motor skill learning has become an important issue , due to the increasing involvement of 
 human movement behaviors in neuroscience , psychology , and physical education1,2,3,4,5 . however , until now , motor learning has 
 puzzled contemporary science , and its mechanisms and contributing factors remaining unclear . how motor skills are acquired and processed by the neuromuscular system has become a topic 
 of interest . furthermore , application methods for improving motor skills have been 
 spotlighted in order to increase the capability of motor performance2 , 6,7,8 . several factors related to 
 motor learning are already well known , in terms of amount of practice , types of feedback , 
 application period of feedback , practice schedule , and so forth9,10,11,12 . it is well known that the effectiveness of motor learning can vary according to the 
 practice schedule . of these schedule types , distributed and massed practice schedules are 
 common in learning studies . distributed practice utilizes temporal spacing intervals between 
 repetition of tasks usually of the order of hours or during a few days , whereas massed 
 practice consists of fewer and shorter inter - trial intervals during training sessions11 , 13,14,15 . 
 in general , motor skills are more effectively learned when there is a long resting time 
 between training intervals , a phenomenon called the spacing effect11 , 13,14,15,16 . based on this , many studies have tried to identify the 
 exact temporal period of inter - trial interval that maximize the effectiveness of motor skill 
 learning14 , 17 . traditionally , research on the spacing effect has been performed in the field of 
 experimental psychology , focusing on areas of memory and verbal learning18 , 19 . to the best of our knowledge , few studies have investigated whether 
 motor skill acquisition is more effective using distributed practice or massed practice . in 
 addition , there has been no consistent finding regarding inter - trial intervals between 
 repetition of tasks among previous studies14 , 15 , 20 . in this study , we compared motor skill learning between 1-day 
 learning and for two days learning formats as well as the effectiveness of motor sequential 
 learning of a serial reaction time ( srt ) task according to practice schedules , i.e. , 
 distributed practice for two days versus massed practice in 1-day learning formats . thirty healthy subjects were recruited based on the following inclusion criteria : ( 1 ) no 
 previous history of neurological or psychiatric disorders , ( 2 ) no pathology of 
 musculoskeletal function in the upper limb , ( 3 ) right - handed as verified by the modified 
 edinburgh handedness inventory , ( 4 ) no previous exposure to other sequential learning 
 experiments within the past 6 months . all subjects were randomly and evenly assigned to the 
 distributed practice group or massed practice group . subjects in the distributed group did 
 not suffer from sleep disturbance during the practice session . all subjects understood the 
 purpose of the study and gave their written , informed consent to participation . this study 
 was approved by the institutional review board of the local ethics committee in accordance 
 with the ethical principles of the declaration of helsinki . the srt task was performed with a computer using stimulus presentation software program 
 ( laxtha , korea ) , which is composed of visual cues that assess the response time and accuracy 
 in performing a task . the visual cues were eight non - colored arabic numbers randomly 
 presented in the center of a computer monitor in consecutive order without auditory cues . when a visual cue ( i.e. one , two , three , four , five , six , seven , and eight ) appeared to a 
 subject , at the start of each trial , the subject selected the appropriate response key , 
 which ended the trial . at the end of each trial , there was a short fixed delay before 
 another cue was displayed . the srt task for training consisted of three sessions , 12 blocks , 
 and 864 trials . one session included a total of four blocks with a resting time of 60 
 seconds between each block . one block consisted of total 72 trials , which were randomly 
 presented with equal probabilities of 12.5% for each of the eight cues . thus , each cue was 
 presented nine times in one block , and it remained on the monitor until 1,000 milliseconds 
 had elapsed regardless of the subject s response . the default inter - stimuli interval was 500 
 milliseconds and was independent of the response time . all subjects sat in front of a table with their right elbow flexed at approximately 90. 
 their right hand was placed on the response keys , which were composed of left , right , up , 
 and down arrows on the computer keyboard . for performance of the srt task , subjects were 
 instructed to respond to each cue with a predetermined set of response keys : the  one  or 
  eight  numbers meant that the subject had to press the button with the index finger , 
 the  two  or  seven  numbers indicated the button should be pressed with the middle 
 finger , the  three  or  six  numbers indicated the button should be pressed with the 
 ring finger , and the  four  or  five  numbers indicated the all subjects were asked to press the 
 button as quickly as possible according to the corresponding cues using the dominant right 
 hand . the experimental paradigm consisted of three consecutive training sessions ( i.e. , 
 first - session , second session , and third session ) and three tests to assess response time 
 and accuracy ( i.e. , pre - test , mid - test immediately after the second session , post - test 
 immediately after the third session ) . all subjects were asked to perform a total of three 
 training sessions , with different spacing gaps according to their designated practice type , 
 that were identical in the composition of the srt task . subjects belonging to the 
 distributed practice group were allowed a resting time of 12 hours between sessions , 
 including sleeping time , whereas subjects in the massed practice group were provided a 
 resting time of 10 minutes between sessions . in addition , subjects in the distributed 
 practice group were instructed not to ingest alcohol or caffeine during the training 
 session . one demonstration and trial for each block of the srt task with a different 
 sequence were presented prior to the actual experiment until they became familiar with the 
 task and experimental procedure . participants were not given the sequence information and were asked for the correct 
 sequence based on memory only after the experiment in order to measure passive motor 
 sequential learning . no one remembered the correct sequence in its entirety after the end of 
 the experiment . the response time from presentation of the visual stimuli to motor response 
 was measured in milliseconds , and the response accuracy was analyzed by percentile as the 
 proportion of correct responses in presented total stimuli in the pre - test , mid - test , and 
 post - test . the  test was used to compare the gender difference of the distributed 
 practice group and the massed practice group . the independent t - test was performed to 
 analyze differences in demographic ( i.e. , age ) and dependent variables ( i.e. , response time 
 and response accuracy ) in the first test between the two groups . separate univariate 
 analyses of variance were carried out to compare between - group and group changes using 2 
 ( groups : distributed practice group , massed practice group )  2 ( test sessions : pre - test and 
 mid - test or pre - test and post - test ) anova with repeated measures . , usa ) was used for all statistical analyses , and differences with a p value lower than 
 0.05 were regarded as significant . improved motor response time and accuracy were observed in both practice groups , indicating 
 successful motor learning during the three training sessions . in the demographic data , no 
 significant differences were observed in terms of gender ( distributed practiced group ; 
 men=7 , women=8 , massed practice group ; men=9 , women=6 , p=0.715 ) or age ( distributd practiced 
 group ; 22.071.94 , distributed practiced group ; 22.472.07 , p=0.589 ) . there were also no 
 significant differences between the groups in the dependent variables of response time ( rt ) 
 and response accuracy ( ra ) in the pre - test ( rt ; p=0.697 , ra ; p=0.748 ) . changes in response time and accuracy in response to visual 
 stimuli in the distributed practice group and the massed practice groupdistributed practicemassed practicepre - testmid - testpost - testpre - testmid - testpost - testresponse time ( ms)674.251.5585.657.8577.369.2675.559.2625.548.1602.952.8response accuracy ( % ) 67.320.090.46.090.44.970.216.785.29.889.08.9 shows the changes in rt and ra in the distributed practice group and massed 
 practice group ( table 1 ) . for rt , univariate analysis showed no significant difference in the between - group 
 comparison ( f(1,28)=1.405 , p=0.246 ) . however , the significant main effects in the 
 within - group comparison of the three tests ( f(2,28)=70.805 , p=0.000 ) and the 
 interaction effect of group  three tests ( f(2,28)=3.310 , p=0.044 ) were observed . 
 in the test of within group contrasts , a significant interaction effect of group  
 pre-/mid - tests was observed ( f(2,28)=7.100 , p=0.013 ) , whereas there was no 
 significant interaction effect of group  mid-/post - tests ( f(2,28)=1.512 , 
 p=0.229 ) . for ra , there was no significant difference in the between - group comparison 
 ( f(1,28)=0.146 , p=0.706 ) nor in the interaction of group  three tests 
 ( f(2,28)=2.085 , p=0.160 ) . however , there was a significant main effect of the 
 within - group comparison of the three tests ( f(2,28)=38.917 , p=0.000 ) . in the test 
 of within group contrasts , a significant main effect in the within - group comparison between 
 the first and second session was found ( f(2,28)=45.533 , p=0.000 ) , but there was 
 no significant effect in the within - group comparison between the second and third session 
 ( f(2,28)=1.128 , p=0.297 ) . in the current study , we compared the effectiveness of motor sequential learning between a 
 distributed practice schedule ( two 12-hour inter - trial intervals ) and a massed practice 
 schedule ( two 10-minute inter - trial intervals ) using srt task . our results show that , 
 distributed practice consisting of inter - trial intervals of 12 hours more effectively 
 enhanced motor skill acquisition than massed practice consisting of inter - trial intervals of 
 10 minutes . distributed practice , in which the third trial was performed on the following 
 day , improved motor sequential learning more than massed practice , in which the trials were 
 performed provided in a 1-day learning format . consequently , we confirmed that a distributed 
 practice schedule better enhances motor sequential learning in both 1-day learning and for 
 two days learning formats than a massed practice schedule . it is well established that distributed 
 practice has advantages in terms of both implicit and explicit memories , basic memory tasks 
 using words , educational materials , motor skill acquisition , and so forth12 , 21 , 22 . despite abundant evidence that 
 distributed learning has benefits for memory and learning , studies of distributed learning 
 in the context of procedural learning or motor sequence acquisition are relatively few . shea 
 et al14 . used three variations of a 
 key - press timing task , similar to our experimental design , and found that the spacing gap in 
 distributed practice scheduled within 1-day and during a few days intervals enhanced motor 
 performance in a delayed retention test . in addition , their other experiment using a dynamic 
 balance task provided the same result regarding the comparison of distributed and massed 
 practice . previous studies have also suggested the benefits of distributed practice in 
 various procedural motor tasks , in terms of surgical skill training using virtual reality , 
 and discrete motor tasks , etc20 , 23 , 24 . likewise , prior studies corroborate our findings , showing that 
 inter - trial intervals play an important role in learning motor skills12,13,14 , 16 , 22 . to explain the benefits of distributed practice , a theory of memory consolidation has been 
 proposed14 , 25 . memory consolidation is a term used as a category of processes in 
 which a memory trace is stabilized from a relatively unstable phase into a permanent 
 form14 , 26 , 
 27 . previous studies have argued that a 
 relatively longer inter - trial interval provides the opportunity for memory consolidation 
 without interruption from additional practice14 , 
 28 . in addition , karni et al29 . suggested that relatively long periods of 
 rest and sleep between training sessions might improve motor performance . thus , our findings 
 that distributed practice within 1-day and for two days intervals led to faster and more 
 accurate motor responses in the srt task than massed practice can be attributed to the 
 improvement of explicit and implicit memory retention upon completion of memory 
 consolidation . however , our results also show that motor performance between the first and 
 second session within 1-day improved more than that between the second and third sessions 
 which were performed on different days . our srt 
 task paradigm was not complex , therefore as a long period of skill acquisition was not 
 required . thus , we assume that our participants almost reached their maximal learning 
 capacity in the second training session . motor skill learning is an essential component for successfully performing physical 
 activities in daily life2 . understanding 
 the mechanisms of motor learning in the human nervous system remains an interesting 
 scientific issue in neuroscience . numerous prior studies have attempted to determine the 
 factors that can enhance the acquisition and retention of motor skills2 , 30 , 31 . in particular , it is important to increase the 
 effectiveness of motor skill learning in the fields of neurological rehabilitation and sport 
 science . therefore , we expect that these findings will help to develop efficient and 
 effective approaches for the facilitation of motor learning . however , this study had some 
 limitations . retention of motor skills was not measured , and the amount of sleep the 
 previous night was not controlled . it is already known that implicit and explicit learning 
 are affected by sleep . in the future , studies will be required to take these elements into 
 consideration .
[ purpose ] the purpose of the current study was to compare the effectiveness of motor 
 sequential learning according to two different types of practice schedules , distributed 
 practice schedule ( two 12-hour inter - trial intervals ) and massed practice schedule ( two 
 10-minute inter - trial intervals ) using a serial reaction time ( srt ) task . 
 [ subjects and 
 methods ] thirty healthy subjects were recruited and then randomly and evenly assigned to 
 either the distributed practice group or the massed practice group . 
 all subjects performed 
 three consecutive sessions of the srt task following one of the two different types of 
 practice schedules . distributed practice was scheduled for two 12-hour inter - session 
 intervals including sleeping time , whereas massed practice was administered for two 
 10-minute inter - session intervals . 
 response time ( rt ) and response accuracy ( ra ) were 
 measured in at pre - test , mid - test , and post - test . 
 [ results ] for rt , univariate analysis 
 demonstrated significant main effects in the within - group comparison of the three tests as 
 well as the interaction effect of two groups  three tests , whereas the between - group 
 comparison showed no significant effect . the results for ra showed no significant 
 differences in neither the between - group comparison nor the interaction effect of two 
 groups  three tests , whereas the within - group comparison of the three tests showed a 
 significant main effect . 
 [ conclusion ] distributed practice led to enhancement of motor 
 skill acquisition at the first inter - session interval as well as at the second 
 inter - interval the following day , compared to massed practice . 
 consequentially , the 
 results of this study suggest that a distributed practice schedule can enhance the 
 effectiveness of motor sequential learning in 1-day learning as well as for two days 
 learning formats compared to massed practice .