neural plasticity is the brain 's ability to change in response to normal developmental processes , experience , and injury .
the mechanisms involved in plasticity in the nervous system are thought to support cognition , meaning that brain function is improved .
there is evidence from neuroimaging that subjects achieving high scores on cognitive tasks compared to low performers complete these tests with lower cortical activation particularly in the frontal brain region [ 1 , 2 ] .
hypothesis , assuming that experts with high cognitive performance are characterized by a more efficient cortical function .
.   suggest that this particular characteristic can only be inferred from brain activation , when experts and nonexperts are compared at similar behavioral performance . within the last years
this hypothesis has been extended to experts of movement - related tasks . in this respect ,
standard eeg techniques have been used to investigate the effect of performance level on the allocation of cortical resources during motor tasks . during the preparation of a shot , haufler et al
.   observed less cortical activity in skilled shooters compared to novice subjects . in line with this finding , del percio et al
.   also found decreased cortical activity in experts prior to a shot requiring high precision and interpreted this observation as index for selective event- or task - related cortical activation .
neural efficiency has also been confirmed in professional piano players , who completed a motor task involving finger movements with lower cortical activity than less skilled controls .
although these findings consistently support the efficiency of cortical function in athletes , some evidences from tasks requiring movements in response to visual stimuli question the general applicability of this hypothesis . in this respect , hung et al .   reported higher lateralized readiness potentials during posner 's visuoattentional task in elite tennis players compared to nonathletes . in line with these results ,
endo et al .   showed that athletes had greater motor cortex activity than nonathletes during a task , in which subjects were asked to abduct the right index finger in response to visual stimuli .
evidence from longitudinal studies investigating effects of motor training consistently supports the efficiency of cortical function in high performers , because motor cortex activity decreased despite improvements on task performance after a training period [ 5 , 10 , 11 ] .
similarly , 4 weeks of endurance training in cyclists led to improved aerobic performance but decreased brain cortical activity during exercise .
based on those findings , the neural efficiency hypothesis might also be applicable to endurance athletes . so far researches have not yet investigated efficacy of cortical function during exercise between subjects of different aerobic performance levels .
however , the current state of research indicates that resting eeg rhythms predict neural efficiency during cognitive and sensorimotor tasks .
alpha synchronization at rest represented by high eeg alpha power is related to increased cognitive and motor performance [ 13 , 14 ] . in line with these findings , elite karate athletes compared to nonathletes show higher alpha-1 power at parietal and occipital regions at rest with eyes closed .
moreover , training levels also influence the reactivity to eyes opening , so that athletes maintain higher alpha power than nonathletes .
whereas alpha is considered the dominant rhythm in the human brain during mental inactivity , beta activity is regarded as index of information processing at the cognitive level . in previous studies increased alpha power and/or decreased beta power , reflected by a higher alpha / beta ratio , were associated with a decreased level of arousal or vigilance [ 18 , 19 ] .
this pattern is considered normal for the resting state due to a lack of task - specific activation demands . however , endurance exercise increases arousal due to preparedness for external input . applying the neural efficiency hypothesis ,
high cortical function despite minimal energy consumption should be indicated by an increased alpha / beta ratio in high performers . in previous trials ,
high and low performers were differentiated by their individual training levels or participation in competitions [ 4 , 15 , 21 ] . for endurance athletes ,
maximal oxygen consumption allows a more appropriate and objective classification , because this variable is considered  gold - standard  and predicts time - trial performance .
this is due to the concept that an athlete 's maximal oxygen consumption is mainly determined by the oxygen delivery to the mitochondria and its utilization , which in turn are limiting the oxidative production of atp required for the energy supply of the working muscles . as neural efficiency
is task - related , it is necessary to study brain cortical activity in athletes directly during endurance exercise .
the majority of studies investigating brain function while moving predominantly examined subjects during cycling exercise , because it does not create stepping impacts that provoke strong neck muscle contractions and electrode movements .
furthermore , experienced cyclists are thought to be able to maintain a stable body position even at demanding workloads , so that the likeliness of movement - related artefacts in the eeg signal is reduced .
the present study investigates whether or not aerobic power has an effect on the alpha / beta ratio during exercise .
additionally , the authors examine if resting eeg predicts brain cortical activity during exercise . in line with the neural efficiency hypothesis subjects with high maximal oxygen consumption
cyclists and triathletes with a minimum cycling training volume of 4 hours per week were directly recruited from local sports clubs .
eligible subjects had to be healthy , between 18 and 35 years , nonsmokers , and right handed . before the trial commenced all participants underwent health screening following the s1-guideline of the dgsp , which included personal anamnesis , orthopaedic assessment , and measurement of resting ecg and blood pressure .
any complications limiting the performance or safety of exercise led to exclusion from the study . following the screening process ,
all study procedures were approved by the local ethics committee and followed the guidelines of the declaration of helsinki .
two laboratory visits separated by 3 to 5 days were scheduled . in the first laboratory session aerobic and anaerobic performance
the second visit included recording of eeg rhythms at rest and during endurance exercise at constant workload .
based on maximal oxygen consumption measured on the first visit , the sample was split into a group of lower aerobic power ( low ; vo2max  49  ml / min / kg ; n = 15 ) and a group of higher aerobic power ( high ; vo2max  50  ml / min / kg ; n = 14 ) .
environmental temperature was held constant at 20c and nutrition was standardized ( last main meal2  h ; 500  ml30  min ) . following the assessment of body weight , subjects completed an incremental test with spirometry ( cortex medical , metamax 3b , germany ) on a high performance cycling ergometer ( fes , germany ) .
after a 5  min warm - up at 80  w  ( f)/100  w  ( m ) , the workload was increased by 25  w/3  min until volitional exhaustion .
this was defined as the inability to maintain a cadence of at least 60  rpm at the given workload .
heart rate and respiratory parameters were recorded continuously . using the enzymatic - amperometric method ( dr . mueller geraetebau , super gl ambulance , germany ) , lactate concentration was analysed in blood taken from an ear lobe after each increment .
collected data were processed with winlactat ( mesics , germany ) . according to the dickhuth model , the individual anaerobic threshold ( pians )
additionally , vo2max was determined as highest value over a 30  s period . on the second laboratory visit eeg
was recorded under the following conditions : resting with eyes closed while sitting on the ergometer ( 2  min ) and during cycling at constant load .
the exercise protocol included a 5  min warm - up ( 80  w  [ f]/100  w  [ m ] ) , 30  min at 100% of the individual anaerobic threshold , and a 5  min cool - down at a low workload ( 80  w  [ f]/100  w  [ m ] ) .
brain cortical activity was recorded over 1  min at 10 and 20 as well as 30  min and subjects maintained a cadence of 90  rpm .
heart rate and cadence were displayed in real - time on a monitor placed in the subjects ' natural viewing direction .
prior to the assessments , all subjects were instructed to stabilize their body position and avoid head movements and contractions of the jaw throughout the cycling exercise . using a breathable cap ,
active agcl electrodes were placed on the subject 's scalp to record eeg from frontal ( f3 , f4 , fz , f7 , and f8 ) , central ( c3 , c4 , and cz ) , and parietal ( p3 , p4 , pz , p7 , and p8 ) brain regions .
the electrodes , which were positioned according to the international 10:20 system , were filled with an electrolyte gel to reduce impedance below 10  k. the eeg signal was amplified by the quickamp system ( brainvision , germany ) and sampled at 512  hz . following the data acquisition , the analyzer 2.0 ( brainvision , germany ) was used for processing the eeg recordings . after a reduction of the sampling rate to 256  hz , data were digitally band - pass filtered ( time constant 0.0318  s ; 24  db / octave ) and a frequency range of 3.0 to 40.0  hz remained for analysis .
the recorded epochs represented the 10th , 20th , and 30th minutes during constant load cycling at pians .
following a manual inspection the artefacts tagged by the software were either rejected or confirmed .
subsequently , those segments were averaged individually within each recording period . by using a hanning window ( 20% ) ,
the eeg data were fast - fourier - transformed ( fft ) to calculate power values in the alpha ( 7.512.49  hz ) and beta ( 12.532  hz ) frequency domain .
subsequently , the alpha / beta ratio was calculated and averaged over the epochs representing the recordings during exercise .
accurate test - retest reliability for the alpha and beta power during cycling at 90  rpm has been reported with the eeg acquisition and processing methods used in the present study .
prior to group comparisons , gaussian distribution of the collected data was verified by the shapiro - wilk test .
one - way anova ( factor : group ) was used to compare anthropometric measures between medium and high fit subjects .
the effect of maximal oxygen consumption on alpha / beta ratio was analysed by applying 2 ( group : low , high )  2 ( condition : rest , exercise )  3 ( region : frontal , central , and parietal ) anova . in a subsequent analysis frequency ( alpha , beta )
was included as additional factor to assess the effect of maximal oxygen consumption on eeg spectral power . in case of nonsphericity
greenhouse - geisser correction was applied . between - subjects and within - subjects effects as well as interactions were reported and further analysed using fischer 's lsd post hoc test .
subsequently , linear regression was used to predict brain cortical activity during exercise and maximal power from resting eeg .
the adjusted pearson 's multivariate coefficient of determination and  as standardized coefficient are provided . for all statistical analyses , an alpha level of p 
brain cortical activity during exercise was recorded at 100% of the individual anaerobic threshold , which was 3.31  0.37  wkg and 2.74  0.45  wkg in high and low ( f(1,27 ) = 13.66 ; p = 0.001 ; eta = 0.336 ) , respectively . subjects with higher maximal oxygen consumption also reached greater maximal power ( 4.67  0.33  wkg versus 4.11  0.49  wkg ; f(1,27 ) = 13.28 ; p = 0.001 ; eta = 0.330 ) than the low group .
the statistical analysis revealed main effects of group ( f(1,27 ) = 12.04 ; p = 0.002 ; eta = 0.308 ) , condition ( f(1,27 ) = 4.24 ; p = 0.049 ; eta = 0.136 ) , and region ( f(2,26 ) = 8.50 ; p = 0.002 ; eta = 0.239 ) on subjects ' alpha / beta ratio .
moreover , there was an interaction of condition and region ( f(2,26 ) = 4.37 ; p = 0.025 ; eta = 0.139 ) . whereas alpha / beta ratio was increased at central brain region compared to parietal ( t(28 ) = 3.36 ; p = 0.002 ) and frontal sites ( t(28 ) = 2.74
; p = 0.010 ) in the exercise condition , no regional differences were confirmed for the resting state .
furthermore , alpha / beta ratio was significantly different between high and low at all electrode sites ( figure 1 ) in both the resting state ( frontal : t(28 ) = 2.86 ; p = 0.008 ; central : t(28 ) = 2.52 ; p = 0.018 ; parietal : t(28 ) = 2.67 ; p = 0.013 ) and the exercise condition ( frontal : t(28 ) = 3.19 ; p = 0.004 ; central : t(28 ) = 2.62 ; p = 0.014 ; parietal : t(28 ) = 2.37 ; p = 0.025 ) .
when frequency was included as additional factor , interactions with condition ( f(1,27 ) = 11.37 ; p = 0.002 ; eta = 0.296 ) and group ( f(1,27 ) = 8.43 ; p = 0.007 ; eta = 0.238 ) were confirmed .
compared to the resting state alpha ( t(28 ) = 7.39 ; p < 0.001 ) and beta power ( t(27 ) = 12.43 ; p < 0.001 ) were significantly increased across electrode sites during exercise . moreover ,
high showed less beta power than low ( t(28 ) = 2.45 ; p = 0.020 ) .
the relation between resting eeg rhythms , brain cortical activity during exercise , and maximal power is shown in table 3 . whereas alpha / beta ratio at rest explained 57% of variance of alpha / beta ratio during exercise at frontal brain region , resting eeg at other electrode sites did not predict brain cortical activity during exercise . when frontal alpha / beta ratio at rest was used as the independent variable for the prediction of maximal power
, the model was able to explain 15% of the variance compared to 18% and 26% for the use of central and parietal alpha / beta ratio , respectively .
compared to the resting state , the alpha / beta ratio was decreased during cycling exercise as beta power increased more than alpha power .
this change towards a higher level of arousal or vigilance has also been reported in a recent review by ludyga et al . .
increased beta activity reflecting higher cortical activation might be the result of greater processing demands during exercise and the tendency of the sensorimotor system to maintain the current network set .
the decrease of the alpha / beta ratio from rest to exercise was most pronounced at central brain region , where inter alia sensorimotor information of lower extremities is integrated . in this respect ,
the maintenance of pedaling movements is associated with increasing feedback from sensory afferents and feed - forward due to the recruitment of motor neurons [ 12 , 30 ] . although the magnitude of the changes in alpha / beta ratio from rest to exercise was similar between groups , high compared to low had a higher index in both conditions .
differences between athletes of high and low training or performance levels have also been observed at rest previously [ 4 , 21 ] .
.   found enhanced cortical synchronization of pyramidal neurons reflected by increased alpha activity in elite karate athletes .
as alpha power serves as an inverse indicator of mental alertness or arousal [ 18 , 19 ] , this implies that athletes are more efficient in posing cortical neurons into a nonoperative mode .
consequently , it can be assumed that a lower level of arousal at rest , which was also observed in the present trial , is due to greater relaxation ability in subjects with higher maximal oxygen consumption .
this is further supported by a lower beta power in high , because a decrease in beta power has been reported to be the eeg response to relaxation . by maintaining a low arousal at rest , attentional resources
may be reserved for a subsequent recruitment in movement - related tasks and associated information processing .
the present results support this assumption as high resting alpha / beta ratio was associated with high alpha / beta ratio at frontal brain region during exercise . in previous studies resting
alpha and beta activity have also been related to different measures of cognitive and motor performance , so that neural efficiency in the resting state is suggested to predict task - related performance [ 13 , 14 ] .
this is also applicable to endurance exercise as arousal at rest was directly related to maximal power during cycling .
similar to the resting state , cyclists in high showed a higher alpha / beta ratio than low during exercise at the individual anaerobic threshold .
this observation indicates that the neural efficiency hypothesis is applicable to endurance - trained athletes .
the present findings are further supported by ludyga et al . , who confirmed a negative correlation between changes of aerobic performance and cyclists ' brain cortical activity after a 4-week training period .
other studies have also shown that training elicits a decrease of cortical activity   and the suppression of task - irrelevant cognitive processes [ 32 , 33 ] during different movement - related tasks .
evidence from animal studies suggests that improved neural processing and increasing influence of the cerebellar  thalamic  cortical circuit are underlying mechanisms of training - induced enhancement of neural efficiency . in this respect , taniwaki et al .  
have proposed that this increased influence reflects a change from internal to increasingly external and automatic processing of motor tasks .
the execution of highly automated movements , such as handwriting , does not require much attentional resources .
therefore , the lower level of arousal during exercise in high could be explained by cycling becoming an increasingly automated process with training .
the reservation of cortical sources in experts is further supported by the results of httermann and memmert , who showed that athletes compared to nonathletes had higher cognitive performance during cycling exercise of similar relative intensity .
furthermore , improvements in circulatory capacity and cerebrovasculature have also been suggested as underlying mechanisms for differences in eeg spectral power between athletes and nonathletes [ 21 , 38 ] .
maximal oxygen consumption is correlated with cerebral blood flow , which is considered as a proxy for assessing neurogenesis .
this indicates an increased formation rate of new neurons and improved cerebral blood flow in high compared to low .
these improvements elicited by endurance exercise are thought to improve energy supply by creating a more supportive and nutritive environment for the surrounding neurons .
however , future research is necessary to investigate whether or not an optimized energy supply accounts for enhanced neural efficiency during exercise . a methodological concern of eeg recordings during exercise is the potential impact of physiologic and mechanical disturbances . to reduce the possibility and impact of artefacts ,
the present study used a data acquisition and processing approach , which has proven reliable .
additionally , only experienced cyclists were included in the study , because their ability to stabilize the upper body at moderate to high intensity is necessary to reduce movement - related artefacts when eeg is recorded during exercise on a stationary bike .
therefore , it remains unclear whether or not the development of the alpha / beta ratio over time was different between groups .
nonetheless , the present results indicate a higher level of arousal in cyclists with lower aerobic fitness . as this
was observed in both conditions , it can not be ruled out that differences in alpha / beta ratio during exercise between high and low are mainly due to the differences observed in the resting state . whereas previous studies also reported differences in alpha activity at rest between elite - athletes and nonathletes [ 15 , 16 , 40 ] , this is not supported by the present findings .
this could be due to a lack of high differences in performance and/or training levels between groups and the low sample size of the present study .
however , the differences in alpha / beta ratio confirmed between groups imply that this index is sensitive to small differences in aerobic power .
nonetheless , further investigations with higher sample size and more aerobic fitness subgroups are warranted to test and extend the current findings .
furthermore , it remains unclear whether or not other variables related to neural efficiency , such as cognitive performance and intelligence , have influenced the observed differences between high and low . in conclusion ,
cyclists with high aerobic fitness compared to peers with less aerobic fitness are able to complete submaximal cycling exercise with a lower level of brain cortical activity .
this indicates enhanced neural efficiency in subjects with high aerobic power , possibly due to the inhibition of task - irrelevant cognitive processes .
moreover the use of resting eeg rhythms as predictor for task performance is also applicable to cycling as resting state arousal predicted maximal power .
 the  neural efficiency  
 hypothesis suggests that experts are characterized by a more efficient cortical function in cognitive tests . 
 although this hypothesis has been extended to a variety of movement - related tasks within the last years , it is unclear whether or not neural efficiency is present in cyclists performing endurance exercise . therefore , this study examined brain cortical activity at rest and during exercise between cyclists of higher ( high ; n = 14 ; 55.6  2.8  ml / min / kg ) and lower ( low ; n = 15 ; 46.4  4.1  ml / min / kg ) maximal oxygen consumption ( vo2max ) . 
 male and female participants performed a graded exercise test with spirometry to assess vo2max . 
 after 3 to 5 days , eeg was recorded at rest with eyes closed and during cycling at the individual anaerobic threshold over a 30  min period . 
 possible differences in alpha / beta ratio as well as alpha and beta power were investigated at frontal , central , and parietal sites . 
 the statistical analysis revealed significant differences between groups ( f = 12.04 ; p = 0.002 ) , as the alpha / beta ratio was increased in high compared to low in both the resting state ( p  0.018 ) and the exercise condition ( p  0.025 ) . 
 the present results indicate enhanced neural efficiency in subjects with high vo2max , possibly due to the inhibition of task - irrelevant cognitive processes . 