stability of the trunk , often referred to as core stability , has gained considerable attention in the recent years . it can be operationally defined as :  the body s ability to control the trunk in response to internal and external disturbances , including the forces generated from distal body segments as well as from expected or unexpected perturbations core stability training has become a major element of training programs in sports as well as rehabilitation ( borghuis et al . it seems plausible that core stability is important for injury prevention and athletic performance , especially in sports with large demands for balance control , such as gymnastics . the importance of core stability for injury prevention in athletes has received some support from epidemiological studies relating low core stability to incidence of injuries of the back ( cholewicki et al . 2005 ) and lower extremities ( zazulak et al . in addition , there is limited evidence to support its role as a determinant of performance . nesser et al . found a moderate correlation between core stability and performance in football players ( nesser et al . it should be noted that , in several of the studies mentioned ( nesser et al . 2004 ) , the measurements used to determine core stability reflect muscle strength and endurance , possible determinants of stability , rather than stability itself . the stability of the spine and trunk depends on contributions of the passive , active and control sub - systems ( panjabi 1992 ) . the passive joint structures such as ligaments and intervertebral discs contribute to stability by providing joint stiffness ( cholewicki et al . the active sub - system , the trunk musculature contributes by further increasing stiffness through cocontraction ( cholewicki et al . 1997 ; gardner - morse and stokes 1998 ; dien et al . 2003 ; stokes and gardner - morse 2003 ) . in addition , the control sub - system , i.e. , the nervous system , controls reflexive and triggered muscle activity contributing to stability ( moorhouse and granata 2007 ) , based on sensory feedback from muscle and joint receptors , and feedback from the visual and vestibular systems ( goodworth and peterka 2009 ) . a model study predicted that the decrease in muscle stiffness that is associated with fatigue ( kirsch and rymer 1987 ; zhang and rymer 2001 ) , may impair trunk stability ( granata et al . 2004 ) . given the importance of reflexive muscle activity for core stability , the reductions in maximum force and rate of force rise that develop with muscle fatigue can also be expected to impair core stability , which may be aggravated by the adverse effects of fatigue on proprioception ( taimela et al . 2004 , 2009 ) , which was attributed to a reduced control over the trunk . granata and gottipati ( 2008 ) showed that fatigue of the extensor muscles had a negative effect on the trunk s local dynamic stability , i.e. , the responses to the small perturbations that are always present due to neuromuscular noise ( dien et al . 2008 ) . herrmann et al . found decreased contact forces during an external perturbation of the trunk induced with a swinging pendulum , which indicates reduced trunk stiffness ( herrmann et al . ( 2010 ) , however , found no change in contact forces . in both of these studies , increased electromyographic amplitudes of the reflex responses indicated compensatory mechanisms to counteract the fatigue effects . moreover , several studies found increased cocontraction in unperturbed standing after inducing trunk muscle fatigue , as a potential compensatory mechanism ( grondin and potvin 2009 ; granata et al . two of these studies also investigated the response to an external perturbation and found no effects of fatigue ( grondin and potvin 2009 ; granata et al . the evidence for the effects of fatigue on trunk stability from the studies reviewed above is not consistent . furthermore , all of these studies involved high fatigue levels ( typically a reduction in force producing capacity by 40% ) induced using isolated , non - functional activity of trunk muscles . it thus remains unclear whether fatigue of the trunk muscles that even well - trained athletes may develop during training or competitive events can induce impairments of trunk stability . the aim of the present study therefore was to investigate the effects of fatigue induced by a set of exercises as performed regularly in training on trunk stability in elite gymnasts . we hypothesized that gymnasts show increased sway amplitudes in a seated balancing task and that they are less able to correct an external perturbation of seated balance after a set of gymnastic exercises . nine gymnasts , all girls , with a mean age of 12.4 ( 2.3 )  years old participated in the study . their mean ( standard deviation ) height and body mass were 1.47    0.12  m and 39.0    12.92  kg , respectively . none of the participants reported any recent history of injuries that did not allow training participation . the experimental protocol , which had been approved by the ethical committee of the faculty of human movement sciences of the vu university amsterdam ( number 2009 - 039 ) , comprised a 10-min fatigue protocol in between the pre- and posttests of trunk stability . the measurements were performed at the beginning of regular training sessions in the afternoon , after school hours . prior activities , either during the day before the training or during warming - up were not controlled . the fatigue protocol contained four series of five dump handstand exercises on the uneven bar . based on subjective report of trainers and gymnasts , this bout of exercises series of dump handstands are a regular part of normal training and the intensity of this bout of exercises was comparable to the more intensive elements of the participant s normal training activities for the measurements of trunk stability , subjects were seated with arms in their lap on an unstable seat , which required them to dynamically balance by trunk movement only ( fig .  1 ) . the seat was mounted over a hemisphere ( radius of hemisphere : 25  cm , height of the seat relative to the lowest point on the hemisphere : 17  cm ) , creating instability in all directions . to trace the center of pressure ( cop ) , the seat was placed on a custom - made strain gauge force plate that was sampled at 200  samples / s . the force plate was calibrated prior to each measurement session , by placing known weights on the plate . the footplate was adjusted to support the feet with the knees and hips at 90 angles . the first task required the subject to sit as still as possible and lasted 30  s. three repetitions of this task were performed before and after the fatiguing exercise , as reliability of single measurements was previously shown to be poor ( dien et al . 2010b ) . in the second task , the subject leaned back on the seat , supported by a strap around the thorax that was attached to two electromagnets on the safety rail in front of the subject . the length of the strap was adjusted to obtain a constant inclination angle for each subject . after a random interval of 37 s after the start of the measurement , the electromagnets were released and the subject had to regain balance as quickly as possible . recording of data was continued for a total of 20  s. nine repetitions were performed within a few minutes before and after the fatigue exercise.fig . 1schematic of the unperturbed seated balancing task ( a ) and the sudden release task ( b ) schematic of the unperturbed seated balancing task ( a ) and the sudden release task ( b ) force plate data were low - pass filtered at 10  hz ( fourth order butterworth ) , as the signals contained only very limited power above 3  hz , while sensitivity analysis showed only minor effects of filtering at either a higher ( 20  hz ) or lower ( 2.5  hz ) cutoff . four independent parameters were derived from the cop time series of the 5th till 30th second of the trial : the rms of the cop in x and y direction ( rmsx , left right ; rmsy , antero - posterior ) and the mean power frequency of the cop time series in both directions ( mpfx and mpfy ) . for the unperturbed trials ( see fig .  2 for an example ) , the mean cop position determined over the first 2  s was first subtracted from the time series . next , the maximum cop position in the y direction was determined , which reflects the maximum backward cop displacement after the sudden release ( maxy ) . subsequently , an exponential decay function was fitted to the cop time series in the y direction from the instant of maxy and the subsequent 7  s ( fig . 2typical examples of : the cop time series in the unperturbed trials with anterior  posterior displacement in black and left  right displacement in gray ( a ) , the spatial distribution of the cop position in the same trial ( b ) , the rmsx ( black ) and rmsy ( gray ) values for all trials of the same subject ( c ) , the mpfx ( black ) and mpfy ( gray ) values for all trials of the same subject ( d ) . 3typical examples of the anterior  posterior cop times series in a sudden release trial with the fitted exponential curve as a thick gray line ( a ) , the spatial distribution of the cop position in the same trial ( b ) , the maxy values for all trials of the same subject ( c ) , the  values for all trials of the same subject ( d ) . the fatiguing exercise was performed between trials nine and ten1\documentclass[12pt]{minimal } 
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				 \begin{document}$$ y(t ) = y^{\prime } + ( { \text{max}}_{y } - y^{\prime } ) \times e^ { - \lambda t } $ $ \end{document}with time t defined as zero at the instant of maxy , y referring to the y coordinate of cop , y referring to the steady state y position of the cop to which the subject converged , estimated as the median position over the fifth till seventh second after the maximum backward displacement , and maxy referring to the maximum displacement . the parameter  was used as an indicator of the rate of recovery after the perturbation , with higher values indicating faster recovery . typical examples of : the cop time series in the unperturbed trials with anterior  posterior displacement in black and left  right displacement in gray ( a ) , the spatial distribution of the cop position in the same trial ( b ) , the rmsx ( black ) and rmsy ( gray ) values for all trials of the same subject ( c ) , the mpfx ( black ) and mpfy ( gray ) values for all trials of the same subject ( d ) . the fatiguing exercise was performed between trials three and four typical examples of the anterior  posterior cop times series in a sudden release trial with the fitted exponential curve as a thick gray line ( a ) , the spatial distribution of the cop position in the same trial ( b ) , the maxy values for all trials of the same subject ( c ) , the  values for all trials of the same subject ( d ) . the fatiguing exercise was performed between trials nine and ten for all six dependent variables , median values over the repeated trials per subject were used for statistical analysis to avoid effects of trials with outlying results . comparisons between the fatigued and unfatigued conditions were made based on averages over subjects , which were tested with paired t tests , with   =  0.05 . the effect of trial order was tested over the trials performed before the fatiguing exercise using repeated measures anova . figure  2 illustrates a typical example of the data obtained in the unperturbed trials . however , the rmsy values were on average higher and mpfy values were on average lower after than before the fatiguing exercise , whereas no difference was discernible in the x direction . the effects in the y direction ( anterior  posterior ) were in line with our hypothesis and were found to be statistically significant for the group ( table  1 ) . no effects were found in the x direction.table  1means and standard deviations of all dependent variables before and after the bout of fatiguing exercise and p values for differences between the fatigued and unfatigued conditions based on paired t testsindependent variablemean    sdprmsx ( cm )  unfatigued0.19    0.080.549  fatigued0.21    0.09rmsy ( cm )  unfatigued0.18    0.080.030  fatigued0.31    0.17mpfx ( hz )  unfatigued0.40    0.150.810  fatigued0.41    0.24mpfy ( hz )  unfatigued0.52    0.130.005  fatigued0.35    0.17maxy ( cm )  unfatigued3.81    0.900.003  fatigued5.10    1.00 ( s )  unfatigued1.73    0.580.012  fatigued0.80    0.27 means and standard deviations of all dependent variables before and after the bout of fatiguing exercise and p values for differences between the fatigued and unfatigued conditions based on paired t tests figure  3 illustrates a typical example of the cop data in the perturbed trials . after the sudden release approximately 4  s after the start of the trial , the subject moved further backward to recover and attain a fairly steady position within 3  s after the release . however , the maxy values were on average higher and  values were on average lower after than before the fatiguing exercise . these effects were in line with our hypothesis and were found to be statistically significant for the group ( table  1 ) . to test whether the differences between the trials before and after the fatiguing exercise could be explained by order effects , we tested for the effect of measurement order over the trials collected before the fatiguing exercise . none of the variables showed a significant order effect ( p  >  0.383 ) . this study was designed to test the hypotheses that gymnasts show increased sway amplitudes in a seated balancing task and that they are less able to correct an external perturbation of seated balance after a set of fatiguing gymnastic exercises . as hypothesized , unperturbed seated balancing performance was decreased in the anterior  posterior direction as evidenced by a larger amplitude and lower frequency . perturbations were performed in the posterior direction only . in line with the hypothesis , the maximum displacement after the perturbation was larger and the recovery of balance was slower after the exercise than before . it is at present unclear why effects were found in the sagittal plane ( anterior  posterior direction ) only . an explanation could be that the dump hand stands fatigued the abdominal muscles selectively given the mechanics of these exercises . while abdominal muscles can contribute to control of lateral bending of the trunk ( seroussi and pope 1987 ) , the extensor muscles have a higher moment producing capacity in the frontal plane ( dien and kingma 1999 ) and probably allow more precise control in view of their abundant insertions on the multi - segmented lumbar spine . thus , while control in frontal plane could be done mainly by the extensor musculature , the abdominal muscles are obviously indispensible for control in the sagittal plane . hence , selective fatigue of these muscles might explain why effects were restricted to the sagittal plane . as reported in the introduction , the previous studies on the effects of fatigue on trunk stability showed inconsistent effects . most of these tested trunk stability by applying perturbations to the trunk in either standing or sitting positions . it was suggested that compensatory mechanisms such as increased cocontraction and increased reflex gains could explain the absence of fatigue effects in some of these studies . in the present study , we used a dynamic balancing task to test trunk stability . in this task , the center of mass unavoidably sways beyond the surface of support , which is the point of contact of the hemisphere on the force plate . trunk moments , accelerating the upper body relative to the pelvis and seat , must then be used to bring the center of mass back over the surface of support , while stiffening the trunk would not be effective ( cf otten 1999 ) . it has previously been shown that cocontraction , which would stiffen the trunk is not effective in this task ( reeves et al . . the present task may therefore be more sensitive to trunk muscle fatigue than tasks in which the support surface is larger . activity of the trunk muscles was not measured and it is therefore unknown whether subjects increased cocontraction , but this would be consistent with the increase in rmsy ( reeves et al . 2006 ) and decrease in mpfy ( dien et al . with fatigue of an agonistic muscle , the activation levels need to be increased to maintain force output . 2009 ) , which would cause increased sway and thus could explain the increase in rmsy . variability of muscle force would increase even more when antagonistic cocontraction would be increased ( selen et al . 2005 ) which , given the fact that stiffening the trunk does not limit kinematic variability in the present task , could further increase sway amplitudes . fatigue will likely slow down muscular responses due to increased proprioceptive thresholds ( taimela et al . 1999 ) and due to the slower force development in the fatigued muscles ( de ruiter et al . 1999 ; perrey et al . slower responses to balance perturbation would increase sway amplitude ( radebold et al . 2001 ) and likely decrease sway frequency in the unperturbed task and would also increase the amplitude and reduce the recovery in the perturbed task ( reeves et al . . increased proprioceptive thresholds might be reflected specifically in an increase in the maximum displacement after the perturbation ( increased maxy ) , while a decreased rate of force development might be more obvious in a decreased rate of recovery ( decreased  ) . finally , fatigue could be associated with an increased respiratory challenge , which might affect trunk stability ( janssens et al . it is to be expected that the respiratory effect would be most obvious in the sagittal plane , which would be in line with the fact that only effects in the y direction were found . unavoidably , trials in the fatigued condition were performed after those in the unfatigued condition and this could lead to order effects , for example due to learning , which might dilute the fatigue effect . the absence of an order effect over the trials performed before the fatiguing exercise suggests that such effects did not occur , possibly due to the fact that the participants were all highly trained athletes . it is unclear to what extent the loss of trunk stability as found in the present study would affect the performance of gymnastic activities . furthermore , impaired trunk stability may limit performance and increase injury risk . therefore , the fact that trunk stability was negatively affected by a series of fatiguing exercises that reflect intensity of typical training activities and competitive events suggests several practical implications . the results suggest that trainers should take fatigue effects into account when planning the order of training activities , e.g. , avoid balance beam exercises after uneven bar or horizontal bar exercises . furthermore , the results may have implications for the intensity of warming up exercises for the competitive events . finally , the results indicate that endurance training of trunk muscles and perhaps balance training may need to be performed not only in rested , but also in fatigued condition . in conclusion , the present study showed that trunk stability in elite gymnasts was negatively affected by a bout of exercises , which reflected normal training activities . both sagittal plane sway in unperturbed balancing and recovery after a backward balance perturbation were affected . these results suggest that fatigue effects on trunk stability should be taken into account in the planning and design of gymnastics training .