human ability to independently move the upper extremity is the result of the combined 
 movement of many musculoskeletal structures and of the increased freedom of movement allowed 
 by the necessary decreased stability1 , 2 .
the shoulder girdle is the main link of the 
 upper extremity to the trunk , and it plays an important role in the movement quality , while 
 providing anatomical and functional cooperation of the shoulder and scapula - thoracic joints 
 as necessary .
scapular upward rotation is an essential component of any lifting of the upper extremity , 
 in combination with humeral elevation at the glenohumeral joint .
these involve 
 force couples formed by the upper ( ut ) and lower ( lt ) trapezius with the serratus anterior 
 ( sa ) muscles .
the result of these combined contractions is upward scapular rotation without 
 any displacement of the scapula in the superior or inferior directions due to cooperation of 
 the ut and lt muscles . at the same time , combined action of the whole trapezius muscle and 
 the sa prevents any displacement of the scapula in the side - to - side direction3 , 4 . as 
 a result , a variety of dysfunctions with pathological or clinical symptoms might develop and 
 need to be assessed and addressed by therapists1 , 
 5 .
consequently , rehabilitation 
 protocols aim for the activation or improvement of muscular cooperation , balance , 
 proprioception , power and strength of the main muscle stabilizers of the scapula3 , 5 .
exercises contributing to the achievement of therapeutic targets are closed kinetic , and 
 they force the whole upper extremity to function as a unit and in a more harmonic manner 
 during daily life6 .
these exercises are 
 recognized as therapeutic tools and have great variety and many levels of difficulty during 
 their performance6 , 7 .
one of the exercises that is used by therapists , and thus , is an 
 object of study , is the push - up plus ( pup ) . in the pup exercise ,
the rest of the body forms a straight line from 
 the head to the ankles and with the torso lying near the floor but without touching it . from 
 this starting position , where the elbows are almost in the fully flexed position
, the 
 subject raises the body upwards as far as possible by extending the elbows and protracting 
 the scapulae .
thus , pup includes two phases : ( 1 ) the push - up ( pu ) phase that involves the 
 first part of the pup which ends at the point of completion of elbow extension without 
 scapular protraction , and ( 2 ) the plus ( p ) phase that is the remaining part of the pup which 
 starts with completion of elbow movement and continues until the end of the scapular 
 protraction .
pup is performed in a closed kinetic chain presupposing muscle cooperation without 
 requiring special equipment .
it elicits high activation of the trapezius and serratus 
 anterior muscles which varies even with small performance alterations7 .
characteristically , during the pup exercise , muscle 
 activation and force response increase significantly with increasing levels of exercise 
 difficulty within safe limits3 , 7 .
different studies have examined the effect of single factors , such as the support 
 surface7 , 9 or the hand orientation on muscular activity during the pup10,11,12,13 . 

due to the complexity of the exercise performance , it is a closed - kinetic activity of the 
 whole body , the effect of different parameters acting simultaneously on the muscle activity 
 must be investigated .
consequently , this study examined the effect of stable and unstable 
 support surfaces , using three different hand positions , on the activities of the trapezius 
 and serratus anterior muscles during the different phases of the push - up plus exercise ( pu 
 and p ) .
thirteen healthy male right - handed subjects who led a sedentary life ( aged 20.5  
 1.0  years , body weight 79.2  12.3 kgr and height 178.8  7.2  cm ) participated in this 
 study .
the tested population was recruited from the department of physiotherapy through 
 requests for volunteers and represented a population of young students .
none of the subjects 
 were professional athletes or were exercising more than five hours per week .
the selection 
 criteria for the sample were to be healthy without a history of neck , back or shoulder pain , 
 injury or operation in these areas ( determined by a self - completed questionnaire ) .
the study 
 was conducted in accordance with the principles of the declaration of helsinki and approved 
 by the department of physiotherapy human ethics committee ( approval number 124/2013 ) . 

confidentiality and anonymity of the participants were maintained at all times , and written 
 informed consent was obtained from all the participants prior to their participation .
the experimental set - up was developed at the human performance & rehabilitation 
 laboratory of the physiotherapy department where the whole procedure was carried out .
data 
 were collected from the dominant arm and all measurements were recorded in a single session . 

initially , the subjects were familiarized with the experimental set - up and the protocol 
 while all exercises were completed in a slow , controlled manner with the aid of a metronome 
 set at a tempo of 60 beats / min .
activities of the ut , lt and sa muscles were recorded using surface electromyography . a 
 laboratory system ,
the datalink ( type nosdlk900 with software version 7.5 , biometrics ltd , 
 uk ) , was used .
three active emg sensors sx230 and a twin axis goniometer ( biometrics ltd ) 
 were employed for the measurements .
emg electrodes were placed on the appropriate locations , 
 using double - sided die - cut tapes provided by the manufacturer , in order to record the 
 activities of the ut , lt and sa muscles according to the recommendations of seniam14 .
a goniometer was placed on the elbow 
 joint following the manufacturer s guidelines to record the motion of the joint at the same 
 time as the emg recording .
a custom - made stop switch was used to determine the end of the 
 exercise .
this was triggered when the maximum distance from the floor was reached by the 
 posterior part of the subject s thoracic spine .
all emg signals were amplified and sampled 
 at 1,000  hz . before testing , anthropometric characteristics were measured and a standardized warm - up 
 procedure13 was used including movement 
 of both arms , such as circumduction and oscillation in the sagittal plane , as well as 
 stretching exercises of the rotator muscles .
subjects then adopted the push - up position by 
 supporting their weight on both hands and feet .
the distance between both hands was equal to 
 120% of the distance between both the acromions , and the hands were placed at the level of 
 the shoulder joints .
the distance between the hands and feet was equal to that measured in 
 the upright position with the shoulders flexed at 90. the measured task was performed on a surface that was parallel to the floor .
the hands and 
 feet were always in contact with the support surface while the body formed a straight line 
 from the head to the ankles .
the subjects performed the pup exercise by placing their hands 
 on a stable wooden surface or on an unstable one that was constructed by adding a wobble 
 board ( rocking side - to - side at a maximum of 16 ) under the stable wooden surface .
they had 
 to balance on those surfaces and not to rock side - to - side during the measured exercise .
the 
 feet of the participants were always placed on a stable wooden surface adjusted to the level 
 of the surface used for the hand placement .
subjects were also asked to perform the exercise with the upper limbs in three different 
 orientations , as follows :  position 1 ( p1 ) : the index fingers were placed parallel to each other on the support 
 surface and each arm was allowed to adopt a self - selected position ;  position 2 ( p2 ) : the index fingers were placed parallel to each other on the support 
 surface while each arm was kept in contact with the trunk of the subject ; and  position 3 ( p3 ) : the index fingers were placed at 45 of internal rotation compared to p1 
 and each arm was kept at 90 of abduction at the shoulder joint .
participants were asked to perform 3 successful repetitions of the tested trials while a 
 rest period of 1  min was allowed between repetitions to avoid fatigue .
each volunteer performed 18 successful 
 repetitions of pup exercises , that is : [ 3 trials ]  [ 2 surfaces ( stable vs unstable ) ]  [ 3 
 hand positions ( p1 , p2 & p3 ) ] .
finally , three successful trials of the maximum voluntary 
 isometric contraction ( mvic ) of each measured muscle were recorded following a muscle 
 testing procedure suggested by previous research15 . during data collection ,
the raw emg recordings were the monitored , full - wave rectified and 
 processed using a root mean square ( rms ) algorithm with a 3-ms moving window .
measurements 
 were expressed in millivolts and the mean value of the three trials for each muscle was used 
 for the estimation of the activation of each tested muscle .
finally , the emg activity values 
 of each measured muscle during the pup exercise were expressed as a percentage of the emg 
 values produced by the muscle during the muscle testing performance , % mvic .
the signal of the goniometer was used to monitor the start and the end of the elbow 
 movement and at the same time to mark the start and the end of the push - up phase of the pup 
 exercise .
the signal of the back - stop was used to monitor the end of the pup exercise . 

consequently , those two signals were the discriminating landmarks of the two phases of the 
 measured pup exercise , that is the pu and the p phase .
the intraclass correlation coefficient ( icc ) was used to determine the same day test - retest 
 reliability of the three emg recordings . in order to determine if there were significant 
 differences in the emg activities of ut , lt , sa , as well as in the ut / lt activity rations 
 among the different phases of the exercise , hand positioning and support surfaces , analysis 
 of variance ( anova ) was performed . a least squares difference ( lsd )
pairwise multiple 
 comparison analysis was performed to determine the significance of the differences between 
 pairs of means .
statistical analysis was performed using spss 20.00 software and 
 significance was accepted in all statistical tests for values of p<0.05 .
the mean value (  sd ) of emg activities the three trials of the muscle testing , with the 
 icc values between these recordings for each tested muscle , were as follows : ut , 1.78  
 0.21  mv ( icc=0.800.87 ) ; lt , 1.75  0.36  mv ( icc=0.960.97 ) and ut , 1.38  0.5  mv 
 ( icc=0.980.98 ) .
the overall activation of the ut showed statistically significant difference with regard to 
 the phase of the exercise ( p<0.05 and f=8.3 ) and the position of the hands ( p<0.05 and 
 f=4.7 ) , but it was not affected by the stability of the support surface ( p>0.05 and 
 f=1.2 ) .
more specifically , the differences between the pu phase ( 30.2  13.6%mvic ) and p 
 phase ( 24.1  8.5%mvic ) , as well as between the hand positions of p1 ( 25.1  8.8%mvic ) and 
 p2 ( 29.5  12.6%mvic ) were statistically significant .
similar to the ut , the overall 
 activation of lt was also significantly altered by the phase of the exercise ( p<0.05 and 
 f=7.1 ) and the position of the hands ( p<0.05 and f=5.2 ) .
the p phase ( 4.1  3.8%mvic ) was 
 significantly different from the pu phase ( 9.8  11.1%mvic ) , while hand position p1 ( 6.6  
 % mvic ) was significantly different from hand position p2 ( 8.7  % mvic ) .
the overall activation of the sa showed statistically significant differences with regard 
 to all tested parameters , such as exercise phase ( p<0.001 and f=72.9 ) , position of the 
 hands ( p<0.05 and f=3.9 ) , and the stable or unstable support surface ( p>0.05 and 
 f=6.4 ) .
more specifically , the differences between the pu phase ( 49.2  17.9%mvic ) and p 
 phase ( 75.9  16.1%mvic ) as well as between the hand positions of p2 ( 59.6  17.6%mvic ) and 
 p3 ( 66.3  22.3%mvic ) , and finally between the stable ( 65.9  21.8%mvic ) and unstable ( 59.9 
  17.3%mvic ) support surfaces , were statistically significant . finally , the ut / lt activity ratio was significantly affected by the phase of the exercise 
 ( p<0.05 and f=5.7 ) , and the p phase ( 8.1  4.3 ) was significantly different from the pu 
 phase ( 6.0  4.5 ) .
table 1 
 table 1.the emg activities ( mean values  sd ) of the measured muscles ( ut , lt and sa ) as 
 a percent of maximum voluntary isometric contraction ( % mvic ) and the ut / lt ratio 
 during the phases of the push - up plus exercise ( push - up and plus ) on stable and 
 unstable supporting surfaces with three different hand positions ( p1 , p2 and 
 p3 ) shows the emg activities ( mean  sd ) of the measured muscles ( ut , lt & sa ) 
 under the tested experimental conditions with the statistical significance of the 
 differences .
the pup exercise is an important tool for therapists , aiming to deal with problems of the 
 upper extremity and specifically to improve the stability of the scapula - thoracic joint by 
 increasing muscular cooperation1 , 13 .
such a demanding task requires 
 well - documented supervision and guidance during its performance resulting from the extensive 
 study of the effect of different parameters on muscle activation8 .
so , the study of the muscle performance , during pup and the 
 simultaneous effect of different conditions to overcome them can be crucial . in the present 

study an effort was made to identify the effects elicited by the support surface , hand 
 positions , and phase of the pup exercise .
different phases of the exercise require different muscle activation with a variety of 
 activation levels , due to different movements taking place and the different ranges of 
 movement used8 .
the pu phase mainly 
 involves elevation of the upper arm with associated movement of the scapula resulting from 
 the activation of the ut and lt .
in contrast , the p phase only involves movement of the 
 scapula leading to activation mainly of the sa . at the same time , cooperation of ut and lt 

this muscle cooperation can be explained by the 
 present findings which are in agreement with the results of similar studies4 , 6 . 

statistically significant differences were recorded for ut , lt and the ut / lt ratio during 
 the pu phase and for sa during the p phase .
the stability of the supporting surface alters the stability of the human body and it is an 
 important factor demanding different patterns of muscular cooperation5 , 6 .
consequently , 
 stable and unstable supports were used in an attempt to identify their effects on muscular 
 activity and the resulting movement patterns .
when subjects tried to avoid rocking 
 side - to - side on the unstable surface , they introduced abduction - adduction to the shoulder 
 joints leading to abduction - adduction of the scapula , altering the muscle involvement .
the 
 present results suggest that an unstable support surface only has a significant effect on 
 the activity of sa , with the stable surface increasing sa muscular activity .
these findings 
 can be explained by the fact that abduction - adduction of the scapula is involved ; therefor 
 the sa must be activated .
why is the sa activity on an unstable supporting surface less than 
 that on a stable one ?
one possible explanation is that there is a decrease in the pu phase 
 of the task resulting from a fear of falling or the mobilization of other muscles such as 
 the horizontal adductors of the shoulder .
pup is a demanding task that requires activation 
 of the whole body and is difficult to supervise and consequently perform properly .
a 
 possible solution to this problem might be the use of optoelectronic techniques to record 
 movement in 3d .
pup on unstable 
 surfaces has been found to increase sa muscle activity4 , 7 , 9 , or sa in combination with ut , and to increase the activity of the 
 trapezius while decrease the activity of sa16 .
considering that the unstable surface had a low level of 
 instability ( rocking of 16 ) , and only in one direction ( side - to - side ) , the effect of 
 instability on muscular activity needs to be investigated further .
the hand position in pup changes the degree of arm elevation in the frontal plane , altering 
 the range of arm movement as well as the movement itsself10,11,12 .
flexion , abduction or horizontal abduction is required at the 
 shoulder joint , leading respectively , to upward rotation with retraction , or adduction of 
 the scapula .
ut and lt demonstrated the same activity patterns in response to hand 
 positioning , and axially showed statistically significant increased involvement in hand 
 position p2 . as this position elicits shoulder flexion during the pup , and consequently 
 upward rotation and protraction of the scapular , increased movement and stability of the 
 scapula was needed in the frontal plane . in contrast , sa showed significant increase in 
 activity in hand position p3 , that is when scapular adduction occurred , replacing scapular 
 movement in the frontal plane . nevertheless , scapular stability in this plane was nt 
 necessary .
the present findings suggest that different phases of the pup exercise require different 
 muscle activities and co - operation .
similarly , the type of supporting surface and the 
 positioning of the hands affect muscle activities in a manner that is dependent on the phase 
 of the exercise making its prescription a highly demanding therapeutic task .
the data was obtained from a small group of young , 
 healthy male adults and only their dominant right hand was measured .
therefore , the 
 extrapolation of the findings to patients or to the general population should be performed 
 with caution .
 [ purpose ] the appropriate exercise prescription is crucial for achieving scapular 
 stability and providing successful rehabilitation , and the push - up plus ( pup ) exercise has 
 an important role in shoulder rehabilitation . 
 consequently , this study examined the effect 
 of support surface stability , hand positioning , and phase of exercise , on the trapezius 
 and serratus anterior muscle contractions as well as on the emg ratio of the upper / lower 
 trapezius . 
 [ subjects and methods ] thirteen healthy male volunteers participated in this 
 study . 
 the subjects performed the pup exercise on stable and unstable supporting surfaces 
 with three different hand orientations . during the pup exercise , 
 the muscle activities of 
 the upper ( ut ) and lower ( lt ) trapezius , as well as the serratus anterior ( sa ) were 
 measured and expressed as percentages of maximum voluntary isometric contraction ( % mvic ) . 
 [ results ] 
 the emg activities of ut and lt were statistically greater during the push - up 
 phase compared to the plus phase of the exercise . 
 the contrary was recorded for the 
 activity of the sa . 
 sa was affected by the support surface as well as by the hand 
 positioning . 
 [ conclusion ] the results suggest that different phases of the pup exercise 
 require different muscle stability actions with corresponding activations of appropriate 
 muscle fibers . 
 a detailed prescription of the required phase of the exercise can more 
 effectively activate the scapula - thoracic musculature . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION