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Data elaboration
Data processing included a 2-D tracking of the markers detected
by each TV camera and a 3-D reconstruction of markers seen by the
two TV cameras Filtering of 3-D markers coordinates and their derivatives
computing were performed by using the algorithms developed by D'Amico
and Ferrigno (1990). The algorithms are based on an autoregressive
model, fitted to the signal, to evaluate the filter bandwidth and
the extrapolation of the data. Then, the components of the coordinates
of each marker are filtered by a linear phase FIR low-pass filter,
with a proper cut-off frequency depending on the frequency content
of the signal.
To account for the bicycle movements, the measured coordinates
were then referred by trigonometry to a reference system fixed to
the bike handlebar (Figure 11)
As showed in Figure 10, the time the rider takes to reach the right
brake lever was defined as the time from the first detectable movement
of the marker m1 until the first occurrence of
the m2 marker movement. In the trials triggered
by an external stimulus the pre-motor time was also measured as
the time from the signal presentation until marker m1
moves.
Statistical analysis
The effect of the different kinds of handlebar hand position on
brake lever reaching times was analyzed with one-way ANOVA. The
existence of significant between-condition differences was then
tested using the Newman Keuls post hoc test. To determine whether
the brake lever access time with the clip-on handlebar position
was different from each of the other conditions, Student’s
t-test for paired data was also performed. All comparisons were
considered significant at an alpha level of P<0.05.
RESULTS AND DISCUSSION
Figure 12 illustrates, as a function of the different riding positions,
the mean brake levers reaching-times (in ms) produced by all the
subjects in the different experimental variants. Data were obtained
analyzing all the 630 trials acquired.
The values ranged from 147 ms of posture 3
(hands on the upper lateral side of the bend, close to the top of
the brake lever mounts) to the 276 ms of position 7
(hands on the top of the handlebar bend, close to the stem), with
a largest difference of 129 ms.
According to the found mean duration values (always less than 300
ms) the examined movement could be classified as a short, pre-programmed
motor responses, controlled via an open-loop process, without any
possibility to change the movement pattern once started.
As expected, postures with hands placed around the brake levers
were characterized by slightly lower brake access times compared
to the others. In concert with some previous studies on human movement,
these results indicate that the movement time, leaving aside other
factors (e.g. movement amplitude and trajectory, number of joint
involved etc.), heavily relies on the spatial distance from the
starting and the final position.
No differences were found between the position with the hands on
the clip-on handlebar and position 5
(hands on the middle section of the top handlebar bend) and 7
(hands on the top of the handlebar bend in proximity of the stem).
Estimate of the effects of the time taken to reach brake levers
on the total braking distance. A comparison between the riding positions
characterized by the minimum and maximum brake lever reaching-times
One of the goal of this study was to evaluate whether the time
spent in reaching the brakes, regardless of the riding position,
is a factor of great or minor importance in influencing the distance
necessary for a bicycle to be brought to a stop in an emergency.
To this end, the following simple calculations were performed:
First, the bicycle travel distances, for three typical riding speeds
(36, 48 e 60 km/h), during the time it took for brake levers to
be reached, are computed and plotted in Figure 13. The results,
obtained from the time values showed in Figure 12, revealed a maximum
difference among the analyzed postures of 1.3, 1.7, and 2.1 m at
36, 48, and 60 Km/h respectively.
Then, these extreme values were related to the total braking distance.
Assuming a maximum braking deceleration of 6m/s2 (hard
braking without causing loss of control), the distance the bike
travels before coming to rest from when the athlete applies the
brakes, was determined for the three chosen speeds.
Finally, the distance the bike will travel during a pre-motor time
of about 180 ms (typical value in laboratory tasks) and a brake
lever reaching-time of 260 ms was added. In doing so, the total
braking distance from the instant the rider perceives the danger
was estimated. The values were found to be 13, 21, and 30 m at 36,
48 and 60 km/h. These results would indicate that the maximum riding
posture induced differences account for a percentage of 10, 8, and
7% of the total braking distance, assuming a bike speed of 36, 48
and 60 km/h, respectively (see Figure 14).
However, it should be noted that the pre-motor time value used
for the calculation (180 ms) refers to a typical laboratory task
with the subject knowing that a trigger signal it is going to be
presented. In real world situations, with the subject required to
react to an unforeseeable stimulus, longer pre-motor times up to
600 ms were found. Given the above, the calculated percentage values
may be even overestimated.
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