document: over the last 25 years , heart rate monitors have been widely used in sports and sports science . since the development of handy heart rate monitors ( hrms ) with the size of a watch and appropriate flexible chest belts with electrodes to transmit signals wirelessly to the watch , measurement of heart rate ( hr ) 
   has become an easily accessible and valuable tool for training and coaching purposes and conducting laboratory or field studies . 
   validity and reliability have been shown to be high for these tools compared to ecg measurements for both hr and heart rate variability ( hrv ) [ 16 ] . owed to its widespread use in physical activity not only for individuals but also for team sports , recent technical development focused on real - time monitoring instead of post - exercise evaluation of recorded data . 
   currently , several different systems are available on the market , of which four were tested in the present study : polar team pro , acentas team monitoring system , suunto pro team pack and activio sport system . 
   all systems allow direct and real - time monitoring of several individuals at the same time . 
   data are transmitted wirelessly to a receiver which is connected with a standard laptop with relevant software . however , a direct comparison of these systems and validity of recorded data are missing in literature 
   . therefore the main purpose of this study was to compare all systems in a field test against each other . 
   operating distance and validity against a commercially available ecg were evaluated as well as the installation and use of software and hardware of the above - mentioned systems . 
   a total of 12 active people ( 11 men and 1 woman ; age : 26.4  3.2  yr ; height : 177.9  8.7  cm ; mass : 78  7  kg ) participated in the study . 
   ten men of an amateur soccer team conducted an online field test during a real football match . 
   one male subject participated for validity of hr monitoring systems against ecg whereas the female subject was only involved in operating distance measurement . because of the low case number of the last two tests , data can only be considered as a pilot study for ecg validation and distance measurement . 
   the following four commercially available systems were used for all tests : polar team pro ( polar electro oy , kempele , finland ) , acentas team monitoring system ( acentas gmbh , hrgertshausen , germany ) , suunto pro team pack ( suunto oy , vantaa , finland ) , and activio sport system ( activio ab , stockholm , sweden ) . a quality rating system similar to rating systems used in trade journals 
   which is specialised on the testing of mobile and electronic communication tools . for this purpose , 
   the authors decide to subdivide the rating system into three main categories : measurement , software , and hardware . at this , the core area of the study focused on the adequate online measurement of the heart rate . 
   therefore , the rating system was set in favour of the measurement which was represented by 200 scores of maximal 500 . 
   the latter 300 were equally divided on quality of the hardware and the usability of the software . 
   subcategories were weighted subjectively by the authors in consideration of heart rate monitoring of team sports including the monitoring of 10 or more subjects in parallel . 
   in addition , the following subcategories were rated once by an independent observer who was not familiar with any of the monitor systems to get a first impression : instruction manual , packing , chest belts ( quality ) , and initial software handling . 
   the remaining categories were judged once by an experienced person . with exception of wearing comfort and failing quota in field , 
   all subcategories were judged and quantified by using a ten - stepped visual analogue scale which is known for pain assessment . 
   visual analogue scaling was weighted by multiplying itself with factor f : ( f = maximal score of subcategory/10 ) . 
   parameters leading the judgement of the subcategories are described in tables 2 and 4 . 
   wearing comfort of the chest belts was evaluated via a short anonymous questionnaire which all 12 participants ( see section 2.1 ) had to complete directly after wearing . 
   the questionnaire contained five questions about the handling and individual fitting of the chest belts . 
   the questions one and two are positively formulated whereas questions three to five are negative ( see table 3 ) . 
   all questions had to be answered by an even scaled rating system including six categories . 
   maximum score for the whole questionnaire was 25 ; hence , each question was scored with maximal 5 points . 
   the main field test was measurement of failure ratio during the half time of a real soccer match . 
   all players apart from goal keeper were equipped with chest belts of the relevant system and measured during first half (= 45  min ) of a league soccer game during four home matches in summer 2009 . 
   temperature range was within 9c , and weather conditions were similar ( no precipitation ) for all measurements . 
   the receiver of the relevant system was always positioned three meters in projection of midline of the soccer ground . in order to improve transmission distance , receiver 
   could not be excluded a priori , a mean failure ratio was determined only with transmitters sending valid data . each deviation / loss of one percent of whole duration measurement of 45  min (= 2700  s ) 
   was subtracted with 10 scores , and a maximal score of 100 could be obtained . 
   if , for example , a total loss time of all ten transmitters of a system was 270  s , a total score of 90 would be credited to this system . 
   a failure ratio of 10 percent of whole duration measurement would be credited with a zero score . 
   distance measurement was conducted on a uphill street with a constant slope and in far distance to high - voltage line or radio tower to reduce possible interference . 
   receiver antenna was placed on highest elevation of the hill one meter above ground , and unit was connected to a laptop ( toshiba satellite series ; toshiba inc , ny , usa ) . 
   the participants wearing the chest belts were equipped with a gps device ( nokia n79 , nokia group , espoo , finland ) to measure distance . by moving backwards 
   additionally , the participants were instructed to move arms to keep hr variable with time . 
   attempts were only started when both valid signals of gps and hr were available and stopped when hr signal was lost . 
   tests were conducted twice , and mean value of both attempts was calculated and taken as maximal range . 
   maximal range distance was credited with 70 scores whereas the system with the absolute maximal covered range was taken as reference system and credited maximum score . 
   percentage deviation of distance of other systems was analogously deducted as percentage of scores . if , for example , reference system had 100  m range , it was credited 70 scores . if a second system had only 50  m range , it would be credited 50% of maximum score , hence 35 . comparison against ecg 
   was conducted in a laboratory by using ecg tool vicardio ecp-6 standard ( energy - lab technologies , hamburg , germany ) . 
   a maximum score of 30 could be obtained at this test . a deviation of 1 beat per minute ( bpm ) 
   was deducted with two scores . only complete accordance of both values was credited with maximum score . 
   a plus ( + ) is a positive feature ; a minus (  ) is a negative one . 
   a plus / minus (  ) indicates aspects which are both positive and negative . in context of the supplied hardware acentas and polar lead the field of monitor systems , both reaching 100 and 106 scores in sum for the hardware . 
   the leading position of polar depends on the storage capacity of the chest belt increasing data safety but also decreasing wearing performance which is indicated by the chest - belt questionnaire presented in table 3 . in case of wearing comfort , it has to be noted that both chest belts of activio and acentas were supplied by the identical manufacture . 
   both polar and acentas delivered high - quality receivers . results of software evaluation are summarized in table 4 . 
   again , similar to hardware evaluation , a plus ( + ) is a positive feature , and a minus (  ) is a negative one . if all systems were similar or identical , there is no separate classification . 
   a plus / minus (  ) indicates aspects which are both positive and negative . here , activio offered the harmonious concept which led to the highest ranking . 
   the installation procedure was easiest with the acentas software , but acentas did not focus on extensive statistical analysis . in this case , 
   polar as well as activia provide a more developed software tool . as data of ecg comparison 
   were only obtained with a single subject , they have to be regarded with caution . 
   suunto 's deviation was 80 to 85  bpm , activio 's deviation was 81 to 85  bpm , and polar 's deviation was 82 to 85  bpm , respectively . 
   same caution has to be taken as only one subject 's data were included in analysis . 
   nevertheless , acentas gave reference with 349  m , followed by activio with 141  m and suunto with 107.5  m. it has to be noted that activio 's system was supposed to reach 300  m. last but not least , polar reached 102  m which corresponded to the manufacturer information for this system . 
   nevertheless , all other systems had a wider range of data acquisition , and hence score distribution was credited as follows : acentas 70 , activio 28 , suunto 21 , and polar 20 . 
   regarding failure quota , average downtime was 148  s ( 0,6% ) for acentas , 286  s ( 1,0% ) for polar , and 552  s ( 2,0% ) for activio . 
   single belts were recognised from time to time , but no consistent data acquisition could be accomplished . 
   similarly , five out of ten polar chest belts had signal loss and could not be reactivated whereas activio managed 10 out of 10 signals , and for the acentas system 9 out of 10 were used for data evaluation as one person was injured during match play . 
   the purpose of the present study was to compare four commercially available online heart rate monitoring systems and especially their use in everyday situation in team sports . 
   the system of polar could score especially with a clear design , useable features of the software , and a convincing product quality . 
   only polar uses wlan for data transmission and also adds a mobile terminal in pocket format which can easily be used outdoor and with bad weather conditions . 
   but the high energy consumption of wlan transmission would terminate the battery capacities and additionally the duration of measurement . 
   the system of activio has a functional and useful software and the instruction manual was very detailed and easily comprehensible . 
   acentas could score with a huge operating distance , a valid heart rate signal compared with ecg , a low failure quota , and an easy handling . 
   in contrast , the system of suunto had a total breakdown during competition measurements and could not be reactivated . 
   prospectively , it would be a great challenge to realise online monitoring for swimmers , possibly leading to new opportunities for the training regime . 
   in this context the system of polar and suunto had the advantage that data are also recorded on the chest belt memory during water immersion and could be evaluated after exercise . 
   meanwhile , some of the tested heart rate monitoring systems are updated and upgraded with new functions , respectively . 
   for example , acentas included an offline memory in the chest belts and added a wrist monitor for direct heart rate analysis from up to twenty athletes by the coach . in summary , 
   acentas system reached the best mark of all systems , but every system has its advantages and disadvantages depending on the using purpose , location , and weather . so this present comparative study can not recommend a single system but rather shows strength and weakness of all systems and additionally can be used for further system improvements . 
   the investigators were responsible for the study design , data collection , data analysis , data interpretation , and submission of the paper for publication , independently of all funding sources . 
   the authors declare that they have no financial conflict of interests to disclose with any of the companies named in this paper .
summary: recent technical development focused on real - time heart rate monitoring instead of postexercise evaluation of recorded data . 
   there are several systems on the market that allow direct and real - time monitoring of several individuals at the same time . 
   the present study compared the systems of polar , acentas , activio , and suunto in a field test with twelve subjects regarding failure quota , operating distance , and ecg validity . moreover , 
   the installation and use of software and hardware were evaluated with a quality rating system . 
   chest belts were evaluated with a questionnaire , too . 
   overall the system of acentas reached the best mark of all systems , but detailed results showed that every system has its advantages and disadvantages depending on using purpose , location , and weather . 
   so this evaluation can not recommend a single system but rather shows strength and weakness of all systems and additionally can be used for further system improvements .