Lactate Inflection Point
We've discussed how to maximise your aerobic engine, but how can we now functionally perform at this power? Your Lactate Inflection Point (LIP) is the exercise intensity at which your body produces lactate at the exact same rate as it can remove it, with any slight increases in exercise intensity causing an exponential rise in blood lactate and subsequent fatigue. LIP is used interchangeably with “Functional Threshold Power”, “anaerobic threshold”, “lactate threshold”, “functional capacity”: they all indicate the same point of physiological stress. These terms are often defined as the maximum intensity you can perform at for events lasting 45-60 minutes.
LIP is important because it is the functional component of endurance performance. Nobody can withstand exercising at 100% of their VO₂ max for any length of time, but those who can perform as close to 100% as possible for long periods of time will be more successful than their less fit counterparts, particularly in events lasting 20-60 minutes. The average athlete generally reaches LIP around 70% VO₂ max, whereas elites can typically withstand intensities of up to 88-94% of their VO₂ max. Let’s say, for example, an amateur and elite athlete both have VO₂ max values of 70ml/kg/min. If all other variables were identical (running economy, nutrition, hydration, fatigue etc), and they were to complete a 10km flat running race, the elite athlete would win comfortably. Why? The elite can hold 90% of 70ml/kg/min (63ml/kg/min) whereas the amateur can only hold 70% of 70ml/kg/min (49ml/kg/min). The elite has more oxygen available to create energy aerobically and will therefore be able to produce higher power outputs without fatiguing.
Why Does Lactate Cause Fatigue?
Well, it doesn’t. Lactic acid is a byproduct of anaerobic metabolism, and is accumulated when there is an imbalance between oxygen supply and oxygen demand. When exercise intensity is low-moderate, oxygen supply is plentiful, so almost 100% of our energy can be made aerobically. When exercise intensity is high, however, our body’s demand for energy is also very high. If we do not have the aerobic capacity to oxidise fats and carbohydrates completely through aerobic energy pathways, our anaerobic energy pathways will pick up the slack and fill this gap. The benefit of anaerobic energy pathways (also known as ‘anaerobic glycolysis’) is that energy is created quickly, however the major consequence of this is that lactic acid is produced.
So, back to the question… Lactic acid is made up of two components: a hydrogen ion (H⁺) and a lactate molecule. Lactate does not cause fatigue (it can actually create energy!), but H⁺ ions do. Therefore, lactate is simply an indicator of H⁺ concentration in the blood. We refer to lactate because it is easily measured during laboratory tests, whereas H⁺ is not. In theory, the concentration of lactate in the blood is directly correlated with the concentration of H⁺.
H⁺ influences the pH of the blood. The pH scale is a measure between 1-14 indicating the acidity or alkalinity of a fluid. 1 is very acidic, 7 is neutral, and 14 is very alkaline. Hydrogen ions are acidic, and push pH value closer towards 1. Human blood (like most things in the body) is tightly regulated, and has an optimal pH range of 7.35-7.45. If we rapidly accumulate H⁺ ions (through exercise above our LIP), without the aerobic ability to clear them, this will decrease our blood pH (because H⁺ is acidic) and push it below this optimal range.
This forces our body to slow down. Why? Our glycolytic enzyme activity (the enzymes responsible for converting carbohydrate into energy) is inhibited and slows down. They slow down because they do not function optimally in an environment below a pH of 7.35. Think of this process like a car engine: a cold engine (blood pH below 7.35) or overheated engine (blood pH above 7.45) will not produce as much power or perform as well as a warm engine (optimal pH 7.35-7.45).
This inhibition of glycolytic enzyme activity means that the body is unable to produce energy to its full potential because many of the enzymes responsible for creating energy are shutting off or not functioning properly. You experience fatigue ('heavy' or 'burning' legs) and your body is forced to slow down in order to clear the excess H⁺. Slowing down allows much of our oxygen supply to refocus on metabolising and clearing out H⁺, rather than creating energy. The oxygen demand has decreased (because exercise intensity has decreased), but our oxygen supply remains similar.
Training to Improve your Functional Capacity
You can improve your functional capacity dependently or independently of VO₂ max. Improving your VO₂ max will proportionately improve your functional capacity, but it won’t necessarily improve the percentage of VO₂ maximum which you can hold for a length of time.
LIP can be delayed, and performance power increased, without any changes to your VO₂ max. This is achieved by improving your muscle efficiency through increasing hydrogen ion buffering capacity, lactate tolerance, lactate clearance, and increasing aerobic enzyme activity. Absorbing, transporting, and metabolising hydrogen and lactate more quickly means we can sustain higher workloads than previously possible, as this exponential accumulation does not occur until a higher exercise intensity.
Sprint Interval Training (SIT) involves completing very high intense anaerobic intervals above 100% of your VO₂ maximum (that is, the velocity/speed/power at which VO₂ max is reached) and is proven to significantly improve your muscle’s buffering capacity, lactate clearance and aerobic enzyme activity.
SIT training can improve muscle buffering capacity in as little as 3 weeks, and is significantly more effective at transporting lactate out of the muscle than the typical steady state endurance trained athlete after 7 weeks of run training consisting of 6x1.5 minute efforts at 112% VO₂ max, with 3 minutes recovery between efforts. SIT training also significantly improves aerobic enzyme activity, another key factor in improving functional capacity.
The take home message? Intensity is important for endurance athletes. Completing sprint interval training will not improve your cardiovascular fitness as well as other training methods, but it will allow you to perform at a higher percentage of your VO₂ maximum during endurance events. The idea is to reach your genetic peak in regards to VO₂ max (which is achieved during your base building phase) and lift up the intensity to improve your peripheral (muscular) adaptations (which improve your muscle’s ability to perform under stress), while maintaining your cardiovascular fitness.
I would encourage athletes to regularly train at intensities at or very close to their functional capacity/LIP in order to improve their lactate tolerance. This threshold intensity is not only race specific, but it also teaches both your muscles and mind to put up with the stressors caused by the presence of lactic acid, so you are able to maintain a high workload even in the presence of this fatiguing byproduct. Threshold sessions may not improve your buffering capacity or lactate clearance as well as SIT sessions, but it will improve the length of time you are able to perform at this intensity.
Incorporating SIT and threshold sessions into your race preparation or peaking training phases will certainly improve the percentage of VO₂ max you can functionally hold during training and racing. The magic number of sessions is very individual, but you should find great benefit by adding two SIT sessions per week with at least 48 hours recovery in between, for perhaps 4-6 weeks in the lead up to a major race. Anything around 20-30 seconds of maximal effort followed by 2 minutes of passive rest (6-10 sets) has been found to be very effective at providing the peripheral adaptations which increase LIP. Longer, slightly less intense (but still over 100% velocity at VO₂ max) intervals of up to 90 seconds show significant improvement in muscle oxidative capacity, and is a good alternative for those of us whose bodies can't handle sprinting. Much longer intervals as close to LIP as possible for durations of 5-10 minutes, with half the rest, is a good goal to aim for when completing threshold sessions. The ideal combination of work:rest ratios for SIT above VO₂ max is yet to be determined; the important thing is that your efforts are at an intensity above 100% VO₂ max and that you have enough rest to ensure the quality of the session is upheld. Quality is more important than quantity during these sessions.
To summarise the process of improving your endurance performance so far: reach your genetic VO₂ peak through long-slow distance training and VO₂ max intervals. Next, maximise the percentage of VO₂ max you can functionally maintain during a race by completing SIT and threshold efforts. Next week we will incorporate economy of movement into the equation and discuss its effect on endurance performance, before exploring the roles of fibre type, heat load and hydration, nutrition, and recovery.
Written by Luke McIlroy – Director of Sport Science at METS Performance Consulting
BEx&SpSci, ESSAM, AES