131 research outputs found
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The impact dynamics of a tennis ball striking a hard surface
The purpose of this research was to study the impact
phenomena of a tennis ball striking a hard surface.
Stroboscopic photography was used to collect the ball's
impact images from seven angles of incidence, ranging from
-23 degrees to -70 degrees with zero, top and back initial
spin respectively.
Through digitization, the image data were converted
and calculated into the experimental parameters which were
composed of the input/output of the translational and angular
velocities, and into the system parameters which included the
coefficient of restitution, coefficient of sliding friction,
ball's dwell time, and ball's dwell distance. Mathematical
models derived from both the differentiation and integration
approach were developed to explore the impact phenomena.
A -23 degree angle of incidence for the data sets (zero
spin, topspin, backspin) was selected to carry out the
mathematical analysis using both experimental and system
parameters. The results were:
1. The successive differentiation approach did not lend
itself well to the investigation of tennis ball impact
phenomena.
2. The successive integration approach based on the Damped
Sin Pulse Model, could be used successfully to describe both
the horizontal and vertical forces, velocities and positions
of ball impact on a surface.
3. In the case of -23 degree incident angle, the effect of
top-spin will produce a high value for the coefficient of
restitution, which provides the ball a chance to rebound
higher.
4. The horizontal component velocity will influence a
shallow angle impact with backspin ball on a surface to have
a smaller sliding friction.
5. The findings of this study will provide the
instructor of tennis skills with information to fully
explain the effects of utilizing the racquet to impart spin
to the tennis ball.
6. This study provides guidelines for future research
that is likely to affect the methodology of teaching tennis
skills
THE STUDY OF THE AREA OF FOOTPATH BETWEEN MALE AND FEMALE ELITE DISTANT RUNNERS UNDER DIFFERENT RUNNING SPEEDS OF A TREADMILL
The purpose of this study was to investigate and to compare the difference of area of footpath between male and female elite distant runners under 3 m/s and 4 m/s running speeds on a treadmill. There were 9 male and 7 female subjects jointed this study. AJVC DVL-9800 digital camera was utilized to record subject's running movement, and the Ariel Performance Analysis System was used for digitizing. The t-test statistics was used to test the difference between variables. The results indicated that both male and female runners' areas of footpaths were greater when the running speed was increased, and there was no significant difference between male and female runners
THE STUDY OF TEAM RESULTANT FORCE VANISHING PERCENTAGE IN ELITE TUG OF WAR PLAYERS
This study was to analyze the factor of team pulling force vanishing percentage between groups with different numbers players and the largest pUlling force with unsteady motion in tug of war after sitting posture between players. The motion of nine female senior high school players(age =16.9 ± 0.6 years, height =163.8 ± 2.7 cm, mass =58.7 ± 4.3 kg) in tug of war was synchronized with a high-speed camera(JVC9800, 60 Hz)and a tensile (TEDEA, 900 Hz). The pulling force was gained by DASYlab6.0. The variance of the angle between the rope and ground was analyzed by Silicon Coach from the film, consisted by two to eight players. Results: The team pUlling resultant was smaller than the sum of individual players. The vanishing percentage of pulling force was increase with the number of players. The factor of force vanishing was the coordination between players. The larger numbers players are, the 'lower coordination was. This is so called Ringlemann effect; the factor was timing of the maximal pulling force with each player, therefore coordination was decreasing with the numbers of players, so was force
KINETIC ANALYSIS OF LOWER EXTREMITY DURING SIDESTEP CUITING UNDER DIFFERENT DIRECTIONS
The purpose of this study was to investigate the ground reaction forces, joint reaction forces, net muscle joint moments of the ankle, knee and hip during elite lhandball athletes executed typical cutting maneuver under four different directions. Four male elite handball players were tested during fast running (4.5-5.5 m/s) and then performed cutting to the right off (30°, 400 50°, 600 A JVC camera (60 Hz) and synchronized with force plate (1200 Hz) were used to collect the relative parameters of braking leg during cutting maneuver. Each participant's braking leg (right leg) was modeled as a system of rigid bodies. The inverse dynamics approach was used to integrate the body segment parameter, kinetic and force plate data, and to solve the resultant joint moments. The results showed that sidestep cutting maneuver would produce a load and according to the increase of cutting angles, the lower extremity could receive a strike force and moment during the braking phase. Besides, while the knee angle of f1exion was small and repeated loading of this magnitude were enough to produce the fatigue of lower extremity joints that would eventually resu'lt in complete disruption and clinical lower extremity joints injury
EFFECT OF DIFFERENT RUNNING SPEED ON VERTICAL OSCILLATION AND STRIDE
The main purpose of this study was to investigate the effect of different running speed (3.5±O.3 and 4.9±O.2 m/si on vertical oscillation of the center of gravity and stride. Ten male elite running players served as subjects. A camera (60Hz) was to collect the parameters during running. From this research, the t-Test is used to analyze the parameters of the kinematics in different running speed. Results indicated that different running speed would not affect the vertical oscillation of human body center of gravity. In addition, a significant difference (p< .05) was found in the same speed of different step, but no significance was found in different running speed of the same step. The duration of the stance phase in lower speed for about 38.34% of the stride and the swing phase for 61.66%, in higher speed for about 32.11 % and 67.89%
KINEMATIC ANALYSIS OF LOWER EXTREMITY ON DIFFERENT RUNNING SPEED
A kinematic analysis of selected variables of human lower extremity was completed during different running speed (3.5±O.3 and 4.9±O.2 m/s)on treadmill.Ten male elite running players served as subjects.It got two dimensions photograph analysis along sagittal plane with video data collected at 60Hz.The purpose of the study was to analyze the kinematic parameters including the range of motion,the joint angle,angular velocity and angular acceleration of lower extremity joint related to the cycle time during horizontal running. According to the experimental result of a research ,we could get the below conclusion:angle of lower-extremityed joints in different running speed reaches significant difference. Angle of lower-extremityed joints is getting more with the increasing of velocity.Then maximum angle, angular velocity and angular acceleration of knee joints stretched are getting more with the increasing of velocity. Maximum stretched angle in joint extension phase of take-off significnce
THE ANALYSIS OF ANGULAR MOMENTUM OF SIDE-VOLLEY SOCCER SHOTS PERFORMED AT THREE BALL-HEIGHT
The purpose of this study was to examine side-volley soccer shots performed at three ballheight.
Five amateur, right leg dominant, male soccer players (height = 173 ±2.86 cm; weight = 68 ±6.04 kg; age = 20.5 ±0.5 yrs) participated in this study. Two electrically synchronized highspeed
video cameras (Redlake) were used to sample the kicking motion at 250 Hz. By the three-dimensional cinematographic technical analysis, the kicking leg was modeled as a threelink
kinetic chain composed of thigh, shank, and foot, from which angular velocities and angular momentum were computed. Significantly greater ball velocity and foot velocity were achieved in kicking lower ball-height, with a shorter leg-swing time. The foot moment of inertia exhibited a high value in side-volley shot performed at the lower ball-height. These results indicated that the side-volley soccer shots performed at the lower ball-height would made higher value of foot angular momentum, and benefited ball velocity
ANALYSIS OF ANGULAR MOMENTUM THE WHOLE BODY DURING GLIDE HITTING AND KICK HITTING IN BASEBALL
The purpose of this study were to analyze the biomechanical characteristics of glide and kick hitting in baseball which exerted by professional baseball players in Taiwan. Five professional baseball players were selected as the subjects. The experiment used two JVC-DV 9800 high-speed digital cameras(120 Hz).The video data was treated by Kwon3D 3.0 motion analysis system. The following are the main results: The kick hitting could get better rotation benefit and optimum. In the stride phase, angUlar momentum of the body is increasing by body inertia. In the rotation phase, the velocity increasing of the body center of gravity makes the angUlar momentum increase. The largest angular momentum appears right before the ball hi!. Because the body inertia in ball hitting moment was very small, the angUlar momentum was mainly affected by angular velocity. In the phase, the average value, largest value and hitting moment value of angular momentum with kick hitting are larger than those with glide hitting. Therefore, the kick hitting could get larger linear and angular momentum
EFFECT OF BACKPACK ON SELECTED GAIT PARAMETERS OF PRIMARY SCHOOL CHilDREN
The purpose of this study was to compare the effects of a backpack load of 15% body weight (BW) on selected gait parameters of primary school children. Ten participants were recruited from primary school (age: 10.3 ± 0.48 yrs; hI: 141.3 ± 0 .41 cm; mass:
38.1 ± 6.1 kg). A JVC 9800 (60 hz) video camera synchronized with an AMTI force plate (1200 Hz) were used to collect data. A repeated measure t-test (p < 0.05) was used for group comparisons. The backpack load did not affect either the proportionate time of the stance phase, swing phase, or the magnitude of selected vertical and anterioposterior ground reaction force parameters. However, the 15% backpack load did cause a significant increase in proportionate double leg support time
Sinus automaticity and sinoatrial conduction in severe symptomatic sick sinus syndrome
AbstractElectrophysiologic studies with recordings of sinus node electrograms were performed in 38 patients with severe symptomatic sick sinus syndrome. Thirty-two of the 38 patients had episodic tachyarrhythmias and 17 presented with syncope. The clinically documented sinus or atrial pause was 5.6 ± 2.8 s (mean ± SD).Patients were divided into three groups according to electrophysiologic findings. Group I consisted of nine patients with complete sinoatrial block. Sinus node electrograms were recorded during the episodes of long pauses. Seven patients had unidirectional exit block, with the atrial impulse being capable of retrograde penetration to the sinus node causing suppression of sinus automaticity; two had bidirectional sinoatrial block.Group II consisted of 22 patients with either 1:1 sinoatrial conduction (group IIa = 13 patients) or second degree sinoatrial exit block (group IIb = 9 patients) during spontaneous sinus rhythm. Sinoatrial exit block, ranging from 1 to >14 sinus beats, was observed during postpacing pauses that ranged from 1,650 to 37,000 ms (mean 7,286 ± 6,989). The maximal sinus node recovery time ranged from 770 to 5,580 ms (mean 3,004 ± 1,686) and was normal in 5 patients and prolonged in 17.Group III consisted of seven patient with no recordable sinus node electrogram, reflecting either a technical failure or a quiescence of sinus activity. The sinus node recovery time in these seven patients ranged from 1,190 to 4,260 ms (mean 2,949 ± 1, 121).Thus, abnormalities in both sinus node automaticity and sinoatrial conduction are responsible for the long sinus or atrial pauses in the sick sinus syndrome. However, complete sinoatrial exit block can occur and cause severe bradycardia with escape rhythm; repetitive sinoatrial exit block plays a major role in producing posttachycardia pauses
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