in general , different direct therapeutic approaches have been used as exercise treatment 
 methods for the neurological forms of some conditions to improve the functions of the 
 affected side ; however , indirect therapeutic approaches aimed at helping the affected side 
 recover using the body parts on the unaffected side have also been said to be effective1 . with regard to indirect treatment , 
 hellebrandt et al.2 reported that maximal 
 exercise of the unilateral limbs triggered muscle tone in the contralateral body part 
 without exercise and that the flexion pattern of the upper limb accompanying contraction of 
 the abdominal muscles indirectly activated the trunk muscles . in addition , devine et 
 al.3 observed that contralateral 
 exercise was conducive to reducing muscular atrophy , maintaining motor coordination , and 
 enhancing the muscle strength of damaged lower extremities , and therapists were able to 
 improve the treatment and rehabilitation of patients with neuromuscular system diseases and 
 musculoskeletal problems . in the clinical field , the application of indirect treatment is 
 explained by the concept of cross education , and it is now being supported by research , as 
 research results indicated that training on the unaffected side influences the functions of 
 the affected side4 , 5 . using electromyography ( emg ) , shima et al.6 verified that resistance training on one side triggered 
 changes in muscle strength on the opposite side , and bemben and murphy7 reported that short - term unilateral side resistance training 
 resulted in increased muscle strength in the untrained extremities . therefore , indirect 
 treatment using cross education has been proposed as a method to help patients with 
 hemiparesis , patients who have undergone hip or knee joint replacement , and patients with 
 plaster fixation of the unilateral limbs for muscular and functional recovery of the paretic 
 side8 . prior studies have noted that 
 muscle training with the paretic side lower limb increased the muscle strength of the 
 contralateral lower limb4,5,6,7,8 , but other studies did not 
 obtain such effects9,10,11,12 . as in previous studies , it is difficult to examine precise and detailed changes in the 
 muscle activity of the lower extremities using emg , and korean studies that look at the 
 muscle activity of the contralateral lower limb through isometric contraction of an upper 
 limb are lacking , and there are conflicting opinions about the effects of indirect 
 treatment . accordingly , this study was conducted to examine changes the lower - limb muscle 
 activity of healthy subjects based on the movement of the upper limbs . its aim was to 
 examine the muscle activity of the contralateral lower limb according to isometric 
 contraction of an upper limb in a supine position and to suggest an efficient means of upper 
 limb movement with the aim of increasing the contraction of the contralateral lower limb 
 muscles , thereby presenting basic data for rehabilitation treatment of early hemiplegic 
 patients who have difficulty with physical postural control . the subjects of this study were 40 healthy adults ( 35 males , 5 females ) in their 20s who 
 were undergraduates at y university , chungcheongbuk - do , republic of korea . their average 
 age , weight , and height were 21.6  years old , 68.6 kg , and 174.4  cm , respectively . prior to 
 the experiment , the subjects were informed about the purpose of this study and exercise 
 method , and they voluntarily consented to participate in this experiment and signed an 
 agreement according to the ethical standards of the declaration of helsinki . those who had 
 received muscle strengthen training in the previous six months or who had orthopedic or 
 neurological damage were excluded . the subjects lay in a supine position , spread their legs shoulder - width apart , placed their 
 toes in a neutral position , and maintained an anatomical position for initiation of the 
 exercise . muscle activity was recorded while the subjects conducted isometric contraction , 
 flexing the shoulder joint at 20  cm from the ground for flexion , extension , abduction , and 
 adduction of the shoulder joint . all the subjects conducted isometric flexion , extension , 
 abduction , and adduction of the shoulder joint using the dominant side upper extremity , to 
 ensure consistency in the experimental method , and the muscle activity of the rectus femoris 
 ( rf ) , biceps femoris ( bf ) , anterior tibialis ( at ) , and medial gastrocnemius ( mg ) was 
 recorded . the four kinds of shoulder joint exercise conditions were randomly applied to the 
 subjects . to provide the same adequate resistance for all subjects , a handheld dynamometer was used . resistance was applied to the proximal part of the front , back , medial , and lateral parts of 
 the arm while the subjects conducted isometric flexion , extension , abduction , and adduction 
 of the shoulder joint . visual feedback was provided to the patients and the evaluator with 
 the handheld dynamometer , so constant resistance against the subjects could be continuously 
 maintained for five seconds . during the measurements , the extension of the elbow joint was 
 maintained , and when other compensation appeared , another measurement was taken . the maximal isometric contraction of each upper extremity 
 movement was obtained using the handheld dynamometer ( commander , jtech medical , salt lake 
 city , ut , usa ) to provide the same appropriate resistance , and resistance was provided to 
 maintain 50% of the maximal contractility13 . surface emg ( mp150 , biopac systems inc . , santa barbara , ca , usa ) was employed to measure 
 changes in muscle activity , and surface electrodes were attached to the rf , bf , at , and mg . data were 
 processed using the acqknowledge software ( version 4.01 ) on a personal computer . the average 
 value of each subject s emg signals was expressed as a percentage of the maximal voluntary 
 isometric contraction . in this study , repeated one - way analysis of variance was conducted in order to examine the 
 muscle activity of the contralateral lower limb following isometric contraction of the upper 
 limb , and the bonferroni method was used as a post hoc test . according to the study results , the muscle activity of the rf of the contralateral lower 
 limb was significantly high when the shoulder joint of an side upper limb was flexed 
 ( p<0.05 ) , and the muscle activity of the mg of the contralateral lower limb was 
 significantly high when the shoulder joint of an limb was adducted ( p<0.05 ) ( table 1table 1.comparison of the opposite side lower extremity muscles according to isometric 
 contraction of the unilateral side upper extremity ( unit : % ) flexion**extensionadduction*abductionrf8.17.71.82.74.77.53.85.1bf1.10.93.75.51.72.82.02.9at3.17.52.44.72.66.72.34.5mg3.53.22.92.85.34.92.61.7 : msd , rf : rectus femoris , bf : biceps femoris , at : anterior tibialis , 
 mg : medial gastrocnemius , * : p<0.05 , * * : p<0.01 , * : repeated one - way anova ) . : msd , rf : rectus femoris , bf : biceps femoris , at : anterior tibialis , 
 mg : medial gastrocnemius , * : p<0.05 , * * : p<0.01 , * : repeated one - way anova in order to examine the effects of isometric contraction of an upper extremity on the 
 muscle activity of contralateral the opposite side lower extremity in a healthy adult , this 
 study measured the muscle activity of the rf , bf , at , and mg of the contralateral site lower 
 extremity in response to flexion , extension , abduction , and adduction of the shoulder joint 
 of the an upper extremity . tarnanen et al.14 examined the trunk 
 muscle activity of the same and opposite sides in response to isometric contraction of one 
 upper extremities , and they reported that the activity of the multifidus muscle of both the 
 same and opposite sides increased during the horizontal abduction of the shoulder joint . in 
 a study of the activity of the spinal erector muscle when subjects supported a weight loaded 
 on the upper extremities in a standing position , crommert et al.15 noted that the activity of the muscle increased more when 
 the subjects supported the loaded weight by horizontally extending the arms at the height of 
 the knee joint than when they supported the loaded weight without horizontally extending the 
 arms , and they asserted that the location of the resistance loaded on the extremities 
 significantly affected the activity of the spinal extensor muscle . park et al.16 asserted that isometric contraction of the 
 hip joint on one side may increase the trunk muscle activity on the opposite side and that 
 the isometric contraction in flexion and abduction of the hip joint on one side influenced 
 the relative activity of the muscles , such as the multifidus muscle engaging in trunk 
 postural control . in a study of patients with a fixed lower extremity resulting from damage it had suffered , arai et al.17 reported 
 that exercise of the contralateral lower extremity comprised of proprioceptive neuromuscular 
 facilitation exercises increased the contractility of the affected side quadriceps femoris 
 muscle more than ordinary exercise consisting of diverse resistance exercises . sato and 
 maruyama18 advised that , among the 
 techniques of proprioceptive neuromuscular facilitation ( pnf ) when isometric resistance 
 exercises were performed by an upper extremity in the final position of the 
 flexion - abduction - external rotation pattern of pnf , the extension force of the lower 
 extremity on the opposite side increased . yoo et al.19 reported that during unilateral lower - limb pnf pattern training , 
 abdominal hollowing significantly increased the muscle activity of the vastus lateralis , the 
 tibialis anterior , the semitendinosus , and the gastrocnemius of contralateral lower 
 extremity . according to the results of the present study , when the subjects flexed the shoulder joint 
 of an upper extremity , the muscle activity of the rf of the contralateral lower extremity 
 was significantly high , and when the subjects adducted the shoulder joint of an upper 
 extremity , the muscle activity of the mg of the contralateral lower extremity was 
 significantly high . regarding this result , myers et al.20 explained that the human body responded as a whole when one part of 
 the body moved , and the only tissue that could perform functional intervention was the 
 connective tissue ; they approached the interactions and the overall functions of different 
 physical systems through the concept of lines , and they expressed interactions using front , 
 back , lateral , arm , spinal , and oblique lines . as for the functional connectivity between 
 the upper extremity and contralateral opposite side lower extremity applied in the present 
 study , flexion and adduction of the shoulder joint of an upper extremity led to high muscle 
 activity in the rf and the mg due to anatomical connectivity between the arm line of the 
 upper extremity and the spinal line , according to the lines proposed by myers et al20 . first , it is difficult to discuss long - term 
 treatment effects with the present study results because isometric contraction of the upper 
 extremity was conducted for a relatively short period of time . second , the possibility of latent signals occurring during surface emg due to 
 other nearby soft tissues can not be excluded because of the characteristics of the 
 measurement method . third , the muscle activity was measured with subjects in a supine 
 position only , and the muscle activity according to different postures and locations was ont 
 considered . thus , future research should examine how isometric contraction of an upper 
 extremity according to different postures and locations influences the muscle activity of 
 healthy adults in the lower extremity on the opposite side .
[ purpose ] the aim of this study was to examine effects of the isometric contraction of an 
 upper limb in a supine position on the muscle activity of a healthy adult in the 
 contralateral lower limb . [ subjects ] the subjects were 40 healthy adults ( 35 males and 5 
 females ) . [ methods ] the muscle activity of the rectus femoris ( rf ) , biceps femoris , 
 anterior tibialis , and medial gastrocnemius ( mg ) of the contralateral lower limb was 
 measured using electromyography while the subjects flexed , extended , abducted , and 
 adducted the shoulder joint of an upper limb . 
 [ results ] the muscle activity of the rf of 
 the contralateral lower limb was significantly high when the subject flexed the shoulder 
 joint of an upper limb , and the muscle activity of the mg of the contralateral lower limb 
 was significantly high when the subject adducted the shoulder joint of an upper limb . 
 [ conclusion ] the isometric contraction that results from flexion and adduction of the 
 shoulder joint of an upper limb in a supine position is considered to selectively affect 
 the rf and mg activity of the contralateral lower limb .