Respiratory Compensatory Point (RCP) was iden- ti ed separately by three researchers as the point where an increase in both VE/ . V O2and VE/ . V CO2occurred [32]. All plots used in the determinations utilized raw breath-by-breath values. Respiratory gases (oxygen uptake ( . V O2), ventilation (VE), and the respiratory exchange ratio (RER)) were continuously measured using a telemetric, portable, breath- by-breath sampling system (K4; Cosmed, Rome, Italy). A GPS watch (Garmin, Olathe, KS, USA) paired with the K4 system was used to measure the HR and the speed response (using 5 s data averages) throughout each trial and its validity has been reported [33]. We used the same cardiac belt for the Garmin Forerunner 645 and K4 because it was compatible with both. The subjects self-paced their run without focusing on the cardio-GPS (the display was hidden). Two weeks later, they completed a marathon wearing the same portable gas exchange system and global positioning system watch (GPS). The incremental test and marathon race were run at the same time (morning), with a 10-day recovery period between the test and the marathon race. The data were collected during France's 2019 Paris marathon (start times were at 9 a.m.). The temperatures ranged between 10 and 13 C (between 9 a.m. and 1 p.m.). There was no precipitation, and the humidity averaged 60%. Blood lactate was measured ( nger) (Lactate PRO2 LT-1730; Arkray, Japan) just after a warm-up (15 min at an easy pace) and then again three minutes after crossing the nish line. During the marathon, refreshment points (water, dry and fresh fruit, and sugar) were offered every 5

a.m. and 1 p.m.). There was no precipitation, and the humidity averaged 60%. Blood lactate was measured ( nger) (Lactate PRO2 LT-1730; Arkray, Japan) just after a warm-up (15 min at an easy pace) and then again three minutes after crossing the nish line. During the marathon, refreshment points (water, dry and fresh fruit, and sugar) were offered every 5 km and at the nish line, and sponge stations were located every 5 km from km 7.5. At the aid stations, the runners were allowed to remove their masks so that they could drink or eat. To improve comfort, the runners used the mask version with inspiratory valves that reduce inspiratory resistance during high-intensity exercise. 2.4. The Variables Used in the Analysis of Results In accordance with the purpose of this study we analyzed the fractional use of the maximal heart rate and . V O2maxand v . V O2max. To show the dissociation between these two fractional uses of HR and . V O2maxwe also analyzed their ratio and compared them at each 5 km section. We also calculated the energy cost of running (Cr in mlO2 kg 1 m 1 ) i.e., the ratio between the . V O2in mL kg 1 min 1 and the speed in m min 1 and the cardiac cost of running in beat m 1 i.e., the ratio between the heart rate (bt min 1 ) and the speed (m min 1 ) for every 500 m section. Given that the runner targets their pace according to a heart rate or speed, we wanted to check that their ratio remained stable by plotting their ratio (%HR/% . V O2maxand %HRmax/%v . VO2max). 2.5. Statistical Analysis All the test variables were reported as the mean SD. For each variable, the normality and homogeneity of the data distribution were examined using Shapiro–Wilks, Lilliefors, Anderson–Darling, and Jarque–Bera tests. For analyzing the effect of repetition (within effect) on data average for each 5 km, and the between (group of performance) effect, we applied a repeated measures ANOVA for % . V O2max, %HRmax, %v . V

the mean SD. For each variable, the normality and homogeneity of the data distribution were examined using Shapiro–Wilks, Lilliefors, Anderson–Darling, and Jarque–Bera tests. For analyzing the effect of repetition (within effect) on data average for each 5 km, and the between (group of performance) effect, we applied a repeated measures ANOVA for % . V O2max, %HRmax, %v . V O2maxand their ratio (%HR/% . VO2maxand %HRmax/%v . VO2max). We used Pearson's correlation coef cient to correlate the performance of the % of HRmaxand . V O2maxat each 5 km of the marathon race. We then determined the signi cance level = 0.05 for interpreting the statistical tests. Given that we clearly had ve runners

Int. J. Environ. Res. Public Health2022,19, 12451 5 of 13 who achieved the marathon in less than 4 h and ve in more than 4 h. Since the slower marathoners (SM) completed their rst marathon in more than 4 h, we decided to analyze the in uence of the performance level on this dissociation between the heart rate and the oxygen uptake relative to their maximal respective values (%HRmaxand % . V O2max). We therefore checked the normality of distribution before applying the ANOVA for repeated measurement with two factors (repetition for every 5 km and the performance level). All statistical analyses were performed using XLSTAT software (version 2019.1.1, Addinsoft, Paris, France). 3. Results 3.1. Maximal Values of . VO2and Heart Rate in the Test UM-TT Table . V O2max, maximal heart rate and v . V O2maxas well as the energy cost of running below the Respiratory Compensatory Point. We can see that the two groups of marathon performance did not have signi cant differences in these maximal values and in Cr and in their speed at the RCP (in %v . VO2max). Table 2. The maximal oxygen uptake ( . V O 2maxin mL kg 1 min 1 ); the speed associated with VO 2max (v . VO 2max(km h 1 )); the maximal heart rate HRmax(bpm) measured in the UM-TT test. Level v . VO2max (km h 1 ) HRmax (bpm) . VO2max (mL kg 1 min 1 ) v@RCP%v . VO2max CR (mL kg 1 km 1 ) %v@RCP High 15.9 176 53 96% 214 High 16.5 179 52 92% 204 High 16.8 174 57 86% 203 High 18.5 169 63 87% 212 High 17.0 170 48 91% 189 Low 16.5 183 49 92% 180 Low 16.5 178 53 81% 191 Low 16.0 188 52 91% 217 Low 16.0 175 45 94% 187 Low 16.0 184 53 83% 203 Mean High Level 16.9 174 55 90% 204 SD Group 1 1.0 4 6 4% 10 Mean Level Low 16.2 182 50 89% 196 SD Level Low 0.3 5 4 6% 15 pvalue 0.2 0.06 0.22 0.840

This research examines the dissociation between heart rate and oxygen uptake in marathon runners.