The cross on rings performed by an Olympic champion

http://dx.doi.org/10.1590/1807-55092016000100071 Paulo CARRARA Alberto Carlos AMADIO Júlio Cerca SERRÃO Gareth IRWIN Luis MOCHIZUKI *Centro Universitário das Faculdades Metropolitanas Unidas, São Paulo, SP, Brasil. **Escola de Educação Física e Esporte, Universidade de São Paulo, São Paulo, SP, Brasil. ***Cardiff School of Sport, Cardiff Metropolitan University, Cardiff, Wales, United Kingdon ****Escola de Artes Ciências e Humanidades, Universidade de São Paulo, São Paulo, SP, Brasil.


Introduction
The cross on rings performed by an Olympic champion CDD.20 e cross is a key skill on rings, which is one event of male artistic gymnastics.It is characterised by maintaining 90 o shoulder abduction in the frontal plane with both limbs for at least two seconds, while the elbows are extended 1 .Moreover, penalties are applicable if a gymnast fails to hold the posture over two seconds or with shoulder angles below 90 o 1 .e cross requires from gymnast to hold his shoulder in an anatomical position that involves the extension of passive structures of the glenohumeral joint, causing shoulder instability [2][3] .
Studies of gymnastics rings rely mainly on handstand [4][5] .Recently, strength skills were studied [6][7] , but no information was provided about muscles cocontraction in the cross 6 .
Considering the kinetics of cross, force plates were used to measure the combined forces which were necessary to the gymnast to perform the cross 8 .
e summed forces should be equivalent or higher as the gymnasts' bodyweight to perform the skills.e time series data showed di erences between limbs; but, none focused about the asymmetry di erences.ere were no papers found regarding the kinematics of this skill.
It is necessary to consider a set of measurements, such as force-instrumented rings 4 , for a comprehensive understanding of a skill 9 .Nonetheless, more knowledge is required about the relation of muscles activity, kinematics and kinetics of cross performed on training or competitions rings, restraining a full understanding of the skill and the drill, and properly compare them biomechanically.
e rst medal for a Latin American gymnast was achieved by a Brazilian on rings event, in 2012

Abstract
The cross is a key skill in Male Artistic Gymnastics rings routines.However, few researches were found about this skill.There is knowledge about the forces needed to perform the cross, or about muscles activation, separately.The aim of this paper was to accomplish a comprehensive research about the biomechanics of cross on rings, in order to obtain a descriptive model about this skill.Therefore, the currently Olympic champion on rings event volunteered in this research.He performed three crosses with the usual apparatus in his training gym.The measurement methods were combined: One digital video camera, one strain gauge in each cable and surface electromyography of nine right shoulder muscles were used.Statistical analyses were performed by parametric and non parametric tests and descriptive statistics.Symmetry values were calculated for shoulder angles and cables of right and left side.Coeffi cient of variation of muscle activation and co contraction were verifi ed.Within gymnast variability was calculated using biological coeffi cient of variation (BCV), discretely for kinematic measures.Low variability values of shoulder angles and cable forces were verifi ed and low values of asymmetry as well.Muscle activation varied according to muscle function, while co-contraction values were different among trials.These results pointed out the characteristics of the cross performed by an elite gymnast.Knowledge about the characteristics of cross can inform coaches, practitioners and clinicians how a successful skill should be presented.One digital camera (model Logitech HD -50 Hz) was placed ve meters away at rings height and facing the frontal plane.e camera was connected via USB to a computer and video recording was controlled with the software MyoResearch (version 3.2, Noraxon, USA).e camera and electromyography (EMG) channels were connected to a data acquisition system controlled by Myoresearch (Noraxon) software for acquisition, synchronizing, analogical/ digital conversion of data and storage.
e calibration frame comprised of six markers made of 20 mm wide re exive tape, xed directly onto the rings frame, forming rectangular solids of three meters high by three meters wide 4 .Re exive markers were placed based in anatomical landmarks according to upper limbs model [10][11] following the International Society of Biomechanics recommendations 11 .Only the anterior view was evaluated for digitalization.
e raw position data was processed and data input into the software Visual 3D (version 5, C-Motion).
For the kinetics, a one dimension strain gauge (EMG system Brazil model 2t) was attached to beginning of each cable ring to measure the cable tension during the task 4 .e longitudinal cable forces of two strain gauges were connected to an analogical/digital converter (EMG system Brazil model 1610).A trigger (EMG system Brazil) was used to synchronize video and force signals.
e EMG signal was measured with bipolarsurface-di erential-active electrodes.e sites for electrode placement were prepared by abrading the skin with ne sandpaper and cleaning with alcohol.Shaving was performed if necessary.e distance between the centres of the disposable electrodes was two centimetres.e placing of electrodes followed the procedures indicated by SENIAN 12 and for muscles not indicated by SENIAN, the electrodes was placed onto the medial line of muscle belly 13 .

Sinal processing
The gymnast has done his warm up exercises, similar to what he usually does before a training section on rings.He performed the cross three times on competition rings 13 .e initial position was when the participant has reached the maintenance position with upper limbs abducted with 90º to the trunk on the transverse plane.e participant has maintained the cross posture for two seconds; then, an oral warning was used to stop the cross.e attempts were considered valid by one gymnastics judge accredited by Fédération Internationale de Gymnastique.Between each repetition, the participant had two minutes to rest.Data collection took place at the gymnast's training gym, with the apparatus he uses to train for competitions.
Videos were digitised and data were ltered with a low-pass Butterworth lter, with appropriate cuto frequency determined by residual analysis 14 .Digitised data of the calibration markers was combined with their known locations to calibrate the camera digitiser system, using the direct linear transformation (DLT) procedure 15 .e known locations of the digitised landmarks on the gymnast and rings apparatus were subsequently reconstructed using the calibrated camera digitiser system based a comprehensive use of biomechanical variables, describing the skill and quantifying the variability of an elite level gymnast performing the cross on rings.

Statistical analysis
Where θ SYM is the symmetry angle; θ left is the gymnast's mean left shoulder angle and θ right is the gymnast's mean right shoulder angle.Symmetry angles were recti ed, allowing the magnitude of those values to be more easily compared between conditions.
EMG time series were compared by means of cross correlation in order to calculate the correlation index R. Cocontraction index is R 2 for lag zero.Cross correlation analysis was performed between all possible muscle pairs.Muscle pairs were grouped according to their function.The agonists (PE, LD, TM and TR), antagonist (DE) and postural (SE, BI, TZ and IF) muscles were grouped into functional groups.e dependent variables were the kinematics, kinetics and EMG variables.e independent variables were side (two levels: left and right sides), and function groups ( ve levels: agonist/ antagonist, agonist/agonist, agonist/postural, antagonist/postural and postural/postural).
e co-contraction index was compared across by means of analysis of variance.Normality tests for the data set were con rmed only for kinematics, by using the Sigmastat Software (version 3.5).
on DLT procedure 15 using Matlab 6.5 (Mathworks Inc) 16 .For those digitizing and reconstruction, a speci c routine "DV5" was run in Matlab 16 .
Image digitalization occurred in Matlab; then, the coordinate data was converted in le converter (C-Motion) and exported into Visual 3D software for calculation of shoulder angle.An upper limbs model 10 was applied to the data points, and the angle between trunk and arm was considered as shoulder angle.Data was related to the gymnast reaching the static posture and the following two seconds of duration.
EMG was normalized by the peak value within the trials.Raw EMG signals was demeaned, recti ed and ltered with a low-pass Butterworth lter of 4th order of 200 Hz.Kinematics, kinetics and EMG data were interpolated to 500Hz.Equation 1Where FR and FL are the right and left forces, respectively.
Percentage di erences between left and right angle values were calculated using the symmetry angle index (θ SYM ) method 17 (Equation 2): Equation 2Symmetry left/right for angles and forces index were evaluated by means of symmetry indexes.Percentage di erence for cable FSYM values were calculated using symmetry index method 17 (Equation 1):

Results
Mean shoulder angles during cross are shown on TABLE 1.Right and left shoulder angles, coefficient of variation (%), asymmetry, standard errors of the mean (%) and biological coefficient of variation (%) values are depicted.Coefficient of variation was lower than 5%, while biological coefficient of variation was lower than 1%.
Mean results of right and left cable forces, coefficient of variation and asymmetry index are showed on TABLE 2. Coefficient of variation was lower than 5%, while Force asymmetry was lower than 10%.
The average electrical activity of upper limbs muscles during the performance of cross at competition rings are presented on TABLE 3. Cocontraction indexes were calculated for all muscle pairs.Those pairs were separated by their functional status agonist, antagonist and postural.Functional relation affected cocontraction (F 5,215 = 2.3 p = 0.04).e gymnast performed the cross with shoulder angle deviations from 90°, which could lead to penalties on competition presentation 1 .is nding may be due to the model that considered the angle between trunk and upper arm 10 .For judging criteria, the set of forearm and upper arm is considered for accounting shoulder angle.Kinematics measurement provides useful information for coaching gymnastics skills, which may subjectively appear to be symmetrical 21 .The variation obtained in static position may occur due to gymnast and rings cables interaction.As the gymnast started from support position, lowering to reach the static position of cross, the action of shoulders abduction may cause the rings swinging, leading to the variations on the shoulder angles measured.
Knowledge of shoulder asymmetry can facilitate the understanding and the development of this gymnastic skill 22 , improving performance and developing more complex skill combinations safely and e ectively 9,23 .During the static cross position, asymmetry directly in uences performance, due to penalties for asymmetrical posture and shoulder angle deviating from 90º 1 .
e experienced gymnast in this study showed reduced biological coe cient variability for shoulder angle.ese could be explained from the performance perspective, due to the fact that this action of abduction is biomechanical key aspect for successful technique performance 1,19 . is is in accordance with the ndings reported by H et al. 24 who showed that in elite gymnasts there was lower variability in the mechanically important aspects of gymnastic performance.
It is possible to observe that, such as angles values, the forces values were not symmetric.However, force asymmetry scores below 10%, as found in the present study, are considered low 25 .Low variability on forces means that a controlled skill is performed 26 .Because the cross is a closed skill, well learned and performed by experts, it is reasonable to assume a stable movement pattern would exist 27 .Asymmetry scores were used to analyse performance in sprint running 28 and to allow for asymmetry comparisons between athletes over time and between asymmetry and performance 21 .In gymnastics, particularly on rings, force asymmetry has direct implications on performance as penalties can be applied for cables swinging 1 .
Muscle activation over trials was not studied in other studies about cross 6 .e coe cient of variation was higher for Pectoralis and Teres Major (agonists) and Infraspinal (postural) muscles.e interaction between anterior and posterior agonists variation was necessary for the maintenance of the position 3 .e cross lasted four seconds, from support until the end of static maintenance, and no indices of skill failures 20 during task repetition were observed, as shoulder angles CV were below 5%.However, co-contraction values were di erent (F 5,215 = 2.3 p = 0.04) among trials, showing that the gymnast employed di erent motor strategies to achieve the same motor task 20,29 .Cocontraction variation may be a strategy used by the motor system to facilitate multi-joint arm skill accuracy 30 .Moreover, any rings swing forward/backward could be present, in a di erent way among trials, due to the unstable apparatus characteristics of construction 4 , in uencing on muscles activation.Furthermore, there are evidences for altered muscle activation associated with shoulder impingement, rotator cu tendinopathy, rotator cu tears, glenohumeral instability, adhesive capsulitis, and sti shoulders 31 .
Besides gymnasts had been questioned about their shoulder conditions and the ability to perform the cross, any of these shoulder clinical conditions could be presented in the participant, and had in uenced on obtained results.Gymnasts performing without clinical evaluation can be a common practice, as they still able to perform even feeling discomfort 32 , and shoulder is the most commonly injured joint in men's gymnastics 5,33 .
is paper brings a high ecological validity about the cross on rings skill, performed by the currently Olympic champion, on his usual apparatus and training gym.e skills were considered similar, with low variability over the measurements, showing the characteristics of performance of elite level on this skill.Values observed for angles and forces symmetry within the skill should be considered as important source of information for coaches, as individual characteristics of performance variation of the cross on rings.Muscular activity suggests that the elite level gymnast enclose different motor strategies to perform the skill.It should be emphasised that coaches need to consider individual capacities when comparing the results depicted here with those found within another gymnasts.

TABLE 2 -
Cable forces (N) mean and SD, CV and asymmetry (F SYM ) values (%) on cross.