among runners , achilles tendonitis is a significant disability that often causes loss of an 
 entire season of competition1 . the 
 occurrence of this disease is related to the individual s activity level2 , and it has been reported to account for 
 79% of sports injuries and 618% of running injuries that elite athletes 
 endure3 . achilles tendinopathy occurs in 
 almost every type of sport , and symptoms develop due to overuse that occurs during 
 repetitive training4 . in a clinical setting , eccentric exercise is used as a part of a therapeutic exercise 
 program for pain relief ; however , the exact pathophysiology of achilles tendinopathy is 
 unknown . recent studies have tried to determine the mechanisms underlying the beneficial 
 effects of eccentric exercise . for instance , sole , milosavljevic , nicholson , and sullivan 
 ( 2011 ) concluded that during eccentric exercise , a difference in electromyography ( emg ) 
 activity is rarely observed between patients with tendinopathy and normal persons when the 
 length of the tendon is short , but the difference becomes appreciable as the length of the 
 tendon increases5 . in another study , 
 henriksen , aaboe , graven - nielsen , bliddal , and langberg ( 2011 ) ascertained that achilles 
 tendon pain reduced emg activity in agonistic , synergistic , and antagonistic muscles6 . however , in that study , pain was induced in 
 normal subjects using intratendinous injections of hypertonic saline . comparisons of changes in lower limb muscle activity 
 during eccentric and concentric exercises between the non - affected side ( nas ) and affected 
 side ( as ) are essential to determining the mechanisms responsible for the beneficial effects 
 of eccentric exercise . this study aims to identify changes in muscle activation by comparing muscle activities of 
 the as and nas during eccentric and concentric exercise in runners with unilateral achilles 
 tendinopathy in order to determine methods for preventing running injury and the 
 pathological characteristics of achilles tendinopathy . the project was approved by the 
 university of sahmyook research ethics review committee ( syuirb2010 - 007 , 27 february 2010 ) , 
 and the study protocol was conducted in strict accordance with the declaration of helsinki . 
 written informed consent this study included 18 participants 
 which consisting of men and women chronic achilles tendinopathy in a single leg and more 
 than 1  year of running experience . subjects were recruited from the k rehabilitation 
 hospital research center located in seoul , south korea . the selection criteria for subjects 
 included a diagnosis of achilles tendinopathy based on structural abnormalities found on 
 only one side by ultrasonography , a diagnosis of achilles tendinopathy for at least 6 
 months , availability for outpatient follow - up , and unaided movement . the mean age , height , and weight were 26.224.14  years old , 
 166.229.33  cm , 63.0612.08  kg , respectively . all subjects had an electrode attached to both 
 the as and nas , and then maximal voluntary contraction ( mvc ) readings were obtained using 
 isometric contraction according to the methods of manual muscle testing . after 1 minute of 
 rest , the subjects performed concentric exercise with the achilles tendon moving from full 
 plantar flexion to full dorsiflexion for 8 seconds . this exercise was repeated 5 times with 
 bare feet in order to acquire emg signals , as suggested for a pre - test7 . after another 1 minute of rest , the researcher collected 
 data by administering eccentric exercise with the achilles tendon moving from full plantar 
 flexion to full dorsiflexion . while subjects were performing concentric and eccentric 
 exercises , the floor behind the heel was removed so that the calcaneus did not touch the 
 ground . for assessment of emg data , we used a trigno wireless system(delsys , 
 boston , ma , usa ) to determine muscle activation during eccentric and concentric exercise . markers were attached to the rectus femoris , tibialis anterior , peroneus longus , medial 
 gastrocnemius , and lateral gastrocnemius , as suggested for a pre - test7 . mvc and peak emg signal data were collected using 
 myoresearch xp me 1.06 , since the emg data measured in the experiment were raw data that 
 were directly obtained from activating muscles . we used a 10380  hz band - passfilter to 
 filter out noise in the raw data and then conducted full - wave rectification to process 
 negative figures into positive ones . in this manner , peak emg signals were collected when 
 each mvc was being performed during eccentric and concentric exercise , and they were later 
 divided by the mvc and multiplied by 100 to determine the % mvc of each muscle . this study identified 
 general characteristics of runners with achilles tendinopathy and calculated means and 
 standard deviations using descriptive statistics to compare the maximum muscle activations 
 of the as and nas during eccentric and concentric exercises . we also used 2-way repeated 
 anovas ( 2  2 ) to compare differences between the as and nas and during ipsilateral 
 eccentric and concentric exercises . a significance level (  ) of 0.05 or less table 1table 1.difference in % mvc during eccentric and concentric exercisesideeccentric exerciseconcentric exerciserectus femoris ( % ) as35.79.361.113.8*nas24.59.232.914.1*tibialis anterior ( % ) as12.26.221.812.0*nas2.81.72.81.8*peroneus longus ( % ) as30.317.648.727.4*nas27.113.730.917.5*lateral gastrocnemius ( % ) as25.420.737.336.1*nas42.011.776.021.4*medial gastrocnemius ( % ) as44.934.413.59.5*nas20.39.524.818.6*values are meanssd . * significant difference between exercise methods ( p<0.05 ) . significant difference between injury sides ( p<0.05 ) . shows the difference between maximum muscle activations of the as and nas 
 during eccentric and concentric exercises performed by runners with achilles tendinopathy . all the muscles examined showed significant increases ( p < 0.05 ) in muscle activation 
 with concentric exercise compared with eccentric exercise . compared with the nas , the as 
 showed a more significant increase ( p < 0.05 ) in activation of the rectus femoris , 
 tibialis anterior , and lateral gastrocnemius . interaction effects between exercise methods 
 and injury side all showed statistically significant changes ( p < 0.05 ) . concentric exercise shows greater emg activation than eccentric exercise for normal muscles 
 because the body rests during the period of cross - bridge formation in eccentric exercise , 
 whereas this is not the case during cross - bridge formation in concentric exercise8 . in addition , concentric exercise recruits 
 more motor units during performance of the same task and leads to 3 times greater energy 
 expenditure than that during eccentric exercise9 . therefore , the present study revealed that concentric exercise 
 induces higher maximum muscle activation in every muscle except the medial gastrocnemius of 
 the as ; specifically , relatively high levels of statistical significance were found for the 
 rectus femoris , tibialis anterior , peroneus longus , and lateral gastrocnemius , and previous 
 studies comparing concentric and eccentric exercises have also reported that peak emg 
 signals increased significantly in all muscles of the lower limbs10 . another study also reported that eccentric exercise of 
 the lower limbs is more effective than concentric exercise in inducing a reduction in emg 
 signals from the knee extensors11 . the results of present study revealed that compared with concentric exercise , eccentric 
 exercise induced a higher % mvc in the medial gastrocnemius of the as . the peak muscle 
 activity of the medial and lateral gastrocnemius changes depending on the mechanical frame 
 of the lower limb . when the lower limb enters the phase of external rotation , the % mvc of 
 the medial gastrocnemius increases as eccentric exercise is performed12 . as the exercise is performed in the upright position , 
 when the lower limb enters the phase of external rotation , the pronation of the ankle 
 increases to compensate for body balance . achilles tendinopathy occurs when pronation of the 
 ankle increases exceptionally so that lateral deviation of the subtalar joint occurs13 . the transformation in lower limb 
 alignment because of this causality reduces the use of the lateral gastrocnemius and 
 increases the % mvc of the medial gastrocnemius . the external rotation of the lower limb 
 during the course of concentric exercise did not show any statistically significant 
 differences between the medial and lateral gastrocnemius , but the % mvc of the medial 
 gastrocnemius showed a significant increase when eccentric exercise was performed12 . in addition , previous studies have 
 reported that separate analyses of medial and lateral gastrocnemius emg findings reveal that 
 when the dorsiflexion range of motion is narrower , the peak emg signal is greater in the 
 medial gastrocnemius14 . this is in 
 accordance with our results , taking into consideration the fact that the ankle range of 
 motion is reduced in patients with achilles tendinopathy compared with normal people . the results of this study revealed that during both eccentric and concentric exercises , the 
 nas has greater % mvcs in the rectus femoris , tibialis anterior , and peroneus longus . however , the % mvc of the gastrocnemius muscle , except the medial gastrocnemius , on the as 
 was higher than that of the nas . a previous study compared emg findings between a patient 
 group consisting of subjects with pain in the achilles tendon and a control group ; this 
 study concluded that the peak achilles tendon strength was weaker in the patient group15 . another study stated that emg signals of 
 the gastrocnemius were significantly decreased in patients with achilles tendinopathy 
 compared with controls16 . as we discussed , when runners with achilles tendinopathy perform eccentric and concentric 
 exercises , differences are observed in lower limb muscle activity ; importantly , runners with 
 achilles tendinopathy experience an increase in medial gastrocnemius activity when 
 performing eccentric exercise . from the clinical viewpoint , extensive effort is required to 
 determine the various muscle changes induced by diseases in order to prevent secondary 
 achilles tendinopathy .
[ purpose ] this study aimed to identify changes in muscle activation by comparing muscle 
 activities of the affected side ( as ) and non - affected side ( nas ) during eccentric and 
 concentric exercises in runners with unilateral achilles tendinopathy . 
 [ subjects ] the 
 study included 18 participants consisting of men and women with chronic achilles 
 tendinopathy in a single leg who had more than 1  year of running experience . [ methods ] 
 all 
 subjects performed concentric and eccentric exercise with the achilles tendon moving from 
 full plantar flexion to full dorsiflexion for 8 seconds , and electromyography data was 
 obtained . 
 [ results ] all muscles examined showed a significant increase in % maximal 
 voluntary contraction ( mvc ) with concentric exercise compared with eccentric exercise . 
 compared with the nas , the as showed significant increases in % mvc of the rectus femoris , 
 tibialis anterior , and lateral gastrocnemius . 
 all interaction effects of exercise methods 
 and injuries showed statistically significant changes . 
 [ conclusion ] runners with achilles 
 tendinopathy show increases in medial gastrocnemius activity when performing eccentric 
 exercise .