How to determine your lactate threshold

Today's topic is: how to determine one's lactate threshold in terms of power and or heart rate. This blog post is not meant to be the authoritative source for this topic, but instead an intro or a good starting point.

"Why would I want to know my lactate threshold?", you may ask. Good Q. Lactate threshold is the point were one's body (while exercising) begins to accumulate more lactate acid than it can process, which leads to eventual shutdown or reduction of said exercise. If you are a recreational rider, knowing your lactate threshold's watt or heart rate number may be of no more value than knowing your IQ score. On the other hand, if you are doing repetitive time trials and/or would like to follow a comprehensive training program it would be extremely valuable to know where your LT (lactate threshold) lands on the heart rate and power chart. Additionally this data accumulated over time can be analyzed to determine the state of one's conditioning (better, same or worse).

The most accurate way to determine LT is in a laboratory setting where blood samples can be taken during exercise to determine LT. What? "Too invasive and probably expensive", you say. Yes, so let's estimate it in a more simple cost effective manor.

In my experience a 40 kilometer flat timetrial is close to ideal for a couple of reasons. One, it's long enough that you simply can not stay above LT without dropping back down in order to continue. This allows averaging to give a more accurate number. Secondly, the 40 kilometer timetrial is typically the gold standard for most state and national championships (this can be variable). Additionally, I find that it's long enough to be a complete workout in itself. It is note worthy to mention that if the temperature is in the 90's, dehydration can occur during the performance and negatively effect the outcome.

An alternative to the 40k method is as follows: warm up for at least 10-15 minutes, ride a 30 minute flat time trial at a maximum controlled effort. Record the last twenty minutes of the ride. The average power and heart rate data during this period will estimate the lactate threshold point.

A third method for estimating LT is the Conconi Test. The premise is that there is a linear relationship between wattage and heart rate during exercise up to the point of LT, where it there after falls off the trend. So if  wattage is increased incrementally over time and plotted against the resulting change in heart rate on a X and Y axis the outcome will look like my chart below:  

I did this Conconi test nearly 15 years ago with a simple polar heart rate monitor and a stationary trainer with digital watt resistance at the gym.  I simply increased the wattage one click every minute and wrote down my heart rate just after increasing wattage (the machine maxed out at 400watts).

Based on this chart my predicted lactate threshold occurred at 169 heart rate and 325 watts (not bad for a category 4 rider).

There is some criticism that the Conconi Test only works with certain fit athletes where LT is close to max heart rate.  Here's link 1 and 2 to studies suggesting that Conconi was smoking crack.  There are studies that say the opposite as well.  At any rate, I subscribe to it's value.

It is very important to note that several rules must be followed for a good Conconi test.  The athlete should:  1. be rested (fatigue effects hr), 2. should eat in a manor appropriate with racing (meal 3-4 hours previous to test), 3. should test in a cool environment and or use cooling fans (rising core temperature can create upward heart rate drift, 4. not consume caffeine (also raises hr), 5. be normally hydrated (dehydration causes elevated hr). (If anyone can think of other factors please tell me)

I would also suggest a thorough warm up to account for heart rate drift as core temperature rises.  I would also suggest a 2 minute interval between wattage change to allow heart rate stabilization.  This will tend to prevent an athlete from going much higher than threshold, but my purpose is only LT and not VO2max.

Below is recent data from a Conconi Test that I tried to produce, using my road bike on my stationary trainer.  Unfortunately, I broke a few rules that I listed above, such as being rested, and not consuming caffeine.  I still want to show it because it shows the strong correlation between exertion and heart rate.   It also illustrates that sometimes the Conconi test can be inconclusive.

Note worthy is that the slope of the line is almost identical to the slope of my chart 15 years previous.  This should be true with most athletic individuals.  As a person improves the line shifts to the right (more wattage at the same heart rate). 

I collected the data above from my SRM software which I downloaded from my watt meter device.

I drew lines to try and show the one minute, 15 watt intervals that I did.  The green is watts and the dark stair-stepping line is heart rate.  It is note worthy that the wattage has some movement (not a fixed watt number).  In my defense it is hard to ride exactly at a specific wattage.  This is another reason that I suggest 2 minute intervals over one minute intervals to allow heart rate stabilization to each new load.  Additionally, my monitor had 3 cadence (3 rotations) smoothing which made my watt numbers appear much more static on my bike monitor than shown here in the above chart.

Note:  My experience shows that cadence (for me personally) within the ranges of 80-100 has no effect on my heart rate's correlation to  wattage.  In other words a cadence of 80 at 300 watts produces 151 heart beats per minute as does a cadence of 100 at 300 watts will also produce the same 151 heart beats.  For me going above or below these cadences can produce inefficiencies which will raise my heart rate.  

The final chart is also from my SRM software.  I included it because it's cool.  It really is.  The squiggly line is the exact same data as the above chart only graphed in a continous line on an X and Y axis, starting with yellow and changing to brown, green, aqua, blue, and finally ending in purple.  

I think it is extremely note worthy that SRM appears to think that the Conconi test is at least worthy enough to include it into their software.  

Please check back in another 15 years for another watt/heartrate chart update.   I predict a shift back to the left.  

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