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Development of Immediate Feedback Software for Optimising Glide Performance and Time of Initiating Post-Glide Actions Roozbeh Naemi1, Serdar Aritan2, Simon Goodwill3, Steve Haake3, Ross Sanders1.
(1) : Centre for Aquatics Research and Education, (2) : Biomechanics Research Group, School of The University of Edinburgh, St Leonard’s Land, Sports Sciences and Technology, Holyrood Road, Edinburgh, UK EH8 8AQ Beytepe, 06800,Ankara, Turkey 0044 131 651 4117 / 0044 131 651 6521 0090 312 297 6893 / 0090 312 299 2167 E-mail :
[email protected] E-mail :
[email protected] (3) : Sports Engineering @ (4) : Sports Engineering @ Centre for Sport and Exercise Science Centre for Sport and Exercise Sciences Sheffield Hallam University, Collegiate Hall Sheffield Hallam University, Collegiate Sheffield, UK S10 2BP Crescent, Sheffield, UK S10 2BP 0044 114 225 4435 0044 114 225 2429 / 0044 114 225 4356 E-mail :
[email protected] E-mail :
[email protected] (5) : Centre for Aquatics Research and Education, The University of Edinburgh, St Leonard’s Land Holyrood Road, Edinburgh, UK EH8 8AQ 0044 131 651 6580 / 0044 131 6516521 E-mail :
[email protected]
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Abstract: Performance in starts and turns is a major contributor to success in swimming and is influenced greatly by the glide efficiency and the timing of commencing the post-glide action (including kick in all strokes and the underwater pull in breaststroke starts and turns). The main aim of this research is to develop and test ‘user friendly’ software for providing immediate feedback to swimmers and coaches to optimise glide performance and time of initiating post-glide actions in starts, turns, and the glide phase of the breaststroke. The developed software reads the video file of a glide followed by the post-glide action (PGA) gathered from a single underwater camera positioned perpendicular to the swimmer’s trajectory path. A set of body markers are digitised to provide the records of displacement data. The mathematical model based on the ‘hydro-kinematic method’ (Naemi, 2007) is coded in MATLAB to calculate the glide efficiency parameters, reconstructed instantaneous velocity, and the actual and optimal times and distances of commencing post-glide action using the raw displacement. A Graphic User Interface with user friendly icon based system enables the results to appear in an aesthetic and effective screen display, which includes a video replay, displacement and velocity graphs, and tabulated results. Early tests revealed that the effectiveness of the ‘Hydro-kinematic’ method in ‘fine-tuning’ performance can potentially be improved by the developed software that enables rapid turnaround of results. The accompanying video replay enables qualitative assessment of postures and orientations enabling refinement in subsequent trials. The information from the software is also particularly important in predicting the exact timing for initiating the post glide action for a particular swimmer with a distinct glide efficiency and post-glide performance. Key words: Glide, PGA Timing, Immediate feedback, Glide Efficiency, Streamlining,
1- Introduction Performance in starts and turns is a major contributor to success in swimming (e.g. Chatard et al., 1990) and is influenced greatly by the glide performance and the timing of commencing the post-glide actions (including kicking in all strokes and the underwater pull in breaststroke starts and turns).The average velocity over a glide as the indicator of performance is dependent on the initial velocity of glide (release velocity from the wall after push-off) as well as to the changes in velocity over the glide period. The characteristic of body that determines its ability to maintain its velocity, and to avoid retardation over time (deceleration) is known as glide efficiency (Naemi, 2007).
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While the initial velocity of a glide is related to the preceding action, such as pushing off the wall in turns, the glide efficiency is dependent on both the virtual mass of the swimmer, being the sum of body mass and the added mass of water, and the passive drag. Due to the differences in the swimmer’s posture during a real glide compared to towing at constant velocity and the inherent difference between the steady state and transient flow conditions, the drag forces found based on constant velocity towing methods can not represent the drag forces in an actual glide under real conditions in which velocity decreases. As the added mass of water for human body during deceleration can not be determined with theoretical and experimental approach, the glide efficiency of body could not be calculated. Recently an innovative method known as ‘Hydro-Kinemtaic’ was proposed by Naemi (2007) that quantifies the glide efficiency in an accurate and reliable way using a ‘Hydro-Kinematic’ Method (HKM) (Naemi, 2007). The HKM requires only a single underwater camera positioned perpendicular to the swimmer’s glide plane, any 2D digitising software, and the mathematical model (Naemi, 2007). The glide efficiency of body depends on the drag force which is affected by the shape and alignment of a body. Thus to maximise glide efficiency one should adopt the most streamlined position, that can be achieved by adopting a body alignment with a minimum angle of attack and maintaining an appropriate posture with suitable joint angles. In addition to the need to increase glide performance, it is highly desirable to optimise timing of the post-glide phase. It has been established that maintaining a passive streamlined posture at higher velocities than a critical value is more beneficial than an active propulsive movement (Sanders and Byatt-Smith, 2001). This has important implications as a swimmer can lose speed unnecessarily if the passive glide is finished prior to or continued beyond an optimum speed. In these cases, additional energy and time is required to regain the optimal velocity. By knowing the initial velocity and glide efficiency, an accurate record of instantaneous velocity can be reconstructed for a glide trial. It is obvious that the instantaneous velocity of a body during the whole period of a glide can be directly calculated from the raw position data. However, due to variations in the velocity as a result of instabilities in flow (Howe et al. 2001; and Wang et al. 2003) that cannot be distinguished from noise, the determined instantaneous velocity data includes excess noise and may not be valid for comparison (Klauck and Daniel, 1976; Bilo and Nachtigall, 1980; and Naemi and Sanders, 2004). The HKM overcomes that problem by enabling precise reconstruction of the velocity variations of the body during a glide. This information is particularly important in predicting the exact timing for initiating the post-glide action for a particular swimmer with a distinct glide efficiency and post-glide performance. For example for each subject the glide should end when the velocity reaches the velocity which can be sustained by the post-glide actions. Although the HKM developed by Naemi (2007) can readily determine the glide efficiency and the time of initiating the PGA, its use in the field was limited by the time required to conduct the analysis (two hours for each trial). It was realised that the effectiveness of the method in ‘fine-tuning’ performance could be increased greatly by developing software that enables rapid turnaround of results so that swimmers can respond to feedback and adjust technique immediately. The main aim of this project was to develop and test ‘user friendly’ software for providing immediate feedback to swimmers and coaches to optimise glide efficiency and time of initiating post-glide actions using ‘Hydro-kinematic’ method. This could have particular applications in improving the performance of starts, turns, and breaststroke mid-pool swim.
2- GlideCoach
Software
The software developed for this purpose ‘GlideCoach’, comprises MATLAB routines for digitising body landmarks and calculating the variables of interest via a Visual Basic user interface. The interface is designed in an aesthetic and user-friendly fashion resembling successful commercial software in the field of sports performance. The digitising module incorporates a tracking algorithm coded specifically for tracking markers underwater. It enables automatic or semi-automatic (allowing user intervention when the point being tracked is ‘lost’) tracking of markers. The calculation routines employ advanced curve fitting and optimisation techniques to calculate all the parameters related to glide performance and post-glide action timing based on the ‘Hydro-kinematic’ method (Naemi, 2007). The parameters of interest are: The parameters related to glide performance including Glide Factor as a measure of glide efficiency as well as the Initial Velocity as a measure of push-off performance. Also an average velocity as a measure of glide performance was calculated based on the values of glide factor and initial velocity that incorporated both the push-off performance and glide efficiency. In addition to these the reconstructed instantaneous glide velocity as well as the sustainable PGA velocity were calculated. This information was used to calculate optimal and current velocity, displacement and time of initiating the PGA. 2-1- Glide Performance and PGA Timing modes (stages)
The software comprises two modes - ‘Glide Performance’ and ‘PGA Timing’ to quantify parameters related to glide performance and timing of initiating PGAs respectively. Having two distinct modes in the software enables the user to focus on improving performance of each independently. The usual sequence is to use the ‘Glide Performance’ mode first as appropriate PGA timing varies according to the level of glide performance. 2.2- Preparation of subjects
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Prior to the pool test, each subject is marked with the permanent black markers at five anatomical landmarks including the styloid process of the ulna, head of the humerus, greater trochanter of the femur, level of the most lateral side of the patella (the lateral epicondyle of the femur), and the lateral malleolus of the fibula. These were marked on the left side of the body to represent the joint centres at the wrist, shoulder, hip, knee and the ankle. A marker on the left side of the head at the eye level on the side of the goggle was used to assess consistency in the head angle during the trials. The markings facilitated the process of automatic tracking as a faster and more desirable option than the manual digitizing. The subject was then instructed to push off the wall and to adopt a streamlined position during the glides. A submersion, preparation and push-off protocol was taught to the subject in order to facilitate glide followed by PGA in a horizontal rectilinear path so that to enable the HKM to be used to quantify parameters related to glide performance and PGA timing in each mode (Naemi, 2007). 2-3- Filming Requirements AVI files of the swimmer performing passive glides only (for Glide Performance mode) or a passive glide followed by a PGA (for PGA Timing mode ) are input to the software. For the calculations to be accurate the followings are required: 1- A camera sampling at either 25 frames per second, (or 50 fields in case of interlaced video) needs to be positioned underwater 10 m from the subject’s intended glide path with its axis perpendicular to the intended glide path. 2- In the ‘Glide Performance’ mode the camera needs to be zoomed to a field of view of approximately 6 meters in the direction of the glide to ensue that it covers adequate duration (1.5 sec) of glide. Although at the beginning of each session the camera view should be adjusted by some trials to make sure that adequate glide duration of 1.5 s is covered. 3-In ‘PGA Timing’ the camera needs to be adjusted to a field of view that covers the swimmers glide and PGA of adequate cycles (3 kick cycles in front crawl, butterfly and backstroke, or a complete long pull in breaststroke). In this case the field of view needs to be approximately 9m wide. This needs to be specifically adjusted for a swimmer depending on the level of performance and height. 4-A second camera needs to be used on a boom at a 5 m height directly above the subject’s line of glide (or PGA) with its axis perpendicular to the intended glide path. This needs to make sure that the subject does not deviate from the ‘T line’ over the period of glide or glide and PGA. 2-4- Capturing and importing The recorded video can be imported to the software. The details are filled in for each subject including name, session date, time, the trial number as well as the sampling frequency of the camera. This information is stored in a database that allows comparison of performance within subject, which allows the current performance of swimmer to be compared with the best and previous performance to assess the improvement and set targets. This feature will be further explained in the ‘Results’. 2-5- Playing and trimming In the player the video of an untrimmed glide including submersion, preparation, push-off and glide (together with PGA in PGA Timing) can be played. The video control panel both allows scrolling and moving frame by frame that facilitates finding the exact frame to start the trim (i.e. from toe-off). The end of trim will be chosen automatically as the frame that comes in 1.5 seconds after the ‘start trim’ frame. As in order to be able to compare the glide performances across trials 1.5 s glide duration was chosen based on the average glide duration after turns for swimmers at elite level. In ‘PGA Timing’ the end trim will be determined as the end of three cycles of kick or the end of long pull in breaststroke. This is the end of the third cycle for kick that is at the start of the forth cycle just before when the legs start to come up for starting the forth cycle. In breaststroke the end of long pull determined as when the swimmers hand reaches the hip marker, is considered as the end trim point. Another point known as ‘switch point’ that is the starts of PGA is determined. The switch point is the start of the first cycle that is determined as the moment when the legs tend to come up (i.e. hyperextension, or knee flexion) in cases where kicking is used as a PGA. In breaststroke when long pull is used as a PGA, the switch point is when hands get apart. The playback feature allows playing both the video at different speeds including slow and fast forward that is particularly useful for visual feedback. 2-6- Analysing The trimmed video is digitized in the ‘Analyser’ that allows both manual and automatic tracking of markers. First the number of joints that needs to be digitised is chosen in the ‘Digitising Points’ panel. The zoom in feature provides a larger view of the skin markers that facilitates exact positioning of the digitising points on the body as well as monitoring the accuracy of tracking in all frames. The hip marker’s horizontal position is found to be more stable compared to other joint markers when the body alignment changes over a glide period. Thus the glide factor calculated based on the hip marker is more reliable in reflecting the changes
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to the body alignment (and angle of attack). For these reasons the hip’s horizontal position data was used for calculating all the parameters related to glide and PGA. Hip marker has also the advantage of being visible from camera view in PGA of all four strokes while remains fairly stable in vertical direction. Therefore all calculations related PGA Timing were based on the hip marker position data which assures consistency of calculations. The hip marker is the minimum that needs to be digitised at all trials. Digitising other markers can be done for finding the joint angles, a feature that is going to be added to the software in future version. After the position of hip was pointed at in the first frame of the trimmed video, the auto analyse next frame button will send the initial positions of the markers in the initial frame as well as the current and the next frames to a tracking code. 2-7- Tracking Code The main problem is to determine the position of a given pattern in an image, so called region of interest (ROI), which in this case, ROI is the skin markers. One basic approach that can be used is template matching. This means that the position of the given pattern is determined by a pixel-wise comparison of the image with a given template that contains the desired pattern. We used normalised cross-correlation (NCC) in our tracking algorithm. In order to utilise the algorithm a code is written in MATLAB that provides the position of the marker in the next frame, knowing the initial marker position using images from the current and the next frame. The same procedure is followed up to the last frame. As the tracking code was specifically developed for underwater video, the code is more reliable tracking than the commercial software available. In early pilot works it was found that tracking frame by frame needs just some minor manual corrections in some cases. Further developments are underway to enable full automatic tracking that needs no intervention in the future version. 2-8- Calibration The tracked coordinates then are converted to the real position data using the two-dimensional Direct Linear Transformation (DLT) techniques (Walton, 1981). The Calibration panel allows a numbers of reference points on the ‘T’ line at the bottom of the pool and on the push-off wall with known coordinates to be digitized and used for calibration Also a feature in the calibration panel allows the use of a previously made calibration file to make calibration easier in consequent testing when the camera is fixed between trials. 2-9- Calculation Core Code The digitized coordinates then will be sent to two Matlab codes developed separately for Glide Performance and PGA Timing. 2-9-1- Glide Performance Core Code
The Glide performance core code receives the position data of glide along with the frequency at which the data was collected at. The code calculates the glide efficiency parameters based on the ‘Hydro-kinematic’ method (Naemi, 2007). This is achieved by fitting a logarithmic function that represents the position changes of a representative body to the raw position data of the swimmer’s body during a rectilinear glide. As the logarithmic function was derived from the equation of motion of the body during a horizontal rectilinear glide, the values of the curve parameter that provides the best fit for the data determine glide performance parameters. These include Glide Factor (m) and Initial Velocity (m/s) from which an average velocity can be calculated. 2-9-2- PGA Timing Core Code
In PGA Timing module in addition to the displacement, and video frequency, the frame number for switching from glide to PGA is sent to the PGA Timing code. In PGA Timing code the raw position data corresponding to glide (the data sequence up to the switch point) is used to calculate the glide parameters including the Glide factor, initial velocity and average velocity using Hydro-kinematic method (Naemi, 2007). The raw position sequence corresponding to PGA performance is used to calculate the sustainable velocity of PGA. combining the reconstructed instantaneous velocity of glide that can be obtained based the initial velocity and glide factor, and the sustainable PGA velocity, the time at which the glide velocity reaches a velocity that can be sustained by kick is quantified. Based on these the parameters related to timing of PGA that includes current and optimal velocity, time and distance of initiating the PGA are calculated (Naemi, 2007). 2-10- Results The results in each mode are presented separately. 2-10-1- Glide Performance
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In the results panel in addition to the video and the parameters related to glide including Glide Factor, Initial Velocity and Average Velocity are displayed (Figure 1- left). The accompanying video replay enables qualitative assessment of postures and orientations enabling refinement in subsequent trials.
Figure 1: Screen shots of the results tab in the Glide Performance (left) and PGA Timing mode (right)
As the higher the glide efficiency is an indicator of a better streamlining position including body posture and alignment (segmental angle of attack), the best streamlined position of the swimmer is achieved using a trial and error approach. In this approach the swimmer is advised to adopt different joint angles and segmental angle of attack, from which the best streamlined position can be chosen based on having highest glide efficiency. In addition to glide factor, the initial glide velocity (wall release velocity) is displayed that represents the effectiveness of push-off both in terms of the push-off force and proper positioning of upper body and trunk during the push-off itself. As the overall glide performance is dependent both on the initial velocity as well as the glide efficiency, the average velocity is calculated and displayed as an indicator of the overall glide performance. The best glide performance as the one with highest average velocity, along with the previous performance appears in a table to allow monitoring progress and setting targets. In case of two glide trails with the same average velocity, the one which has higher glide factor is chosen as a better one, as the advantage was gained with less physiological cost (Naemi, 2007) All the three parameters related to glide performance are displayed in an average velocity contour graph with initial velocity and glide factor displayed in the horizontal and vertical axes respectively (figure 1- left). The contour graph is coloured in shades ranging from red (dark in grayscale) at lower left corner that represent poor performance, to green ( light in grayscale ) at upper right corner that represents high performances. This facilitates initial interpretation of results and enables the coach to determine areas that need to be worked on in order to increase the glide performance e.g. push-off or streamlining. 2-10-2- PGA Timing
In PGA Timing in addition to the parameters related to glide performance of the glide part of the trial, the time difference between the current and optimal time and distance (from wall) of initiating PGA is to advise swimmer on either delaying the initiation of PGA to a later time, or initiating it earlier than the current time. The better trial is determined as the one which has close to zero time/distance difference. A criterion for the overall glide and PGA is under development, that is intended to appear in overall performance bar in future version. In addition to these a velocity-time diagram of the performance is shown in the ‘Results’ section that allows the changes of velocity over the period of glide as well as the current and optimal velocities of initiating PGA to be displayed graphically ( figure 1-right). Two dashed lines show the current and optimal time of initiating PGA on the time axis. Two distance lines were also reproduced on the video in the positions corresponding to real distances to allow the swimmer to realize the optimal and current positions of initiating the PGA. The PGA Timing panel allows displaying both the optimal and the current time, velocity and distance in a table that facilitates more insight into the timing parameters. As it can be seen in this sample the PGA was initiated 0.2 sec earlier than an optimal time. The kicking over this 0.2 sec causes the velocity to drop faster as compared to if the swimmer continues to passive glide. This is due to the fact that the net decelerating force (the difference between the active drag and the propulsive force) is greater than the passive drag for a gliding position (Lyttle, et al., 2000).
3- Discussion and further developments The development of the ‘GlideCoach’ software provides for the first time the ability to apply the Hydro-kinematic method to improve the streamlining posture and optimising the time of initiating PGA on the poolside. Initial pilot works with the software revealed that feedback to the swimmer can be provided in less than 5 minutes from the start of the trial. This enables
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the swimmer to be benefited from the developed method without being affected by the prolonged procedures of trimming, tracking, digitising, importing and exporting files from and to different packages which was approximated to last more than two hours for each trial not using the software. The software is also one of the few in the field that provides immediate quantitative feedback on the kinemetaic parameters related to performance and indeed the only available software in the field of sports performance that provides immediate feedback on a complex parameter such as glide factor that has both a kinematic and a hydrodynamic entity. Further developments are underway specifically with regards to fine tuning the tracking code so that the tracking could be done with minimal manual intervention. Also for the future version the values of joint angles is going to be included in the result section of Glide Performance software. Further improvement in the software is planned after its use for a larger numbers of swimmers on the poolside. In addition providing overall performance in PGA Timing and adding other features like calculating the ‘Glide Coefficient’ (Naemi,2007) that allows comparison between subjects in terms of their glide efficiency can be added to the software.
4- Conclusion The ‘GlideCoach’ software was developed that provides immediate feedback on the glide efficiency and timing of PGA initiation using ‘Hydro-kinemtaic’ method. The software was developed as a ‘user friendly’ menu driven interface. The autotracking algorithm was developed in Matlab using advance image processing technique that enabled tracking markers to be fast and reliable. The calculations were based on the Hydro-kinematic method and done in two core codes for each mode in Matlab that provides the important parameters related to glide performance and timing of PGA initiation in each mode. Output the results in an aesthetic and effective screen display that include a video replay, graphs, and tabulated results can potentially improve the effectiveness of the software and its use in the field. By facilitating immediate feedback in applying the ‘Hydrokinematic’ method to quantify the swimmer’s glide performance and timing of PGA initiation, the ‘GlideCoach’ software can provide an opportunity, for the first time, to ‘fine tune’ swimmers’ streamlining posture as well as to optimise timing of initiating post-glide action in the field. References [BN1]Bilo, D. & Nachtigall, W. (1980). A simple method to determine drag coefficients in aquatic animals. Journal of Experimental Biology, 87, 357-359. [CL1]Chatard, J. C., Lavoie, J. M., Bourgoin, B., & Lacour, J. R. (1990). The contribution of passive drag as a determinant of swimming performance. International Journal of Sports Medicine, 11(5), 367-372. [HL1]Howe, M.S., Lauchle, G. C. & Wang, J. (2001). Aerodynamic lift and drag fluctuations of a sphere. Journal of Fluid Mechanics 436, 41–57. [KD1]Klauck, J. & Daniel, K.(1976) Determination of man’s drag coefficient and effective propelling forces in swimming by means of chronocyclography. In Komi.P.V. ed. Biomechanics VB, Intentional Ser. on Biomechanics. Vol. 1 B, PP.250257 [LB1]Lyttle, A. D., Blanksby, B. A., Elliott, B. C., & Lloyd, D. G. (2000). Net forces during tethered simulation of underwater streamlined gliding and kicking techniques of the freestyle turn. Journal of Sport Sciences, 18, 801-807. [N1]Naemi, R. (2007). A Hydro-kinematic Method for Quantifying Glide Efficiency of Swimmers. Doctoral Thesis, The University of Edinburgh, Edinburgh, U.K. [NS1]Naemi, R. & Sanders, R.H., (2004). A Comparison of Two Functions Representing Velocity of a Human Body Subject to Passive Drag. In Proceedings of XXII International Symposium on Biomechanics in Sports, Ed. Lamontagne, M; Gordon, D., Roberstson, E. and Sveistrup, H., pp. 430-433, University of Ottawa, Canada. [SB1]Sanders, R.H., & Byatt-Smith, J. (2001). Improving feedback on swimming turns and starts exponentially. In J. Blackwell and R.H. Sanders (Eds.) Proceedings of Swim Sessions XIX International Symposium on Biomechanics in Sports (pp. 91-94). San Francisco, California, June 26, 2001. [W1]Walton, J. S. (1981). Close-range Cine-photogrammetry: A Generalized Techniques for Quantifying Gross Human Motion. Unpublished dissertation at Pennsylvania State University [WL1]Wang, J., Lauchle, G. C., & Howe, M. S. (2003). Flow-induced force fluctuations on a sphere at high Strouhal number. Journal of fluid and structure. 17, 365-380 Acknowledgement We thank funding support provided through EPSRC and UKsport as well as the support of Scott Drawer, Ian Wright, Andrei Vorontsov, Tom Bruce, Brian Blanksby, Gareth Irwin, Bill Easson, Barry Wilson and Khelil Sefiane.
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