Biomedical Engineering Reference
In-Depth Information
situation by stopping the virtual attacker. Defenders' movements were recorded with
a Qualisys ProReflex motion capture cameras. Thirty-eight reflective markers were
placed on key anatomical landmarks on the defenders' body. The external markers
attached to the body of the participants were used to compute the 3D positions of the
joint centers and then to obtain the 3D position of the global center of mass [
65
]. This
latter was used to determine if the defender initiated his action by an early move-
ment bias in the wrong direction. The initiation of the displacement was taken as
effective when the COM mediolateral linear velocity get over a 0.5 m.s-1 threshold.
Four parameters were then calculated to compare novices and experts: (i) movement
initiation time (ms) of the defending action, (ii) percentage of early movement bias,
(iii) displacement amplitude (cm) of the early bias, and (iv) the minimal distance
(cm) observed between the defender and the attacker during the duel.
The real defender is also equipped with a head mounted display (Cybermind
Visette pro, 45deg field of view, resolution 1280 * 1024) to have stereoscopic vision.
His viewpoint is changed in real time (120 Hz) thanks to an Intersense head tracker
sensor mounted on the front of the headset.
13.2.3 Results
The different results illustrate the defending strategies differences between experts
and novices. Firstly, results show that experts wait significantly longer before ini-
tiating movement (Experts 267.74
±
±
36.18 ms vs. Novices 192.71
63.82 ms;
t(22)
.002) (Fig.
13.3
). Secondly, regarding the percentage of early
bias, authors note that novices initiates more often in the wrong direction (Novices
41.9
=
3.54, p
=
.01). These two results
can be logically linked. The third parameter, namely the amplitude of the early bias,
has significantly greater values for novices (14.99
±
20.5% vs. Experts 14.62
±
9.8%; t(12)
=−
4
.
219, p
=
±
2.68 cm) compared to experts
(11.74
.98) (Fig.
13.3
). Finally, the minimal
distance between defender and attacker in front of DM, which highlights the real
performance of the defender, appears to be logically smaller for experts compared
to novices (Novices 70.8
±
3.81 cm) (t(22)
=
2.41; p
=
.025, d
=
±
7.6 cm; Experts 49.2
±
11.4 cm, p
<
.001).
13.2.4 Discussion
The aim of this study was to explore the differences of displacement strategies
between novices and experts defender in front of DM. The results highlight the
fact that experts, compared to novices, wait much longer before initiating a dis-
placement to intercept the virtual attacker. Consequently, experts are able to make
a significantly lower number of early bias movement in the wrong direction as well
as a significantly lower amplitude in the wrong direction during an early bias. This
type of results has already been observed in the literature [
23
]. Dessing and Craig
