the purpose of all sports training is the maximization of the physical and mental elements 
 required to perform exercises or play games .
therefore , the application of specialized and 
 specific training suitable for sports is important . in particular
, the acquisition of 
 maximum muscle strength and power is an essential element of all sports .
plyometric training 
 is widely used as a method of developing explosive power capacity in those sports that 
 require jumping ability such as athletics , basketball , and volleyball1 .
most sports require various kinds of fast and slow muscle strength and speed2 . although typical training methods for 
 increasing the muscle strength of athletes include weight training , such as resistance 
 exercises and plyometric training3 , the 
 power necessary for explosive jumping refers to the power exhibited in dynamic states4 .
plyometric training is a type of muscle strength exercise that can improve basic physical 
 strength and it has been extensively studied for the improvement of exercise performance 
 ability .
it is an explosive and repetitive rebounding load type exercise that uses the 
 muscles stretch reflexes and stretch - shortening cycles to develop lower extremity 
 muscles5 , 6 .
stretch reflexes are used to promote motor unit mobilization in 
 order to reduce differences between speeds and muscle strength7 . in athletics
, diverse efforts have been made to try and enhance records through active 
 studies and the development of training methods . with increasing interest in athletics , the 
 importance of athletics scientification has been recognized .
desired records in athletics 
 can not be achieved only with natural movements and efforts , but also require more reasonable 
 and scientific training content and methods8 .
the physical strength elements required for athletic throwing 
 events include muscle strength , swiftness , agility , speed , flexibility , and physical 
 balance .
although plyometric training and weight training are implemented as representative 
 training methods for improving swiftness and agility , most studies of it have been conducted 
 with players of other sports .
therefore , the purpose of the present study was to identify the effects of lower extremity 
 plyometric training for 12 weeks , performed by high school throwing event athletes , on 
 isokinetic lower extremity muscle strength and lumbar muscle strength in order to develop 
 more efficient training methods .
the study subjects were 10 throwing event athletes attending k physical education high 
 school .
the subjects were randomly assigned to a control group of five subjects and an 
 experimental group of five subjects .
the experimental group performed 12 weeks of lower 
 extremity plyometric training in addition to general physical strength training programs 
 implemented during school hours .
the control group ( n=5 ) was 17.600.54  years old in age , 
 1.74.584.60  cm in height , and 73.229.90  kg in weight , and had 14.525.67% body fat , and  kg 
 lean body mass 32.924.81 .
the plyometric training group was 17.600.89  years old in age , 
 173.587.78  cm in height , and 77.2211.66  kg in weight , and had 15.346.30% body fat , and 
 35.1626.85 81  kg lbm . to analyze the body composition ,
an inbody 3.0 instrument ( biospace , korea ) was used as 
 experimental equipment to measure the subjects heights , weight , body fat percentages , and 
 muscle masses and a biodex system 4.0 ( biodex , usa ) was used to measure isokinetic 
 muscle - joint and lumbar muscle strengths . the plyometric training consisted of 15 techniques out of the training methods introduced 
 in the  power up plyometric training9 . 

lateral jump with one foot10315315511 . jump and dead run10315315512 . running up the stairs with one foot10315315513 . running up the stairs with both feet10315315514 . running up the stairs aside10315315515 . running fast and light103153155 .
the plyometric program was implemented without any training load three times 
 per week during daybreak exercises performed by the experimental group .
the number of times 
 and the number of sets were changed over time as follows : three sets of 10 times in the 
 1st4th weeks , three sets of 15 times in the 5th8th weeks , and five sets of 15 times in the 
 9th12th weeks . to measure kinetic lumbar muscle strength ,
a biodex system 4.0 was used to measure average 
 power . when stabilized after warm - up exercises
, the subject s body regions other than the 
 lumbar region were fixed using adjustable pads so that they would not be subject to external 
 force during extension and flexion movements .
the lower extremities and the chest were fixed 
 using straps to restrict the actions of the other muscles .
the rotation axis of the joint 
 being tested was aligned with the rotation axis of the dynamometer , and the subjects were 
 encouraged to exert maximum efforts .
the average power of flexion and extension was 
 calculated from the values of 4 repeated measurements of lumbar extension and flexion at 
 60/sec .
the data obtained were processed using the sas ver 9.12 statistics program . the means and 
 standard deviations of the two groups were calculated . to examine differences between the 
 groups and between measurement time points ,
the results of the prior test were set as 
 covariates and analyses of covariance ( ancova ) were conducted .
post hoc tests of differences 
 between the groups were conducted using the least squares mean ( lsm ) method only when the 
 assumption that the gradients of regression lines were statistically the same within the 
 groups was established , and improvement rate independent t - tests were conducted to examine 
 differences in records between athletes of different items .
all the subjects understood the purpose of this study and provided their written informed 
 consent prior to their participation in the study in accordance with the ethical standards 
 of the declaration of helsinki .
according to the ancova results of lumbar extensor muscle strength at 60/sec , the overall 
 reliability of the model was significant ( f=27.28 p<0.001 ) . in the results of the prior 
 test ,
the common gradient of regression straight lines of lumbar extensor muscle strength at 
 60/sec was significant within each treatment , indicating that the initial value of the 
 ancova did not significantly affect actions after the treatment ( f=26.19 p<0.001 ) .
the 
 main effect , i.e. , the differences between the groups resulting from the experimental 
 treatment , was significant ( f=22.15 , p<0.002 ) . in the lumbar extensor muscle strength at 60/sec ,
the standard deviation of the control 
 group decreased from 327.2258.41 nm in the prior test to 274.6249.95 nm in the 
 post - treatment test , and the standard deviation of the experimental group improved from 
 341.8481.96 nm in the prior test to 382.5877.65 nm in the post - treatment test ( table 2table 2 .
least mean square ( lsm ) of lumbar extensor muscle strength at 60/sec and 
 comparison of differences between the groups ( nm)grouppre_msdpost_msdmeansdcontrol group327.2258.41274.6249.95280.5714.38*experimental group341.8481.96382.5877.65376.6214.38*p<0.01 ) .
the lsms of both groups were compared and the results shown in ( table 2 ) indicated that there were differences when seen based on 
 two - tailed tests ( p<0.002 ) .
however , one - tailed tests were appropriate in the present 
 study , and the significance level was eventually shown to be 0.001 ( 0.0021/2 ) , indicating 
 that the lumbar extensor muscle strength at 60/sec was significantly different between the 
 two groups . according to the ancova results of lumbar flexor muscle strength at 60/sec , the overall 
 reliability of the model was significant ( f=6.21 p<0.02 ) .
the common gradient of linear 
 regression of lumbar flexor muscle strength at 60/sec was significant within each 
 treatment , indicating that the initial value of the ancova did not significantly affect 
 post - treatment actions ( f=6.58 p<0.003 ) .
the main effect , i.e. , the differences between 
 the groups made by the experimental treatment , was significant ( f=6.26 , p<0.004 ) . in the lumbar flexor muscle strength at 60/sec ,
the standard deviation of the control 
 group decreased from 170.1244.75 nm in the pre - test to 153.9420.24 nm in the post - test and 
 the standard deviation of the experimental group improved from 167.4445.25 nm in the 
 pre - test to 185.6832.45 nm in the post - test ( table 
 3table 3 .
least mean square ( lsm ) of lumbar flexor muscle strength at 60/sec and 
 comparison of differences between the groups ( nm)grouppre_msdpost_msdmeansdcontrol group170.1244.75153.9420.24153.379.28*experimental group167.4445.25185.6832.45186.249.28*p<0.01 ) .
the lsms of both groups were compared and the results indicate that there were differences based on two - tailed tests 
 ( p<0.004 ) .
however , one - tailed tests were appropriate for the present study , and the 
 significance level was eventually shown to be 0.002 ( 0.0041/2 ) , indicating that the lumbar 
 flexor muscle strength at 60/sec was significantly different between the two groups .
since measurement of peak torque is very accurate and is highly reproducible , all 
 isokinetic contractile force values can be standardized and utilized as reference data .
lumbar flexor / extensor muscle strength at 60/sec in two - tailed tests 
 showed significant differences between the two groups ( p<0.01 ) .
this result is consistent 
 with those of youm and colleagues10 , who 
 reported that after implementing abs and back training , the expression ratio of abdominal 
 and lumbar muscle strengths significantly increased ( 14.53% ) , and also with those of 
 brittenham11 , who reported that 
 abdominal / lumbar training evenly developed abdominal , spinal , and gluteal muscles conferring 
 positive effects on power expression .
the differences in isokinetic muscle strength centered 
 on the waist between sports activities are similar to as the results of a previous study 
 which indicated that the major reason is the development of the muscles around the 
 waist12 .
although plyometric training mainly jumping consist of exercises , appropriate levels of 
 stimulation are required for back muscle strength to increase lower extremity muscle 
 strength .
ju13 reported that plyometric 
 training was correlated with the improvement of lumbar joint flexor / extensor muscle 
 strength .
standaert and colleagues14 reported that 
 when posture swayed due to lumbar stabilization exercises , the transversus abdominis muscle 
 ( tra ) was activated and that muscle activity was reduced when lumbar stability was deficient 
 or there was a problem in motor control .
gresswell and colleagues15 reported that functional activities caused changes in 
 intra - abdominal pressure and that the tra was activated before the erector spinae muscle was 
 activated when loads were imposed on the spine .
maffiuletti et al.16 reported that four 
 weeks of plyometric training performed by 10 female volleyball players improved lower 
 extremity muscle strength by 20% .
although most studies of plyometric training examined the 
 degree of improvement of lower extremity muscle strength , the present study revealed good 
 results from an examination of lumbar flexor / extensor muscle strength .
we consider that the 
 increase in core muscle strength improved the athletes motor abilities .
plyometric training positively affected the performance of high school throwing event 
 athletes . to summarize the study findings ,
the application of plyometric training with high 
 intensity and loads improved the performance of athletes who perform highly intensive 
 exercises at normal times .
   [ purpose ] the physical strength elements required for athletic throwing events include 
 muscle strength , swiftness , agility , speed , flexibility , and physical balance . 
 although 
 plyometric training and weight training are implemented as representative training methods 
 for improving swiftness and agility , most studies of it have been conducted with players 
 of other sports . 
 [ subjects ] the study subjects were 10 throwing event athletes attending k 
 physical education high school . 
 the subjects were randomly assigned to a control group of 
 five subjects and an experimental group of five subjects . to analyze the body composition , 
 
 an inbody 3.0 instrument ( biospace , korea ) was used as experimental equipment to measure 
 heights , weight , body fat percentages , and muscle masses and a biodex system 4.0 ( biodex , 
 usa ) was used to measure isokinetic muscle - joint and lumbar muscle strengths . the 
 plyometric training consisted of 15 techniques out of the training methods introduced in 
 the  power up plyometric training. the plyometric program 
 was implemented without any 
 training load three times per week during daybreak exercises for the experimental group . 
 
 the number of times and the number of sets were changed over time as follows : three sets 
 of 10 times in the 1st 4th weeks , three sets of 15 times in the 5th8th weeks , and five 
 sets of 15 times in the 9th12th weeks . 
 [ results ] according to the ancova results of 
 lumbar extensor muscle strength at 60/sec , the overall reliability of the model was 
 significant . according to the ancova results of lumbar flexor muscle strength at 60/sec , 
 the overall reliability of the model was significant . 
 [ conclusion ] plyometric training 
 positively affected high school throwing event athletes . to summarize the study findings 
 , 
 the application of plyometric training with high intensity and loads improved the results 
 of athletes who perform highly intensive exercises at normal times . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION