josh neely
DESCRIPTION
ÂTRANSCRIPT
An investigation into the differences in stroke parameters
through statistical and movement analysis
Josh Neely
Falmouth Marine School
Marine Sport Science
Year 2
Abstract
The aims to discover the differences in stroke parameters, speed and body posture
between the Impact vest and the Buoyancy aid through movement and statistical
analysis and provide a recommendation to water users about which one is better for
swimming in. the method was to swim 20 meters in both the buoyancy aid and
impact vest. The times of the swims were recorded and they were videoed as a
source for movement analysis and collecting other data. The data collected was then
analysed through calculations and statistical analysis in the form of a paired t-test.
The results showed that there was no significant difference in stroke rate, distance
per stroke, swimming speed and time taken to swim 20 meters. The movement
analysis showed that the body posture and technique different when in the different
PFD’s with the impact vest having a more horizontal position in the water than the
buoyancy aid
Key words: impact vest, buoyancy aid, stroke rate, personal flotation device
INTRODUCTION
While participating in water sports of any kind safety is one of the main
considerations that must be observed and wearing a Personal Flotation Device
(PFD) can significantly increase chances of survival. ‘In 2009, the U.S. Coast Guard
received reports for 4,730 boating incidents; 3,358 boaters were reported injured,
and 736 died. Among those who drowned, 9 out of 10 were not wearing life jackets’
(U.S. Coast Guard, 2009). Personal flotation devices are defined as ‘any device
designed to keep a person afloat in the water’. (Sayour, 2011). ‘A variety of
buoyancy aids have therefore been constructed to support the body in water and to
maintain the body afloat without difficulty’ (Dowdeswell, 1988). These devices are
majorly important in keeping us safe when partaking in water sports. However, the
ways in which they affect our ability to swim are questionable
This experiment aims to discover the differences in stroke parameters between two
of these devices the Impact vest and the Buoyancy aid through movement and
statistical analysis and provide a recommendation to water users about which one is
better for swimming in. The stroke parameters being measured are stroke rate (SR),
distance per stroke (DPS) and speed. The impact vest ‘is designed with tough
materials including multiple heavy-duty straps/buckles and with a longer design that
will sit down to your waist covering and protecting your vital organs.’ (Personal
Watercraft Partnership, anon). The buoyancy aid however is of a more bulky design
and is designed mainly for floatation rather than impact protection. This must mean
that there will be a difference in how they affect the way we swim.
The buoyancy provided by the PFD’s in the torso area may affect the swimmers
body posture. Having a horizontal body position is extremely important when trying to
swim, as it minimizes drag and makes the body more streamline. When swimming
with a bad posture it can cause people to swim ‘slower than they are capable or work
MUCH harder to swim at the same speed.’ (Michael Collins, anon)
H0: There will be no significant difference in stroke parameters between the two
PFD’s
H1: There will be a significant difference in stroke parameters between the two PFD’s
Methods and materials
Location: Falmouth Water Sports Centre
Date: 21st March 2012
Time: 9:00am -
Temperature:
Air: 9-10°C
Water: 10.2°C
Cloud cover: 66%
Wind strength and direction: Easterly 9 mph
The experiment was designed to test stroke rate over a 20 meters in a straight line
front crawl swim. The subjects are 4 asymptomatic college students aged 20 to 26
years of age. They were selected from a class of students at Falmouth marine
school. The equipment will be collected and brought to the Falmouth Water Sports
centre for the experiment. A distance of 20 meters between the pontoon and harbour
wall will be measured with a 15 metre (m) Ronson tape measure. For validity, this
will be done by 2 assistants chosen from the participants. The subject will put on a
5mm wetsuit and Naish Impact vest. The subject will then enter the water at a
temperature of 10.2 degrees centigrade. Once in the water the subject will prepare
themselves to swim and from a resting position and without pushing off the wall the
subject the will swim the distance of 20 metres in a straight line as fast as possible
using the front crawl stroke. The subjects will then give the impact vest to the next
person and the process will be repeated for each participant. Whilst the subject is
swimming the 2 assistants will be positioned at the midpoint of the course on the
slipway. One of the assistants will use a Sony handy cam to film the subject whilst
swimming to provide a source for data collection(stroke rate.) The camera will follow
the swimmer by moving round on a pivot controlled by the assistant. Each swim will
be filmed and timed separately. The other assistant will time each swim from start to
finish with a Casio stopwatch and record the results in the table provided. Once all
the impact vest swims have been completed the first subject will put on the Gill
buoyancy aid. The swims will then be repeated with the same process and distance
as done with the impact vest.
the data collected will be formatted into tables and different results will be calculated.
These results will then be statistically tested using the Paired T test.
Results
Participant Impact Vest Buoyancy Aid
A 26.18 27.62
B 22.39 22.63
C 22.39 23.70
D 25.00 25.44
Mean 23.99 24.85 Table 1: showing time in seconds of each swim and means
This table shows the times of each of the swims done in both the impact vest and the
buoyancy aid. The mean times show that the buoyancy aid is 0.86 seconds slower
than the impact vest. A paired t-test was performed on the two means. The paired t
test show that the mean time of the impact vest and is not significantly different from
the mean time of the buoyancy aid at the 0.05 significance level
Participant Impact Vest Buoyancy Aid
A 0.764 0.724
B 0.893 0.884
C 0.893 0.844
D 0.8 0.786
Mean 0.838 0.809 Table 2: showing speed in metres per second (m/s) of each swim and means
This table shows the speed in m/s for all the swims in both PFD’s and the mean
speeds show that the Impact vest is quicker than the Buoyancy aid at a speed of
0.838 m/s. A paired t-test was performed on the two means. The paired t test shows
that the mean time of the impact vest and is not significantly different from the mean
time of the buoyancy aid at the 0.05 significance level
Participant Impact Vest Buoyancy Aid
A 47.12042 45.685
B 48 51.136
C 50.420 53.097
D 57.508 50.847
Mean 50.455 50.847 Table 3: showing strokes per minute (strokes.min
-1) and means
This table shows stroke rate in stroke.min-1 and the means show that when wearing
the impact vest the swimmer uses less strokes.min-1 (50.455) than when wearing the
buoyancy aid (50.847). A paired t-test was performed on the two means. The paired
t test shows that the mean time of the impact vest and is not significantly different
from the mean time of the buoyancy aid at the 0.05 significance level
Participant Impact Vest Buoyancy Aid
A 0.191 0.197
B 0.19 0.176
C 0.178 0.169
D 0.156 0.165
Mean 0.18 1.177 Table 4: showing distance per stroke in meters per stroke (mps)
This table shows the distance each stroke (when one arm enters the water and does
a full 360 degrees until it enters the water again) takes the swimmer in meres. It
shows that there is a higher mean DPS for the impact vest than the buoyancy aid. A
paired t-test was performed on the two means. The paired t test shows that the
mean DPS of the impact vest and is not significantly different from the mean DPS of
the buoyancy aid at the 0.05 significance level
Discussion and movement analysis
In order to reach the aim of discovering which PFD is the better to swim in,
movement analysis of the body posture and stroke movement must be done
Figure 1: impact vest mid stroke Figure 2: buoyancy aid mid stroke
Body posture
As can be seen in the figures 1 and 2, the postural angle when wearing the impact
vest is at a more horizontal angle to the water when compared with the buoyancy aid
which is at a more vertical angle. Body position ‘affects the whole stroke’ (British
Swimming Association, anon) when swimming so it is very important to get it right.
When swimming with a bad posture it can cause people to swim ‘slower than they
are capable or work MUCH harder to swim at the same speed.’ (Michael Collins,
anon). A more horizontal swimming posture will make swimming easier. Huijing et al
1988 found a high degree of correlation between body cross section and active drag
which means that the greater the angle of the body to the water surface the more
drag created
As the body posture when wearing the impact vest is more horizontal to the water
surface it would suggest that it would be faster to swim in and use less effort. This
can be seen in the results as the mean time for the swims done in the impact vest is
lower than the mean time for the buoyancy aid. There is also a slightly lower mean
stroke rate when wearing the impact vest at 50.455 stroke.min-1 showing that fewer
strokes are used than the buoyancy aid which has a mean of 50.847 stroke.min-1.
The difference in body posture could be explained by the difference in the aesthetics
of the jackets. The buoyancy aid is much more bulky and provides more buoyancy
whereas the impact vest is much more compact and provides a little less buoyancy.
This means that the buoyancy aid will be pushing the torso of the body up upwards
with more force than the impact vest causing a noticeable difference in posture
Stroke movement
‘During front crawl the continuous alternating arm action provides the majority of the
power and propulsion of the entire swimming stroke.’ (www.swim-teach.com, 2012).
This makes it a very important part of the stroke. In figures 1 and 2 measurements of
the distance from the wrist to the water have been taken (measurements to picture
scale). The impact vest measured at a size of 0.9 cm and the buoyancy aid
measured at a scale size of 1.1 cm. From this it can be seen that there is a
difference of 0.2cm between the 2 devices. This could be due to the amount of
buoyancy in the vests. As the buoyancy aid provides more flotation it lifts the torso
further out of the water causing the clavicle, scapula and the humerus to be higher
thus creating a gap > 0.9cm. The body posture will also add to this as a more upright
position will cause the top half of the torso to be further out of the water. The impact
vest has less buoyancy than the buoyancy aid therefore the torso is lower in the
water causing there to be less distance between the Glenohurmeral joint (shoulder
joint) and the water
In figures 3 and 4 the angle of the arm to the water and the flexion of the arm have
been observed. In figure 3 wearing the impact vest, the arm enters the water at a
Figure 3: impact vest end of stroke Figure 4: buoyancy aid end of stroke
much more downward angle than in figure 4.the arm is also fully extended meaning
that there will be a greater DPS which can be seen in the results as the mean DPS
for the impact vest is 1.863mps and 1.938mps. This shows that the impact vest
provides a wider range of movement causing the arm to move freely and in a more
natural movement. In figure 4 the angle of the arm is more upward and there is
flexion at the elbow joint. This shows that there is less range of movement in the
buoyancy aid and it causes a less natural stroke. This is due to the extra buoyancy
that is provided, changing the posture of the body and affecting the stroke
movement. The flexion at the elbow means that the stroke length will be decreased
thus causing the DPS to be reduced. This means that more effort will have to be
used to travel a distance than if the full stroke length was used
Time and speed
The results show that the impact vest has a mean time of 23.99 secs and the
buoyancy aid has a mean time of 24.85 secs. This impact vest has a faster mean
time than the buoyancy aid meaning that it is quicker through the water. This is
shown in the mean speeds of the impact vest at 1.20mps and the buoyancy aid had
a mean speed of 1.24mps. These differences were shown not to be significantly
different. However the small differences that there may due to the fact that the
buoyancy aid is considerably more bulky than the impact vest which is very small
and compact in comparison. The bulkiness of the buoyancy aid means that there will
be more drag created in the water. This lack of laminar flow means that the
buoyancy aid will be slower through the water and need more effort to propel the
user through the water. This is also because of the bad posture that it puts the user
in whilst swimming with the legs lower in the water
Stroke rate (SR)
The mean stroke rate for the buoyancy aid is 50.45 stroke.min-1 showing that fewer
strokes are used than the buoyancy aid which has a mean SR of 50.85 stroke.min-1.
The paired t-test showed that these results were not significantly different there are
still small differences. These differences show that when wearing the buoyancy aid
strokes are taken within a minute than the impact vest. ‘This increase in SR will be
detrimental to the stroke length’ (Dekerle J, 2005) which will affect DPS. These
results suggest that a person wearing the buoyancy aid is likely to use more energy
to push themselves through the water.
Distance per Stroke (DPS)
The mean DPS for the impact vest is 0.180mps and the mean DPS for the buoyancy
aid is 0.177mps. Distance per stroke is a good measure of how well a person is
swimming. Craig et al found that on average the better swimmer distinguishes
themselves from the poorer by distance per stroke rather than stroke frequency. The
paired t-test showed that there was no significant difference between the two PFD’s.
the small differences however show that with one stroke a person wearing the
impact vest will travel slightly further than a person wearing a buoyancy aid. This is
due to a number of factors that have been mentioned previously such as body
posture, range of movement and drag that is created
Evaluation of experiment
If doing the experiment again things that would be done differently would be to use
more participants and do more swims in each PFD. This would provide a larger
amount of data to statistically analyse. This would mean that there would be more
likely hood of finding a difference between the two different PFDs. The timing of the
experiment would also be changed to when the tide is high as there was very little
water for the experiment to take part in. using buoys instead of harbour walls as
markers for the start and finish would be better as it would reduce the factor of
subjects pushing of the wall. The location could be changed to a swimming pool to
reduce variables further and provide more accurate results.
Conclusion
To conclude this report there is no significant difference in stroke parameters speed
or time. This means that there is not enough evidence to reject the H0. The
movement analysis however showed differences in body posture and explained the
small differences that did occur. The recommendation that come out of this is that for
swimming short distances either PFD will work, but if you’re looking for more
protection rather than buoyancy for sports such as windsurfing for example go for an
impact vest as it protects your vital organs. If more buoyancy is what you need for
sports like kayaking for example then buoyancy aid is the correct choice
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