the relationship between heart rate deflection point ...

THE RELATIONSHIP BETWEEN HEART RATE DEFLECTION POINT AND THE VENTILATORY ANAEROBIC THRESHOLD IN RUNNERS WHIT DIFFERENT AEROBIC CAPACITY Vlatko Vučetić1, Davor Šentija1 Faculty of Kinesiology, Zagreb, Croatia ABSTRACT The purpose of this study was to examine the relationship between the heart rate deflection point (HRDP) and the gas exchange anaerobic threshold (AnT) and to determine whether changes in heart rate to workload linearity can be used to accurately estimate anaerobic threshold in runners. Forty-eight male runners competing in different running disciplines performed a graded maximal exercise test on a motor-driven treadmill. The anaerobic threshold was assessed by a nonlinear increase in carbon dioxide output to oxygen consumption ratio (V-slope method). The HRDP was determined using the method of deflection of linearity by visual inspection. There was no statistically significant difference between the heart rate at the AnT (176.5±9.9 bpm) and HRDP (177.5±9.9 bpm, p>0.05), as well as between running speed (v) at the AnT (15.0±2.2 km/h) and HRDP (15.0±2.0 km/h). We conclude that the intensity at the HRDP, based on HR response during graded exercise in runners, can be recommended as a valid tool for non-invasive detection of the intensity at the AnT. Key words: Anaerobic threshold, heart rate deflection point, runners INTRODUCTION The ‘anaerobic threshold’, also termed respiratory compensation point, is defined as the exercise intensity, or speed of locomotion, where the processes of glycogen oxidation and lactate removal reach their ceilings and above which blood [La-] ad [H+] rise inexorably and cannot be stabilized, with a disproportionate increase in VCO2 and VE in relation to VO2, and decrease in arterial pH and CO2 pressure (15, 16). Arterial blood lactates and/or gas exchange measurements are conventionally used for determination of this metabolic threshold. The AnT has been shown to be highly correlated to performance in aerobic events and is valuable in determination of optimal training loads and fitness level in competitive athletes. The determination of the anaerobic threshold has traditionally required laboratory exercise and blood sampling procedures for the assessment of the maximal exercise intensity above which blood [La-] rise inexorably and cannot be stabilized (maximal lactate steady state, MLSS), or sophisticated gas analysis systems if the anaerobic threshold is defined by non-invasive gas exchange methods. Laboratory assessment of the AnT by gas exchange measurement represents an accurate but expensive procedure, and is thus out of reach for many athletes and sport coaches. Therefore, there is a need in sports diagnostics for simple and inexpensive, but nevertheless valid and accurate tests to assess the AnT. An alternative method to indentify the AnT using HR alone was originally suggested by Conconi et al. (1982). They described a non-invasive field test for AnT determination, hypothesizing that anaerobic energy production would ‘spare’ aerobic demand and result in a reduced rate of increase in VO2 (and therefore in a reduced increase, or deflection of heart rate) above the anaerobic threshold in relation to running speed. This test, which was to 1

become known in sports science and clinical exercise laboratories as the ‘Conconi test’, was based upon the loss of linearity in the relationship between heart rate and running velocity during an incremental exercise protocol. The relevant parameters at this point of deflection are the running speed (vDP) and the heart rate, i.e., the heart rate deflection point (HRDP) (Fig.1). HRDP, as a marker of exercise intensity related to the AnT, is used to evaluate aerobic endurance, prescribe and monitor exercise intensity of healthy subjects and patients (Conconi et al., 1982; Bodner and Rhodes, 2000; Bunc et al., 1995). It is performed either as a field or as a laboratory test, with numerous modifications for different modes of exercise (field running, treadmill running, cycling, swimming, etc.). Conconi et al. (1982) and other researchers (Bunc et al., 1995; Hofmann et al., 1994) report a high correlation between vDP and the lactate threshold (LT) and AnT, and recommend its use to evaluate endurance capacity and to assess training programs. Since the original work (Conconi et al., 1982), several modifications of the test have been proposed (Ribeiro et al., 1985; Zacharogiannis and Farrally, 1993; Hofmann et al., 1994; Bunc et al., 1995; Pokan et al., 1995). The relationship between HRDP and AnT is the subject of research in the last 2 decades; recently, a number of studies have independently assessed the validity of the Conconi test, but producing contradictory results. Some studies have demonstrated disparity between the exercise intensities corresponding to a deviation in heart rate from linearity (HRDP) and to the ventilatory or lactate anaerobic threshold (Tokmakidis and Leger, 1992; Zacharogiannis and Farrally, 1993), whereas others have confirmed the validity of the method (Hofmann et al., 1994; Bunc et al., 1995). However, the blood lactate or gas exchange criteria used to define the anaerobic threshold have differed between studies, as different authors use the same term (anaerobic threshold) to describe different metabolic thresholds, or use different methods in their comparisons (Hofmann et al., 1994; Bunc et al., 1995; Pokan et al., 1995). In our laboratory, when evaluating aerobic capacity with gas exchange data collection, we perform a standard treadmill test with fixed stage duration, and a protocol that allows simultaneous determination of both, Conconi and gas exchange thresholds. The aim of this study was to investigate the relationship between the parameters at the gas exchange anaerobic threshold and at the heart rate deflection point, derived from the same incremental treadmill test, in trained runners competing in different running disciplines. METHODS Forty-eight Croatian runners of national rank participated in the study (10 sprinters, 15 400m runners, 10 middle distance runners and 13 long distance runners) (Table 1). Table 1. Physical characteristics Variables

mean±SD

Age (years)

21.7±5.1

Weight (kg)

181.1±5.7

Height (cm)

71.9±6.9

2

The measurement procedures and potential risks were verbally explained to each subject prior to obtaining a written informed consent according to the Helsinki Declaration. The study was approved by the institutional Ethics Committee. Subjects were admitted in the study if they had a minimum training experience of 3 yrs, with 10 training hours per week and were currently active in national or international competitions. All subjects performed an incremental maximal exercise test to volitional fatigue on a motor-driven treadmill (Run race, Technogym, Italy). The test started with running at 7 km/h and with 1 km/h speed increments every minute, at a constant inclination of 1.5%. A „breath-by-breath“ gas analysis system (Quark b2, Cosmed, Italy) was used for respiratory gas exchange recording. Heart rate was monitored using a Polar Vantage NV (Polar ElectroOi, Finland) heart rate monitor. HR, metabolic and ventilatory parameters were averaged at 30 second intervals. The anaerobic threshold was estimated by the V-slope method, using a second disproportionate increase of the volume of carbon dioxide expired in relation to the volume of oxygen consumption (respiratory compensation point) (Beaver et al., 1986). Heart rate and running speed at AnT were interpolated from test data. Using the same test data, the heart rate vs. time was plotted and evaluated by visual inspection for detecting the HR breakpoint. The subjects whose HRDP could not be indentified, were excluded from further analysis. Paired t-test for dependent samples was used to evaluate the statistical significance of differences between the ventilatory AnT and the HRDP method. The Pearson product moment correlation coefficients were used to determine the relationship between AnT and HRDP parameters. The significance level was set at p < 0.05.

RESULTS AND DISCUSSION The mean peak values of the treadmill incremental test are presented in Table 2. The time necessary to perform the test was relatively short (approximately 10 to 20 minutes), which means that it can be incorporated within or as part of a training session. Table 2. Peak values of the treadmill test Variables

mean±SD

VO2max (ml/kg/min)

62.0±6.0

HRmax (beat/min)

194.4±8.7

RQmax

1.18±0.04

vmax

19.99±2.05

Abbreviations: VO2max: maximal oxygen uptake, HRmax: maximal heart rate, RQmax: maximal respiratory quotient, vmax: maximal running speed

Table 2 presents the mean values of parameters measured at the gas exchange threshold and HRDP. No

&O , %V & O , HR, %HRmax, at AnT and HRDP. significant differences were noted between mean V 2 2 3

Table 3. VO2, heart rate and running speed at peak values, AnT and HRDP and corresponding correlation coefficients (r) VT

HRDP*

r

VO2 (ml/kg/min)

53.5±5.8

53.1±5.7

0.77*

HR (beat/min)

176.5±9.9

177.5±9.9

0.83*

v (km/h)

15.0±2.2 15.0±2.0 0.85* Values are means ± SD; *correlation coefficient significantly different from zero; Abbreviations: VO2: oxygen uptake, HR: heart rate, v: running speed, VT: ventilatory threshold, HRDP: heart rate deflection point

In our study, the HR deflection and the gas exchange anaerobic thresholds were evident in all 48 subjects. Some others studies have documented the same (100%) success discerning the HRDP (Conconi et al., 1982; Pokan et al., 1995) but others reported lower values (46 – 94%) of successfulness in HRDP detection (Hofmann at al., 1997; Ribeiro et al., 1985; Kuipers et al., 1988; Bodner et al., 2000), suggesting that the HRDP may not be reproducible across dissimilar populations. A deficiency of HRDP detection, in the literature has been attributed to differences among the training or fitness status (Ribeiro et al., 1985; Francis et al., 1989). The HR/speed curve and the VCO2/VO2 relationship, used to determine HRDP and AnT in one subject, are shown in Fig.1 and Fig.2. The HR achieved at AnT and HRDP (176.5±9.9 and 177.5±9.9 bpm, respectively, p=0.61), equaled 90.7% and 91.3% of maximal heart rate, respectively, similarly to other studies reporting values in the range of 88 to 94% of HRmax (Bodner et al., 2000). Identical average values for heart rate at the Conconi and gas exchange thresholds (177.0±6.0 and 176.0±6.0 bpm) were measured in the study of Bunc et al. (1988) on 28 trained runners. The HR values at the two thresholds in this study were highly correlated (r=0.83; p