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%HRmax target heart rate is dependent on heart rate performance curve deflection

HOFMANN, PETER; VON DUVILLARD, SERGE P.; SEIBERT, FRANZ-JOSEF; POKAN, ROCHUS; WONISCH, MANFRED; LEMURA, LINDA M.; SCHWABERGER, GÜNTHER

Medicine & Science in Sports & Exercise: October 2001 - Volume 33 - Issue 10 - p 1726-1731
APPLIED SCIENCES: Physical Fitness and Performance
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HOFMANN, P., S. P. VON DUVILLARD, F-J. SEIBERT, R. POKAN, M. WONISCH, L. M. LEMURA, and G. SCHWABERGER. %HRmax target heart rate is dependent on heart rate performance curve deflection. Med. Sci. Sports Exerc., Vol. 33, No. 10, 2001, pp. 1726–1731. The percent of maximal heart rate (%HRmax) model is widely used to determine training intensities in healthy subjects and patients when prescribing training intensities in these groups of subjects. Purpose: The aim of the study was to investigate the influence of the time course of the heart rate performance curve (HRPC) on the accuracy of target training heart rate. Methods: Sixty-two young healthy male subjects performed an incremental cycle ergometer exercise test until voluntary exhaustion. Subjects were then divided into four groups according to the time course of the HRPC. Groups were classified in regular HR response (kHR2 > 0.2), indifferent HR response (0 < kHR2 < 0.2), linear HR response (kHR2 = 0), and inverted HR response (kHR2 < 0). The first and the second lactate turn point (LTP1, LTP2) as well as the heart rate turn point (HRTP) were determined as submaximal markers of performance. Linear regression lines were calculated for HR in the three regions of energy supply defined by LTP1 and LTP2. Results: HR at LTP1 and HRmax was not significantly different between all four groups. HR at LTP2 was dependent on the time course of the HRPC and was significantly lower (P < 0.05) as kHR2 decreased. Power output and blood lactate concentration at LTP1, LTP2 and maximal workload (Pmax) were not significantly different between the groups. Conclusion: From our data, we conclude that target training HR detected by means of the %HRmax method may be overestimated in cases where the HR response is not regular, as it was found in many of our subjects.

Institute of Sports Sciences, Department of Internal Medicine and Department of Physiology, School of Medicine, Karl-Franzens-University Graz, Graz, AUSTRIA; Human Performance Laboratory, Department of PEXS, University of North Dakota, Grand Forks, ND; Institute of Sport Science, University Vienna, Vienna, AUSTRIA; and Bloomsburg University of Pennsylvania, PA

September 2000

January 2001

The use of heart rate as an index for describing training intensity has been widely accepted for the general population (25). Recent technological advances have made HR monitoring during training highly accurate and inexpensive (10), and it is therefore a tool easily applied to regulating training intensities in various groups of subjects. The percent of maximal heart rate (%HRmax) method is widely used to describe training intensities in athletes, healthy sedentary subjects, and patients (1). It is one of the oldest methods of prescribing the target HR range and uses a straight percent of the HRmax. Training intensity is the key factor to improve VO2max, and several recommendations have been made on a %HRmax basis. Referring to Pollock et al. (25) the American College of Sports Medicine (1) has recommended training intensity between 55 and 65% to 90% of maximal heart rate (HRmax), or between 40 and 50% to 85% of oxygen uptake reserve (VO2R), or HR reserve (HRR). These guidelines reflect the most up-to-date textbooks, with upper limits for %HRmax set at 85–90% (4,7,24,33).

The basis for these guidelines for exercise intensity prescription is a body of knowledge which indicates that individuals who train in accordance with these guidelines favorably alter VO2max, body composition, and health parameters (1). Although these guidelines are similar for healthy young and older subjects as well as patients (with some restrictions), the accuracy of the %HRmax method strongly relies on a functional relationship of metabolic markers such as the lactate threshold (35) and may therefore not be applicable under certain circumstances. Recently, the validity of the percent maximum concept for equating training intensity has been questioned (9). Data indicate that the use of standard HR intensity guidelines result in differing levels of metabolic stress across subjects (22). This was also reported by Weltman et al. (34), who suggested that sedentary women and men (35) showed widely varying metabolic responses to exercise intensities of specified percent of HRmax and HRR.

Bunc and Heller (5) reported that the anaerobic threshold was found to be in a narrow range at about 90% HRmax (see also 15), and, therefore, these authors suggested %HRmax should be used as a simple and objective tool for noninvasive determination of anaerobic threshold. However, this relationship was confirmed only for a typical s-shaped HR response pattern in incremental exercise; no uniform HR response during incremental exercise can be expected in all individuals (16). Three different HR response patterns have been identified in cycle ergometer exercise and running (16) where, in about 16% of the young healthy male and female subjects, this response was not regular. Older subjects (14,28) and most cardiac patients (29) show a higher degree of nonregular HR performance curve. In cases of a nonregular HR response, which was linear and even inverted above the LTP2, the percent HRmax concept would overestimate subjects systematically. Gilman and Wells (9) discussed that metabolic reference points (usually used as the main criterion for exercise prescription) occur not only at different percentages of VO2max but also at different percent HRmax.

Therefore, the aim of the study was to investigate the influence of the time course of the HRPC on the accuracy of target training heart rate calculated as a percentage of maximal heart rate.

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MATERIALS AND METHODS

Subjects.

Results from standard incremental cycle ergometry tests of 62 young healthy male subjects were randomly selected for this investigation. Subject characteristics are presented in Table 1. Before all testing, subjects were informed of the nature, risks, and benefits of the investigation, and written consent was obtained. The study was approved by the ministry of education and science of the Austrian Republic. All subjects completed a maximal incremental cycle ergometer test to voluntary exhaustion. Before testing, subjects were asked to sit quietly for 1 min on the cycle ergometer followed by an exercise workload of 40 W and an increase of 20 W every minute thereafter. HR was measured continuously and stored in 5s intervals (Sporttester PE4000, Polar Electro, Kempele, Finland). A 12-lead ECG (Schiller, Germany) was recorded and supervised by a physician. Blood lactate concentration (La) was measured at rest, after every workload step and at maximal performance via Eppendorf fully enzymatic analysis (EBIO+, Eppendorf, Germany).

Table 1

Table 1

Turn points in the time course of HR (HRTP) and La (LTP1, LTP2) were assessed by means of linear regression break point analysis using software PA7000 (21). The principle of determination has been presented previously (12,16). To quantify the degree and the direction of the HRPC, second degree polynomial fitting was used, and the differences of angles between the tangents at 40 W and LTP2 and LTP1 and maximal performance (Pmax) were defined as factors kHR1 and kHR2 (26) (Fig. 1A). Additionally, linear regression analysis was performed for three segments of the HRPC between 40 W and LTP1 (S1), LTP1 and LTP2 (S2), and LTP2 and Pmax (S3) (Fig. 1B), and the differences between S1 and S2 and S2 and S3 were calculated to quantify the changes in the direction of the HRPC.

FIGURE 1

FIGURE 1

From kHR2, subjects were divided into four groups: group 1 (DEF;N = 24) with a regular HR response (kHR2 > 0.3); group 2 (IND;N = 12) with an indifferent HR response (−0.2 < kHR2 < 0.2); group 3 (NON;N = 14) with a linear HR response (kHR2 = 0); and group 4 (NEG;N = 12) with an inverted HR response (kHR2 < −0.2) (Table 2). Different from our earlier study where we used three groups (19), we indicated an additional group with only a slight deflection of the HRPC (kHR2) but without detectable HRTP.

Table 2

Table 2

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Statistics.

Analysis of variance (ANOVA) and Tukey post hoc tests were performed to calculate group mean differences. Linear regression analysis and Pearson product moment correlation were calculated where appropriate. Level of significance was set at P < 0.05.

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RESULTS

Subject characteristics (Table 1) were not significantly different between groups. Maximal and submaximal values of measured variables are presented in Table 2 and Figure 2 A–C. No significant differences were found for power output at LTP1, LTP2, and Pmax, respectively (Table 2). Blood lactate concentration was not significantly different at LTP1 (except between DEF and IND), LTP2, and Pmax (Table 2). HR was not significantly different at LTP1 and HRmax. HR at LTP2 was significantly different between DEF and all other groups (Fig. 2A).

FIGURE 2

FIGURE 2

The deflection of the HRPC between 40 W and LTP2 (kHR1) was not significantly different between groups (Fig. 2B). Factor kHR2 the marker of the degree and the deflection of the HRPC were significantly different between all groups. Similar to kHR2, the inclination of the HRPC in the three segments showed a similar pattern. No significant differences were found between groups for the increase in HR between 40 W and LTP1 (S1), but significant differences were found between LTP1 and LTP2 (S2) and between LTP2 and Pmax (S3) as shown in Figure 2C. The greater the HRPC’s linearity or inversion, the lower the increase in HR between LTP1 and LTP2 and the higher the increase between LTP2 and Pmax. The difference between the inclinations in S2 and S3 was significantly related to kHR2 (R = 0.961;P < 0.001) (Fig. 3A).

FIGURE 3

FIGURE 3

Percent of Pmax at LTP1 and LTP2 was not significantly different between groups (Table 3). Percent of HRmax was not significantly different at LTP1 but significantly different at LTP2 between DEF and all other groups. The lower the kHR2, the lower the %HRmax at LTP2. This relationship was significant (R = 0.660;P < 0.001) (Fig. 4A), and a similar pattern was found for the relationship between the difference of inclinations in S2 and S3 and %HRmax at LTP2 (R = 0.715, P < 0.001) (Fig. 4B). Absolute HR at LTP2 was significantly lower as the kHR2 (Fig. 4C), or S2–S3 was lowered (Fig. 4D).

Table 3

Table 3

FIGURE 4

FIGURE 4

The difference between S1 and S2 was not significant, but the difference between S2 and S3 was significant for all groups, except between IND and NON. In cases where HRTP could be determined (N = 34), power output at HRTP and LTP2 was not significantly different and highly correlated (R = 0.90, P < 0.001) (Fig. 3B), similar to HR at HRTP and LTP2 (R = 0.93, P < 0.001) (Fig. 3C).

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DISCUSSION

As reported previously (16), we found that HR response during incremental cycle ergometer exercise presented no uniform HRPC in a homogenous group of young healthy male subjects of comparable age, performance, and metabolic response. HR showed three typical phases in most of the subjects where the increase up to LTP1 was almost uniform. Although subjects reached similar maximal heart rate, the heart rate response between LTP1 and Pmax was significantly different. Only the DEF group presented the typical s-shaped HRPC described earlier (16). Both kHR2 and the difference of inclination in S2–S3 gave similar information. The lower the increase in HR between LTP1 and LTP2, the higher the increase above LTP2 reflecting an inverted HR response and an obviously limited myocardial function described earlier for this kind of exercise in healthy young male subjects (13,26), older healthy subjects (14,28) and cardiovascular diseased patients (29) where the number of subjects presenting this nonregular response was increased with age (28) and cardiac malfunction (29).

Several attempts have been undertaken to identify possible causes for these differences in HR response pattern, but neither plasma catecholamine response (27), parasympathetic receptor blockade (30), nor pH (20) or lactate response (27) was able to explain the phenomenon of HR deflection in young healthy subjects. Additionally, the HR response pattern was shown to be highly reproducible independent of the methodology applied (18,31). A review of the concept of the heart rate deflection point was presented previously (3).

A possible explanation may be found in studies showing a similar HR response by intervention, as reported by Gaesser and Rich in 1985 (8), who described an unexpected finding that caffeine significantly reduced submaximal exercise HR without changes in maximal HR. These authors were unable to explain the discrepancy; however, the results were consistent. The submaximal heart rates have been suggested to be mediated by baroreceptors, which is in agreement with a simulation study by Pessenhofer et al. (23). It was also reported by Graham (11) that caffeine may enhance the release of calcium or to lessen the accumulation of potassium, which was shown to be related to the kHR2 (19). This effect was suggested to be related to the ability of caffeine to interfere or block the action of adenosine. Andros and Gerber (2) studied the effect of adenosine on the reduced heart rate in the elderly. They suggested that the cardiac tissue in older human either produces more adenosine to β-adrenoceptor stimuli or is more responsive to adenosine, but from their data they were not able to demonstrate a significant effect as described by Gaesser and Rich (8). These results are rather speculative regarding the nonregular HR response in young and healthy subjects.

However, as HRmax was not significantly different among groups, %HRmax and HR at LTP2 must be different between groups, implying that the %HRmax method, based on a straight percent of HRmax as recommended by the ACSM (1) and other investigators (4,7,24,33), was inaccurate to determine a comparable target HR related to a metabolic threshold such as the LTP2, which has been shown to represent the maximal lactate steady state (12). Brooks et al. (4) critically noted the implications of the curvilinear relationship between HR and VO2 during heavy exercise, but most authors, such as Squires (in 24), recommend a training intensity of 70–85% HRmax, which was shown to have beneficial effects on various groups of cardiac patients. However, Miller et al. (in 33) set the upper limit for cardiac patients at a lower limit (85% HRR). Froelicher and Myers (7) described the appropriate intensity for most patients in rehabilitation programs to be 60–70% of maximal capacity, which may be more accurate when HR response is nonregular. Similar to our homogeneous group of subjects, %Pmax values were found much more stable, and this was also previously shown in patients (29,32), using LTP2 as the reference set point.

Although the concept of heart rate turn point, better known as the Conconi threshold (6), is critically discussed in the literature (3,17), HRTP was not significantly different from the LTP2 in cases where clear deflection of HRPC was present. This was previously shown in a great number of healthy subjects (16), older subjects (28), and cardiac patients (29). HRTP and LTP2 both were shown to represent the maximal lactate steady state (12).

In conclusion, the HR response in incremental cycle ergometer exercise is neither linear nor uniform between LTP1 and Pmax. The more the HR response is linear or even inverted, especially in older subjects and patients, the more the linear %HRmax method overestimates target HR for training. Therefore, the HRmax method should be adapted to more accurately reflect the relationship between HR and power output. The use of turn points such as the LTP2 or the HRTP may be recommended for exercise training prescription.

Address for correspondence: Peter Hofmann, Ph.D., Institute of Sports Sciences, KF-University of Graz, Mozartgasse 14/1, A-8010 Graz, Austria; E-mail: Peter.Hofmann@kfunigraz.ac.at.

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Keywords:

HEART RATE TURN POINT; LACTATE TURN POINT; CYCLE ERGOMETRY

© 2001 Lippincott Williams & Wilkins, Inc.