Comparison of Active Drag Using the MRT-Method and the MAD-System in Front Crawl Swimming

Abstract

The purpose of this study was to compare the active drag values estimated by the MRT-method and the MAD-system. Six male competitive swimmers participated in this study and performed front crawl with arms only condition. The drag was compared at six-staged velocities ranged from 0.9 to 1.4 m/s between MRT-method and MAD-system. The drag estimated by MRT-method showed larger values than that obtained using MAD-system at each velocity (MRT-method/MAD-system: 119% at 1.0 m/s; 133% at 1.2 m/s; 147% at 1.4 m/s). In addition, the stroke length in MRT-method condition decreased with swimming velocity being increased, while that in MAD-system condition was constant. Therefore, swimmers had to increase their stroke frequency in MRT-method condition in order to achieve the same swimming velocities as MAD-system condition, especially at high velocities. It was concluded that the difference in the way of exerting propulsion between MAD-system and MRT-method influenced the active drag which were estimated in two methods.

Full text

Proceedings 2018, 2, 287; doi:10.3390/proceedings2060287 www.mdpi.com/journal/proceedings
Proceedings
Comparison of Active Drag Using the MRT-Method
and the MAD-System in Front Crawl Swimming †
Kenzo Narita 1,*, Futoshi Ogita 2, Motomu Nakashima 3 and Hideki Takagi 4
1 Doctoral Program in Physical Education, Health and Sport Sciences, University of Tsukuba,
Tsukuba 305-8574, Japan
2 Department of Sports and Life Sciences, National Institute of Fitness and Sports in Kanoya,
Kanoya 891-2393, Japan; ogita@nifs-k.ac.jp
3 Department of Systems and Control Engineering, Tokyo Institute of Technology, Tokyo 152-8550, Japan;
motomu@sc.e.titech.ac.jp
4 Faculty of Health and Sport Sciences, University of Tsukuba, Tsukuba 305-8574, Japan;
takagi.hideki.ga@u.tsukuba.ac.jp
* Correspondence: ninzen7@gmail.com; Tel.: +81-80-4337-4699
† Presented at the 12th Conference of the International Sports Engineering Association, Brisbane,
Queensland, Australia, 26–28 March 2018.
Published: 11 February 2018
Abstract: The purpose of this study was to compare the active drag values estimated by the
MRT-method and the MAD-system. Six male competitive swimmers participated in this study and
performed front crawl with arms only condition. The drag was compared at six-staged velocities
ranged from 0.9 to 1.4 m/s between MRT-method and MAD-system. The drag estimated by
MRT-method showed larger values than that obtained using MAD-system at each velocity
(MRT-method/MAD-system: 119% at 1.0 m/s; 133% at 1.2 m/s; 147% at 1.4 m/s). In addition, the
stroke length in MRT-method condition decreased with swimming velocity being increased, while
that in MAD-system condition was constant. Therefore, swimmers had to increase their stroke
frequency in MRT-method condition in order to achieve the same swimming velocities as MAD-
system condition, especially at high velocities. It was concluded that the difference in the way of
exerting propulsion between MAD-system and MRT-method influenced the active drag which were
estimated in two methods.
Keywords: active drag; front crawl; stroke frequency; stroke length; propulsion
1. Introduction
A resistive force in swimming, i.e., active drag, is a main factor of determining the swimming
performance. Swimming velocity depends on a balance between propulsive force and active drag.
When active drag being larger than propulsive force in swimming, the velocity of the swimmer
decreases. Hence, reducing active drag is important to achieve high swimming velocity. Although
various methodologies have been suggested, it is currently impossible to measure active drag
accurately during swimming, and there has been no standard method for measuring active drag.
In front-crawl swimming, which is the fastest human swimming stroke, distances varied from
50 m to 1500 m are adopted as official events in competitions. Since swimmers alter their swimming
velocity depending on the distance, it is necessary to evaluate active drag at various swimming
velocities to provide detailed kinetic information on swimming performances to athletes and coaches.
As a methodology for evaluating active drag at various velocities, Hollander et al. [1] developed the
measuring active drag system (MAD-system). In this method, the swimmer propelled him/herself
forward by pushing the pads which were fixed under the water, and the pushing forces exerted by
the swimmer was measured by a load cell. Then, under the assumption that the swimmer’s

Proceedings 2018, 2, 287 2 of 6
swimming velocity during the measurement is constant, the active drag was evaluated from the
principle that the mean propulsive force exerted by the swimmer (that is, the force for pushing the
pad) was equal to the mean drag in swimming. The drag estimation using MAD-system can only be
applied to front crawl swimming without legs motion due to its methodological characteristics.
Therefore, active drag acting on the swimmer during whole body swimming cannot be estimated
with this method. Another problem in MAD-system is that swimmers change their velocity in a
different manner compared with actual swimming condition. Swimming velocity is calculated by the
product of stroke frequency and length. In MAD-system condition, however, stroke length of the
swimmer is constant and the swimming velocity is affected only by the stroke frequency, which is
not the case in reality [2,3]. On the other hand, the drag in various swimming styles can be assessed
by other two methods [4,5]. However, they can only evaluate the drag during maximal effort trials.
The drag during swimming has been mainly assessed by those three methods, and MAD-system is
the only method which is able to evaluate active drag at various velocities among the three.
A methodology for estimating the drag in swimming using measured values of residual thrust
(MRT-method) has recently been developed by Narita et al. [6]. The MRT-method has no restriction
on swimming style and velocity, therefore, the method can evaluate active drag at any velocities as
with MAD-system. Furthermore, unlike the MAD-system approach, this method allows researchers
to verify an influence of stroke frequency and length on active drag. In MRT-method condition, the
swimmer propels his/her body forward by sweeping their arms through the water without the
restriction by the pads. Therefore, with this method, it is possible to investigate the active drag at
various velocities without neglecting the influence of the stroke length. By comparing active drag
evaluated by both MAD-system and MRT-method with the same swimmers, the effect of the way of
generating propulsion on the active drag can be investigated.
The purpose of this study was to compare the active drag values in front crawl swimming with
arms only condition between the MRT-method and the MAD-system.
2. Methods
2.1. Participants
Six male competitive swimmers (age: 20.0 ± 1.0 years; height: 1.71 ± 0.03 m; weight: 67.6 ± 6.2 kg)
participated in this study. They all trained six days a week and had experience in participating in
Japanese national competitions. The test procedures were approved by the University of Tsukuba
Ethics Committee and each participant signed an informed-consent form.
2.2. Experimental Design
Each swimmer performed front crawl using arms only in MRT-method and MAD-system. To
restrict the movement of the swimmer’s legs, we attached a buoyant buoy with the thigh of the
swimmer and fastened a band to the ankle. In all experiments, the swimmers used a snorkel to
eliminate an influence of the breathing motion. Moreover, the swimmers were instructed to wear the
same type of swimsuit in both testing conditions to avoid any potential effects on resistance caused
by different types of swimsuit.
2.2.1. MRT-Method
Trials using MRT-method were conducted in a water flume (Igarashi Industrial Works Co., Ltd.,
Chiba, Japan), which allowed the flow velocity to be controlled precisely. Prior to the measurements,
the swimmers had familiarization period for the flume.
The MRT-method evaluates the active drag from the relationship between the residual thrusts
and the flow velocities. The residual thrust is the difference between the propulsion and drag. This
can be calculated by measuring towing forces exerted by the swimmer by two load cells connected
to wires which are attached to the waist of the swimmer at various flow velocities (U), while the
swimmer maintains the same techniques and kinematics. Thus, to estimate the active drag at a given
velocity = i m/s (VSi), the swimmer has to maintain his/her stroke motion and body position required

Proceedings 2018, 2, 287 3 of 6
to swim at V
Si
even when U was varied. Prior to measuring the residual thrust, each swimmer
self-propelled in the flume with the flow velocity U being set to i m/s. To make it eas y for the swimmer
to maintain his/her stroke at different U, the stroke time (s/stroke) that the swimmer used to propel
himself at i m/s was provided using a small audible waterproof metronome (Tempo trainer Pro;
FINIS, Inc., Livermore, LA, USA). To measure the residual thrust at each U, a belt wrapped around
the swimmer’s waist was connected to load cells using wires (LUX-B-2KN-ID, Kyowa Electronic
Instruments Co. Ltd., Tokyo, Japan). The load cells were located at the front and back of the flume
(Figure 1). The forward and backward towing forces were measured for 10 s, and then the residual
thrust was calculated from their difference between the forces. We measured the residual thrust at
eight points within the range of ±0.2 m/s around V
Si
, changing U by 0.05 m/s each time. Thereafter,
we derived best-fit regression curves for the measured values of residual thrust and U and used them
to calculate the active drag (for further details, see Narita et al. [6]). The drag was estimated in
six-staged velocities from 0.9 to 1.4 m/s. The stroke frequency (SF [Hz]) was calculated from the
inverse of the stroke time (s/stroke), and the stroke length SL was computed by dividing the velocities
U
Tre0
derived from the regression curve fitted to the SF.
Figure 1. Top view of MRT-method.
2.2.2. MAD-System
In testing using MAD-system, each swimmer swam 25 m with pushing pads. The pads were
attached to a 23 m rod, which was mounted 0.8 m below the water surface and connected to a force
transducer, and a 1.30 m interval (Figure 2). The force by pushing off pads were measured at a
frequency of 100 Hz. Mean active drag was determined to be equal to the mean propulsive force
under the assumption that the velocity of the swimmer was constant throughout the trial. To establish
the relationship between the active drag and the velocity, each subject completed ten trials at different
selected velocities with approximately 3 min rest between trials. We chose six out of the ten trials
which showed similar velocity as U
Tre0
that was calculated for the same swimmer in MRT-method.
The SF was calculated by dividing the swimming velocity by the SL (which is constantly 2.6 m with
MAD-system).
Figure 2. Top view of MAD-system.

Proceedings 2018, 2, 287 4 of 6
2.3. Data Analysis
To compare the values of active drag which were evaluated with MRT-method and
MAD-system at various velocities, the drag/velocity data were fitted to the function: D = k vn (D: drag,
v: velocity) to obtain coefficient k and degree n for each swimmer, and the active drag for 1.0, 1.2 and
1.4 m/s were calculated by the aforementioned equation with obtained k and n values being
substituted.
2.4. Statistical Analysis
We compared the active drag calculated by MRT-method and MAD-system using a paired t-
test. To investigate the influence of stroke parameters on the swimming velocity and the active drag
in MRT-method and MAD-system, we obtained by Pearson’s correlation coefficients (r) between
swimming velocity/active drag and SF/SL of all swimmers. All statistical analyses were conducted at
a significance level of p < 0.05 using SPSS ver. 22.0 (SPSS, Inc., Chicago, IL, USA).
3. Results
Table 1 indicates the values of k and n obtained in MRT-method and MAD-system. Significant
differences between the two methods were observed in k (t (5) = 4.96, p < 0.01) and n (t (5) = 2.76,
p = 0.04). The average values of all swimmer’s active drag at 1.0, 1.2 and 1.4 m/s were shown in
Figure 3. There were also significant differences in active drag values at 1.0 (t (5) = 4.96, p < 0.01), 1.2
(t (5) = 5.00, p < 0.01) and 1.4 m/s (t (5) = 3.90, p = 0.01).
Table 1. Coefficient k and degree n in each swimmer obtained by MRT-method and MAD-system.
Swimmer MRT-Method MAD-System
k n k n
A 37.8 3.28 30.5 1.73
B 34.1 2.30 28.2 1.85
C 32.2 2.34 27.1 2.06
D 38.9 2.16 29.6 2.10
E 33.0 2.83 31.3 1.85
F 31.1 2.23 27.6 1.84
Mean 34.5 2.53 29.1 1.90
SD 2.9 0.40 1.5 0.13
Figure 3. The average values of active drag at 1.0, 1.2 and 1.4 m/s in MRT-method (blue) and MAD-
system (orange). Asterisk (*) indicates a significant difference between MRT-method and MAD-
system, p < 0.05.
0
20
40
60
80
100
120
1.0 1.2 1.4
Active drag (N)
Velocity (m/s)
∗
: MRT-method : MAD-system
∗
∗

Proceedings 2018, 2, 287 5 of 6
Figure 4 present the relationships between swimming velocity/active drag and stroke frequency.
In MRT-method, SF had a significant positive correlation with swimming velocity (r = 0.797, p < 0.01)
and active drag (r = 0.808, p < 0.01), whereas SL showed a significant negative correlation with
swimming velocity (r = −0.401, p = 0.02) and active drag (r = −0.452, p < 0.01). On the other hand, in
MAD-system, there was a significant positive correlation between SF and swimming velocity (r =
0.999, p < 0.01)/active drag (r = 0.970, p < 0.01), besides, the relationship between SL and active
drag/velocity could not be evaluated because SL was constant.
Figure. 4 The relationship between each variables in MRT-method (blue) and MAD-system (orange)
for all swimmers. (a) The relationship between swimming velocity and stroke frequency; (b) the
relationship between stroke frequency and active drag.
4. Discussion
Active drag in front-crawl swimming without legs motion assessed by MRT-method was larger
than the drag estimated using MAD-system at all velocities. In MAD-system, due to its mechanical
structure, swimmers propels their body by pushing fixed pads under the water. Thus, swimmers can
utilize all reaction force acquired by pushing the fixed pads by the hands as the propulsive force. On
the other hand, during the actual swimming, the force swimmers obtain from the water are divided
into propulsive force and force which does not contribute to propulsion [7]. Therefore, propulsive
force is probably generated more efficiently by swimmers with MAD-system compared with the
actual swimming and MRT-method condition at the same velocities. The swimming velocity is
determined by the balance between propulsive force and active drag. It means that when the
swimmer achieves a given swimming velocity with a small propulsive force, the drag acting on the
swimmer is also small. In this study, the SL, which was adopted as a simple index of swimming
efficiency, was constant regardless the swimming velocity in MAD-system condition. However, in
MRT-method, it decreased with the velocity being increased. Therefore, MRT-method required the
swimmers to achieve the same swimming velocities as they did with MAD-system by increasing their
SF, especially at high velocity conditions. On the other hand, SF and active drag were positively
correlated in both methods. However, the resistance force is strongly influenced by the velocity, as
well as SF. Hence, in order to investigate a relationship between SF and the drag with minimizing
the influence of the velocity, we calculated an active drag coefficient and conducted a correlation
analysis. A significant positive correlation was observed between these variables in MRT-method
(r = 0.44, p < 0.01), while no significant correlation was found in MAD-system (r = −0.10, p = 0.57). In
front-crawl swimming, swimmers repeatedly move their arms and legs around the water surface.
Therefore, it is expected that additional drags from waves and splashes are generated in each stroke
cycle. Therefore, it is possible that the active drag in high SF conditions was affected by those drags
more than that in low SF conditions.
0.2
0.4
0.6
0.8
1.0
0.8 1.0 1.2 1.4
Stroke frequency (Hz)
Velocity (m/s)
●MRT-method
r= .797, p< .01
▲MAD-system
r= .999, p< .01
(
a
)
0
20
40
60
80
100
120
0.2 0.4 0.6 0.8 1.0
Active drag (N)
Stroke frequency (Hz)
●MRT-method
r= .808, p< .01
▲MAD-system
r= .970, p< .01
(
b
)

Proceedings 2018, 2, 287 6 of 6
On the other hand, since present study did not conduct a motion analysis, the influence of the
difference in the motion of the swimmer between each method on the active drag was unclear.
Obtaining information on the path and speed of upper limbs and pitch/yaw of the body of swimmers
in each method will be helpful for better understanding of the influence of these variables on the
difference of the drag between the two methods. Therefore, further investigations including the
motion analysis are needed for detailed analysis of the effect of the difference of each methodology
on active drag.
5. Conclusions
The present study compared the active drag using the MRT-method and the MAD-system in
front crawl swimming without kicking motion. As a result, the active drag values estimated using
MRT-method was higher than those obtained by MAD-system. In addition, SL in MAD-system
condition was constant, while that in MRT-method condition decreased with swimming velocity
being increased. Therefore, swimmers had to increase their SF in MRT-method condition in order to
achieve the same swimming velocities as MAD-system condition, especially in high velocity
conditions. It is probable that the different ways to generate propulsive force by upper limbs between
the two methods influence the stroke parameters, consequently, the active drag.
Acknowledgments: This research was supported in part by the Ministry of Education, Culture, Sports, Science
and Technology (MEXT) for the Human High Performance Project (2014–2018) and by a grant from Advanced
Research Initiative for Human High Performance (ARIHHP), University of Tsukuba.
Conflicts of Interest: There are no conflicts of interest to declare.
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