Motion and trajectory are sometimes misused. Motion is the headway in which we move. It is always a straight line determining a starting point and a destination point. Trajectory is the mean used to move on, employing as many trajectories as we need while displacing to the destination point.
Types of skiing trajectories
To walk to a destination, we normally orient our body directly towards a particular point adopting a linear trajectory. To change directions when driving a vehicle, we generally guide it performing a curvilinear trajectory. In skiing, direction depends on diminishing the angle between our skis and our destination point, i.e., aligning them towards a specific place of the slope, and this can be executed using both types of trajectories.
Connecting linear trajectories is an improvised behavior that we may adopt when skiing. We tend to abruptly turn our skis because it is difficult for us to plan curvilinear trajectories, which we will get with training. Thus, we tend to perform “Z” shape turns with a marked steering action at the beginning and a linear trajectory until the next direction change. In this reactive procedure, turns are not connected, losing skiing efficiency. Thus, we do not follow the natural arch of the curvilinear trajectory but tend to close the turning angle in a hurried action to rapidly orient our skis toward the desired direction, rather than doing it gradually. Instead, if we choose the curvilinear path is because, in general, we have a built-in linked “S” shape turns which promotes skiing flow.
Exemplifying both models, the “Z” type turn in skiing resembles as driving a vehicle in a city, suddenly turning at each corner, connecting two linear paths through tight turns. The advanced “S” type turns looks like connecting turns as driving on a road with gradual and successive curves. From the efficiency point of view, rather than adopting a turning action that first moves the skis and continues then in a straight line, we should adopt a more efficient motor behavior like steering a vehicle on a winding road, which we gradually orient our vehicle towards the desired direction.
Trajectory determination
Slope configuration imposes us to select, between several possible paths, a particular trajectory to a determined place. If we do not know to precisely establish our trajectory due to lack of training and visual anticipation, we will suffer higher cognitive and muscle effort having to constantly strive while improvising our descent. This is not the case, because of our experience, if we know how to plan our motions in advance.
Most adult skiers are used to drive their vehicles over defined trajectories on roads with lateral reference lines. In skiing, slope space is relatively wide and there are not trajectories demarcations, except for signs marking slope limits. All skiers have the freedom to choose their paths. The beginner, according to inexperience and/or anxiety, chooses to quickly turn the skis, locating them perpendicular to the slope, location that he perceives as safe. The advanced skier, due to know-how and patience, chooses curvilinear trajectories by diminishing energy expenditure in taking advantage of skis’ geometry.
Trajectory planning
Establishing destination points and direction changes means having to plan the trajectory that we must follow to reach that place. Setting a path leads to proactive skiing and depends on factors such as terrain accessibility, slope conditions, snow, traffic, shortcuts choice, speed, and visual anticipation.
On the contrary, if we do not visualize our trajectory and depend on obstacles and conditions that are constantly displayed, we act based on the current situation, acquiring a reactive behavior. If we do not plan our trajectories, we generally slow down or stop before an obstacle rather than avoiding it without speed modification.
Direction change and skier’s intention
Turning the skis is modifying the direction of our trajectory. For such purpose, it is observed the use of two strategies according to our intention. Because of the linear paths in which we are moving, our intention in turning may not only be changing direction but also reduce speed using a ‘turning-to-slow down’ strategy.
On the other hand, if we ski mostly in curvilinear trajectories acquiring speed control at each point of the turn, our intention at changing direction may be not just to slow down but to change our headway (in wide turns) through a ‘turning-to-guide’ strategy.
Direction change and speed control
In deciding to change direction, we must take into account our motion speed since this must be consistent with our skill to turn the skis. Speed choice may be a problem as we are still failing to determine the proper speed we should maintain. A reduced speed makes turning more demanding because of the greater skis-snow friction, but a higher speed may compromise skis’ control and our balance.
Visual strategies, levels of steering control, and the skier’s extrapersonal space
Based on the propositions of some authors (Donges 1978; Salvucci & Gray 2004; Lappi 2013; Lehtonen et al. (2013); Vansteenkiste 2015) and adapted to skiing, our steering behavior could be divided into three path sections related to visual strategies, levels of control, and extrapersonal space:
- An immediate path section referred to the proximate space just in front of us, providing an instantaneous control in our near extrapersonal space (the space which is out of reach for our extremities).
- In the pursuit path section, visual information is buffered (a function of our memory) just a few seconds for a pursuit control mode in the far extrapersonal space.
- The anticipatory path section where visual information of more than a few seconds supplies future path information as an anticipatory control in our extrapersonal space and visual background.
