Cross country skiing is an endurance sport. No matter if one’s goal is to be able to ski the outer loop of Teacup without stopping to rest or turning in a pleasing performance at the Birkie. Effective preparation has fundamentals the world’s elite use and the rest us can learn from.
The Physiology of Endurance Training
Not just about racing but a little endurance training improves all outdoor endurance fun– skiing, paddling, cycling, backpacking. Any sustained activity that lasts longer than about 1 ½ to 3 minutes is defined as endurance regarding muscle cell metabolism.
Lets start with this – C3H6O3, C3H5O3. The first one is Lactic acid and the second one is the conjugate base, Lactate. Lactic acid is something nearly everyone has heard of, as it is attributed to the muscle “burn” felt in hard efforts, or as the culprit in fatigue, or the muscular soreness the days following a strenuous event, but this is not necessarily true.
You may be familiar with the idea that glycogen is the primary fuel for our bodies. In the process of glycolysis, a glycogen molecule produces two ATP (the actual energy source) and two pyruvate molecules. Doesn’t stop there, as the pyruvate can enter the muscle cell mitochondria (cellular organelle of respiration and energy production) and along with oxygen will convert to a lot more ATP but can also
produce Lactate (but not Lactic Acid). Lots of things happen next, but for Lactate there are several pathways. It can be combined with even more oxygen and converted into another energy substrate. It can also pick up any free Hydrogen ion, become Lactic Acid, and leave the cell into the bloodstream which will buffer the acid – this process can delay acidosis in the muscle cell, which can indeed cause fatigue or even collapse. And with enough oxygen and rest the lactate and lactic acid will be metabolized in a very short time – probably by the time you sit down for a pint of beer after a great day of skiing, lactate/lactic acid will be cleared.
Cool, huh? Lactate is about energy production, and a mop for too much acid.
Although lactate does come from glycolysis the anaerobic process, it is another misconception that our bodies only “switch” from aerobic to anaerobic at a certain intensity level. Lactate from anaerobic metabolism happens constantly (the conversion to energy is just too amazingly fast) but will then continue as an energy substrate via an aerobic process until an intensity level is reached where the
aerobic capacity to eliminate it is exceeded. At that level lactate will then start to accumulate. This concentration level is an indicator of muscle cell energy system pathways and limitations. This intensity level is highly trainable and is a major goal of endurance training.
When lactate concentration in the blood is sampled, the result gives us a very clear window into what is happening in the muscles (ever seen videos of athletes getting their fingers pricked for a blood sample after hard efforts?). What is the process at that point? What is well established by exercise physiologists
is that there are two turn point, Ventilatory Threshold One (VT1) and Ventilatory Threshold Two (VT2). These are marked by blood lactate concentrations of 2mM at VT1 and 4nM at VT2 and this establishes three training intensity zones, Z1, Z2, and Z3, each of which provide specific and different training stimuli. Z1 is super easy to figure out, if one is in that zone, it should be very easy to carry on a conversation (some years ago I had some high school boys train using Kinesio tape over their mouths –breathing through the tape is possible but difficult and makes one breathe through the nose which will force down the workout intensity. The HS girls were easy, they would ski and sing simultaneously as a group). Z3 is also easy to identify, it is impossible to say more than one word! Every breath is for oxygen, not talking. And Z2 is where most uncoached athletes seem to spend their time, and that can be identified by the talk/breathe pattern “I’m….breathe…really getting….huff, puff…a great…wheeze…workout”.
Here is the zinger of this entire primer.
For decades, empirical evidence – that is, what coaches found produced best results - has shown that the best stimulus for endurance happens in Z1. Not Z2, not Z3. While training programs for endurance sport will vary in the distribution of time spent in Z1/2/3, most of the programs vary most in time spent in Z2 and Z3, with 80% of time in Z1 being common. In her last two years of international Nordic ski competition the amazing Marit Bjorgen spent 95% of her non-strength training in Z1. Remember that in Z1 talking is easy, as is the workout itself. That’s a lot of conversation!
How is this possible?
This distribution of training intensities has been established by practical means, in that over decades it continues to produce the best results. It is only recently that science has been able to come up with some reasons why.
Remember the mitochondria? The cellular “processing plant” for aerobic metabolism? Testing has now shown that the density and concentration of mitochondria is optimally stimulated in Z1 exercise, with consistent volume over weeks, months, and years continuing to improve capacity. The efficiency of
mitochondria is optimized in exercise in Z3, with results occurring in a matter of weeks. With extended inactivity the processes will reverse in the same way, efficiency first and density second. The commonly used “I’m getting a good workout” perceived exertion of Z2 will stimulate both mitochondrial density and efficiency, but not as well as specifically using Z1 and Z3 to target improvement in physical improvement.
Three into Five
We don’t have the opportunity – or the desire – to have our blood sampled frequently during exercise to confirm where what intensity level we are at. This results in the use of indirect methods, all of which have compromises. These indirect methods can be as simple as the talk test, a chart (10 scale or 20
scale) to assign personally experienced perception of effort called Rate of Perceived Exertion (RPE), or Heart Rate monitors (HR).
The use by elite athletes of HR monitors led coaches to come up with levels of intensity based on percentages of heart rate (commonly five levels) for training that have produced solid results. The HR five zone scale has been a popular scale for decades now. While popular – and useful – the five-zone scale is a bit
arbitrary. But not so different than the well-established physiological (VT1, VT2) turn points. For HR Level 1 and 2, this matches Z1. L1 is often called “very easy” or “recovery” level, L2 is termed “easy”. HR L3 is the same as Z2, “moderately hard”. HR L4 and L5 are the same as Z3, and for some athletes the difference between L4 and L5 are difficult to discern, it is simply really, really hard.
The HR Five zone scale is popular but accurately establishing the actual HR levels has proven to be problematic. The 5 levels are described as percentages starting from the maximum possible HR, and the common way is to start is with an assumption of one’s max by using the equation of 220-age. Level one (L1), by one method, is between 60 and 72%, L2 is 72-82%, L3 is 82-87%, and so on.
Here’s the problem with using HR. First up, the 220-age to determine maximum HR has long been proven to be wildly inaccurate. Even elite athletes of the same age can have significant differences in HR Max (and Max HR for athletes of similar
age can’t be compared as an indicator of performance). The most accurate way to determine usable HR Levels is to find the HR that occurs at VT2 and use that as the reference for the other zones. This will involve getting a blood sample during exercise.
Next problem is that the actual levels will change with fitness; the critical change between L2 and L3 (same as Z1/Z2) can be as low as 70% of HR Max in an untrained person, and as high as 90% for highly trained athletes.
More problems. If used for shorter interval training the HR may not rise to the actual exertion level before the work interval is over! Here’s the big one – the scale is different for different sports. Think of it this way, the heart and the HR is
not the goal of the training, it is muscular metabolism. The heart is in some way a short order cook – a request for one egg sunny side up and a piece of toast takes less involvement than eggs/bacon/hash browns, and a side of pancakes. The heart doesn’t know if it just the arms and shoulders engaged in
kayaking, or the legs, back, arms, and shoulders in a rowing stroke; the heart receives an order for oxygen delivery and complies. More muscles engaged require more oxygen, and the heart just takes the
order and delivers what is necessary. So, if one can’t seem to “get their HR rate up” in one discipline compared to another, in no way does that mean that the movement using fewer muscles and lower HR is easier.
A subtle but important issue when using HR as an exercise intensity gauge is Cardiac Drift. Cardiac Drift is when the HR will increase without a corresponding increase in movement speed or load. Usually this occurs about 10 minutes or so after initiating exercise – the speed or load isn’t different, but the HR
rises. One might consider this an example of why a warm-up is necessary but is cautionary about starting in one’s target HR level and not adjusting after the HR rises. More importantly is that Cardiac drift will occur after an extended period of exercise, usually when it is more than one’s body usually experiences. If one intends to work out at an L1 or L2 (Z1) intensity but for a significantly longer period of time, Cardiac drift will imply that this particular “easy” day may not be all that easy.
Enter the RPE, or Rate of Perceived Exertion. Originally a 20-point scale (the Borg RPE), it was meant to approximate the actual Heart Rate. This implies a 0 on the 20 point scale would be one isn’t alive! 60 could be one’s base resting rate, 200 their maximum heart rate. Problem being, maximum heart rate is't an individual attribute at any age (and a higher max HR isn’t a predictor of success!). Also, max HR will decrease with age. The 10-point RPE (Modified Borg or Modified RPE) scale is easier to use and can line up effectively with the five zone HR scale. The combination can be more useful if one can’t establish actual HR zone from blood lactate testing.
| RPE | Description | HR L1-5 | HR % Max | VT Zones |
| 1 | very light | L1 | 50 | Z1 |
| 2 | light | L1 | 60 | Z1 |
| 3 | easy exercise | L2 | 61 | Z1 |
| 4 | Deeper breathing but conversation is easy | L2 | 70 | Z1 |
| 5 | Somewhat hard. Deep breathing, talking is possible but more difficult | L2 | 71 | Z2 |
| 6 | Getting harder, but sustainable for an extended time with ensuing fatigue. |
L3 | 80 | Z2 |
| 7 | Hard. Breathing is more important than talking. “Threshold” |
L4 | 81 | Z3 |
| 8 | Very Hard. Getting a word out is very difficult | L4 | 90 | Z3 |
| 9 | Extremely Hard | L5 | 91 | Z3 |
| 10 | Difficult to maintain, minimal duration measured in seconds. | L5 | 100 | Z3 |
Recipes for Success
The distribution of training intensity varies from sport to sport, country to country, coach to coach. Using the Z1/2/3 level, many highly successful international programs have employed a distribution termed “polarized” training, with about 80% in Z1, 20% in Z3, and almost nothing in Z2 (Z2 might be considered as the “weekend warrior’s” training intensity). Successful programs vary, as example 70% Z1 and the remaining 30% using Z2 and Z3 in varying percentages.
What has been missing until recently is the value to endurance athletes who cannot train huge volumes per year. It had been conjectured that for those with only a few hours per week, substituting intensity could make up for volume. More Z2 than Z1. A few years ago, the only study done for non-elite athletes with very few training hours showed that those using a Polarized (lots of Z1 some Z3, no Z2) did better after a few weeks than those using as much Z2 as Z1 – but it was just one study. No more, quite a bit of recent research has been done that establishes the stimulus to mitochondria density is best below VT1 (Z1 or HR L1/L2) at any training volume.
The big takeaway here is that if one has limited hours to train every week, keeping things easy is still better! Optimal improvement will require a small number of workouts using high intensity Z3 intervals.
Is Z2 training (HR L3) a waste of time? That is not true either. Woody Allen was once quoted as saying “90% of success is just showing up”, and this is true of training as well. If working out at this intensity is what gets one motivated, there will be benefits. And there are some workouts, such as specific strength
(I think of using rollerskis to double pole up Mt Tabor) that are great at that middle zone.
There have been a few studies that seem to indicate that for people in low physical condition, working out in the Z2 (HR L3, RPE 4.5-6) will bring rapid results – but after about 6-8 weeks everything starts to plateau. Not a bad plan for those of us a bit behind in preparing for ski season but for continued improvement a de-emphasis on Z2 training is a better choice.
Takeaways
First up, higher volume of training hours is desirable. Training hours add up, so a half hour several times a week is better than just one longer session. Hard interval training isn’t just for those with competition goals. For competition level the time “in the zone” is tightly controlled as the stimulus has to be sufficient to meet needs of race stress but not create an overtraining load. For non-competition skiers the use of HIIT workouts in Z3 (HR L4-5) doesn’t have to be very long – indeed, most of the time for a HIIT workout will be spent in the warm-up and cool down. Intervals of 30 seconds at L5 (that’s really hard!) and a long rest, with as little as 5 minutes total at that level may be enough stimulus to have a positive effect.
If working out at Z2 (HR L3) is what gets you out the door, then getting out the door and working out is a good thing. Be careful about increasing training volume using only this type of training as not only will improvement stagnate but there’s a possibility of overtraining.
Set a goal! If competition is a goal, having a coach directed training schedule can be beneficial simply because athletes when left on their own tend to do the type of workout they like to do (I don’t like indoor strength training even though I know I would benefit). For myself, I no longer compete, yet I train for an enjoyable day of skiing at Z2 (HR L3) because that’s where I can sustain a speed that feels athletically fun and fast. My best preparation for days like these is to be disciplined at maintaining Z1 in my off-season workouts but including a little bit of hard intervals. A lot of workouts at Z2 hasn’t improved my days of enjoyable skiing at that intensity.
A final note.
For skiers taking part in the Teacup Team programs, the use of training intensity zones will be used for
workouts. Any coach might use Z1/Z2/Z3 as a descriptor, or L1/2/3/4/5. Or RPE 1-10 (although don’t expect much RPE 10!).
Thanks for reading,
Coach Karl
Comments will be approved before showing up.
Tom Jeanne
December 02, 2022
This is a fantastic summary, Karl! It aligns with everything I’ve learned in the past few years. Training for the Uphill Athlete is an excellent book for those who are interested in learning more, but your overview here captures the key points.