Case study: performance testing in Cross-Country (XC) Skiing

This case study is about a National Cross-Country (XC) Skier who is not able to keep up with his teammate during training and racing. Field observations and lactate measurements seem to indicate he needs to improve his aerobic energy system, but this turns out to be a false assumption.

COMPARING 2 CROSS COUNTRY SKI ATHLETES: VO2MAX

A National Coach works with 2 cross country skiers (Nordic Skiing) who both compete at the highest level. According to the coach, athlete A has a significantly higher aerobic performance (VO2max) than athlete B.

The difference between the athletes become very clear during training and racing. As soon as the intensity increases, athlete B cannot keep up with athlete A.

RESULTS LACTATE TEST IN XC SKIING

A conventional lactate test seems to confirm the assumption of the coach, that athlete A indeed has a higher aerobic capacity than athlete B. The lactate curves of the two athletes differ significantly.

Here are the results of the lactate step test, performed on a ski treadmill with a 2° incline and a step duration of 6 minutes:

You can easily see that athlete A has a lactate curve that is shifted to the right. For many coaches, this would be enough to conclude that athlete A indeed has a better aerobic base or aerobic capacity (VO2max).

What we know:

Athlete A performs much better in training and racing, compared to athlete B.
A lactate step test shows athlete A has a lactate curve that is shifted to the right.

A potential conclusion based on this input:

Athlete A has a better aerobic capacity than athlete B.

A corresponding practical implication:

Athlete B needs to follow a training program that aims to increase the aerobic capacity.

To better understand the (aerobic) physiological differences between the two athletes, the coach decides to use the INSCYD software. It all starts with a Cross-Country (XC) ski specific performance test.

THE SKI SPECIFIC TEST PROTOCOL

To get a full 360 metabolic profile, you need to first perform an exercise test. INSCYD is known for the wide possibilities in test protocols. Here’s the protocol that was used in this specific case study:

Before the examination, the weight and body composition were determined. These values, just like gender, are included in the INSCYD analysis.

The protocol in detail:

Warm up
2x8min, incline: 1.5°, technique: V2 alternate
1x5min, gradient 4°, technique: V1 skate
1x4min, gradient 6°, technique: V2 skate
1x approx. 3min, incline 8°, technique: V2 skate, all out until exhaustion

Blood lactate was measures before and after each interval. VO2 data was collected during exercise.

As shown, INSCYD enables you to combine different ski techniques (e.g. mountain step, pendulum run, etc.) as well as different inclines of the treadmill within one performance assessment. This results in further advantages in diagnostics. You can for instance quantify movement economy (the energy expenditure in relation to the speed) in different ski techniques.

Learn more about assessing the physiological (metabolic) performance determinants in XC skiing via our virtual lecture in INSCYD College

THE (PERFORMANCE DETERMINING) SKI METRICS PROVIDED BY INSCYD

Here are the metrics you can get when performing an INSCYD test:

Aerobic energy – VO2max

Maximum aerobic power.

Anaerobic energy – VLamax

Maximum glycolytic power.

MLSS & MMSS

Maximum lactate steady state & maximum metabolic (VO2) steady state.

Maximum lactate concentration

In all-out efforts.

Economy

Energy expenditure expressed in oxygen demand as a function of speed. Available for different technique and incline.

Lactate accumulation - and recovery rates

At any given exercise intensity, in steady state conditions.

Fat- and carbohydrate combustion

At any given exercise intensity, in steady state conditions.

Aerobic - and anaerobic energy contribution

At any given exercise intensity, in steady state and non-steady state conditions

THE RESULTS OF THE SKI METABOLIC TEST

Remember we said it would make sense to conclude that athlete A is aerobically stronger than athlete B? Here are the facts:

Aerobic energy – VO2max

Athlete A has a VO2max of 71.30 ml/min/kg, with a body weight of 76.3 kg.

Athlete B has a VO2max of 73.30 ml/min/kg, with a body weight of 75.9 kg.

In other words, contrary to what you might expected, athlete B has a slightly higher VO2max. How come these results contradict what many coaches would assume? Let’s continue looking at the results.

Anaerobic energy - VLamax

Theoretically, it would be possible that the two athletes differ significantly in terms of their glycolytic performance. This could explain why the lactate curves look significantly different. However, only small differences were measured:

Athlete A has a VLamax of 0.36 mmol/l/s.

Athlete B has a VLamax of 0.41 mmol/l/s.

Again we see athlete B is slightly superior to athlete A, not only in terms of aerobic- but also in glycolytic performance.

So far, we have no explanation why athlete B performs worse in training and racing, compared to athlete A. All we know is that the initial assumption that it’s because athlete B has a less well developed aerobic base, is not true. We need to dive a little deeper.

LOOKING DEEPER INTO THE SKI exercise TEST RESULTS

Anaerobic threshold or Maximum lactate steady state (MLSS)

Athlete A has a MLSS at 7.04 m/s.

Athlete B has a MLSS at 5.50 m/s.

This seems to confirm A has a better aerobic engine, which we know is not true..

Aerobic vs Anaerobic energy contribution

The INSCYD results show that – at the same speed – athlete A has a significantly higher energy contribution from the aerobic energy system, compared to athlete B (Fig. 5). At a speed of 8 m/s, for example, the aerobic energy contribution for athlete A is around 87% (solid blue line), while the aerobic energy contribution for athlete B is only 67% (dashed blue line).

It is easy to see that at a speed above 5 m/s, athlete B can produce significantly less energy via the aerobic metabolism than athlete A. This again seems to confirm A has a better aerobic engine, which is contradicted by the VO2max values.

Let’s continue our search. Hang on!

Fatigue and lactate accumulation

Figure 6 shows why athletes A and B cannot ski together in competition or training, if higher and lower intensities alternate. Let’s look what happens at a speed of 7.5 m/s. Athlete A accumulates approx. 0.56 lactate per mmol/L/min. After 15 minutes, he would have a lactate concentration of about 10 mmol/L. Athlete B, on the other hand, accumulates 7.33 mmol/l lactate per minute at this speed. He would be exhausted after only 1 minute.

Lactate recovery

We see a similar pattern when it comes to the ability to “clear” (recover from) lactate. Athlete A can already recover from a previous lactate accumulation at 6.5 m/s. At this rate, he can clear about 0.44 mmol/l per minute. Athlete B on the other hand, still accumulates approx. 2.62 mmol/l/min lactate at this level of intensity. This means he would not be able to recover at all, but instead continue to accumulate lactate.

Connecting the dots: XC Ski economy

As mentioned in the protocol section, both VO2 and lactate samples were collected during the test. By combining these two markers for aerobic metabolism and anaerobic metabolism, you get the total energy demand. If you compare the energy demand with the ski speed, you get the economy.

This is where we see significant differences between athlete A and B. Although athlete B is slightly stronger both aerobically and anaerobically, he needs significantly more energy for the same speed.

Figure 7 shows the economy of athlete A (green) and athlete B (red). The bars show how much faster or slower the athlete goes compared to a control group, based on energy expenditure. In other words: if your bar is above the dashed line, you go faster with a given energy expenditure. This means your economy is better than the control group. If your bar is below the dashed line, you go slower with a given energy expenditure. This means you economy is worse than the control group.

In other words, athlete B is not able to use his metabolic abilities (input) to turn it into speed on the skis (output). So we’re not looking at a metabolic problem, but at an issue in ski technique.

This translates into all other metrics and graphs, since they (anaerobic threshold, energy contribution, lactate accumulation and recovery) are all expressed in speed or power, which is about the output.

A low ski economy connects the dots between what the coach sees in practice and what the test results show.

Conclusion and training recommendation

If we had only compared the lactate concentration curves of a conventional lactate step test, we would have easily concluded that athlete A has a better aerobic engine than athlete B. We would tell athlete B to focus on increasing the aerobic energy system.

The 360 metabolic profile from INSCYD shows that this approach is based on false assumptions. It showed that athlete B even has a slightly higher aerobic and anaerobic performance than athlete A. Instead, the real cause of athlete B’s underperformance is his ski economy.

Without the INSCYD report, athlete B would spend a lot of time trying to increasing his aerobic performance and waste precious time. With the knowledge of INSCYD, he now knows what would effectively increase his performance: an improvement in ski economy.

In this case study, we confirmed the observation of the coach that athlete A is better in training and racing. However, the reason turned out to be not about endurance performance, but the ineffective conversion of existing metabolic power into speed on the skis.

Watch our virtual lecture to learn more about physiological (metabolic) performance determinants in XC skiing. We’ll learn which metrics to look for and how to assess them, in a lab or in a field test (independent of speed or power).

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