Lactate threshold (LT) is considered an important indicator of endurance performance and can be useful to define the training intensities and optimize training (15). For these reasons, the determination of the LT in endurance athletes is common (2,6,26). Typically, the LTs are calculated from the power- or speed-lactate curves obtained in the laboratory using incremental protocols with steps of fixed-time duration (FixT). However, athletes prefer to exercise on the field than in a laboratory. Therefore, the use of fixed-distance (FixD) intervals is another widespread but not extensively investigated form of incremental test protocol for determining the LTs in field settings. In 1996, Usaj and Starc proposed a protocol consisting of 1,200-m intervals starting at 12.2 or 15.1 km·h−1 (depending on the athlete's level) with increments of 0.5 km·h−1 (24). A similar protocol (1,200-m intervals with increments of 1 km·h−1) has been used in the last 10 years by the Italian Track and Field Federation for testing middle and long distance runners and race walkers. The advantage of this protocol is essentially that the athletes stop for the blood sample in the same place. However, the use of FixD produces a curvilinear protocol because the time to complete the step decreases on increasing the running speed. The progressive reduction in step duration may be a confounding factor because the LT can be influenced not only by the training and fitness level of the athlete but also by interindividual differences in lactate kinetics that may affect the time needed to reach an equilibrium between muscular and blood lactate (3,13,25). This may be particularly problematic at high speeds (i.e., shorter step times) and when assessing athletes longitudinally. Furthermore, although a step duration of 6 minutes (300–360 seconds) has been suggested to be optimal for LT determination (13), with the aforementioned protocols, the step duration starts at about 330 seconds but decreases to about 200 seconds (3 minutes) in the last stages.
Irrespective of these theoretical considerations and potential problems, to be valid, FixD incremental tests should provide LTs correlated to those obtained using the traditional and validated FixT protocols performed in the laboratory. Moreover, another important measurement property of a new measure that needs to be examined is the so-called longitudinal validity (or external responsiveness) (8,22). The longitudinal validity would indicate to what extent the changes in LTs determined with FixD tests on the field reflect changes in the LTs obtained with FixT protocols in the laboratory. Indeed, a correlation between 2 measures at 1 time point not necessarily reflects a relationship between their changes over time.
Therefore, the aim of this study was to examine the criterion validity of the LTs determined with FixD tests by assessing their correlations with those obtained using FixT laboratory protocols (criterion). A second aim was to verify the longitudinal validity by examining the relationships between the changes over time in LTs obtained with the 2 protocols. We hypothesized that the LTs determined using the 2 protocols, and their changes were highly correlated.
Experimental Approach to the Problem
The construct validity refers to whether a measure correlates with the construct that it purports to measure. In this study, we used the LTs determined with FixT protocols as the reference indicators of the construct (criterion): indicator of endurance capacity. Therefore, criterion evidence of construct validity (i.e., convergent validity) was examined by assessing the correlations between the LTs obtained using the FixD test and the same LTs determined with a traditional FixT laboratory protocols (1). Furthermore, to examine the longitudinal validity, we assessed the correlations between the changes over time of the LTs determined with the 2 methods, where the changes of FixT LTs were the criteria. The dependent variables of this study were the LTs, and the independent variable was the test protocol. Because this study focused on construct validity, methods for assessing agreement between the 2 protocols were not used. Every point of the lactate speed curve is correlated to endurance performance, so there is no superior LT (5,23). For this study, we decided to use a low- and high-intensity threshold, one calculated from the individual basal values and from a fixed blood lactate concentration, respectively.
The participants completed in random order and on separate days (48–72 hours apart) an incremental treadmill test in the laboratory (FixT protocol) and an incremental test on the field (FixD protocol). Outdoor tests were performed on a tartan track. The 2 assessments were completed at the same time of the day to avoid circadian influences. The tests were repeated in the same order after 6–12 weeks but in 10 participants only. The tests were planned during the tapering week and after at least 3 days of low volume and intensity training. The laboratory temperature was about 21°C on both occasions (humidity from 50 to 60%), whereas in the field, the temperature was 15–19°C during the first test session (humidity from 42 to 67%) and from 19 to 24°C during the second session (humidity from 56 to 84%).
Twelve well-trained male middle and long distance amateur and competitive runners (age 25  years, height 178.1 [5.2] cm, body mass 66  kg, SD in parentheses) participated in the study. Their V̇O2max (58.6 [4.9] ml·min−1·kg−1) was estimated using a shuttle run test consisting of 20-m shuttles covered at increasing speeds (0.5 km·h−1·min−1) until exhaustion (19). The participants trained from 4 to 7 d·wk−1. After being informed of the aims and procedures of the study, all the subjects gave their written consent. This study was approved by the local Ethics Committee. The subjects were asked not to eat for at least 2 hours before testing, not to drink coffee or beverages containing caffeine for at least 8 hours before testing, and to avoid intense exercise for 48 hours before testing. The subjects were asked to maintain their nutritional habits and were allowed to drink ad libitum; however, the hydration status was not controlled. Although the hydration status is rarely measured in studies involving LTs, we cannot exclude the influence on the results considering that previous studies have reported both an alteration (12) and unchanged (16) LTs in hypohydrated conditions.
The laboratory test consisted of an incremental test on a motorized treadmill (RunRace, Technogym, Gambettola, Italy) starting at 12 km·h−1 with increments of 1 km·h−1 every 6 minutes until volitional exhaustion (Figure 1B). The inclination was set at 1% (11). Between steps, the runners rested 1 minute for allowing blood samples to be collected. The field test was performed on a track (400 m) and consisted of an incremental test starting at 12 km·h−1 with increments of 1 km·h−1 every 1,200 m until volitional exhaustion (Figure 1A). As per the laboratory test, the runners rested 1 minute for the blood sample collection. The peak speed was calculated according to Kuipers et al. (14). The running pace was indicated to the participants using an acoustic signal and markers (cones) on the track every 50 m. The athletes were instructed to be close to the cones (adjusting their running speed) when hearing each acoustic signal. The acoustic signal sequence was created using a PC software and subsequently converted into mp3 format and played on an Mp3 player connected to preamplified multimedia speakers (Harman Kardon, Stamford, CT, USA). The heart rate (HR) was recorded every 5 seconds during each testing session using short-range telemetry HR monitor systems (model 710, Polar Electro, Kempele, Finland).
Blood lactate samples were taken within 30 seconds from the end of each step. Capillary blood samples (5 μL) were collected from the ear lobe and immediately analyzed using a portable amperometric microvolume lactate analyzers (LactatePro, Arkray, Kyoto, Japan). The portable blood lactate analyzer used in this study was reported to be reliable and valid (17). However, to further increase the reliability of the measures, the blood samples were taken twice, and the mean value of the 2 measures was used for the analyses. For this study, we arbitrary selected 2 LTs: (a) LT, the intensity eliciting a 1-mmol·L−1 increase in [La−] above values measured during exercise at about 40–60% of peak speed (7); (b) onset of blood lactate accumulation (OBLA), the intensity corresponding to a fixed [La−] value of 4 mmol·L−1 (20).
The OBLAs were determined from the equation: (ln [4 mmol·L−1 − a] − ln b)/c, where a, b, and c are constants obtained by solving the continuous nonlinear regression y = a + b exp (cx), where x is the speed (23). The LT was computed using the same calculations but substituting to 4 mmol·L−1 the blood lactate value corresponding to 1 mmol·L−1 above the mean value measured during exercise at about 40–60% of the peak speed according to the definition of Hagberg and Coyle (7).
Unless otherwise noted, all data are presented as mean (SD). Before using parametric tests, the assumption of normality was verified using the Shapiro-Wilk W test. Correlations between speed at LT and OBLA obtained with the 2 protocols and between absolute changes were examined using Pearson's product moment correlation. The 95% confidence intervals of these correlations were also reported. The following scale of magnitudes proposed by Hopkins (www.sportsci.org) was used to interpret the correlation coefficients: <0.1, trivial: 0.1–0.3, small: 0.3–0.5, moderate: 0.5–0.7, large: 0.7–0.9, very large: >0.9, nearly perfect. Differences in speed and HR (30-second average) at LT and OBLA between protocols were examined using a 2-way analysis of variance (time × protocol) followed by the Bonferroni post hoc test. The probability of type 1 error (alpha) was set a priori at 0.05 in all statistical analyses.
The maximal HR reached during the tests between FixD and FixT was similar both at the first testing session (195  and 194  b·min−1, respectively) and the second testing session (197  and 195  b·min−1, respectively). Heart rates at LT and OBLA were similar between protocols (p > 0.66). The speed at LT and OBLA was not significantly different but FixD showed trends toward higher speeds (p level from 0.051 to 0.21, Table 1). The peak speed reached during the FixT test was lower than that reached in the FixD test (18.4 [0.8] and 19.3 [0.9] km·h−1, respectively; p < 0.001). In the first testing session, the mean peak lactate reached in the FixT test was higher than that reached in the FixD test (11.5 [1.8] vs. 9.7 [2.5] mmol·L−1, respectively; p = 0.04). Similarly, in the second testing session, the mean peak lactate of the FixT test was higher than that in the FixD test (12.5 [1.6] vs. 9.9 [2.4] mmol·L−1, respectively; p = 0.002). The peak lactate values reached in the 2 protocols in the 2 testing sessions were not different (p = 0.186 and p = 0.218 for FixT and FixD, respectively).
A nearly perfect relationship was found between LT speeds determined with the 2 protocols (r = 0.97 [CI 95% 0.91–0.99]; p < 0.001) (Figure 2). Similarly, a nearly perfect correlation was found between OBLA values (r = 0.95 [0.83–0.99]; p < 0.001) (Figure 3). In the second testing session, correlations calculated on 10 participants were similar (data not shown). The relationship between changes in LT and OBLA was very large: 0.79 (0.32–0.95; p = 0.007) and 0.78 (0.32–0.95; p = 0.006), respectively (Figures 4 and 5). Confidence intervals ranged from moderate to nearly perfect.
The findings of this study demonstrated that the LTs obtained from a protocol performed on the field and consisting of fixed-distance intervals showed acceptable criterion and longitudinal validity. Indeed, significant and very large correlations were found between the LTs determined with FixD and FixT protocols and between their longitudinal changes.
Although the advantage of FixD tests is mainly practical, their use on the field is quite common not only for running but also for other disciplines such as swimming (18). In the past, the use of FixD field tests has been questioned. However, the concerns were mainly related to the use of hear rate for estimating the LT (9,10). Nevertheless, when using FixD test for determining the lactate–workload relationship, the steps should be long enough to allow reaching an equilibrium between muscle and blood lactate. Indeed, step durations longer than 3 minutes are necessary for reaching steady blood lactate concentrations and for obtaining a lactate–workload curve. Furthermore, 6–8 minutes has been suggested as the optimal durations for identifying the intensity corresponding to the maximal lactate in the steady state (4,13,21,25). When determining the LTs, the main aim of sport scientists and coaches alike is to obtain a measure that can predict aerobic performance and can be used as a reference tool for monitoring training-induced changes and prescribing training intensities (6). In this regard, various studies have shown that LTs determined with steps of 3–4 minutes are correlated to endurance performance (6,23). However, the use of fixed-distance intervals determines a progressive decrease of the time at disposal for reaching blood–muscle lactate equilibrium. Although to our knowledge the lactate kinetics of each step of incremental tests has not been purposely investigated, the influence of individual lactate kinetics (i.e., differences in the time necessary for diffusing in the blood) cannot be excluded. Furthermore, individual peripheral adaptations induced by training may further influence the longitudinal changes in the LTs determined with the 2 protocols. On the contrary, the use in the FixT test with 6-minute steps (as the reference test of this study) would ensure more time for allowing the steady blood lactate concentrations. The FixT protocol used in this study started at 12 km·h−1 meaning the first step was 331 seconds long (5 minutes and 31 seconds), that is close to the 6 minutes of the FixT protocol. However, the step duration decreases to 3–4 minutes at a higher intensity (>16 km·h−1). This may also be a reason why the speed at OBLA was higher for FixD than that for FixT (as per the maximal speed). Despite these physiological speculations, evidence is needed to understand whether these potential limitations preclude the use of FixD tests for determining the LTs. The significant and high correlations between the LTs determined with FixD and FixT protocols support the use of this field test protocol for determining the aerobic capacity of the athletes. Besides, the changes in LT and OBLA obtained during the FixD test explained about 60% of the variance of changes in the LTs determined with the traditional incremental laboratory tests. The unexplained variance may be due to the aforementioned individual differences in lactate kinetics, the variability due to the reliability of the LTs themselves, and changes in ambient conditions. It should be noted that 2 subjects improved in the FixD test but showed a decrease in the FixT protocol indicating that some substantial differences may exist at individual level.
The HRs corresponding to the LT and OBLA were similar between protocols suggesting that the HR is an indicator of intensity less influenced by protocol than speed. These results are similar to those found by Bentley et al. (3) who showed that the HRs at OBLA were similar between 3-minute and 8-minute step incremental protocols. However, they found different HRs at LT. Given the importance of HR at LTs for training prescription, quantification of training load and exercise intensity, the influence of testing protocols on HR–lactate relationship warrants further investigation. The peak speed and lactate value reached in the FixT test was lower than those reached during the FixD test, probably as a consequence of the overall shorter test duration. This result is similar to that reported by Kuipers et al. (13) and is therefore expected (4). Although this study provided some evidence of validity of this Fix-D test, future studies should examine the relationships between changes in the LTs and changes in endurance performance, which is the most appropriate validation method (external responsiveness) not only for the FixD test but also for the LTs in general (8). A test that is not reliable is not valid, but reliability is not sufficient to validate a test (1). Therefore, before assessing the reliability of a test, some evidence of validity is necessary. Based on the results of this study, it is worthwhile to determine in future studies both the absolute and relative reliabilities of FixD.
The findings of this study support the use of FixD tests for assessing the LT and OBLA. Coaches may therefore use protocols consisting of fixed-distance intervals performed in field setting for determining the LTs in distance runners, thus substituting the laboratory assessment. This test should not be seen as a new test in addition to the plethora of existing assessments but as an easier alternative from a practical point of view and potentially more motivating for the athletes. This test like those performed in the laboratory can allow one to obtain a measure for longitudinal monitoring and for determining training intensities. Interestingly, the HRs corresponding to the LT and OBLA were similar between protocols suggesting that this physiological parameter is less protocol sensitive than speed is. However, these findings can be only generalized to FixD protocols with stage durations ranging from 6 to 3 minutes that are probably long enough for obtaining a stable blood lactate-workload relationship. Similarly, these results can be applied to amateur runners, and therefore, future studies should examine the validity of this protocol in elite high-level runners. However, it is important to underline that laboratory tests would ensure better control on testing conditions being performed in a controlled environment.
No grant support was provided for this study. The results of this study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association.
1. Ary, D, Cheser Jacobs, L, Razavieh, A, and Sorensen, C. Introduction to Research in Education
. Belmont, CA: Wadsworth, 2006.
2. Australian-Sports-Commission. Physiological Tests for Elite Athletes
. C.J. Gore, ed. Champaign, IL: Human Kinetics Publishers, Inc., 2000.
3. Bentley, DJ, McNaughton, LR, and Batterham, AM. Prolonged stage duration during incremental cycle exercise: Effects on the lactate threshold and onset of blood lactate accumulation. Eur J Appl Physiol
85: 351–357, 2001.
4. Bentley, DJ, Newell, J, and Bishop, D. Incremental exercise test design and analysis: Implications for performance diagnostics in endurance athletes. Sports Med
37: 575–586, 2007.
5. Bosquet, L, Leger, L, and Legros, P. Methods to determine aerobic endurance. Sports Med
32: 675–700, 2002.
6. Faude, O, Kindermann, W, and Meyer, T. Lactate threshold concepts: How valid are they? Sports Med
39: 469–490, 2009.
7. Hagberg, JM and Coyle, EF. Physiological determinants of endurance performance as studied in competitive racewalkers. Med Sci Sports Exerc
15: 287–289, 1983.
8. Impellizzeri, FM and Marcora, SM. Test validation in sport physiology: Lessons learned from clinimetrics. Int J Sports Physiol Perform
4: 269–277, 2009.
9. Jeukendrup, AE, Hesselink, MK, Kuipers, H, and Keizer, HA. The Conconi test. Int J Sports Med
18: 393–396, 1997.
10. Jones, AM and Doust, JH. Conconi's heart rate deviation is an artefact of fixed distance protocol
. J Sports Sci
6: 559, 1992.
11. Jones, AM and Doust, JH. A 1% treadmill grade most accurately reflects the energetic cost of outdoor running. J Sports Sci
14: 321–327, 1996.
12. Kenefick, RW, Mahood, NV, Mattern, CO, Kertzer, R, and Quinn, TJ. Hypohydration adversely affects lactate threshold in endurance athletes. J Strength Cond Res/Natl Strength Cond Assoc
16: 38–43, 2002.
13. Kuipers, H, Rietjens, G, Verstappen, F, Schoenmakers, H, and Hofman, G. Effects of stage duration in incremental running tests on physiological variables. Int J Sports Med
24: 486–491, 2003.
14. Kuipers, H, Verstappen, FT, Keizer, HA, Geurten, P, and van Kranenburg, G. Variability of aerobic performance in the laboratory and its physiologic correlates. Int J Sports Med
6: 197–201, 1985.
15. Londeree, BR. Effect of training on lactate/ventilatory thresholds: A meta-analysis. Med Sci Sports Exerc
29: 837–843, 1997.
16. Papadopoulos, C, Doyle, J, Rupp, J, Brandon, L, Benardot, D, and Thompson, W. The effect of the hypohydration on the lactate threshold in a hot and humid environment. J Sports Med Phys Fitness
48: 293–239, 2008.
17. Pyne, DB, Boston, T, Martin, DT, and Logan, A. Evaluation of the lactate pro blood lactate analyser. Eur J Appl Physiol
82: 112–116, 2000.
18. Pyne, DB, Lee, H, and Swanwick, KM. Monitoring the lactate threshold in world-ranked swimmers. Med Sci Sports Exerc
33: 291–297, 2001.
19. Ramsbottom, R, Brewer, J, and Williams, C. A progressive shuttle run test to estimate maximal oxygen uptake. Br J Sports Med
22: 141–144, 1988.
20. Sjodin, B, Jacobs, I, and Karlsson, J. Onset of blood lactate accumulation and enzyme activities in m. vastus lateralis in man. Int J Sports Med
2: 166–170, 1981.
21. Stockhausen, W, Grathwohl, D, Burklin, C, Spranz, P, and Keul, J. Stage duration and increase of work load in incremental testing on a cycle ergometer. Eur J Appl Physiol Occup Physiol
76: 295–301, 1997.
22. Terwee, CB, Dekker, FW, Wiersinga, WM, Prummel, MF, and Bossuyt, PM. On assessing responsiveness of health-related quality of life instruments: guidelines for instrument evaluation. Qual Life Res
12: 349–362, 2003.
23. Tokmakidis, SP, Leger, LA, and Pilianidis, TC. Failure to obtain a unique threshold on the blood lactate concentration curve during exercise. Eur J Appl Physiol Occup Physiol
77: 333–342, 1998.
24. Usaj, A and Starc, V. Blood pH and lactate kinetics in the assessment
of running endurance. Int J Sports Med
17: 34–40, 1996.
25. Yoshida, T. Effect of exercise duration during incremental exercise on the determination of anaerobic threshold and the onset of blood lactate accumulation. Eur J Appl Physiol Occup Physiol
53: 196–199, 1984.
26. Yoshida, T, Chida, M, Ichioka, M, and Suda, Y. Blood lactate parameters related to aerobic capacity and endurance performance. Eur J Appl Physiol Occup Physiol
56: 7–11, 1987.
Keywords:Copyright © 2012 by the National Strength & Conditioning Association.
assessment; correlation; protocol; LT; OBLA; external responsiveness