Exercise Thresholds

A crash course.

Introduction

‘Threshold’ is a term that gets thrown around a lot in endurance circles. Physiologically, there are certain observable ‘thresholds’ that demarcate certain patterns of fatigue. However, most athletes and coaches often use the term both ambiguously and inconsistently. In my own experience, ‘threshold’ seems to denote a level of intensity that can be maintained for a prescribed time/distance. Clearly, under this loose definition, a marathon ‘threshold’ and an 800m ‘threshold’ refer to very different intensities. What’s more, this ambiguous ‘threshold’ intensity is used synonymously with ‘lactate threshold’, confounding matters further.

exercise intensity domains A schematic of exercise intensity domains and associated thresholds.

Lactate Thresholds

One method for identifying the different intensity domains is via the measurement of blood lactate concentration.

NB: In case you haven’t read my previous post, I would just clarify: while blood lactate is measured in conjunction with fatigue, it has no direct role in causing that fatigue. In other words, it is a “marker not a maker” of fatigue.

When we measure blood lactate concentration during an incremental exercise test (that is, a test in which the exercise intensity gradually increases until exhaustion), we get an exponential curve. On this curve, it is generally accepted that there are two typical breakpoints.

Lactate Threshold (LT)

This represents the speed/power at which blood lactate rises above resting levels, and marks the upper limit of nearly exclusive aerobic metabolism1. Exercise at this intensity is actually relatively comfortable, and can be maintained for ~4 hours2. The speed at which the LT occurs is a strong predictor of marathon performance8 (and indeed, any event lasting over 2hrs)5,6. While determining this point may sound straightforward, in practice it is not as simple as you might think. Identifying it visually has been shown to be unreliable3, so more objective criteria must be used. Faude et al.1 identified 10 different criteria for the determination of LT.

Maximal Lactate Steady State (MLSS)

a.k.a Lactate threshold (!), Lactate Turnpoint (LTP), Onset of Blood Lactate Accumuation (OBLA), Anaerobic Threshold

Perhaps what most people think of as the ‘lactate threshold’, this represents the highest constant workload at which lactate values can eventually stabilise (i.e. achieve a ‘steady state’)1. The MLSS represents the maximal sustainable rate of exercise13. When exercise intensity is increased beyond this point, there is a considerable contribution from anaerobic metabolism. As such, some have defined this breakpoint as the ‘anaerobic threshold’1.

Note that the term ‘anaerobic threshold’ may be misleading, as the contribution of the aerobic and anaerobic pathways is more transitional in nature (rather than an abrupt shift, as the term suggests)1.

While there are numerous criteria for identifying the MLSS from a single incremental test1, the gold standard for identifying this intensity is through the use of several constant intensity exercise bouts of at least 30 mins—all completed on separate days. From this protocol, the MLSS is established as the highest intensity that leads to no substantial fluctuation in blood lactate after the 10th minute4.

lactate curve annotated with the lower and upper thresholds Faude et al.1

Critical Power

a.k.a Critical Speed/Velocity (CS/CV), Critical Pace (CP), Critical Swim Speed (CSS)

This represents a purely mathematical, performance-based threshold. It is derived from a highly predictable relationship between high-intensity power (or speed/pace) and the time that power can be maintained for. Interestingly, this relationship (termed the Power-time relationship, or P-t) is highly conserved across different species, contraction types and exercise modalities (i.e. running, cycling, rowing etc)9,11.

Put very simply, to calculate your CP you need at least 4 (but ideally more) maximal performance tests spanning durations of 2–15 min, all completed on separate days to ensure maximal output9,19. If you were to plot these on a graph (with time on the x-axis, and power or speed/pace on the y-axis) you would get a hyperbola (i.e. a curve). If you know the formula for the plotted curve, this would allow you to predict the power/pace that could (theoretically) be sustained indefinitely11. This ‘infinitely’ sustainable pace is your critical power, and is supposedly the highest intensity at which you can achieve a steady-state9.

I labor the point of the CP being indefinitely sustainable in theory, as this isn’t necessarily the case in practice. In reality, CP intensity is seldom maintained for longer than ~30–40 minutes12,13,19 (though the tolerable duration of CP intensity will hinge on how it is estimated)17. This is due, in part, to the fact that the power-time relationship doesn’t account for certain fatiguing factors (e.g. body temperature, ‘fuel reserves’, hydration levels etc.), that become increasingly significant during prolonged exercise. The critical power is, however, a very strong predictor of performance in events of roughly 30 mins (10 mile cycling TT, 5 km run etc)5.

Note that the CP may occur at a similar intensity to the MLSS10. However, the CP will likely overestimate the MLSS13.

the critical power model Jones et al.9

Ventilatory Thresholds

The same incremental test used to determine an athlete’s VO2max can also be used to identify certain intensity thresholds. This involves the analysis of expired gases; specifically the volume of expired (i.e. ‘blown out’) air, the amount of oxygen consumed by the body (VO2), and the amount of CO2 produced (VCO2). All of the aforementioned values are measured in litres, with ‘V’ standing for volume.

First Ventilatory Threshold (VT1)

This corresponds to the first disproportionate increase in expired air volume relative to oxygen uptake (VO2)14. Put simply, if you could happily recite the works of Shakespeare while training, you’re probably below the first ventilatory threshold. If, however, you can only string short sentences together, you are likely above VT1, but below VT215.

The VT1 roughly corresponds to the lactate threshold15.

Second Ventilatory Threshold (VT2)

a.k.a Respiratory Compensation Point (RCP)

This corresponds to the first disproportionate increase in expired air volume relative to CO2 production (VCO2)14. If you are working at an intensity at which you can’t/don’t want to talk to anyone, you are probably above the second ventilatory threshold. Note, however, that you should allow ~4 minutes at a given high-intensity work rate before making this assessment15.

The VT2 very roughly approximates the CP/MLSS5,13.

Methodological Issues

Athletes should be aware that there are a lot of confounding factors when it comes to determining the above thresholds. For example, blood lactate measures can be affected by muscle glycogen levels, the blood sampling site, gender, and the test protocol itself1,7. Similarly, the power-time curve (used to derive CP) will be influenced by the trial durations18 and the motivation of the subject. Finally, ventilatory thresholds are particularly test-dependent, and will vary substantially (if not disappear altogether) depending on the protocol5,16. The take home message? Comparing your threshold values to your mate’s is not all that useful (unless, of course, you both perform very similar tests under very similar conditions).

References

  1. Faude, O., Kindermann, W. and Meyer, T. Lactate threshold concepts. Sports Med. 39(6): 469–490, 2009.
  2. Meyer, T., Gabriel, H., Auracher, M., Scharhag, J. and Kindermann, W. Metabolic profile of 4 h cycling in the field with varying amounts of carbohydrate supply. European Journal of Applied Physiology 88(4–5): 431–437, 2003.
  3. Yeh, M.P., Gardner, R.M., Adams, T.D., Yanowitz, F.G. and Crapo, R.O. “Anaerobic threshold”: problems of determination and validation. Journal of Applied Physiology: Respiratory, Environmental and Exercise Physiology 55(4): 1178–1186, 1983.
  4. Beneke, R. Methodological aspects of maximal lactate steady state-implications for performance testing. European Journal of Applied Physiology 89(1): 95–99, 2003.
  5. Black, M.I., Durant, J., Jones, A.M. and Vanhatalo, A. Critical power derived from a 3-min all-out test predicts 16.1-km road time-trial performance. European Journal of Sport Science 14(3): 217–23, 2013.
  6. Burnley, M. and Jones, A.M. Oxygen uptake kinetics as a determinant of sports performance. European Journal of Sport Science 7(2): 63–79, 2007.
  7. Plato, P., Mcnulty, M., Crunk, S. and Ergun, A.T. Predicting lactate threshold using ventilatory threshold. International Journal of Sports Medicine 29(09): 732–737, 2008.
  8. Jones, A.M. The physiology of the world record holder for the women’s marathon. International Journal of Sports Science and Coaching 1(2): 101–116, 2006.
  9. Jones, A.M., Vanhatalo, A., Burnley, M., Morton, R.H. and Poole, D.C. Critical power: implications for determination of VO2max and exercise tolerance. Med Sci Sports Exerc. 42(10): 1876–1890, 2010.
  10. Chidnok, W., Fulford, J., Bailey, S.J., Dimenna, F.J., Skiba, P.F., Vanhatalo, A. and Jones, A.M. Muscle metabolic determinants of exercise tolerance following exhaustion: relationship to the “critical power”. J Appl Physiol. 115(2): 243–250, 2013.
  11. Skiba, P.F., Chidnok, W., Vanhatalo, A. and Jones, A.M. Modeling the expenditure and reconstitution of work capacity above critical power. Med Sci Sports Exerc. 44(8): 1526–1532, 2012.
  12. Brickley, G., Doust, J. and Williams, C. Physiological responses during exercise to exhaustion at critical power. European Journal of Applied Physiology 88(1-2): 146–151, 2002.
  13. Dekerle, J., Baron, B., Dupont, L., Vanvelcenaher, J. and Pelayo, P. Maximal lactate steady state, respiratory compensation threshold and critical power. European Journal of Applied Physiology 89(3–4): 281–288, 2003.
  14. Pettitt, R.W., Clark, I.E., Ebner, S.M., Sedgeman, D.T. and Murray, S.R. Gas exchange threshold and VO2max testing for athletes: an update. J Strength Cond Res. 27(2): 549–555, 2013.
  15. Jeanes, E.M., Foster, C., Porcari, J.P., Gibson, M. and Doberstein, S. Translation of exercise testing to exercise prescription using the talk test. J Strength Cond Res. 25(3): 590–596, 2011.
  16. Wasserman, K., Hansen, J. E., Sue, D. Y., Whipp, B. J., and Casaburi, R., 1994. Principles of exercise testing and interpretation (2nd ed.). Philadelphia, PA: Lea & Febiger.
  17. Pringle, J.S. and Jones, A.M. Maximal lactate steady state, critical power and EMG during cycling. European Journal of Applied Physiology 88(3): 214–226, 2002.
  18. Jenkins, D., Kretek, K. and Bishop, D. The duration of predicting trials influences time to fatigue at critical power. Journal of Science and Medicine in Sport 1(4): 213–218, 1998.
  19. Hill, D.W. The critical power concept. Sports Med. 16(4): 237–254, 1993.