Workload Mechanics of Nordic Skiing

todd lodwick, cross country skiing, nordic, sochi

US Cross Country Skiing Olympian Todd Lodwick (right) was the American flag bearer at 2014 Winter Games in Sochi. A study finds that increases in double pole cross country ski work rate come from the legs, not the arms.

Cross country skiing is unique in that it requires enormous aerobic capacity. Aerobic capacity not only is a requirement of cross country skiing, but it also is a powerful predictor of nordic performance. In fact, a study done in 1991 found that VO2 Max, a measure of aerobic capacity or, more specifically, the maximum amount of oxygen that can be consumed by a subject during strenuous exercise, can predict the success of elite cross country skiers. VO2 Max could predict if an elite cross country skier was less successful, more successful or world class (Maximal oxygen uptake as a predictor of performance ability in women and men elite cross-country skiers, 1991.  Ingjer F).

In cross country skiing there are two primary techniques of movement: diagonal stride and double poling. Diagonal stride (see Todd Lodwick above) is used on uphill sections and requires mostly leg power. On the other hand, double poling is done on flat terrain and utilizes a combination of leg power and arm power.  The physiological mechanics of double poling were investigated in a study published in the Scandinavian Journal of Medicine & Science in Sports (Metabolic and Mechanical involvement of arms and legs in simulated double pole skiing, 2013. Bjarne Rud, et al.).

Double pole cross country skiing poses a problem for the arms. The arms rely on increased blood pressure during exercise, but when the legs are working their increase in circulation drops the blood pressure. Furthermore, the legs maintain a high arterial-venous oxygen difference that is generally not produced by working arms. So when a nordic skier increases his work rate while double poling, which limbs take up the additional work?

Subjects on a cross country 
skiing ergometer that mimics
cross country skiing.

The answer appears to be the legs. Bjarne Rud and colleagues found that when recreational cross country skiers were put on a machine that mimics cross country skiing, a cross country ergometer, the subjects increased work rate using their legs. The ergometer used by the researchers is pictured to the left. The subjects used the cross country skiing ergometer at a low and moderate work rate. A variety of physiological parameters were measured from the eight subjects.

One of the parameter measured in the subjects was the degree of motion of different joints. The degree of motion of the shoulder, elbow, knee and trunk were each measured. The knee was the only joint that showed a significant difference in range of motion between low and moderate work rate. The range of motion of the knee significantly increased when the work rate was increased from low to moderate on the cross country ski ergometer. The figure below shows the results of measuring the degree of motion in the shoulder, elbow, knee and trunk joints.  

Shoulder, elbow, knee, and trunk ranges of motion during low and moderate work rate double pole ergometer skiing. *P < 0.05 between work rates; n = 8.

In addition, the researchers measured blood flow and VO2 in the subjects at rest, at a low work rate and at a moderate work rate. Blood flow increased significantly in both the arm and leg when the subject work rate increased from a low to moderate work rate. However, VO2, which is a measure of oxygen consumption, showed a significant increase in the legs relative to the arms at moderate exercise. The figure below shows this blood flow and VO2 data.

Variables during double pole ergometer skiing. Arm and leg blood flow (a) and oxygen uptake (VO2) (b) at rest and low and moderate (Mod) work rates. *P < 0.05, between arm and leg; †P < 0.05, between work rates. BP; n = 8 or as indicated during rest and moderate work rate for VO2 and blood flow.

The researchers also investigated potassium and lactate balance. Potassium is released when the muscles and motor neurons repolarize between contractions.  The arms were found to have a net release of potassium at a low and moderate work rate. This suggests that the muscle fibers are firing to fast for the Na+/K+ ATPase pumps in the sarcolemma and neuromuscular junction to recover the released potassium. The legs had a neutral potassium balance. With regards to lactate, the arms had a negative lactate balance at a low and moderate work rate. The legs had, on the other hand, a positive lactate balance at a low and moderate work rate. Recall that lactate is a product of anaerobic metabolism.  Lactate can be used as fuel if sufficient oxygen is present to oxidize it. Thus, the legs burn the lactate that the arms were unable to oxidize.

All this data suggests that the arms are at maximal aerobic capacity under a low work rate during double poling. It is important to note that the subjects were recreational skiers, and this conclusion may not hold with elite cross country skiers. In addition, the data was collected by subjects on an ergometer in a lab. One can only speculate at how these results would change if the experiments were performed on snow with actual cross country skis. 

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