electromyography ( emg ) is a recognized recording tool that is commonly used for measuring 
 the electrical activity of a contracting muscle1 . a number of research studies have investigated the relationships 
 between emg and other parameters . before conducting this study , we conducted a review of the 
 literature concerning relationships regarding emg activities of the upper limb muscles . commonly assessed relationships are those of : emg - force , emg - torque , emg - time , emg - angle and 
 emg - other parameters . for example , munteanu et al . studied the relationship between emg and 
 muscle temperature during dynamic contraction of the forearm muscles2 . these researchers found that the disruption of the physiological 
 signal ( emg ) caused by the innervation zone alters the reliability of the force - emg 
 relationship on a single bipolar channel level3 . investigated how altering the wrist posture 
 influences the relationship between the time and frequency measures of the surface emg 
 signal of the wrist muscles . their results suggested that changes in the relationship 
 between the time and frequency measures should be considered in studies using emg with 
 different wrist postures4 . a previous study of the biceps brachii ( bb ) tested the relationship between the emg median 
 frequency and the low - frequency - band amplitude of the surface emg profile as a 
 representation of muscle fatigue . the results showed that there is a relationship between 
 the median frequency shift and the amplitude of the 15- to 45-hz bandwidth and the high - low 
 frequency amplitude ratio5 . similarly , 
 doheny et al . examined the effect of the joint angle on the relationship between the force 
 and emg amplitude and the median frequency in the bb , brachioradialis and triceps brachii 
 muscles6 . some other emg studies have 
 also examined force , exercise and movement . according to the definition by previous 
 gerontological studies , the age range of adolescence is 13 to 19 . in the literature we 
 reviewed , most of the previous studies had investigated the emg signal of subjects older 
 than 20  years of age . we could not find any report of the emg - time relationship during mvc 
 of adolescent s muscle . the major goal of this study was to fill this gap by analyzing this 
 relationship in order to characterize the bb muscle activity of adolescents . in other words , 
 to investigate the emg - time relationship in order to evaluate the endurance time of an 
 adolescent s bb using emg under three contraction conditions . a male adolescent ( age=17  years , weight=60  kg , height=171  cm ) participated in this study 
 and provided written informed consent prior to the experiment . all of the experimental 
 procedures conformed to the principles of the declaration of helsinki and were approved by 
 the local human research ethics committee of the university . before the test , the subject 
 was told to sit in a chair and relax as much as possible . dynamic ( concentric ( up ) and 
 eccentric ( down ) ) contractions were then induced by lifting and lowering a weight . during 
 the dynamic contractions , the subject was instructed to move his forearm between elbow 
 angles of 0 and 90. in contrast , during static ( isometric ) contractions , the subject was 
 instructed to hold the same load with an elbow angle fixed at 90. three trials of each type 
 of contraction were performed for 10  s with a rest period of 5  min provided between each 
 trial . a wireless sensor was used to record the emg signal at the belly of the bb muscle using two 
 foam adhesive electrodes ( ag / agcl ) . the inter - electrode distance , electrode placement 
 procedure and skin preparation followed the descriptions of seniam7 . the raw signals were recorded at a sampling rate of 1 khz 
 before analog to digital conversion . fourth - order bandpass butterworth filter was used to 
 remove skin movement artefacts and high - frequency noises ( cutoff frequency between 10 and 
 500  hz ) . the digitized emg datasets were processed offline ( filtering , windowing , and signal 
 extraction ) using matlab software . the emg signals were divided into four parts and analyzed 
 as 2,500-ms time windows . emg amplitude data were normalized to the root mean square ( rms ) 
 values : i.e. , the individual rms values during the contraction were considered 100% mvc . the 
 filtered emg activity was normalized by dividing the observed emg value by the maximum value 
 recorded during the three mvc trials . the mean ( rms ) normalized emg activity was then 
 calculated as the mean of the sum of the normalized emg percentages during each contraction . the maximum 
 peak - to - peak value of the emg was considered a relative measure of the motor activity . significant 
 differences in the time lag and the emg amplitudes ( rms ) between the three contractions were 
 detected using repeated - measures analysis of variance and post - hoc tests were applied to 
 test the significance of differences of the variables at a significance level of =0.05 and 
 95% ( p<0.05 ) confidence intervals . linear regression ( r ) analysis was used to 
 analyze the relationship among the variables . the results show that the regression slope of emg versus time was very close to 1.00 during 
 concentric contraction : r = 0.93 , f - value = 456.7 . in contrast , a significant 
 decrease in the emg signal was observed during eccentric contraction in terms of the time 
 lag : r = 0.69 , f - value=77.5 . the slope of the regression line between emg and 
 time decreased during isometric contraction , and the predictability under this condition was 
 poor : r=0.24 , f - value=10.46 . significant differences ( p<0.05 ) existed between 
 the emg amplitude and the time lag during concentric , eccentric and isometric 
 contractions . it is important to analyze the firing rates of the bb muscle during voluntary contractions 
 at different time intervals8 . in this 
 study , three types of contraction were selected , and the duration was sliced into time 
 windows to determine the relationships of emg with contractions . the time series of the 
 measurable tension after the onset of electrical activity gradually increased during 
 concentric and eccentric contractions , and this increase is collated with emg as a function 
 of the endurance time . the findings of our present study agree with the results reported by 
 komi et al9 . prior to this study , no 
 research study had investigated the emg - time relationship of an adolescent s bb muscle 
 during contractions . the results of this study will be value to researchers who are interested in emg analysis 
 over time during contractions . in addition , our results could be used in various practical 
 applications associated with the non - invasive evaluation of the bb muscle . this study 
 characterized the emg responses , which may aid further research and demonstrate the clinical 
 importance of predicting the emg - time relationship . our most significant contribution to 
 research is that our findings note the differences between the three types of contraction in 
 terms of emg - time relationships of the muscle during the early stage of the contraction . like , only a single muscle was selected , and the emg data 
 were recorded from a single subject . future research should focus on defining the 
 relationship between the emg - moment , emg - force and other parameters of the bb muscle of 
 individuals in different age groups .
[ purpose ] this study investigated the changes in the slope of emg - time curves 
 ( relationship ) at the maximal and different levels of dynamic ( eccentric and concentric ) 
 and static ( isometric ) contractions . [ subjects and methods ] the subject was a 17  year - old 
 male adolescent . 
 the surface emg signal of the dominant arm s biceps brachii ( bb ) was 
 recorded through electrodes placed on the muscle belly . 
 [ results ] the results obtained 
 during the contractions show that the regression slope was very close to 1.00 during 
 concentric contraction , whereas those of eccentric and isometric contractions were lower . 
 significant differences were found for the emg amplitude and time lags among the 
 contractions . 
 [ conclusion ] the results show that the emg signal of the bb varies among the 
 three modes of contraction and the relationship of the emg amplitude with a time lag gives 
 the best fit during concentric contraction .