Energetic Considerations in Cross-Country Skiing

Fig. 20.1

Cost of transport (ml O₂/m) for a horse walking, trotting and galloping at increasing speeds of locomotion. Note that the transition from one gait pattern to the next occurs at speeds in the vicinity of intersection between the cost of transport curves for gait patterns, indicating that gait transitions might occur to minimize the energy required for locomotion at a given speed (Adapted from Hoyt and Taylor 1981, with permission)

If ski racers indeed choose their gait pattern in skate skiing in a manner to minimize the energy requirements for skiing at a particular speed, then it is surprising that they choose the 2-skate technique (at slow to intermediate), and then the 1-skate (at intermediate to fast) speeds, but then revert back to the 2-skate technique for fast to very fast (sprinting) speeds. This is equivalent to saying that a four legged animal, such as a horse, changes from a trot to a gallop and then back to a trot for increasing speeds of running, something that would never happen. Therefore, the question arises: why do cross-country skiers revert back to a technique (or gait pattern – 2-skate skiing) at very fast speeds, that was rejected at a slower speed in favour of another gait pattern (the 1-skate technique)?

The purpose of this study was to answer this question. We hypothesized that the reason for choosing the 2-skate over the 1-skate at slow and very fast speeds, but not at intermediate speeds, was associated with the cost of transport. In other words, we hypothesized that the 2-skate technique was more efficient at slow and very high speeds of skiing compared to the 1-skate technique, while the reverse was true for intermediate to fast speeds of skiing. A secondary question then became: is it possible to explain why the 2-skate technique is more efficient at slow and very high, but not at intermediate to fast speeds compared to the 1-skate technique? This, we felt, was an intriguing question, as it is, to our knowledge, the only form of locomotion, two-legged or four-legged, in which a gait pattern that was rejected at a slow speed is re-introduced at a high speed.

Cross-country skiing is a four-legged gait with arms and legs contributing to propulsion and the speed of locomotion. In four-legged gaits of dogs, horses and rabbits, it has been observed that breathing is coordinated with the footfall patterns (Ainsworth 1997; Attenburrow and Goss 1994; Bramble and Carrier 1983; Bramble and Jenkins 1993). Specifically, in galloping animals, inhaling is associated with an expansion of the chest cavity when the forelimbs are moved forward relative to the body, while exhaling is associated with compression of the chest cavity as occurs when the forelimbs are swung backwards relative to the body. 2-skate skiing has a footfall pattern similar to a galloping horse, and the arms swing backwards and forwards in unison in their double pole action. Therefore, it seems feasible to assume, and it has been suggested, that inhaling and exhaling in 2-skate skiing is directly tied to the arm movements (Faria 2008). It has been argued that this “respiration coupling” in animals results in a reduced metabolic requirement for locomotion, as the movement of the forelimbs in galloping animals causes natural chest expansion and compression that assists the respiratory muscles and thus reduces the metabolic cost of breathing (Ainsworth 1997; Bramble and Carrier 1983). However, whether this assertion is correct, and what magnitude this effect might have, cannot be tested in animals, as animals cannot be asked to abandon the natural respiration coupling pattern while galloping. However, in cross-country skiers performing the 2-skate technique, it is easy to measure the oxygen uptake when breathing occurs naturally with respiration coupled to the skiing motion, and when skiers are asked to abandon the natural coupled breathing patterns. Thus, the purpose of a second study described here was to test the hypothesis that respiration coupling inferred a distinct energetic advantage to cross-country skiers using the 2-skate skiing technique.

20.2 Methods

In order to accomplish the aims of this study, it was necessary to measure the kinematics and kinetics of cross-country skiing using the 1- and 2-skate techniques, and measure the oxygen uptake required for these two gait patterns. It was also necessary to make these measurements in a subset of skiers for the 2-skate technique while using their normal respiration coupling patterns and while skiing at the same speed and effort while abandoning the normal respiration coupling pattern. All measurements were performed using roller skiing on a motor driven skiing treadmill at the facilities of the National Centre for Excellence in Sport, Calgary and Canmore.

Kinematic and Kinetic Measurements

Kinematic measurements were performed using two high speed video cameras, one placed with the optical axis perpendicular to the sagittal plane of skiing, the other aligned with the optical axis in the direction of skiing, filming the skiers from behind. Forces exerted by each roller ski and each pole were obtained using instrumented skiing poles and roller skis. Force transducers inserted into the shaft of the poles measured the forces along the axis of the poles while compensating for bending forces at the pole. Forces in the roller skis were measured by replacing the normal roller ski braces that connect the ski part with the rollers with strain gauged braces that measured the forces perpendicular to the roller skis in the vertical direction, and in the medial-lateral (horizontal) plane relative to the roller skis. Anterior-posterior forces were not measured as roller skis are essentially frictionless and effective propulsion in the anterior-posterior direction is not possible in skate skiing, in contrast to classic skiing where anterior-posterior forces are essential for effective striding.

Oxygen Uptake Measurements

Oxygen uptake measurements were obtained while skiers skied at steady-state, typically after 4–5 min into a specific experimental condition (i.e. speed of skiing and slope) using a ParvoMedics TrueOne 2400 Metabolic Measurement System. Oxygen uptake was collected and expired air was analyzed every thirty seconds. Expired air was converted to standard temperature, pressure and dry (STPD) conditions, and analyzed to determine the rate of oxygen consumption and metabolic energy consumed (WEIR 1949).

Protocols

For comparison of the efficiency of the 1-skate and 2-skate techniques across a range of speeds, skiers (n = 8, young, active, elite level provincial and national skiers) skied using the 1- and 2-skate techniques at speeds of 6–33 km/h at increments of 3 km/h. They were asked to ski at each speed below the anaerobic threshold for 3 min with data collection occurring once the metabolic steady-state was achieved (typically after 2–2.5 min) (Solberg et al. 2005). For speeds above the anaerobic threshold, skiers just skied for 1 min at each speed in a precisely timed manner so that conditions for the 1- and 2-skate tests were identical, and thus comparable.

In order to estimate the oxygen cost for just the upper body and poling action for skate skiing using the 1- and 2-skate techniques, skiers performed an additional series of tests with oxygen uptake measurements. In these tests, the skiers stood on fixed skis and pulled on a cross-country arm ergometer at the frequency and excursion of their own skiing at low, intermediate and high speeds (6, 15, and 30 km/h). They did this by viewing a video of their own skiing, and pretending to follow the displayed skier in a perfectly matched manner. In order to obtain the proper forces, the pole forces measured during the actual skiing test were fed back to the skiers, and the resistance on the ergometer was adjusted until they satisfactorily matched the actual skiing forces. The legs were braced for this experiment so that no leg propulsion was possible, ensuring that we measured exclusively the oxygen cost of the arm and upper body motions at the target speeds.

In order to derive the functional force-velocity and power-velocity relationships of the poling action for the 1- and 2-skate techniques, skiers were fixed on a roller board on top of the skiing treadmill. The treadmill was then run below the fixed skiers at speeds ranging from 6 to 42 km/h at increments of 6 km/h and skiers were asked to perform double poling actions at maximal effort, controlled by the rhythm of a metronome to give them the frequency of poling in the 1- and 2-skate techniques. The forces for ten consecutive maximal effort poling actions at each speed were measured, and the corresponding impulses calculated. From these force/impulse-velocity relationships of the poling action, the corresponding power output as a function of skiing speeds was calculated.

For comparison of the efficiency of skiing with and without respiration coupling in the 2-skate technique, skiers (n = 9, young, active, elite level provincial and national skiers) were asked to ski at a pre-determined fast but sub-anaerobic threshold (Solberg et al. 2005) speed for 5 min on the motor driven treadmill. The speeds ranged from 18 to 22 km/h on a zero slope. Skiers performed the test first using their normal breathing patterns and respiration coupling was confirmed with the breath by breath analyzer. They then repeated this test twice more, first with respiration coupling consciously abolished and breathing in a reverse coupled manner: that is inhaling occurred during the push phase of the poles when the arms swung backwards and compressed the chest cavity, and exhaling occurred in the recovery phase, when the arms were swung forwards thereby expanding the chest cavity, and second, in the control condition with breathing coupling re-established.

20.3 Results

As hypothesized, the cost of transport (or equivalently, the oxygen requirement for a given speed of skiing) for the 1-skate and 2-skate techniques had two intersections, with the 2-skate technique being the more efficient gait pattern at the slow (6, 9 km/h) and high speeds (>24 km/h), while the 1-skate technique was the more efficient technique at the intermediate speeds (9–21 km/h) (Fig. 20.2). Out of the eight skiers, four showed precisely this double intersection of the cost of transport curves (Fig. 20.3), while the remaining four skiers, although not vastly different from the first four, showed an approximation of those curves, but not the double intersection.

Fig. 20.2

Mean (n = 8) cost of transport values as a function of skiing speed for the 1-skate (black dots) and the 2-skate (grey squares) skiing techniques. Note that the 2-skate technique was more efficient (lower values) at the slowest (6, 9 km/h) and the highest (>27 km/h) speeds, while the 1-skate technique was metabolically more efficient at intermediate speeds (12–21 km/h)

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