Energy Systems and Training for Cross-Country Ski Race Courses

It is well known that XC ski racers have to be incredibly “fit” to be competitive. But how do we measure “fitness”? For decades, we’ve known that a high VO2max is necessary to be a competitive ski racer. Training to improve VO2max involves a large volume of training at a low intensity and a comparatively smaller volume of training at a very high intensity. Over the past 20 years, changes in equipment technology, course preparation, and a change in race formats from primarily individual start events to a mix of individual starts and head to head racing have necessitated a change in our view of “fitness.” It’s no longer all about VO2max.

This spring Thomas Losnegard wrote a review titled “Energy system contribution during competitive cross-country skiing” in the European Journal of Applied Physiology. In this review, Thomas compares the energy demands of skiing to other sports and summarizes the available research into what type of “fitness” is necessary to compete in modern XC skiing. Before I dive into a summary of the review article, I think it’s important to explain metabolism in a nutshell, so I’ll borrow an analogy from my friend and colleague Joe Howdyshell of the Summit Endurance Academy – The Financial Analogy:

Your body has 3 pathways for generating energy in the form of ATP. ATP powers all muscle contractions. 

1. The Phosphagen system, also known as ATP/CP. CP stands for Creatine Phosphate. Your body stores enough energy to power exercise for up to 15-20 seconds… with training you can increase this to maybe 25-30 seconds of stored energy in the muscles. Think of the phosphagen system as the cash in your wallet.
2.  The anaerobic system, also known as non-oxidative or glycolytic systems, is the conversion of Glucose to Lactate. This process does not require oxygen and hence is not measured by VO2max testing. It is quick to produce energy (not as quick as the phospagen system) to meet the demands of training and racing. Think of the anaerobic system as your credit card. 
3.  The aerobic system produces the bulk of energy for events lasting longer than 2-3 minutes. Think of the aerobic system as your bank account, with your paycheck deposited regularly by your employer. You could perhaps think of recovery as your employer in this analogy… recovery puts the money back in the bank!

You can train to increase the size of your wallet. You can train to lower the interest rate on your credit card. And you can train to get a bigger paycheck! During an XC ski race. You are constantly balancing the cash in your wallet (ATP/CP) and the amount of debt you can take on (glycolytic) with the amount in your bank account (aerobic). You have to use the aerobic system to pay off the credit card and put cash back in your wallet before you have to pay too much interest.

Here are my most important take-home points from Losnegard’s review:

• In XC skiing, there are large variances in speed, power output, and mechanics over the course of a race. The most similar physiological analog to XC ski racing would be short track XC-mountain biking. To adequately prepare for XC ski racing, we must first look into the specific demands of ski races. In the review, Losnegard cites research stating that elite level 10 or 15 km distance races may last 25-35 minutes, of which about 50% of time is spent going uphill, with 25% of time spent on the flats, and 25% on downhills. However, the time spent in any one type of terrain is usually no more than 10 to 35 seconds at a time; sometimes longer, but rarely much over one minute. This means that while racing, skiers must constantly be assessing speed and changing between techniques and sub-techniques approximately 25 times per km in order to maintain the highest speed possible around the course.

 

•  In uphill terrain, a skier may work at 120-160% of VO2max for short periods of time, followed by periods of lower VO2max to recover. As you likely know, VO2max is the maximal amount of oxygen the body can use during exercise. It seems odd that an athlete can work at greater than 100% of VO2max. This does not mean they are using more oxygen, it means that anaerobic (aka non-oxidative) metabolism is contributing to the workload. Think of workload as speed in a given terrain and conditions. At workloads above VO2max, a significant amount of energy is derived from anaerobic metabolism. This results in an oxygen deficit. Oxygen deficit is essentially, how much energy had to be produced by anaerobic methods to sustain the workload. Oxygen deficit is difficult to measure and quantify. Typically, exercise at high intensities resulting in oxygen deficit results in the production of Lactate, so the measurement of blood lactate can be used to estimate the extent of oxygen deficit (higher blood lactate levels indicate a greater oxygen deficit incurred).

 

• In general, sprint skiers have a higher anaerobic capacity than distance skiers. This is directly correlated to the fact that sprint specialists tend to have a higher muscle mass than distance skiers. Elite men typically have a 10-30% greater anaerobic capacity than women, largely correlated to women typically having less muscle mass than men. The performance difference between men and women is greatest in techniques relying largely on upper body (e.g. double poling and V2) and least in techniques that are not as upper body dependent (e.g. striding and V1). 

 

• The contributions of the arms to XC skiing are obvious, but how training of the upper body impacts performance is not fully understood. Arms generally constitute a much smaller muscle mass than legs. Muscles in the arms have a lower oxidative capacity than leg muscles, even in elite XC skiers.  During double poling, the arms release lactate into the blood, where the legs are actually a net importer of lactate- meaning the arms are utilizing a higher percentage of anaerobic metabolism thus producing lactate that must leave the muscle, while the legs are working at an intensity where they are capable of taking lactate from the blood and utilizing it for energy through aerobic pathways. At high intensities in double poling, the energy contribution from the arms plateaus, and any further increase in intensity must come from the legs which increase power output leading to greater vertical changes in the center of mass in order to increase speed (you jump onto your poles more at higher intensities). 

 

• It appears that pole contact time may be the ultimate limiter of energy input and thereby speed in flat terrain. With this in mind, training at high speeds to minimize pole contact time (the amount of time the poles are in contact with the ground) may be of great benefit.

Now for my interpretations and some practical implications.

In order to repeatedly perform at workloads above VO2max, an athlete must have a high level of aerobic fitness in order to recover from efforts and regenerate stores of ATP/CP during the short periods between supra-VO2max efforts. Instead of looking at increasing VO2max as the ultimate goal of training, perhaps training to increase an athletes’ maximal accumulated oxygen deficit would yield better results. 

The Principle of Specificity is a foundation of exercise physiology. Essentially, you can improve only what you train.  It is important that we as coaches (and parents, and skiers), recognize the energy demands of XC ski racing and train our athletes (and ourselves) appropriately. We need to design workouts that mimic the demands of racing to prepare our athletes to race. We need to train to move fast in a variety of terrain utilizing both techniques (skate and classic) and all sub-techniques (herringbone, double pole, striding, V2, V1, etc.)

We need to look at skiing as a non-steady state exercise. There are few instances in ski racing where a skier can grind away for 3-15 minutes at a steady effort/intensity, so why do many coaches (myself included) tend to focus training on this type of workout? Instead, a focus on workouts that mimic the actual demands of XC ski racing- 10-60 sec maximal efforts with 5-30 sec of recovery, may lead to better results in XC ski racing. 

Here are my practical recommendations based on this review article and the research summarized in it:

1. Train to the demands of your event. While workouts like 4 by 10 min L3 certainly improve lactate threshold and VO2max, a workout like 4 by 10 min of 45 sec fast/15 sec recovery may achieve similar aerobic gains and allow for faster movements and greater anaerobic contribution during the workout, perhaps making it more similar to racing. See below for some possible workout progressions. Also, the training necessary for a Wave 1 Birkie skier is different from a Wave 8 Birkie skier which is different from a junior which is different from a World Cup athlete. Identify the demands of your event(s) and target your training towards them.
2. Include “gear shifts” in intervals. Most intervals should include some portion uphill, but not all. Have you ever heard “transitions are important”? Practice them at speed!
3. Use ski specific modalities for high intensity work- bounding/ski walking, rollerskiing, or (obviously) on-snow. Intervals on bike or swimming are good for fitness, but not nearly as specific as similar workouts in ski specific modalities, especially given the increased utilization of upper body power.
4. Train fast movements. Athletes need to be able to move quickly and apply power quickly in racing. In order to do this in races, we need to do it in training. Incorporate short sprints, explosive strength training, and plyometrics into your training. Andy Newell has some great video resources on his website( www.nordicteamsolutions.com ) that can be modified for any level of athlete!

Here is a progression of workouts you may find beneficial to add to your training. These workouts should be done on skis, rollerskis, or bounding/running with poles. Each workout is preceded by a 20-25-ish min warm-up consisting of: 

5 min easy, 3 min L3/Threshold, 2 min easy, 2 min L4, 2 min easy, 1 min L5, 5-10 min easy. In an ideal world, this warm-up would be preceded by some mobility and light plyometric work!

Each Main Set consists of 12-20 min of total “on” time, all “on” time is done at sprint race pace/effort. HR will not be useful in guiding the intervals because they are so short, though looking at HR recovery between intervals and the response of HR to the workouts over time can be interesting). I also prefer to have a timer of some sort so you don’t have to look at your watch to know when to start and stop:

• 24 by 30/30 (24 min, 12 min on time)
• 5 by 6 by 30/30 w/ 4 min between sets (46 min, 15 min on time)
• 5 by 8 by 30/30 w/ 4 min between sets (56 min, 20 min on time)
• 30 by 30/30 (30 min, 15 min on time)
• 21 by 40/20 (20 min, 14 min on time)
• 5 by 6 by 40/20 w/ 4 min between sets (46 min, 20 min on time)
• 5 by 6 by 40/20 w/ 2 min between sets (38 min, 20 min on time)
• 30 by 40/20 (30 min, 20 min on time)
• 15 by 45/15 (15 min, 12 min on time)
• 3 sets of 8 by 45/15 w/ 4 min between sets (32 min, 18 min on time
• 3 by 10 by 45/15 w/ 5 min between sets (40 min, 22.5 min on time)
• 20 by 45/15 (20 min, 15 min on time)
• 12 by 60/30 (18 min, 12 min on time)
• 4 sets of 4 by 60/30 w/ 2 min between sets (30 min, 16 min on time)
• 5 sets of 4 by 60/30 w/ 4 min between sets (36 min, 20 min on the time)
• 20 by 60/30 (30 min, 20 min on time)
• 10 by 30/30, 5 min easy, 8 by 40/20, 4 min easy, 6 by 45/15 (33 min, 15.5 min on time)

End each workout with a 5-20 min cool-down

I recommend doing one session like this every other week in the summer, one per week in the fall, and one-two per week in the winter (depending on race schedule). For skiers living/training at altitude, it may be necessary to increase the rest between intervals to ensure that the short “on” periods are of sufficient quality to train really quick and powerful movements. 

I’d like to give thanks to a few friends and colleagues who aided in the editing of this article: Corrine Malcolm, Duncan Callahan, and Joe Howdyshell.