The Center for Human Movement Science - UNC Chapel Hill |
Biomechanics on Human Performance in Sports |
Bing Yu, PhD |
| Biomechanics of
human performance in sports is an important area in biomechanics and a long time interests
of the Center for Human Movement Science at University of North Carolina at Chapel Hill.
Several research projects have been conducted to study techniques of triple jump, discus
throw, and soccer kicking at the Center for Human Movement Science. These studies were
supported by the United States Olympic Committee, USA Track & Field, and US Soccer. We
also provide scientific service to US elite discus throwers. |
| Research on
Triple Jump Techniques |
| Triple jump is one
of the field events in track and field. A triple jump consists an approach run followed by
a hop in which the athlete takes off and lands on the same foot, a step in which the
athlete takes off and lands on different feet, and a jump in which athletes takes off on
one foot and lands on both feet in the sand pit (Figure 1). Unlike other field events in
which only one maximum effort is required, triple jump requires three jumping efforts at
high speed. This makes the triple jump physically and technically very demanding. Although
many debates on triple jump techniques can be found in coaching literaure, scientific
support to those debates on triple jump techniques is very limited. |
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Hop |
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Step |
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Jump |
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| Optimum Phase
Ratio in the Triple Jump
As previously described, the triple jump requires a series of hop, step, and jump at
high speed. The shortest distance measured from the front edge of the takeoff board to the
nearest mark the athlete made in the sand pit is referred to as official distance (Figure
2), and used as the measure of the athlete's performance in a triple jump. The official
distance of the triple jump is mainly determined by the actual distance that is defined as
the shortest distance from the toe of the takeoff foot of the hop to the nearest mark the
athlete made in the sand pit (Figure 2). The actual distance is the sum of the hop, step,
and jump distances. The ratios of the hop, step, and jump distances to the actual distance
are referred to as relative phase distances. The ratio of the three relative phase
distances is referred to as phase ratio. The phase ratio is a key technical factor that
affects the performance of the triple jump. From the point of view of phase ratio, triple
jump techniques were divided into three categories: (1) hop-dominated technique in which
the relative hop distance is at least 2% of the actual distance longer than the relative
jump distance; (2) jump-dominated technique in which the relative jump distance is at
least 2% of the actual distance longer than the relative hop distance, and (3) balanced
technique in which the difference between the relative hop and jump distances is no
greater than 2% of the actual distance. One of the key technical factors and long-standing
debates on the triple jump techniques in coaching literature is which of these three
techniques is the optimum triple jump technique. |
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Figure 2. Phase and actual
distances . |
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| To determine the optimum triple jump
technique in terms of phase ratio, a study was conducted to determine the relationship of
the loss in the horizontal velocity and the gain in the vertical velocity in the three
stance phases of the triple jump (Yu, 1999). A triple jumper inevitably loses horizontal
velocity while gaining vertical velocity for takeoff during each of the stance phases of
the hop, step, and jump. Based on a theoretical analysis, whether there is an optimum
triple jump technique in terms of phase ratio and which technique in terms of phase ratio
is the optimum triple jump technique mainly depends on the relationship between the loss
in the horizontal velocity and the gain in the vertical velocity during the three stance
phases of the triple jump. To determine this relationship, elite US male and female triple
jumpers' videographic data were obtained in the actual competitions. Horizontal and
vertical velocities of the center of mass (COM) at the landing of the last step of
approach run, the hop, and the step, and at the takeoff of hop, step, and jump were
obtained for at least 4 trials in the same competition for each athlete. The loss in the
horizontal velocity and the gain in the vertical velocity were calculated for each phase
in each trial for each athlete. A multiple regression analysis with dummy variables was
conducted to determine the relationship between the loss in the horizontal velocity and
the gain in the vertical velocity for each athlete.
The results show the loss in the horizontal velocity is linearly correlated with the
gain in the vertical velocity during each of the three stance phases of the triple jump.
The interception of the loss in the horizontal velocity as a linear function of the gain
in the vertical velocity during the stance phase of the hop is different from that during
the stance phases of the step and jump. The results also show that the relationship
between the loss in the horizontal velocity and the gain in the vertical velocity is
different among athletes. The slop of the loss in the horizontal velocity as a linear
function of the gain in the vertical velocity is an important parameter in the
relationship between the loss in the horizontal velocity and the gain in the vertical
velocity. This parameter is referred to as the horizontal-to-vertical velocity conversion
coefficient. These results indicate that there is likely an optimum phase ratio for a
given athlete.
Based on these results, another study was conducted to determine the optimum phase
ratio for a given athlete (Yu and Hay, 1996). An optimization model was developed to
express the actual distance as a function of the horizontal and vertical velocities at the
landing of the last step of the approach run and gains in the vertical velocity during
three stance phases. The relationship between the loss in the horizontal velocity and the
gain in the vertical velocity of a given athlete obtained in the previous study was used
to estimate the loss in the horizontal velocity during each stance phase for the given
athlete.
The results of this study show that there is indeed an optimum phase ratio for a given
triple jumper. The horizontal-to-vertical velocity conversion coefficient is the key
parameter for determination of optimum phase ratio. The jump-dominated technique is
optimum for triple jumpers with a horizontal-to-vertical velocity conversion coefficient
greater than 0.9. Hop-dominated technique is optimum for triple jumpers with a
horizontal-to-vertical velocity conversion coefficient lower than 0.7. The jump-dominated
technique is the optimum technique for most of the male triple jumpers while the
hop-dominated technique is the optimum technique for most of the female triple jumpers. An
inappropriate phase ratio could result in a loss of 5% of the longest actual distance. The
horizontal-to-vertical velocity conversion coefficient is also an important parameter that
differentiates triple jumpers from long jumpers. Elite triple jumpers should have a great
magnitude of the horizontal-to-vertical velocity conversion coefficient while elite long
jumpers should have a low magnitude of this coefficient.
The model developed in this study was applied to predict the performances of elite
athletes and obtained accurate prediction results. The results of these studies provide
not only a useful tool for determining optimum phase ratio for triple jumpers, but also
significant information for technical training of elite triple jumpers and future
scientific studies on triple jump techniques.
Arm Swing Techniques in the Triple Jump
There are three arm swing techniques commonly used in the triple jump. They are:
alternate-arm swing technique, double-arm swing technique, and arm-and-half swing
technique. An alternate-arm swing is simply an exaggeration of the arm motions in walking
and running. It merely requires raising the opposite arm and leg simultaneously. A
double-arm swing requires that both arms be moved backward before touchdown and driven
forward and upward together during the support phase. An arm-and-half swing requires that
the arm on the takeoff leg side be moved backward while the other arm be positioned beside
the trunk before touchdown, and that both arms be driven forward and upward during the
support phase. Although there are many debates in coaching literature about the use of
different arm swing techniques, there is essentially no scientific support to any
persistent argument in these debates.
To determine the optimum arm swing techniques, a study was conducted to obtain the
basic understanding of the functions of arm motions during the triple jump. A Direct
Linear Transformation procedure with panning cameras (Yu et al., 1993) was used to collect
three-dimensional coordinate data of 13 elite male triple jumpers during the actual
competition. A biomechanical model was developed to determine the changes of the
velocities of the center of mass during each stance phase of the triple jump.
The results suggest that arm swing motions during each support phase are responsible
for up to 19% of the total loss in the whole body horizontal velocity. The total loss in
the whole body horizontal velocity is associated with the loss in the horizontal velocity
due to arm swing motions during three support phases of the triple jump. The more the loss
in the horizontal velocity due to arm swing motions, the more the total loss in the whole
body horizontal velocity. The results also show that the arm swing motions in the triple
jump are responsible for 9% of the total gain in the whole body vertical velocity. The
total gain in the whole body vertical velocity is associated with the gain in the vertical
velocity due to arm swing motions during the triple jump. The more the vertical velocity
generated by arm swing motions, the more the gain in the whole body vertical velocity. The
results further show that loss in the horizontal velocity due to arm swing motions is
associated with the gain in the vertical velocity due to arm swing motions. The more the
vertical velocity generated by arm swing motions, the more the loss in the horizontal
velocity due to arm swing motions. These results combined together suggest that one of the
functions of arm swing motions in the triple jump is to assist athlete in generating
vertical velocity by converting horizontal velocity to vertical velocity.
With the basic understanding of the functions of arm motions during the triple jump,
the effects of arm motions on the whole body horizontal and vertical velocities were
compared between arm swing techniques. The results of the comparisons show that the
alternate arm swing technique has the lowest ratio of the loss in horizontal velocity to
the gain in the vertical velocity while the double arm swing technique has the greatest
gain in the vertical velocity. These results combined with the results of studies on the
optimum phase ratio suggest that the alternate arm swing technique is the optimum for the
hop and step while the double arm swing technique is the optimum for the jump. |
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| Research
on Discus Throwing Techniques
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| Discus throwing is
one of the four throwing events in track and field. Complicated movements performed at
high speed in a limited space make the discus throwing technically and physically
demanding. Thus, the discus throw requires thorough biomechanical analysis to have a good
understanding of the techniques and training of elite discus throwers. A recent extensive
literature review, however, revealed that, although there are many debates on different
aspects of the techniques of throwing discus, the biomechanical studies on this topic are
very limited. The primary reason for the lack of biomechanical studies appears to be the
complexity of the technique of throwing the discus. Because of the complexity of the
techniques, three-dimensional image analysis techniques are essential for kinematic
analysis, multiple force plates are required for kinetic analysis, and sophisticated data
reduction techniques are needed to obtain meaningful biomechanical parameters. |
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| Figure 3. Discus throwing. |
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The Center for Human Movement Science was
involved in a Scientific Service Program for Elite Discus Throwers and High Performance
Program of USA Track & Field since 1997. Three-dimensional coordinate data of discus
throwing were regular collected in USA Track & Field Outdoor Championships. The discus
throwing biomechanical database we have is the largest in the world. Technical
characteristics of elite US discus throwers were analyzed. Technical reports were
developed athletes and their coaches every year since 1997. Dr. Bing Yu regularly met
elite US discus throwers and their coaches to discuss their techniques and training. Our
database, technical reports, and regular meeting with elite athletes and coaches helped
elite US discus throwers identify critical factor in their techniques, and provided
significant information that assisted elite US discus throwers in improving their
performances. Besides scientific services, we also conducted research on discus throwing
techniques. Our research provided significant information for elite athletes and coaches
to understand discus throwing techniques, and scientific basis for our service program.
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Kinetics of Discus
Throwing |
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As a coach initiated project, we conducted a study on the
kinetics of discus throwing techniques in collaboration with former men's discus throw
world record holder and US men's discus throw coach, Dr. Jay Silvester, and biomechanist
at US Olympic Committee, Dr. Jeffery Broker. The purpose of this study were to investigate
(1) the relationship between official distance and selected ground reaction force measures
during discus throwing; and (2) the relationship between selected ground reaction force
measures and selected lower extremity joint kinetics. An understanding of these
relationships may provide critical information for technical and physical training of
elite discus throwers. Eight elite male discus throwers in a 1998 discus training camp at
the US Olympic Training Center in San Diego were recruited as the subjects for this study.
Three high speed video cameras were used to collect three-dimensional coordinate data
while three force plates were used to collect ground reaction force data of discus throw
for each subject during discus throw. Ground reaction forces and impulses and lower
extremity joint resultant forces and moments at selected critical instants and phases of
the discus throw were reduced for each subject in each trial. The results of this study
showed significant correlations of official distance and selected ground reaction force
measures and joint resultant measures. These results have the following important
implications to coaches and athletes: (1) a discus thrower should drive his or her
body-plus-discus system as vigorously as possible towards the throwing direction during
the first single support phase, (2) a discus thrower should also generate a certain amount
of vertical thrust during the first single support phase to have a certain height of
flight, (3) the concept that discus throwers should jump as low as possible for the flight
is not supported by these results, (4) a hard right foot landing after the flight may
assist discus throwers to generate ground reaction impulses on the right foot during the
second single support phase and delivery phase for long official distance, (5) a discus
thrower should drive his or her right leg forward and rightward during the second single
support phase and delivery phase for long official distance, (6) a discus throwers should
also drive his or her left leg upwards and backwards as vigorously as possible during
delivery phase, and (7) hip and knee extension strengths are critical for right and left
legs' drives during the second single support and delivery phases. These results indicate
a possibility to use force plates as quick feedback tool for technical training of the
discus throw. A new force plate formation for measurement of ground reaction forces in
discus throw and corresponding computer programs were designed at the Center for Human
Movement Science and proposed to the US Olympic Committee.
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Optimum Release Angle |
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Currently we are conducting a study on the optimum release
angle of discus throw. The angle of release is an important release parameter that affects
the official distance. Although recent studies reported the release conditions of elite
discus throwers similar to those reported in early literature, there was no evidence that
those reported actual release conditions were indeed optimal. It has also been noticed
that the studies on the optimal release conditions for discus throwing were based on many
assumptions. The validity of these assumptions may have significant effects on the
validity of the results in previous studies. One of the assumptions in previous studies on
the optimal release conditions in discus throwing and other projectile motions that have
not been noticed is the assumption that the speed of the discus at release is a constant
that is not affected by the angle of release. The validity of this assumption threatens
the validity of the optimal angle reported in literature, and is an obstacle for our
further understanding of discus throwing techniques as well as other human projectile
motions. The speed of the discus at release should be correlated to the angle of release
if the speed of the discus at release is not a constant as the angle of release changes in
maximum effort throws. The relationships of the speed of the discus at the release with
the angle of release and the relationship of the aerodynamic distance with selected
release conditions would provide basis for determining optimal angle of release. The
purposes of this study were (1) examine the relationship of the speed of release with the
angle of release and horizontal and vertical speed of the release for selected individual
men and women's discus throwers, (2) examine the relationship of the aerodynamic distance
with speed of the discus at the release and the angle of release, and (3) determine the
optimal angle of release for these individual discus throwers.
Three elite male and four elite female discus throwers who had at least eight trials in
our discus throw database were used as the subjects for this study. Two high speed video
camcorders were used to record subjects' performances at a frame rate of 60 frames/second
in each competition. The flight distances, aerodynamic distances, discus horizontal,
vertical, and resultant speeds at the release, height of release, and angle of release
were reduced.
The results of this study showed that the magnitude of the resultant speed of the
discus at the release is a function of the angle of release. The effects of angle of
release on the magnitude of the resultant speed of the discus are different from athletes
to athletes. The optimum angle of release, therefore, is an individualized release
parameter. Although that the optimal angle of release for some athletes is between 35°
and 40° as literature show, the optimal angle of release for some other athletes could be
less than 35° or greater than 40°. The optimal angle of release for women's discus
throwers tends to be greater than that for men's discus throwers. In addition, an angle of
release no more than 2° away from the optimal angle of release would affect the actual
distance no more than 0.2 m, but an angle of release more than 5° away from the optimal
angle of release would decrease the actual distance by up to 1.26 m. Further more, an
angle of release smaller than the optimal angle of release decreases the actual distance
more than an angle of release greater than the optimal angle of release does. These
findings indicate that the angle of release is indeed an important release condition with
a narrow margin for error in discus throw, and that the optimal angles of release for
discus throw recommended in literature may not necessarily be appropriate references for
every athlete.
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