A Wireless, Unobtrusive Kayak Sensor Network Enabling Feedback ...

2010 International Conference on Body Sensor Networks

A wireless, unobtrusive Kayak Sensor Network enabling Feedback Solutions Dennis Sturm

Khurram Yousaf

Martin Eriksson

School of Technology and Health Royal Institute of Technology Stockholm, Sweden Email: [email protected]

School of Technology and Health Royal Institute of Technology Stockholm, Sweden Email: [email protected]

School of Technology and Health Royal Institute of Technology Stockholm, Sweden Email: [email protected]

a new movement. After such a training period, the overall performance should increase to a level higher than before the relearning was initiated. During this training period, though, it is essential to receive feedback based not only on the result but also on performance in order to motivate and control the learning process as well as to increase its effectiveness [4]. The system presented in the following provides data for real-time feedback to facilitate KP in kayaking. Athletes can manipulate the magnitude and slope of the developed force [1]. The athlete and/or the coach will thereby be able to immediately react upon suboptimal training. If a coach is given additional real-time data that can easily be interpreted. The coach will be able to provide better suggestions and motivation to the athlete. A coach outside the laboratory has very little data to use in order to assess the athlete’s performance. Furthermore, our design can contribute to ongoing research in making training on a kayak ergometer more similar to training on water. Research building on previous work such as [5]–[7] can gain leverage from a flexible system that is applicable both on water and to an ergometer. We have not been able to identify any previous studies examining the specific paddle and foot stretcher forces in a kayak with a wireless sensor setup. Specifically, [6] state in their review paper the need to analyse the technique and movement patterns in an on-water environment to derive a scientific description of the kayak paddling technique. The necessity and importance of this description is derived from the conclusion that at a given output by the athlete the force on the paddle should be maximized [6], [8], regardless of the boat design.

Abstract—Canoeing is a very competitive sport involving a non-trivial pattern of motion. A group of athletes and coaches approached the authors for aid in quantifying what until today only is qualitative, personal and thereby subjective data. The objective of this work is to present a measurement tool that records paddle and foot stretcher force in a flatwater kayak training situation, i.e. when training on the water. The system facilitates a wireless (Bluetooth) star network link with three sensor nodes and one central unit. Validation data was obtained from a kayak ergometer that is equipped with analysis software. The stroke power obtained from this ergometer system is compared to the force data measured by the presented wireless sensor nodes. We have not been able to find any similar systems that would provide better data for performance analysis.

I. I NTRODUCTION Canoeing/kayaking is an Olympic sport where a boat is propelled through water by paddling. In the Olympic context there are two disciplines: slalom and flatwater canoeing/kayaking. This paper presents a wireless system primarily for use in flatwater kayaking to measure performance in real-time in terms of paddle force and force exerted to the foot stretcher. As these two are the main sources of force input to the system [1] it should be extremely helpful to get information about their magnitude and interaction in real-time. Such realtime information is rarely used in sports training today. So far, in an on-water training situation, a trainer has to mainly rely on his or her visual impression and can only measure time and distance travelled. However, no information on force measurements in the paddle or on the foot stretcher or other parameters is available in real-time. Video analysis post-training can reveal further information but even this information is mostly qualitative. There are indications, though, that live feedback enhances the results from learning/relearning of a motor task [2]–[4]. In motor learning, there is a distinction between knowledge of result (KR) and knowledge of performance (KP). KR means that the athlete rates the quality of a trial based on the overall outcome. In KP the success of a trial is based on how the result was achieved. In other words, the athlete can receive positive feedback even though the overall result of the trial was not outstanding. This may seem to be counterproductive at first, but when changing a technique, the overall result will initially drop before the athlete has properly relearned 978-0-7695-4065-8/10 $26.00 © 2010 IEEE DOI 10.1109/BSN.2010.24

II. S YSTEM R EQUIREMENTS The data recorded by the sensors are used to describe the major contributing factors for the kinematics and kinetics in a kayak/athlete system. Secondly, the data has to be relevant for the purpose of implementing a feedback channel to the athlete. The aim of this work is to present a wireless system with a user-centred design in terms of physical measures and usability. The development has been driven in close cooperation between elite athletes, trainers and researchers. It has been the foremost important design intention to create a system that 159

In literature [5], [9] a range of typical stroke frequencies is given. In determining a standard paddling frequency for the validation of the presented system we have chosen to specify higher sampling frequency requirements than in the reviewed literature in order to record even more time details for a stroke: As the duration of the pull-phase is approximately 0.5 s, we chose a sampling frequency of 100 Hz, as it produces enough data-points to properly analyse the force profile during a stroke. B. User considerations Fig. 1.

To meet the desired user friendliness, several aspects were regarded as being crucial. Obviously, the system must be very light in order to not affect the technique. Furthermore, the setup of the system has to be easy, quick and must not require technical skills. To the targeted user it should solely appear as a device that will help and improve a training session. Therefore the system’s installation and operation has to conform to intuitive handling procedures. In addition, the recording and feedback unit should be a system that the user is already familiar with. The battery, which in most mobile applications is of a considerable weight and volume in order to produce a satisfactory operation time, had to be carefully chosen so that the power consumption of each node can be accounted for within a reasonable amount of time. Choosing a wireless connection between the sensor nodes makes the system less obtrusive as there are no cables that could disturb the athlete or that would pose a liability for the system’s functionality. Nowadays, radio standards are available that can assure the necessary range and quality of a connection. Consumers today are very acquainted with wireless links (such as GSM, UMTS, WLAN, Bluetooth, wireless USB etc), primarily due to the evolution of mobile phones. Therefore we reason that the additional complexity of this form of data transmission should not be experienced as barrier. To cater for the generally tight budgets of athletes and sports clubs the system cost should be kept as low as possible.

One stroke, visualised for the left side of the paddle.

can easily be used in daily training and that conforms to an athlete’s expectation from a technical aid. A. Data considerations A stroke is generally defined as one cycle of the periodic movement that includes paddle entry (catch), pull phase, paddle exit and air phase [8], [9]. Figure 1 illustrates a stroke for the left arm being the lower arm. The process follows analogously for the right side. The beginning of a stroke is defined as the instant of the paddle entering the water. A propulsive force is generated in the motion during the pull phase and until the paddle exits the water. It has been our intention to investigate the profile of this force and to present this data to athletes and coaches in a quantitative manner in order to better understand and improve the training process and thus enhance the effect of the training. The paddle force Fpaddle accelerates the athlete in the boat during the stroke phase and is therefore the most important parameter for the competitiveness of an athlete-paddle-kayak system Sapk . For the straight paddle position it can be stated that: Fpaddle = mab · aathlete+boat (1) with mab = mathlete + mboat being the mass of the athlete and the kayak. The paddle mass is negligible. To be more general, only the component aligned with the long axis of the boat contributes to the propulsion of Sapk so that Equation 1 has to be converted into a vector notation in order to describe any other instant of the pull phase. It has been our interest, however, to focus on the athletes biomechanics rather than on the kinematics of the boat. With this in mind we have decided to measure the force perpendicular to the paddle blade. As the kinetics of kayaking has a lot in common with that of rowing, we assume the the conclusions from [10] which state that rowing effectiveness critically depends on the right position and usage of the foot stretcher, is applicable in this work as well. The boat is “kicked” forward during the pull phase of a stroke and therefore we have designed the system to also measure the force exerted perpendicular to the foot stretcher for each foot independently.

III. S YSTEM D ESIGN The presented system for the kayak encompasses two identical, easily detachable nodes measuring the bend of the kayak paddle and a modified foot stretcher measuring the centroid of pressure for each foot. A central unit (a mobile phone or a personal computer) is used to collect and store the data. Communication between the four components is implemented by a Bluetooth radio in a star shaped network. The central unit is responsible for initiating the connection. Figure 2 shows one of the sensor nodes designed to measure the flexure of the paddle during the pull phase of a stroke. Because the blades of a paddle are tilted, one node is aligned with each side for optimised measurement performance. A plastic screw in an aluminium U-shaped arm accounts for the fixation onto paddles of different diameters. This ensures quick fixation and detachment of the sensor node without the

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Fig. 2.

Fig. 3.

Paddle sensors.

Foot stretcher force measurement.

TABLE I M ASS AND DIMENSIONS OF THE SYSTEM ’ S COMPONENTS

requirement for any tools and further alteration of the sports equipment. The electronics, which provide the functionality discussed, are located in a shock- and waterproof housing. The prototype still makes use of an off-the-shelf plastic box, but will be custom designed for volume and mass reduction in the future. All optical indicators, switches and contacts conform to industry standards for water resistivity (IP-65 to IP-68). This user interface complies with the requirements described in the previous section and offers possibilities for further development. To measure the flexion of the paddle evoked through perpendicular force on one of the blades during the pull phase of a stroke, four strain gages (HBM 1-LY-48-6/350, Darmstadt Germany) are mounted in a Wheatstone full-bridge onto a 50 mm long lever. The strain gages are covered with a special thermoplastic to seal the contacts. A plastic screw at the end of the lever is used to adjust the distance of the lever end to the paddle shaft and thereby configures the preload of the Wheatstone bridge to a defined level. A light emitting diode (LED) indicates proper positioning and adjustment. The induced imbalance in the Wheatstone bridge is fed into an instrumentation amplifier and the signal amplified in two stages. It is also possible to tune the sensitivity of the system by adjusting the amplifications. The signal is then fed into a 10 bit analogue-to-digital converter (ADC). The data is sampled at 150 Hz by each of the two paddle sensor nodes, which exceeds the aspired time resolution of 100 Hz stated as a requirement in the previous section. The ADC, which transforms the analogue signals into binary data, is embedded in a Renesas microcontroller (M16C/62P) which itself is part of a commercially available Bluetooth platform (EISTEC Mulle V3.1, Lulea Sweden). This platform also provides a real time clock, 2 MB of memory for temporary data storage and a battery monitor. On the foot stretcher, force sensitive resistors (TekScan A201-100, Boston USA) transfer the force of each foot into an electrically measurable quantity. For this purpose a custom foot stretcher was built that replaces the original wooden piece with a carbon-sandwich and aluminium construction at approximately the same weight and with only a slightly

System Component Left paddle node Right paddle node Foot stretcher control unit Modified foot stretcher



Dimensions mm3



100 x 80 x 80 100 x 80 x 80 100 x 60 x 70 210 x 140 x 25

Mass [g] 184 187 192 390

increased thickness. The sensors on this custom foot stretcher are allocated in a way to enable measurement of the centroid of every foot’s pressure force, see Figure 3. The main processing unit of the foot stretcher is the same Bluetooth platform (EISTEC Mulle V3.1, Lulea Sweden) as in the paddle nodes. Each of the force sensitive resistors is coupled to a single stage operational amplifier. Once the signal from the 8 sensors in the foot stretcher has been amplified it is sampled at 100 Hz for all channels by the microcontrollers internal ADC. The Bluetooth link transmits the data to the host of the system via a Serial Port Profile (SPP) protocol. Because virtually every athlete and coach has access to a mobile phone, and is familiar with its use, we have targeted these handheld devices as being the central host unit of the presented system. All recent mobile phone models feature a colour display, a Bluetooth radio and the ability to run third party designed code. Some of today’s more advanced models (e.g. the Apple Inc. iPhone series and many products by HTC) even provide a touch screen interface, and very powerful processing power with clocking speeds of up to more than 1 GHz. The dimensions and mass of each of the nodes has been measured and is listed in Table I. It should be noted that the paddle node dimensions include the lever, electronic housing and mounting device. The next development stage will have considerably reduced size and weight. IV. E XPERIMENTAL R ESULTS For the measurements presented in this paper, a personal computer running a custom program written with Matlab (V7.8.0.347, The MathWorks, USA) emulates the mobile phone both to provide benchmarking capabilities and for generalisation purposes. The three Bluetooth SPP links were 161

TABLE II S ENSOR CHARACTERISTICS Sensor

max. load [N]

linearity error [%]

noise [bit]

380 380 1500 1500

1.6 4.0 2.5 1.8

2.4 0.7 3.6 3.2

Left paddle node Right paddle node Left foot Right foot

Fig. 4. Sketch of the exercise scenario. The dashed blue line represents the rope to the flywheel. Angles and dimensions may deviate in the sketch.

established via an external USB Bluetooth dongle (CSR, Cambridge UK). A. Static Behaviour

in rowing machines, was set to level 8 (of 10) to provide a defined resistance for the training subject on the ergometer. The two sensor nodes dedicated to measure the force on each side of the paddle were mounted on the rod, i.e. the virtual paddle, of the ergometer at an angle of 30 ◦ . This angle was chosen to simulate the twist, which can be seen on paddle blades when used on water. The placement of each of the two nodes was chosen to be in between, and 120 mm away from, the inner end of the gripping areas of each hand. The distance between the gripping areas was standardised to 800 mm. With respect to Figure 2, the distal end of the paddle is on the right side of the picture and the grip area for the left hand is located to the right of the box in this figure. (Please, note that the photo was taken from the virtual bow, i.e. the front of the ergometer.) Figure 4 is a simplified sketch of the ergometer setup. The position of the blades in an on-water scenario is indicated by thick, dashed lines. During a training set the athlete was supposed to row on three different power output levels: 80 W, 120 W and 160 W. The transition time was not specified. The athlete was asked to try to keep a constant stroke frequency. After a training set, the data recorded by our system was compared to the data provided by the ergometer. Figure 5 visualises a 90 s window of the data recorded from one training set. Light grey, dash-dotted horizontal lines mark the three power levels.1 The ergometer’s power measurement (dashed, blue line) is presented along both paddle forces (right side: solid, red line; left side: dash-dotted, black line). The graphs nicely show the correlation of power measurement and the stroke forces and the transitions in power output levels. Figure 6 shows a more detailed view on the data of one stroke. At the presented sample rate it is possible to analyse details of the stroke force profile and to measure and compare timing between arm and foot action, as the foot data has been added in this view. The graph shows two strokes, one stroke on the left side followed one stroke with the right arm and foot. We are looking forward to investigating profile charac-

Every paddle or pole on a kayak ergometer is unique and hence the sensor nodes need to be calibrated for every use. At the presented development stage manual calibration is still needed before every measurement. This procedure, which contradicts the usability aspect of being able to quickly mount the sensors to any paddle, will be considered in future improvements and should become redundant. Calibration of the pole was done similar to the method used by [11]: While one end of the pole was fixed, the other end was loaded with calibrated weights. The middle of the pole was supported to assure a defined pivot point. This procedure was done at both ends, i.e. both sensors. Calibration data was obtained to guarantee reliable data up to the maximum expected load. Table II shows that both paddle sensors can measure loads of up to 380 N. The prototypes differ slightly in their accuracy and linearity characteristics. The foot stretcher measurement unit replaces the original foot stretcher, that is normally mounted on the ergometer (or in a kayak). Because this measuring unit is an independent system it was pre-calibrated with an electromechanical testing system (Instron 5567). The four sensors mounted for each foot can detect the pressure and its centre for each foot. Each foot pressure force has been calibrated to reliably measure up to 1500 N. This range will be decreased for future studies to improve noise characteristics. Table II lists the sensor’s performance parameters. B. Dynamic Scenario The system has been tested against a kayak ergometer (Dansprint, Hvidovre Denmark) equipped with a rowing computer that outputs performance data (speed, stroke rate, power for each side separately, stroke distance and water distance over time as well as air time and water section) via a serial port and a special software (Dansprint Analyser V1.10, Hvidovre Denmark) to a computer. This data lacks force profile information and was therefore only used to assure a consistent performance. Two recreational kayakers provided the test data by training on the ergometer. Training data was recorded after a short warm-up phase over a 250 m rowing distance (measured by the computer) with a stroke rate of approximately 84 spm (strokes per minute), which is a realistic frequency for competitive paddling [5]. A flywheel, an airbrake identical to those found

1 Please note that the power scale is 0.6 W in order to improve the visual information.

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V. D ISCUSSION AND F UTURE W ORK In this paper we have presented a non-obtrusive system intended for kinematic measurements during kayaking on water. The functionality has been verified in the laboratory. The next step is to use the system in real-world situations in order to further understand the interaction between kinematics and kinetics during kayaking. Most importantly, though, will be the evaluation of live-feedback during training. As this system will serve as a means to control the athletes technique in a systematic fashion, technical changes can be assessed in a cheap and simple manner. ACKNOWLEDGMENT The authors would like to thank the Olympic Performance Centre for the financial support and the Swedish Sports Confederation for providing testing facilities.

Fig. 5. Force on the paddle measured by sensors and power by the ergometer.

R EFERENCES

Fig. 6.

[1] R. M. Smith and C. Loschner, “Biomechanics feedback for rowing,” Journal of Sports Sciences, vol. 20, no. 10, pp. 783 – 791, October 2002, not used. [2] R. A. Schmidt and T. D. Lee, Motor Control and Learning, 4th ed., J. P. Wright, Ed. Human Kinetics, 2005. [3] A. W. Salmonia, R. A. Schmidt, and C. B. Walter, “Knowledge of results and motor learning: A review and critical reappraisal,” Psychological Bulletin, vol. 95, no. 3, pp. 355–386, May 1984. [4] K. Newell and C. Walter, “Kinematic and kinetic parameters as information feedback in motor skill acquisition,” Journal of Human Movement Studies, vol. 7, pp. 235–254, 1981. [5] M. Begon, O. Mourasse, and P. Lacouture, “A method of providing accurate velocity feedback of performance on an instrumented kayak ergometer,” Sports Engineering, vol. 11, pp. 57–65, 2008. [6] J. S. Michael, R. Smith, and K. B. Rooney, “Determinants of kayak paddling performance,” Sports Biomechanics, vol. 8, no. 2, pp. 167–179, June 2009. [Online]. Available: http://dx.doi.org/10.1080/ 14763140902745019 [7] N. Petrone, M. Quaresimin, and S. Spina, “A load acquisition device for the paddling action on olympic kayak,” in Experimental mechanics, advances in design, testing and analysis: proceedings of XI ICEM, Allison, Ed., vol. 2, Balkerna, Rotterdam, 1998, pp. 817–822. [8] S. J. Kendal and R. H. Sanders, “The technique of elite flatwater kayak paddlers using the wing paddle,” International Journal of Sport Biomechanics, vol. 8, pp. 233–250, 1992. [9] R. v. Mann and J. T. Kearney, “A biomechanical analysis of the olympic-style flatwater kayak stroke,” Medicine and Science in Sports and Exercise, vol. 12, no. 3, pp. 183–188, June 1980. [Online]. Available: http://dx.doi.org/10.1080/14763140902745019 [10] N. Caplan and T. Gardner, “The influence of stretcher height on the mechanical effectiveness of rowing,” Journal of applied Biomechanics, vol. 21, no. 3, pp. 286–296, August 2005. [11] D. Henderson. (2009, February 18) Dan henderson’s canoe/kayak. blogg. Visited: January 15, 2010. [Online]. Available: http://dancanoekayak. blogspot.com/

Force on the paddle and foot stretcher during two strokes.

teristics of the forces. The second, relative maximum in the decrease of the force after a stroke, as shown for both strokes with the left arm in Figure 6 has been an often seen characteristic. It could be due to the rubber band loaded line being retracted by the machine after each stroke, but further research is necessary to analyse this feature. Secondly, the development of foot pressure force obviously has a slower rate than of the upper body. For a recreational athletes this can be expected due to a better trained dynamic response in the upper extremities and trunk musculature. It is our goal to do further research on this matter and to compare recreational athlete’s force profiles against elite athlete’s characteristics.

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