Biomedical Engineering Reference
In-Depth Information
The size of the screens can also be a problem. Sometimes, simple television screens
were indeed used to view the video clips. The field of view is then limited and
it reduces the access to depth information [
21
]. For example, in tennis, Fery and
Crognier [
25
] have shown that the presentation of video clips on a TV screen lead
to overestimation of the length of short ball trajectories and underestimation of
long ball trajectories. These errors were not made in real situations.
Video-based methodology mainly provides the instant when visual information
are important but not the location of these information. This lack can be overcome
by combining the technique of time occlusion with an occlusion of spatial visual
information presented to subjects [
1
,
6
]. However, few studies have used this cou-
pling between time and space occlusions because the modification of the video is
not an easy task. Other methods can be used to determine the position of pertinent
visual information. The most used tool is the eye-tracker: this technique consists
in analyzing the subject's point of gaze during an action. In duels for example, the
subject must focus his attention on the most relevant sources of visual information
on his opponent's gesture to react accordingly. It is therefore important to know
where an athlete looks at [
61
]. It was indeed demonstrated that experts used spe-
cific visual search strategies during different actions [
8
]. This spatial dimension
of the visual information gathered by athletes in front of a video clip has been
analyzed with an eye-tracker [
50
]. The focus of the subject is then deduced from
the position, the duration and the frequency of the fixations [
47
]. Many studies
using this coupling between eye-tracking and video clips techniques have shown
differences between expert and novice players, in soccer [
29
,
46
,
47
,
59
,
63
],
tennis [
28
,
50
], baseball [
49
], badminton [
6
] and combat sport [
44
,
62
]. However,
conflicting conclusions emerged about the visual strategies used by the same group
level [
61
]. There are indeed several limitations to the use of eye-trackers. The main
problem is that observing a visual fixation during an action does not necessarily
mean that it is linked to pertinent information used to react [
61
]. Thus, the location
of the gaze is not necessarily related to the visual information uptake [
3
,
22
,
58
].
If these two methodologies can be used to define the relationship between the
decision of the player and his level of expertise, or to define the moment when the
player makes his decision, it seems very difficult to differentiate the influence of one
kinematic parameter compared to another. In the case of sports without human inter-
actions, it is easier to standardize situations and to control their modifications, even
in the field by the means of throwing machines for example. But in the case of human
interactive sports, it becomes very difficult to standardize situations since the athlete
cannot perform twice the same movement and further less modify only one part of
his motion without modifying the other parameters. For example in the case of duel
between a goalkeeper and a thrower in handball, it is very difficult to demonstrate
the influence of the wrist of the thrower on the reaction of the goalkeeper. Indeed,
the thrower cannot change the position of his wrist at ball release without modify-
ing the rest of his movement. The experimental conditions are thus not controlled.
To understand and determine the weight of each kinematical parameter of the move-
ment of an opponent in the decision of a player, it is necessary to control all these
