|
|
PHIL WATTS, Ph.D.
Contributing Editor for The Master Skier
Phil Watts, Ph.D. is a professor and exercise physiologist at Northern Michigan University in Marquette, Michigan. A native of North Carolina, Phil received his Ph.D. from the University of Maryland. He transplanted to the north woods of upper Michigan during the winter of 1978 and immediately purchased his first pair of cross-country skis. Phil is also author of the book, Rock Climbing, from Human Kinetics Publishers.
|
 |
In the previous two articles of this series we began building a physiological model for cross country ski race performance based upon common laboratory tests.
For events of 3-20 minutes duration, maximum oxygen uptake (VO2max) is very predictive of performance.
As event duration extends beyond 20 minutes, the ability to sustain a high submaximal, or steady-state, VO2 becomes important.
A skier’s maximum sustainable steady-state VO2 is usually associated with the lactate threshold point (TLA).
With events that last over an hour, additional factors can begin to negatively influence the maximum steady-state VO2.
These include thermal balance between the body and the environment, hydration status and liver and muscle glycogen stores.
A third factor, economy, can also have a significant impact on performance.
In the laboratory, we measure economy as the steady-state VO2 or energy expenditure rate (kcals/minute) at a set performance power output.
For treadmill tests, the performance power output is controlled as a set speed, gradient or some combination of the two.
The lower the VO2 or energy expenditure for the set power output, the more economical the performance.
Better economy could translate into less “wasted” energy, less glycogen depletion, and lower physiological stress for a given exercise intensity.
Looking at the concept of economy from a different angle, it becomes evident that, for a given VO2, the more economical athlete has a higher power output.
This should lead to a faster racing pace relative to physiological effort. Thus, economy can lead to endurance and economy can lead to speed.
Studies on runners with similar VO2max have found that up to 64% of the variance in 10-km race times (range of 30-33 mins) could be explained by differences in running economy among the subjects.
However, economy can vary among athletes of similar performance abilities. Sjodin and Svedenhag reported running economy differences of 17-21% among sub 3-hour marathoners (see: Sports Medicine, 2:83-89, 1985).
The chart presents data for two cross country skiers roller-skiing with V1 technique on a motorized treadmill at 6.5 miles/hour over different grades.
Skier A has a lower VO2 than skier B at all grades, indicating better economy.
At gradients between 8% and 10%, skier A can perform the work at a VO2 that is about 8 ml/kg/min lower than skier B.
Adjusting for body mass, this difference could account for an estimated 200 kcal difference in energy expenditure over a 2-hour event.
Also, from the “different angle,” we can see that, if these two skiers worked at the same VO2, say around 53-54 ml/kg/ min, skier A could deliver enough power to climb a 10% grade, while skier B would be limited to around an 8% grade.
So what determines an individual’s economy?
What I am calling economy is a function of mechanical efficiency and muscle efficiency - two variables that are difficult to measure.
Mechanical efficiency is defined as the mechanical power output divided by the required metabolic energy expenditure.
Many factors have potential to affect mechanical efficiency.
These include stride length and frequency relationships; body and equipment mass; degree of vertical oscillation (or movement up and down of the body and appendages (such as the legs and the skis); ground reaction forces between the ski (and the pole) and the snow; the mean power vector (opposite to the direction of forces applied); and a host of other biomechanical aspects.
Human mechanical efficiency has been most studied for running and cycling and usually ranges between 20 and 30% in those activities.
Muscular efficiency is related to the development of tension within an individual muscle relative to the muscle’s energy expenditure rate.
Muscular efficiency can be influenced by factors such as the primary substrates or fuels used for energy (carbohydrates yield more energy per unit of VO2 than fats), fiber type recruitment (slow twitch act with greater efficiency than fast twitch) and contributions from the elastic properties of muscle.
Thus, when we measure the steady-state VO2 during V1 roller-skiing at a set pace in the lab, we are evaluating the combined effects of all factors that influence mechanical and muscular efficiency.
This is why there is still a lot to learn relative to economy in such a technique-intensive sport as cross country skiing.
And this does not even begin to consider equipment design, waxing, and snow conditions.
The mystery is enough to keep sport scientists, coaches, athletes … and equipment designers busy for decades!
So Many Ways To Win!
The logic of the physiological model tells us to combine the top “bragging rights” VO2max, the highest possible TLA, and a fantastic economy in a single individual to get the ultimate cross country racer.
In an interesting article, M.J. Joyner models optimal marathon running performance on the basis of the three physiological tests we have studied in this series; VO2max, %VO2max at TLA and running economy at a set pace.
By using the reported “best” values for each factor, his equation estimated a “best pace” of 21.46 km/hour or a 1:57:58 (hr:min:sec) finishing time – a sub two-hour marathon! (See: Joyner, J. Appl. Physiol. 70:683-687,1991.)
This looks great in theory. However, in reality, it has not happened … yet.
One problem is that there is a tendency for athletes with the highest economies to have lower VO2max values and vice versa.
Elite middle distance runners often have higher VO2max values (ex. 85 ml/kg/min) than elite marathoners (ex. 72 ml/kg/min), but exhibit lower economies.
Take a look again at our skiers’ data in the chart.
While skier B presents an inferior economy compared with skier A, her VO2max is significantly higher.
So which skier will race the fastest?
In our lab at Northern Michigan University, we feel that expressing economy relative to VO2max can be important.
Sjodin and Svedenhag did this for marathon running by expressing steady-state VO2 at a set pace (economy) as a percentage of the individual subject’s VO2max.
They found a very high relationship between this calculated variable and competition pace in the marathon.
The calculated value for %VO2max at a given power output or pace combines the effects of both economy and VO2max.
For the two skiers in the chart, skiing with V1 on a 9% grade, we obtain steady-state VO2 values of 48 ml/kg/min for A and 56 ml/kg/min for B. VO2max values are 54 ml/kg/min for skier A and 62 ml/kg/min for skier B.
The calculated values for %VO2max at the set 9% grade would be 90% for skier B and 89% for skier A.
Thus, these skiers may perform very close to one another in a longer race.
What one gains in VO2max, the other makes up in economy.
Sjodin and Svedenhag found that elite runners (2:10-2:30 hr:min for the marathon) only differed within 5% for calculated %VO2max at a test pace of 15 km/hr even though VO2max ranged from 63 to 78 ml/kg/min in this group.
Can Economy Be Improved?
In theory, improvement in any of the factors of mechanical or muscle efficiency should improve economy. Improvements, however, may be too small to measure until they begin to accumulate over time.
Short-term technique training does not always produce measurable improvement in economy, at least with running.
Most research today indicates that improvements in economy come over the long term and are related to training volume and years of training.
It may be that identifying and mastering correct technique, and then applying correct technique over long periods of training is necessary.
The complex nature and evolving status of cross country ski technique suggest that there are significant performance gains to be had from technique training in this sport.
Unfortunately, I am unaware of any short- or long-term studies that have objectively evaluated changes in technical performance by a skier and related these to measured changes in economy; or race performance, for that matter.
At least such data are not readily available in the published literature. I suspect a large part of the problem is that we have no standard, objective tool to evaluate and rate technique.
A reasonable strategy may be to keep up with the evolution of skiing by reading articles like Pete Vordenberg’s “The Better Way to Skate” (The Master Skier, Mid-Season, 2001/2002) and work with a knowledgeable coach to put the information into practice.
Periodic critical video analysis can be helpful with this.
Consider physiological testing in a lab that specializes in testing skiers.
Remember that single test sessions may have little to offer the master skier other than bragging rights.
Repeat testing is required to evaluate changes relative to your training and performance records.
Be open to change. There is still a lot to be learned about skiing.
Yes, this wonderful sport could keep all of us … sport scientists, coaches, athletes, designers … busy for a long time.
|
For more articles like this one, subscribe today to The Master Skier. |
|
