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A Test For Determining Critical Heart Rate Using The Critical Power Model

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A Test For Determining Critical Heart Rate Using The Critical Power Model
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   A T EST FOR   D ETERMINING  C RITICAL  H EART  R   ATE U SING THE  C RITICAL  P OWER   M ODEL M ICHELLE M IELKE , 1 T ERRY  J.H OUSH , 2 C.R  USSELL H ENDRIX  , 2 J ORGE Z UNIGA  , 2 C LAYTON L.C  AMIC , 2 R  ICHARD  J. S CHMIDT , 2  AND  G LEN  O. J OHNSON 2 1 Department of Sport Sciences, University of the Pacific, Stockton, California; and   2  Department of Nutrition and Health Sciences, Human Performance Laboratory, University of Nebraska—Lincoln, Lincoln, Nebraska   A  BSTRACT Mielke, M, Housh, TJ, Hendrix, CR, Zuniga, J, Camic, CL,Schmidt, RJ, and Johnson, GO. A test for determining criticalheart rate using the critical power model.  J Strength Cond Res 25(2): 504–510, 2011–The purposes of this study were to (a)determine if the mathematical model that has previously beenusedtoestimatethecriticalpower (CP)wasapplicable toheartrate (HR) to estimate the critical heart rate (CHR), and (b)compare the CHR to the HR values at the CP (CP HR ),ventilatory threshold (VT HR ), and respiratory compensationpoint (RCP HR ). Fifteen women (mean age 6 SD  = 21.7 6 2.1years)performedanincrementaltesttoexhaustiontodetermine _ V O 2 peak, VT HR , and RCP HR . The subjects also performed 4exhaustive workbouts at different power outputs for the deter-mination of CP and CHR. For each power output, the totalnumber of heart beats (HB lim ) was calculated as the product ofthe average 5-second HR (bpm) and total time to exhaustion(T lim  in minutes). The HB lim  and total work (W lim  in kilograms-meters) were plotted as a function of the T lim  at each poweroutput, and the slope coefficients of the regression linesbetween HB lim  or W lim  and T lim  were defined as the CHRand CP, respectively. A 1-way repeated-measures analysis ofvariance (ANOVA) indicated that CHR (172 6 11 bpm, 92.9 6 2.7%HR max ) was similar to RCP HR  (172  6  9 bpm, 92.9  6 2.2%HR max )butwashigher(  p , 0.05)thanCP HR (154 6 10bpm,83.2  6  4.0%HR max ) and VT HR  (152  6  12 bpm, 82.1  6 4.3%HR max ). The relationship between HR and T lim  from theCHR test can be described by the CP model. The CHR testmay be a practical method for estimating RCP without the needto measure expired gas samples. Furthermore, like the RCP,the CHR test may be used to demarcate the heavy fromsevere exercise intensity domains, predict endurance exerciseperformance, and prescribe a training intensity for competitivecyclists. K  EY  W ORDS  heart rate, fatigue threshold, ventilatory threshold,respiratory compensation point, exercise intensity domain,endurance exercise I NTRODUCTION  V  arious fatigue thresholds have been used todetermine the onset of metabolic acidosis (10),prescribe exercise training intensity (2,8,18), pre-dict endurance exercise performance (2,8,12), andexamine the mechanisms associated with neuromuscularfatigue (16). For example, the lactate threshold has been usedto determine the workload at which the net blood lactateconcentration increases from the onset of exercise (10).In addition, Casaburi et al. (8) suggested that the lactatethreshold be used as a training intensity. The respiratorycompensation point (RCP) (18) and maximal lactate steadystate (12) have also been prescribed as training intensities forcyclists. Furthermore, Rhodes and McKenzie (31) reporteda correlation of   r   = 0.94 between actual marathon time andpredicted marathon time estimated from the ventilatorythreshold (VT). Amann et al. (2) also reported that theVT was a better predictor of a 40-km cycling time trial thanthe onset of blood lactate accumulation or individualanaerobic threshold.Fatiguethresholdssuchasthelactatethreshold,VT,criticalpower (CP), maximal lactate steady state, and RCP have alsobeen used to estimate the exercise intensity that demarcatesfatiguing from nonfatiguing work (2,8,12,26,28). The de-marcation between fatiguing and nonfatiguing work isusually based on the boundaries of the 3 exercise intensitydomains (moderate, heavy, and severe) as described byGaesser and Poole (13). These boundaries are based on  _ V O 2 and blood lactate responses during constant-intensityexercise. The moderate exercise domain includes intensitiesthat can be maintained without an increase in  _ V O 2  or bloodlactate levels; therefore, it has been suggested that the lactatethreshold and/or VT demarcates the moderate from heavyexercise domains. The lower boundary of the heavy exercisedomain is the lowest work intensity that elicits an increase in Address correspondence to Michelle Mielke, mmielke@pacific.edu. 25(2)/504–510 Journal of Strength and Conditioning Research   2011 National Strength and Conditioning Association  504  Journal of Strength and Conditioning Research the  TM Coriht © National Strenth and Conditionin Association Unauthorized reroduction of this article is rohibited.  blood lactate, whereas the upper boundary is the highestwork intensity at which blood lactate eventually stabilizes.Within the heavy exercise domain,  _ V O 2  increases buteventually stabilizes without reaching   _ V O 2 max. Gaesserand Poole (13) have suggested that the CP or the maximallactate steady state demarcates the heavy from severeexercise domains. The RCP has been defined as the exerciseintensity at which  _ V E  increases disproportionately to  _ V CO 2 (Figure 1) and has been used to noninvasively demarcate theheavy from severe exercise intensity domains (4,34).The CP concept, of Monod and Scherer (25) for individualand synergistic muscle groups, relates the amount of work accomplished at exhaustion or work limit (W lim ) and thetime to exhaustion or time limit (T lim ) (Figure 2A) and alsoprovides estimates of 2 independent parameters called theCP and anaerobic work capacity (AWC). The CP is the slopeand AWC is the y-intercept of the W lim  vs. T lim  relationshipand they have been defined, theoretically, as the maximumpower output (P) that can be maintained for an extendedperiod without fatigue (i.e., the asymptote of the P vs. T lim relationship [Figure 2B]) and the total amount of work that canbe performed using only stored energy reserves (i.e., inde-pendent of oxygen supply), respectively (25). Moritani et al.(26) extended the CP concept to cycle ergometry, and CP wasdetermined from a series of 4 continuous, fatiguing workboutsat varying   P   values. The W lim  (P 3 T lim ) was then plotted as a function of the T lim  for each  P   value. The relationship betweenW lim  and T lim  was highly linear ( r  . 0.98) and described by theequation W lim  = AWC + CP(T lim ) (Figure 2B) (26).The CP model is estimated from the linear relationshipbetween W lim  and T lim  (Figure 2A) and has been successfullyapplied to other modes of exercise such as running (17),kayaking (9), rowing (20), and swimming (33). This modelhas not, however, been applied to a physiological parametersuch as heart rate (HR). Heart rate is a simple, easilymeasured, and practical method of determining exerciseintensity. Theoretically, the CP model can be applied to HRto develop a HR-based analog of the CP test called thecritical heart rate (CHR) test (Figure 3). The slope of the totalnumber of heart beats (HB lim ) vs. T lim  relationship from theCHR test would provide a physiological measure (i.e., HR)which represents a threshold exercise intensity that, theore-tically, could be maintained for an extended period.Therefore, the purposes of this study were to (a) determineif the mathematical model that has previously been used toestimate the CP (25,26) was applicable to HR to estimate theCHR, and (b) compare the CHR to the HRvalues at the CP(CP HR ), VT (VT HR ), and RCP (RCP HR ). Based on previousstudies, it was hypothesized that (a) the mathematical modelused for the estimation of CP would be applicable to HRmeasurements to derive the CHR (25,26), and (b) the meanCHR, CP HR , and RCP HR  would not be significantly differentbut would be greater than the VT HR  (26). M ETHODS Experimental Approach to the Problem In this study,  _ V O 2  and HR values were measured during anincremental test to exhaustion on a cycle ergometer todetermine peak oxygen consumption (  _ V O 2 peak), HR max  ,VT HR , and RCP HR . In addition, HR was measured during 4 continuous workbouts to exhaustion at different poweroutputs to determine the CHR and CP HR . The mathematicalmodel that has previously been used to estimate CP (25,26)was applied to the heart rate data to derive the CHR (a heartrate analog of the CP test). The mean CHR was thenstatistically compared to VT HR , RCP HR , and CP HR . Subjects Fifteen women volunteered for the study. Their mean ( 6 SD  )age, body weight, height, and  _ V O 2 peak were 21.7 6 2.1 years,65.7 6 9.9 kg, 168.1 6 5.7 cm, and 2.7 6 0.4 L  min 2 1 (40.7 6 4.6 mL  kg  2 1  min 2 1 ), respectively. The physical activity levelsof the subjects’ ranged fromsedentary ( n   = 4) to moderatelyactive ( n   = 11). Sedentary wasdefined as not currently partici-pating in aerobic and/or re-sistance training, whereasmoderately active was definedas participating in aerobic an-d/or resistance training for 4 to5 hours per week. All subjectswere instructed to avoid exer-cising the day prior to the testand to not eat for approxi-mately 4 hours prior to testing.The subjects had no knowncardiovascular, pulmonary,metabolic, muscular, and/orcoronary heart disease and didnot regularly use prescription Figure 1.  The method used for determining the RCP. VOLUME 25 | NUMBER 2 | FEBRUARY 2011 |  505  Journal of Strength and Conditioning Research the  TM |  www.nsca-jscr.org Coriht © National Strenth and Conditionin Association Unauthorized reroduction of this article is rohibited.  medication. The study was approved by the UniversityInstitutional Review Board for Human Subjects, and allsubjects completed a health history questionnaire (3) andsigned a written informed consent prior to any testing. Determination of  _ V O 2 peak  Each subject performed an incremental test to exhaustion onaCalibrated Quinton(Corval400)electronically brakedcycleergometer (Quinton Instruments Inc., Seattle, Washington,U.S.A.) at a pedal cadence of 70 rev  min 2 1 . Seat height wasadjusted so that the subject’s legs were at near full extensionduring each pedal revolution. The subjects wore a nose clipand breathed through a mouthpiece (2700; Hans Rudolph,Kansas City, Missouri, U.S.A). Expired gas samples werecollected (20-second average) and analyzed using a cali-brated TrueMax 2400 metabolic measurement system(Parvo Medics, Sandy, Utah, U.S.A.). Prior to all testing sessions the gas analyzers were calibrated with room air andgases of known concentration. The subjects were fitted witha Polar Heart Watch system (Polar Electro Inc., LakeSuccess, New York, U.S.A.) to monitor HR throughout thetest. The test began with a 3-minute warm-up at a poweroutput of 50 watts and a pedaling rate of 70 rev  min 2 1 . Thepower output was increased by 30 watts every 2 minutesuntil voluntary exhaustion or the subject could no longermaintain a pedal cadence of 70 rev  min 2 1 despite strong verbal encouragement.  _ V O 2 peak was defined as the highest _ V O 2  value during the last 30 seconds of the exercise test. Asubject’s  _ V O 2 peak data were used if they met at least 2 of thefollowing 3 criteria (1,11): (a) 90% of age-predicted heartrate, (b) respiratory exchange ratio . 1.1, and (c) a plateau-ing of oxygen uptake (less than 150mL  min 2 1 in  _ V O 2 over thelast 30 seconds of the test). At the completion of the test, thesubjects performed a cool-down period on the cycle ergometerat a lower power output for as long as they wished. Assessmentof   _ V O 2 peak has been shown to be highly reliable in a similarpopulation, with an intraclass correlation . 0.80 (27) and SEM , 5% of the means. Determination of the Ventilatory Threshold (VT) and VT HR The VT was determined by noninvasive gas exchange mea-surements using the V-slope method of Beaver et al. (4). TheVTwas defined as the  _ V O 2  value corresponding to the inter-section of 2 linear regression lines derived separately from thedata points below and above the breakpoint in the  _ V CO 2  vs. _ V O 2  relationship (Figure 4). Heart rate values from the incre-mental test were plotted against  _ V O 2  values, and the regres-sion equation derived was usedto determine the VT HR . Determination of theRespiratory CompensationPoint (RCP) and RCP HR The RCP was determined bynoninvasive gas exchangemeasurements using the  _ V E - _ V CO 2  plot as described byBeaver et al. (4). The RCPwas defined as the  _ V O 2  valuecorresponding to the point of departure from linearity of   _ V E and  _ V CO 2  (Figure 1). Heart ratevalues from the incrementaltest were plotted against  _ V O 2 values, and the regression equa-tion derived was used to de-termine the RCP HR . Figure2. (A). Schematic diagram of the relationships for work limit (W lim )versus time limit T lim  for the estimation of the critical power (CP) andanaerobic work capacity (AWC). (B). Schematic diagram of therelationships for power output (P) versus T lim . Figure 3.  Relationship for HB lim  versus T lim  for 1 subject. 506  Journal of Strength and Conditioning Research the  TM Critical Heart Rate Test Coriht © National Strenth and Conditionin Association Unauthorized reroduction of this article is rohibited.  Determination of CHR, CP, and CP HR Onseparatedays,atleast48hoursfollowingthe  _ V O 2 peak test,the subjects performed 4 randomly ordered rides (1 ride oneach day) to exhaustion at 70 rev  min 2 1 at 4 different poweroutputs. The seat height, toe clips, and warm-up procedureswere the same as for the incremental test. The power outputswere selected by experienced investigators and were basedon the fitness level of the subject (23,24). The power outputswere selected so that the subjects could complete approx-imately 8 to 20 minutes before exhaustion (23,24). Heart ratevalues were continuously monitored and recorded every 5seconds. For each power output, the HB lim  was calculated asthe product of the average 5-second HR (bpm) and totalT lim  (minutes). The HB lim  for each of the 4 power outputswere plotted as a function of the T lim  at each power output(Figure 2). The CHR was defined as the slope coefficient of the regression line between HB lim  and T lim .To determine CP, the total work (W lim  in kilogram-meters)performed at each power output was calculated asthe productof the power output (kg   min 2 1 )and time to exhaustion (min-utes). The W lim  for each of the4 power outputs was plotted asa function of T lim  at each poweroutput (Figure 2A). The CPwas defined as the slope coeffi-cient of the regression linebetween W lim  and T lim . Heartrate values from the incremen-tal test were plotted againstpower output values, and theregression equation derived wasused to determine the CP HR . Statistical Analyses Means and standard deviationswere calculated for CHR,CP HR , VT HR , and RCP HR . A1-way repeated-measures anal-ysis of variance (ANOVA) was used to determine if therewere significant differences among the thresholds. An alpha level of   p  # 0.05 was selected for all statistical comparisons. Azero-order correlation matrix was used to determine therelationships among the thresholds. The analyses wereconducted using the Statistical Package for the SocialSciences software (v.17.0, SPSS Inc., Chicago, Illinois, USA). R  ESULTS Table 1 includes the mean ( 6 SD  ) and range values for CHR,CP HR , VT HR , and RCP HR . The mean CHR (172 6 11 bpm,92.9  6  2.7%HR max  ) was not significantly different fromRCP HR  (172  6  9 bpm, 92.9  6  2.2%HR max  ) but was higher(  p  , 0.05) than the CP HR  (154 6 10 bpm, 83.2 6 3.7%HR max  )and VT HR  (152 6 12 bpm, 82.1 6 4.3%HR max  ). The  r  2 valuesfor HB lim  vs. T lim  relationships ranged from 0.985 to 1.0 .  The r  2 values for the W lim  versus T lim  relationships for theestimation of CP ranged from 0.866 to 0.999 .  The  r  2 values for T ABLE  1.  Mean ( 6 SD ) and range values for CHR,RCP, CP HR , and VT HR .Mean  6 SD  Range (bpm) %HR max CHR 172 6 11 153–193 92.9 6 2.7RCP HR  172 6 9 162–195 92.9 6 2.2CP HR  154 6 10* 135–177 83.2 6 3.7*VT HR  152 6 12* 129–178 82.1 6 4.3* %HR max  = percentage of maximum heart rate; CHR =critical heart rate; RCP = respiratory compensation point;CP HR  = heart rate at critical power; VT HR  = heart rate atventilatory threshold.*Significantly different from CHR and RCP HR . T ABLE  2.  Correlational matrix for the fatiguethresholds.CHR RCP HR  CP HR  VT HR CHR 1.00 — — —RCP HR  0.83* 1.00 — —CP HR  0.63* 0.58* 1.00 —VT HR  0.76* 0.80* 0.60* 1.00 CHR = critical heart rate; RCP = respiratorycompensation point; CP HR  = heart rate at critical power;VT HR  = heart rate at ventilatory threshold.*  p  ,  0.05 Figure 4.  The method used for determining VT. VOLUME 25 | NUMBER 2 | FEBRUARY 2011 |  507  Journal of Strength and Conditioning Research the  TM |  www.nsca-jscr.org Coriht © National Strenth and Conditionin Association Unauthorized reroduction of this article is rohibited.  the HR vs. power output, from the incremental test, rangedfrom 0.966 to 0.999. Table 2 is a zero-order correlation matrixamong CHR, CP HR , VT HR , and RCP HR . All the fatiguethresholds were significantly intercorrelated at  r   = 0.58 to0.83 (Table 2). D ISCUSSION One purpose of this study was to determine if the mathe-maticalmodelthathasbeenusedtoestimateCP(25,26)couldbe applied to HR measurements to derive a new fatiguethreshold called the critical heart rate (CHR). The HB lim during each exhaustive workbout was substituted for W lim  inthe W lim  vs. T lim  relationship used to estimate CP. For eachsubject, the relationship between HB lim  and T lim  wasdescribed by the equation HB lim  = a + CHR(T lim ), whichindicated that the total number of heart beats accumulatedduring each exhaustive workbout increased linearly with T lim ( r  2 = 0.985–1.0). These  r  2 values were similar to those for theW lim  vs. T lim  relationship for the estimation of CP in thepresent study ( r  2 = 0.866–0.999) and to those from previousstudies ( r  2 = 0.982–1.0) (6,7,19,26) .  The high correlations( r  2 = 0.985–1.0) for the HB lim  vs. T lim  relationships found inthe present study indicated that the mathematical modelused for the determination of CP could be applied to HRmeasurements to estimate the CHR. Thus, theoretically,both the CP and CHR tests provide estimates of the maximalintensity of exercise that can be maintained for an extendedperiod without fatigue. The tests differ, however, in that theCP test estimates the maximal nonfatiguing power output,whereas the CHR test estimates the maximal nonfatiguing HR. Thus, the physiological responses during a continuousworkbout at a constant power output differ from those ata constant HR. For example, previous studies (6,7,14,15,29)have shown that during continuous exercise at the CP,  _ V O 2 and HR increased and did not reach steady state. Main-tenance of a constant HR during a continuous workbout,however, requires a reduction in power output (5). Therefore,it is unclear if the threshold associated with the maximalnonfatiguing intensity of exercise should be based on a specific power output or a physiological variable such as HR.Future studies should examine this issue by comparing theT lim  values for continuous cycle ergometer workbouts at theCP vs. those at the CHR.The findings of the present study indicated that there wasno mean difference between CHR (172  6  11 bpm, 92.9  6 2.7%HR max  , 85.5 6 8.8%  _ V O 2 peak) and RCP HR  (172 6 9 bpm,92.9  6  2.2%HR max  , 84.3  6  6.4%  _ V O 2 peak), but both weregreater (  p   ,  0.05) than CP HR  (154  6  10 bpm, 83.2  6 4.0%HR max  ) and VT HR  (152 6 12 bpm, 82.1 6 4.3%HR max  ).The mean CHR and RCP HR  values in the current studywere also highly correlated ( r   = 0.834) and similar to therelative exercise intensities for CP (85.4  6  4.8%  _ V O 2 max),RCP (85.3 6 5.6%  _ V O 2 max), and HR at RCP (174 6 10 bpm,90 %HR max  ) reported by Dekerle et al. (12), and the meanRCP (91.8%HR max  , 87.5%  _ V O 2 max) reported by Impellizzeriet al. (18). Furthermore, in the present study, there was nomean difference between CP HR  and VT HR , and they weremoderately correlated ( r   = 0.598). These findings were con-sistent with those of Moritani et al. (26), who reported a highcorrelation ( r   = 0.927) and no mean difference betweenthe  _ V O 2  at CP (2.48  6  0.54 L  min 2 1 ) and VT (2.30  6 0.44 L  min 2 1 ). In addition, Le Chevalier et al. (21) andVautier et al. (32) have reported that CP corresponded to theVTand lactate threshold, respectively. In contrast, Pooleet al.(29) reported that CP (197 6 12 W, 79 6 8.1%  _ V O 2 max) was64% higher than VT (120 6 8 W, 46 6 4.6 %  _ V O 2 max) andrepresented an ‘‘upper limit for sustainable power’’ (p. 421).In addition, Dekerle et al. (12) reported that CP (278 6 22 W,85.4  6  4.8%  _ V O 2 max) coincided with RCP (286  6  28 W,85.3  6  5.6%  _ V O 2 max). Thus, previous studies (12,18,21,26,29,32) have provided conflicting results regarding the asso-ciations among the VT, RCP, CP, and lactate threshold,whereas the present study indicated that CHR and RCP HR were greater than CP HR  and VT HR . The differences betweenstudies may have been a result of the training status of thesubjects, the durations and intensities of the workbouts usedto estimate CP, and/or the procedures used to determine theVTand RCP. For example, the subjects in the present studywere sedentary to moderately active, whereas those of Dekerle et al. (12) were well trained (  _ V O 2 max = 50.8 6  2.7mL  kg  2 1  min 2 1 ) and those of Impellizzeri et al. (18) wereinternationally competitive mountain bikers. In addition, thedurations of the workbouts (63.3–95.7%  _ V O 2 peak) used toestimate CP in the current study ranged from approximately8 to 20 minutes, whereas other studies used workbouts (90.0–110.0%  _ V O 2 max) that led to exhaustion in 2 to 15 (12) or 4 to8 minutes (29). Furthermore, in the present study, VT andRCP were determined using the procedures of Beaver et al.(4). In contrast, Moritani et al. (26) determined the VT byvisual inspection of the changes in  _ V E ,  _ V CO 2 , and  _ V E /  _ V CO 2 across time, whereas Poole et al. (29) used the changes inventilatory equivalents along with the end tidal partialpressures of O 2  and CO 2 .The close similarity between the CHR and RCP HR  in thepresent study suggests that (a) the RCP can be estimatedfrom the CHR test without measuring expired gas samples,and (b) the CHR can be used for the same purposes as theRCP such as demarcating the heavy from severe exerciseintensity domains (4,35), predicting endurance exerciseperformance (30), and prescribing training intensity forcompetitive cyclists (22). For example, Yamamoto et al.(35) demonstrated that the RCP coincided with thedemarcation of the heavy and severe exercise intensitydomains and that exercise above, but not at, the RCPelicited an increase in lactate from the 15th to the 30thminute of exercise. In addition, Reybrouck et al. (30)reported that the RCP was a better predictor ( r   = 0.82) of performance in a 12-minute run than the VT ( r   = 0.73) (30)and that the RCP accounted for 67% of the variance inendurance exercise performance. Furthermore, Lucia et al. 508  Journal of Strength and Conditioning Research the  TM Critical Heart Rate Test Coriht © National Strenth and Conditionin Association Unauthorized reroduction of this article is rohibited.
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