resistance exercise helps improve the strength of all the muscles of the body and can be 
 performed continuously at any location .
well - designed resistance exercise programs have been 
 reported to effectively improve muscle strength and the ability to perform exercise , 
 increase fat - free mass , and decrease body fat1 .
the most important aspect of resistance exercise is identifying the 
 appropriate intensity for the exercise , and repetition maximum ( rm ) is most frequently 
 determined for identifying the appropriate intensity . during resistance exercises ,
1rm 
 should be changed depending on exercise intensities in order for the exercise to be 
 effective2 .
resistance exercise at 80% 
 rm or higher are classified as muscle strengthening exercise , that at 75% rm as muscle 
 strength exercise , and that at 60% rm as muscle endurance exercise .
although resistance 
 exercises at 7080% rm are effective for muscle strength enhancement in beginners , those at 
 85% rm or higher are effective for muscle strength enhancement in experts3 . however , resistance exercises performed 
 without considering a participant s ability can result in injury .
the degrees of muscle 
 damage are similar during high- , medium- , and low - intensity resistance exercises4 .
however , different degrees of muscle damage 
 have been reported to occur according to the type of muscle resistance exercise , the degree 
 of muscle contraction , and exercise load5 . 

based on the findings of studies on muscle damage , muscle fatigue , and changes in hormone 
 levels after muscle resistance exercise , studies that aim to minimize side effects after 
 exercise should be performed .
vibrations of muscles before exercise have positive effects on 
 motor skill improvement , and are used to increase the metabolic and blood flow rates of 
 individuals in the fields of sports and rehabilitation6 .
the effects of vibrations vary with frequency , and frequencies of 
 1550  hz have been reported to promote oxidation , increase oxygen uptake , help blood 
 circulation , and raise skin temperature7 .
one study reported that muscle pain reduced after stretching on a machine vibrating at a 
 frequency of 35  hz before resistance exercises8 .
another study with 20 adults showed that blood flow through the 
 quadriceps femoris and gastrocnemius increased with the application of vibrations , which 
 caused the inflow of blood into the muscles9 .
additionally , a study reported that muscle pain and creatine 
 phosphokinase ( cpk ) level decreased significantly with the application of vibrations at a 
 frequency of 50  hz before treadmill exercise10 .
although studies on the positive effects of vibrations on muscles have been performed , no 
 study has evaluated the effects of vibrations before resistance exercise at different 
 intensities on the levels of stress hormones .
therefore , the purpose of the present study 
 was to determine whether vibrations should be applied before resistance exercise by 
 examining changes in stress hormones levels with the application of vibrations resistance 
 exercises at different intensities .
the present study included 18 healthy male subjects in their 20s , having less than 6 months 
 of experience in weight training and residing in k city , south korea .
the subjects were 
 randomly divided into 1rm 50% group ( 50%rmg , n = 6 ) , 1rm 70% group ( 70%rmg , n = 6 ) , and 1rm 
 90% group ( 90%rmg , n = 6 ) . subjects in the 50%rmg had a mean age , height , and weight of 21.5 
  2.42  years , 170.2  6.68  cm , and 71.1  14.11  kg , respectively .
those in the 70%rmg had a 
 mean age , height , and weight of 22.3  2.40  years , 172.9  6.14  cm , and 70.5  7.23  kg , 
 respectively . additionally , those in the 90%rmg had a mean age , height , and weight of 21.12 
  2.13  years , 172.1  5.35  cm , and 70.38  6.35  kg , respectively .
the subjects voluntarily 
 participated in the study after receiving explanations about the intent and purpose of the 
 study .
the three groups had no statistically significant differences of physical 
 characteristic of subjects such as age , height , body weight and body fat ( p < 0.05 ) ; 
 therefore , the groups were homogeneous .
the smith squat exercise for resistance training was 
 used to examine changes in the levels of stress hormones with the application of vibrations 
 before resistance exercises at the following intensities : 1rm 50% ( maximum number of 
 repetitions 30 ) , 1rm 70% ( maximum number of repetitions 1215 ) , and 1rm 90% ( maximum number 
 of repetitions 34 ) .
the subjects performed three sets of smith squats , and vibrations were 
 applied for 1  min before each set .
a vibratory apparatus sky life - sm290 ( maxton , korea ) 
 capable of frequencies of 260  hz was used .
the frequency was fixed at 30  hz , and vibrations 
 were applied at knee flexion to sufficiently stimulate the quadriceps femoris and 
 gastrocnemius .
blood was collected at rest , immediately after exercise , and 24  h after 
 recovery from exercise to analyze levels of the stress hormones epinephrine , norepinephrine , 
 and dopamine . to examine changes in the levels of the stress hormones in each group ,
one - way 
 repeated anova was performed using spss 20.0 for windows on the basis of the levels at rest 
 in each group .
all of the subjects understood the 
 purpose of this study and provided written informed consent prior to their participation in 
 the study in accordance with the ethical standards of the declaration of helsinki .
epinephrine levels were significantly higher immediately after exercise than at rest in the 
 50%rmg ( p < 0.05 ) ; however , no significant changes were noted in these levels in the 
 70%rmg and 90%rmg .
the levels of norepinephrine were significantly higher immediately after 
 exercise than at rest in all the three groups ( p < 0.05 ) .
however , no significant changes 
 in the levels of dopamine were noted in any of the three groups ( table 1table 1.changes in stress hormones ( in  pg / ml ) after the application of vibrations during 
 resistance exercises at different intensitiesgroupat restimmediately after exerciseat 24 hours after exerciseepinephrine50%rm44.7  16.599.9  30.0 * 52.2  28.470%rm70.2  27.3112.3  48.062.7  20.390%rm56.5  13.868.5  26.659.6  30.4norepinephrine50%rm403.8  192.31196.1  659.6 * 362.0  156.670%rm466.9  174.01219.7  542.4 * 336.9  93.390%rm489.7  255.4815.6  434.4 * 295.6  89.5dopamine50%rm32.7  6.045.4  12.431.7  3.270%rm33.6  5.932.0  3.232.2  3.590%rm34.8  5.839.1  15.531.5  2.6 * statistically significant difference between rest ( p<0.05 ) ) .
physiological stress in the body is determined by assessing changes in catecholamines and 
 cortisol levels , and epinephrine and norepinephrine are considered representative 
 catecholamines11 .
epinephrine and norepinephrine levels increase when the sympathetic system is activated by 
 physical activities . during exercise
, epinephrine is secreted from the adrenal medulla and 
 norepinephrine from the sympathetic terminal12 .
the major function of epinephrine includes promoting 
 glycogenolysis and lipolysis during energy metabolism , and norepinephrine is a precursor of 
 epinephrine , which is secreted from the sympathetic terminal to function as a 
 neurotransmitter .
the arterial plasma levels of these two hormones are known to continuously 
 increase with exercise time and intensity .
epinephrine levels do not increase during 
 low - intensity exercise not involving mental stress , and its levels increase when oxygen 
 uptake rises to at least 60% of the maximum oxygen uptake .
norepinephrine levels increase at 
 exercise intensities that increase oxygen uptake to > 50% of the maximum oxygen uptake , 
 and its levels have been reported to increase by up to six times those at rest during 
 maximum intensity exercise13 .
therefore , 
 measurement of the levels of stress hormones help evaluate the degree of physiological 
 stress in the body according to the intensity of resistance exercise , and enable the 
 identification of changes in the levels after vibrations are applied . in the present study , 

epinephrine levels were significantly higher immediately after exercises than at rest in the 
 50%rmg ( p < 0.05 ) , and no significant changes in these levels were noted in the 70%rmg 
 and 90%rmg .
therefore , the application of vibrations at a frequency of 30  hz before the 
 70%rm and 90%rm resistance exercises did not result in significant changes in epinephrine 
 levels immediately after exercise .
the use of vibrations at frequencies of 2050  hz is recommended because vibrations at 
 frequencies below 20  hz induce excessive relaxation of muscles and those at frequencies over 
 50  hz cause muscle pain14 . in general , 
 vibration of muscles at 2545  hz is used to increase muscle strength and mass15 , and the application of vibrations has 
 been reported to have positive effects on the muscle function recovery of delayed - onset 
 muscle soreness16 .
vibrations suppress 
 interneuronal activities of the skin and spinal cord receptors that reduce pain signals 
 crossing -exercise nerves and c fibers and instead cause the migration of these signals to 
 the spinal cord and brain8 .
in addition , 
 vibrations have been shown to affect capillary vessels and reduce lactic acid levels9 , and a study has presented the mechanism 
 underlying the activation of the neuromuscular system that may improve the ability of the 
 neuromuscular system to function appropriately18 .
the activation of muscle spindle receptors with vibrations has 
 been reported to have positive effects not only on the directly stimulated muscles but also 
 on the surrounding muscles.19 a study of 
 29 male subjects who ran downhill for 40  min reported that the levels of the inflammatory 
 markers il-6 and histamine , as well as muscle pain in the femoral region decreased 
 significantly in the group in which vibrations were applied before the run20 .
on the basis of the results , vibrations 
 of muscles before activities may help reduce epinephrine levels . in the present study , 
 vibrations applied at a frequency of 30  hz before 70%rm and 90%rm resistance exercises 
 suppressed increases in epinephrine levels immediately after exercise . in the future , 

studies on the effects of vibrations on muscles and the areas of application should be 
 performed by using different intensities of resistance , frequencies of vibration , and 
 physiological variables .
 [ purpose ] the purpose of the present study was to determine whether vibrations should be 
 applied before resistance exercises by examining changes in stress hormone levels with 
 vibrations applied before different exercise intensities . 
 [ subjects ] eighteen male 
 subjects in their 20s were included , and were randomly divided into one - repetition maximum 
 ( 1rm ) 50% group ( 50% rmg , n = 6 ) , 1rm 70% group ( 70%rmg , n = 6 ) , and 1rm 90% group ( 90% 
 rmg , n = 6 ) . 
 [ methods ] three sets of smith squats were performed at 1rm 50% , 70% , and 90% 
 according to resistance intensities , and vibrations were applied for 1  min at a fixed 
 frequency of 30  hz before each set . 
 epinephrine , norepinephrine , and dopamine stress 
 hormone levels were analyzed . 
 [ results ] epinephrine levels were significantly higher 
 immediately after exercise than at rest in the 50%rmg ; however , no significant changes 
 were noted in the 70%rmg and 90%rmg . 
 norepinephrine levels were significantly higher 
 immediately after exercise than at rest in all three groups . 
 however , no significant 
 changes in dopamine levels were noted in the three groups . 
 [ conclusion ] the application of 
 vibrations at a frequency of 30  hz before 70%rm and 90%rm resistance exercises suppressed 
 increases in the stress hormone epinephrine levels immediately after exercise . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION