muscle strengthening exercise is effective in preventing and improving musculoskeletal and 
 cardiovascular system problems1 . it 
 increases sympathetic activity and decreases parasympathetic activity , thereby heightening 
 the heart rate . when the exercise is finished , the sympathetic activity decreases and the 
 parasympathetic activity increases , reducing the heart rate and returning it to normal2 . this type of autonomic nervous system 
 balance plays an important role in maintaining the health of the cardiovascular system3 . high - intensity muscle contraction performed within a short time may affect the activity of 
 the autonomic nervous system . in particular , resistive exercise conducted for a short time 
 is known to decrease cardiac vagal control4 . cardiac stress may be triggered by a continued increase in 
 sympathetic activity and a simultaneous decrease in parasympathetic activity , causing health 
 problems5 , 6 . a physical therapist may enhance muscle strength by triggering muscle contraction through 
 neuromuscular electrical stimulation7 . muscle strengthening by nmes may affect a person s autonomic nervous system balance in a 
 similar way to resistance exercise . current research on electrical stimulation and autonomic 
 nervous system balance mostly concerns transcutaneous electrical stimulation , and research 
 on the effects of nmes on muscle strengthening and autonomic nervous system balance is 
 insufficient8 . given that there are many 
 patients who should not be exposed to cardiac stress in a physical therapy setting , the 
 effects of nmes on autonomic nervous system balance need to be examined . accordingly , this 
 study contracted the quadriceps muscle using nmes and measured the variability in the heart 
 rate to examine the effects of nmes on autonomic nervous system balance . this study included 10 healthy adult males who had no history of disease related to the 
 cardiovascular system and autonomic nervous system ( ans ) and had the same daily routines . the average age , weight , and height of the subjects were 222  years , 653.2  kg , and 
 164.24.1  cm , respectively . all the subjects signed an informed consent form , and the study 
 was approved by the institutional review board of the catholic university of pusan ( document 
 number : cupirb-2013 - 012 ) . the temperature and humidity of the experimental room were maintained at 23  c and 65% , 
 respectively , outside noise was blocked , and the experimental area was enclosed with a 
 curtain to minimize environmental influences . prior to the experiment , the subjects were forbidden to 
 perform exercise , overeat , or drink alcohol . from 4 hours prior to the experiment , they were 
 required to fast and restricted from smoking or consuming drinks containing caffeine . prior to starting the nmes stimulation , they 
 rested on a chair for 10 minutes . for nmes stimulation , a subject sat on a chair and flexed 
 the hip joint and the knee joint at 90 degrees . the electrodes were attached to the motor 
 points of the vastus lateralis and vastus medialis . using an es-420 low - frequency 
 electrotherapy unit ( ito , tokyo , japan ) , asymmetrical biphasic rectangular wave stimulation 
 was applied to the motor points for 15 minutes . the wave was set at a pulse width of 300 us 
 with an on - time of 10 sec , and an off - time of 10 sec . the intensity of the stimulation was 
 adjusted in the range of 20 to 30 ma when the subject s knee joint was fully extended . in a sitting position , the subject s right hand was placed at the same height as the 
 subject s heart , and a ubplus t1 pulse oximeter ( laxtha inc . , the following items were analyzed : heart rate ( hr ) , high - frequency ( hf ) oscillation 
 power , low - frequency ( lf ) oscillation power , standard deviation of all normal r - r intervals 
 ( sdnn ) , total power ( tp ) , and the hrv index . the wilcoxon test was conducted on the obtained 
 data with spss 18.0 , and the significance level was set at =0.05 . after the nmes stimulation , the heart rate , lf , hf , sdnn , tp , and hrv index all slightly 
 increased , but the increases were not significant ( table 1table 1 . comparison of hrv between beforeand after the 
 interventionbefore nmesafter nmeshr ( bpm)73.32.373.62.6lf ( ms)6.40.16.80.2hf ( ms)5.80.260.2sdnn ( ms)47.24.155.15tp ( ms)7.30.17.70.1hrv index130.814.31.1all data represents the meanse ) . pulse oximeters process biological signals in a safe and precise way and monitor them on a 
 real - time basis9 . this study measured the hrv using a pulse oximeter , and consistent 
 data with a small standard error were obtained . strengthening exercise may affect the activities of the ans by increasing sympathetic and 
 decreasing parasympathetic activity2 . this 
 study attempted to apply nmes training similar to strengthening exercise to the subjects . we 
 used a 30  hz biphasic pulse , which may trigger tetanic contractions , with an on - time of 10 
 sec and an off - time of 10 sec . the pulse duration , which 
 greatly affects the formation of quadriceps strength , was set at 300 sec11 , 12 . the 
 intensity of the stimulation was adjusted continuously to maintain full extension during the 
 on - time between a minimum of 25 ma and a maximum of 35 ma . the electrical stimulation slightly increased the lf oscillations and the hf oscillations , 
 reflecting the activity of the sympathetic and the parasympathetic nerves , respectively . thus , the muscle strengthening exercise using nmes did not affect the sympathetic 
 or parasympathetic nervous systems . after the application of nmes , the sdnn , reflecting the 
 ability to resist stress , and the tp , reflecting the ability to adjust the ans , slightly 
 increased . however , these changes were not significant . nmes did not influence the stress 
 resistance ability or the ability to adjust the ans . the above 
 results show that nmes can potentially be used as a safe muscle strengthening tool . however , electrical stimulation may modulate the activity of the sympathetic and the 
 parasympathetic nerves . stein et al.8 
 asserted that 10  hz of electrical stimulation to the paravertebral ganglionar region may 
 decrease the lf and increase the hf and that 100  hz of electrical stimulation may increase 
 the lf and decrease the hf . the present study used 60  hz , which is a relatively high 
 frequency , but the results were not consistent with those of stein et al8 . in several studies , changes in sympathetic 
 tone have been observed as a result of spinal cord electrical stmulation13,14,15 . hsin et al.16 reported that electrical stimulation at the nuchal region 
 may increase sympathetic tone and decrease parasympathetic activity . because the 
 paravertebral regions are important sites of an anatomical autonomic organization , 
 electrical stimulation can easily influence the autonomic nervous system17 , 18 . wong et al.19 applied electrical 
 stimulation to acupuncture points of the hand and forearm and observed increased activity of 
 the sympathetic nervous system . the sympathetic activity slightly increased in the present 
 study , but the increase was not significant . although the physiological mechanism has not 
 yet been discovered , acupuncture points are known to affect the autonomic nervous system . syuu et al.20 reported that electrical 
 stimulation of the neiguan point of the forearm flap may influence cardiovascular function . the discord between the results of this study and the other studies is likely due to the 
 differences in the areas where the electrical stimulation was applied . in the previous 
 studies , the electrodes were attached to the paravertebral ganglionar region or the 
 acupuncture point , whereas they were attached to the motor points in the present study . the 
 quadriceps muscle strengthening exercise using nmes did not change the balance of the ans . this finding suggests that nmes may be safely applied to those who have diseases that affect 
 the ans . future research to examine long - term nmes in a large number of subjects with 
 diverse diseases who are undergoing long - term nmes is considered necessary .
[ purpose ] the purpose of this study was to examine the effect of neuromuscular 
 electrical stimulation ( nmes ) on heart rate variability ( hrv ) . 
 [ subjects and methods ] ten 
 healthy subjects participated in the study . 
 all subjects received nmes with a pulse 
 duration of 300 us and frequency of 30  hz at the vastus lateralis and vastus medialis for 
 15 minutes . 
 the stimulation intensity was adjusted in the range of 20 to 30 ma . 
 hrv using 
 a pulse oximeter was measured in the sitting position before and after nmes . 
 [ results ] 
 after the nmes , all hrv data slightly increased , but there was no significance between 
 before and after data . 
 [ conclusion ] we suggest that strengthening exercises using nmes may 
 be undertaken safely .