In the current study, we tested a prototype of an isokinetic exercise device for the lower limbs, named the ERIK. The ERIK enables a type of single-limb squat exercise with a translational load on the swing leg in a closed kinetic chain, putting load on the muscles of the stance leg in the standing position. This training applies load to the gluteal muscles, which is effective for avoiding excessive knee valgus moment, a major factor in anterior cruciate ligament injuries. To enhance the quality of the load, an electro-rheological (ER) fluid brake system is implemented in the ERIK. The ER brake can reversibly control resistive torque with a rapid response. This paper reports a prototype of the device with four training modes, verifying its performance through basic experiments. Although high resistance is created within a wide motion area and requires isokinetic training by controlling the velocity of the trainee’s legs, the ERIK has the advantage of a high level of safety because of its passive resistive function. Keywords: 23rd Robotics Symposia, Isokinetic exercise, Lower limb, Closed kinetic chain training, ER fluid Introduction gluteal muscles, and is effective for preventing knee val - Background and aim gus moment, a major factor in ACL injuries. The pri - Anterior cruciate ligament (ACL) injuries are a common mary goal of this style of training is to rehabilitate and/ knee injury during sporting activities. The number of or preserve the ACL, and to provide a useful method ACL injuries per year in Japan has not been investigated for athletic strength training. To enhance the quality of on a national level, but is estimated to be approximately 4 the load used in this type of exercise, we have developed in 10,000 individuals. In addition, approximately 10,000 an electro-rheological isokinetic exercise device pro- ACL reconstructions are performed each year in Japan totype, called the “ERIK” [4–6]. The ERIK includes an . Similarly, the number of ACL injuries in the United electro-rheological (ER) fluid brake system, which pro - States is estimated to be approximately 3 in 10,000 indi- vides apparent viscosity that can be reversibly controlled viduals per year . by varying the applied voltage. In the current study, we In the current study, we tested a prototype of an isoki- tested the ERIK to verify its performance through repro- netic exercise device for the lower limbs. The proposed ducible experiments using a servomotor, in contrast with device involves a type of single-limb squat, which applies previous studies using only a basic control. Moreover, a translational load on the swing leg in the closed kinetic several new training modes were also tested, including chain (CKC) style, placing load by the body weight on the initial resistance mode and the terminal resistance the muscles of the stance leg in the standing position. mode. We refer to this training style as a resistive leg reach exercise . This type of training applies load to the Training styles Strength training styles include an isotonic exercise per- formed with a constant load, an isometric exercise per- formed in a constant posture, and an isokinetic exercise *Correspondence: firstname.lastname@example.org Faculty of Engineering, Toyama Prefectural University, 5180 Kurokawa, performed at a constant angular velocity of the joint. Imizu, Toyama 939-0398, Japan However, because of the requirement of a real-time Full list of author information is available at the end of the article © The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat iveco mmons .org/licen ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Koyanagi et al. Robomech J (2018) 5:15 Page 2 of 12 controller to apply resistance, very few commercial- using magneto-rheological fluid was included in a pro - ized products are available for isokinetic exercise train- posed stepper device , the equipment enabled limited ing. Most of the currently available products target only trainee postures and motion types because of its recum- rotational movements of a joint in a sitting position, bent design. such as the Cybex , HUMAC-NORM , and MARES Closed kinetic chain training creates a closed-loop  devices. Although a real-time speed control system linkage with both legs and the floor. Importantly, because this type of exercise decreases the anterior shear and valgus forces on the knee joint, it has a high level of safety. Kimura et al. analyzed CKC training in the stand- ing (loaded) posture for rehabilitation after ACL recon- struction, and applied it clinically to evaluate its safety and effects with a constant resistive force, as shown in Fig. 1, during a single-leg squat using a device with a slid- ing table and a rubber band [3, 11]. The single leg squat Internal hip abduction shown in Fig. 2 includes the right knee bending during moment was increased by b gluteus med activity the lateral opening motion of the left leg. The reversal motion is conducted during the lift up. However, because the device used a rubber band, the resistant force could External knee valgus not be controlled. Thus, improved training equipment is Moment (-) needed for isokinetic exercise, enabling easy and variable setting of the resistant force to provide acceptable levels of safety and effectiveness. Isokinetic exercise has a number of benefits: (1) ade - quate resistance can be loaded for the whole range of Pushed the floor laterallyReaction force Resistance force with ERIK motion; (2) the load can be easily tuned in coordina- Fig. 1 Schematic image of the resistive leg reach exercise in CKC tion with the reference angular velocity because the training style joint rotates at a constant velocity; (3) incidental muscle Fig. 2 Strobe of a single leg squat motion Koyanagi et al. Robomech J (2018) 5:15 Page 3 of 12 injuries are unlikely to occur; and (4) quantitative eval- Driven Shaft uations of clinical tests and the effects of training are Rotational available, because the controller included in the training Cylinder system enables the measurement of joint torque [12, 13]. The ERIK has the advantage of a high level of safety in ER Fluid isokinetic exercise, which conventionally requires con- trollable actuators, such as servomotors. In addition, the ERIK can load higher resistance within a wide motion Applied Voltage area compared with conventional equipment. Unlike other devices, the ERIK enables control of the velocity of Fixed Cylinder the leg during translational motion in isokinetic training, such as the open–close leg exercise. The single leg squat Fig. 3 Sectional view of the ER brake using the ERIK provides the constant velocity motion of the sliding leg; therefore the knee joint flexion at a con - stant velocity as mentioned as the above benefit (2) is not strictly produced. Thus, whether the isokinetic mode of the ERIK has adequate training effectiveness is confirmed in “Experimental verification ” section. E=3[kV/mm] Importantly, the ERIK has unique two training modes: the initial resistance mode, in which resistant force is E=2[kV/mm] maximal at the initial position of motion and decreases according to the movement distance of the leg, and the terminal resistance mode, in which the resistant force is minimal at the initial position of the leg and increases according to the moving distance (opposite to the the E=1[kV/mm] initial resistance mode). Although this type of resist- ance pattern is the same as that of devices using a rubber band, the ERIK enables easy adjustment of the maxi- E=0[kV/mm] mum resistant force. The trainee can choose either of the modes for their particular training purpose. 0 200 400 −1 Shear rate [s ] ERIK: an isokinetic exercise device for the lower limbs in the standing position Fig. 4 Shear stress characteristics of the ER fluid ER fluid brake Particle-type ER fluid is colloidal fluid consisting of sus - pended particles, and is commonly referred to as ER The maximum braking torque depends on the character - fluid. The fluid’s apparent viscosity increases when an istics of the ER fluid used. electric field is applied. Thus, ER fluid is suitable as a working fluid in clutch or brake mechanisms for the spe - cific requirements of control systems, possessing a rapid Design and prototype of ERIK response speed and no hysteresis . The basic design of the ERIK is shown in Fig. 5. The ERIK Figure 3 shows the basic structure of an ER brake  prototype is shown in Fig. 6. Its specifications are listed consisting of a fixed cylinder, a rotational cylinder with a in Table 1. The device has a slidable platform on the top, driven shaft, and ER fluid filling the inside. Each cylinder which is connected to a timing belt. The belt engages with can have a multi-cylindrical body, and the two cylinders the ER brake via a pulley, transmitting the brake torque to also serve as a pair of electrodes. When voltage is applied the platform. The position and speed of the platform are between them, the output component is forced to slow calculated from the rotational angle of the brake, which is down or halt. The braking torque does not depend on measured with an encoder. The controller, which runs on the shear rate (rotational speed), corresponding only to a PC, calculates the desired torque according to the situa- the magnitude of the applied voltage as shown in Fig. 4, tion and outputs this torque value to the ER brake. Quan- −1 especially the range between 60 and 240 s referring the titative evaluation of the training can be performed with desired velocity in “Experimental verification ” section. these values because power, work, and other measures Shear stress [Pa] Koyanagi et al. Robomech J (2018) 5:15 Page 4 of 12 However, the other training modes of the ERIK produce resistive forces like frictions against the motion of the trainee. Moreover, it is substantially easier to maintain the safety of the trainee with an ER brake system com- pared with a servomotor system, particularly when an Encoder Pulley ER Fluid Platform Timing Belt unexpected situation occurs. Magneto-rheological (MR) Brake brakes, which also use a type of functional fluid, have the advantage of a higher torque-weight ratio than ER Roller brakes, because of greater generative shear stress. How- ever, unlike ER brakes with response times of around 5 ms, large torque type MR brakes slow response times Fig. 5 Schematic image of the isokinetic exercise device for the of more than 30 ms in 100% response, and hysteresis . lower limbs, ERIK The goal of exercising with the ERIK device is to strengthen leg squatting on a stage outside of the ERIK. The sliding leg on the platform of the ERIK meets resist - ance from the brake during squatting, then attempts the open–close exercise in the forward–backward or left– right direction, as shown in Fig. 7. These movements can be used to train muscles of the lower limbs loaded in both the longitudinal and vertical directions. The movable range of the platform is 0.7 m, but this can be restricted further with pins. A high voltage amplifier and a microcomputer are located inside the main compartment. The ER brake was designed to generate 20 Nm torque at an applied voltage of 1 kV for a restricting force of over 200 N. The high volt - age amplifier (ER-1.5k8PX, Chubu-R&D Co., Japan) can supply a maximum of 1.5 kV–8 mA with a response time of approximately 6 ms. The resolution of the encoder Fig. 6 Prototype of the isokinetic exercise device for the lower limbs, (E6B2, OMRON Co., Japan) was increased to 8000 ppr ERIK by the belt–pulley transmission mechanism. The micro - computer (iMCs01, iXs Research Co., Japan) transmits the command from the controller (PC), via USB, to the high voltage amplifier after digital to analog conversion. Table 1 Specifications of the ERIK prototype The microcomputer also counts pulses from the encoder, Parameter Value then transmits the information to the PC via USB. On the controller PC, the software for control and eval- Size ( W × H × L) 0.32 m × 0.26 m × 1.4 m uation ran during each sampling time of 10 ms. Although (From ground level to the platform top) 0.12 m Windows does not ensure real-time responsiveness, the Movable range of platform 0.7 m actual sample period was measured using the software. Generative maximum force 250 N The PC provides a detailed description of the training Desired velocity range ≤ 1.2 m/s conditions, a display for training results, and a mecha- nism for recording the results. Information about the trainee, the target number of can also be calculated from these values. For optimal per- strokes, and the training period is set on the input screen, formance, the resistance must be able to respond rapid in as shown in Fig. 8. The training mode, the reference value response, exhibit high stability, and have a high torque to of the training mode, and other details are set on the run- volume ratio, making devices using ER fluids preferable. ning screen, as shown in Fig. 9. Values for the resistant The ERIK cannot move until the trainee moves it them - force or the desired velocity can be set as constants. The selves, which inherently improves the safety of the device. desired velocity of the platform is set to have a maximum Training systems using servomotors, which are in com- value of 1.2 m/s, which is specified empirically in accord mon use and are superior to present ER brakes in terms with previous studies [3, 11]. of cost and availability, can enable isokinetic training. Koyanagi et al. Robomech J (2018) 5:15 Page 5 of 12 Fig. 7 Examples of practical exercise styles Force characteristics of the prototype A wire-driven system with a high output motor as shown in Fig. 10 was used to display the force characteristics of the ERIK. The force was measured with a load cell (LUR- A-500NSA1, Kyowa Electronic Instruments Co., Ltd., Japan) by pulling the platform via the wire by the servo- motor (NX1040AS, Oriental Motor Co., Ltd., Japan). The servomotor controlled the rotational velocity, converted to the sliding speed of the platform at 0.20 m/s, except in Fig. 8 Input screen for the trainee’s data the experiments testing the isokinetic exercise mode, in which the servomotor rotates with a constant torque of approximately 15 Nm. The running screen also provides trigger informa - The relationship between the energizing voltage V tion for the training motion and graphs of the speed or (kV) and resistant force F (N) of the ERIK prototype was the resistant force during training. The results of each experimentally determined as shown in Fig. 11. Although training session are recorded in a CSV file for in-depth the shear stress of the ER fluids is known to depend on analysis. Fig. 9 Screen during exercise. Users can input the training condition on the same screen Koyanagi et al. Robomech J (2018) 5:15 Page 6 of 12 Fig. 10 Experimental system for measuring the braking force of the ERIK Voltage < 0.8kV 0100 200 Reference [N] Fig. 12 Reference and measured force relation of the prototype Voltage > 0.8kV Fig. 13 Block diagram of the IPD controller for the isokinetic mode 0 0.5 1 1.5 Voltage [kV] Fig. 11 Voltage and braking force relation of the prototype Control methods The isotonic mode The isotonic mode enables isotonic exercise by loading the square of the electric field, the experimental result is a constant resistant (braking) force by the ERIK. The ER approximated using the following quadratic equations: brake torque was calculated from the target value and Eq. (1) using feed-forward control. F = 126V + 39.2V + 22.5 (V < 0.8 kV) F =−97.3V + 358V − 89.5 (V ≥ 0.8 kV) The isokinetic mode (1) Real-time controlled resistance against the maximum Here, the coefficients of determination are over 0.96. effort of the trainee to slide the platform is generated for According to Eq. (1), the ERIK can generate a braking the isokinetic mode to keep the sliding velocity of the force of almost 250 N at 1.5 kV, which is the maximum platform at the desired constant value. voltage for the prototype. The ER brake torque was controlled by a I-PD speed con - The voltage calculated with the reference force and the troller as shown in Fig. 13. Here, the velocity v, the target inverse function of Eq. (1) was applied to the ER brake velocity v , the resistant force F , the model of the ER brake r r to ensure that the ERIK could generate the desired resis- G , the model of the ERIK G and feedback gains K , ERB ERIK I tive force. The measured values and the line of direct pro - K , K and K are used. The value of v is decided by the P1 P2 D r portion are shown in Fig. 12. When the reference is over trainer according to the condition of the trainee. The driv - 22.5 N, the measured values increase proportionally. The ing force for the platform by the trainee F was treated trainee coefficient of determination of the results from the line as a disturbance. The speed controller using passive ele - was 0.998, confirming that the ERIK was able to repre - ments was empirically selected; the simple IP controller sent the reference force. was avoided because the IP controller did not produce Force [N] Force [N] Koyanagi et al. Robomech J (2018) 5:15 Page 7 of 12 In the current study, x = 0.70 m and F = 50 N were max rb 400 2600 fixed. Using Eq. (1), V was calculated and applied to the ER brake. K The terminal resistance mode P2 The terminal resistance mode can augment the activ - ity of major muscles and the internal hip abduction moment with deep knee bend of the squatting leg: a sporting activity often require the posture. In this P1 mode, F can be written as same as in the previous section: F = F r rmax max (4) 0 0.5 1 if F < F , then F = F . r rb r rb Target Velocity [m/s] Fig. 14 Feedback gains of the isokinetic mode Experimental verification The initial resistance mode This section describes the experimental verification of the ERIK and the controllers for the initial resist- appropriate results . Thus, adding degrees of freedom to ance modes using the measurement system, as shown the IP controller for following the velocity and suppression in Fig. 10. The motor pulled the platform at a constant of vibration, and the resistant force F can be expressed as speed of approximately 0.20 m/s while the ERIK con- follows: trolled and loaded the resistance. Figure 15 shows the results of trials for a target F = K (v − v)dt + K (v − v) r I r P1 r force of 50, 100, 150 and 200 N. The resistance force (2) decreases according as Eq. (3) under 150 N. In the case − K v − K v P2 D of the target force of 200 N, the force in the starting if F < 0, then F = 0. r r phase is larger than the target value because of stretch- ing of the wire and the stick–slip phenomenon of the The driving force for the platform depends on the trainee, ER brake. and modeling a trainee’s body is complex. For this rea- son, despite the platform’s position being restricted to the external floor by the trainee’s leg, the feedback gains were experimentally tuned, as shown in Fig. 14. The feed - back gain K contributed suppression of vibration of the P2 platform’s motion. Because the feedback gains cannot be treated as optimized, they could potentially be improved 200N with other tuning methods. The initial resistance mode Because ACL injuries often occur at shallow knee bend- 150N ing, the initial resistance mode is expected to improve the posture control ability and prevent knee from injuries in 100N shallow knee bending postures. Given the position of the 100 platform x, the preset stroke length x , the target force max value F shown in Fig. 9 and the base resistive force F , rmax rb 50N F can be expressed as follows: F = 1 − F r rmax 0246 max (3) Time[s] if F < F , then F = F . r rb r rb Fig. 15 Force response in initial resistance mode exercise K , K , K P1 P2 D Force[N] Koyanagi et al. Robomech J (2018) 5:15 Page 8 of 12 1.2m/s 1.2 200N 0.6m/s 0.6 150N 0.3m/s 100N 50N 12 3 024 6 Time [s] Time[s] Fig. 17 Results of isokinetic exercises Fig. 16 Force response in terminal resistance mode exercise The terminal resistance mode 1.2 Figure 16 shows the results of trials in the terminal resistance mode, under the same conditions described in the previous section. Command Force The resistance forces increased according to Eq. (4 ). 0.8 The isokinetic mode Figure 17 shows the results of trials in the isokinetic mode for target speeds of 0.30, 0.60 and 1.2 m/s. The platform was operated by one of the authors. 0.4 The results revealed that isokinetic motion was Speed almost realized, although the response at 1.2 m/s did not show a sufficient time period for evaluation while achieving the constant controlled speed. The mean speed values during the constant speed 0 0.4 0.81.2 shown in Fig. 18 indicated that the ERIK was able to Reference Speed [m/s] follow the target velocity. The bullet shows the mean Fig. 18 Mean speed values and command values of resistant force in speed value for each reference speed. The coefficient of isokinetic exercises determination of the results from the dashed propor- tional line is 0.998. The white circle shows the mean command force to the ERIK during the constant speed. up. The subject was instructed to bend his right knee This value would be expected to vary in each trainee, until about 80 , and practiced before experiments. In and could be used for evaluation of muscular function experiments, a bar was set in the front of the bend- or motion function. ing knee for a guide to control the knee bending angle. When sliding to the left, the resistance was loaded by EMG of a healthy subject the ERIK, while no resistance was loaded when slid- This section describes the experimental verification of ing to the right. Because the particular muscles to be the ERIK and the controllers for the training modes strengthened for preventing ACL injuries include the using a healthy subject as a trainee. The trainee slid quadriceps femoris, gluteus medius (Gmed), gluteus their left leg on the platform to the left while squatting maximus (Gmax), electromyograms (EMGs) of these down, then slid the same leg to the right while standing Force[N] Speed [m/s] Speed [m/s] Command Force [N] Koyanagi et al. Robomech J (2018) 5:15 Page 9 of 12 muscles were measured. Vastus medialis (VM) was selected among the quadriceps femoris. For example, VM we conducted isotonic exercises for target forces of 0 Gmed and 120 N and a target speed of 0.25 m/s. Gmax Root-mean-square (RMS) in 50 ms processed EMGs are shown in Figs. 19, 20 and 21. Each graph shows data during one squatting motion. Comparing with the EMG in the 0 N isotonic mode, exercises in the other modes required greater muscular activity, particularly VM Gmed Gmax 01 2 Time [s] Fig. 21 RMS EMG during isokinetic exercise. The target speed was 0.25 m/s in Gmed and Gmax. Exercise in the isokinetic mode requires slower motion under this condition compared with isotonic exercises. However, isokinetic exercise places a greater load on the gluteal muscles than iso- tonic exercise. This indicates that the effects of using 00.5 1 the ERIK were as planned. Time [s] Fig. 19 RMS EMG during isotonic exercise. The target force was 0 N EMG of ACL reconstructed subject A male subject 1 year after an ACL reconstruction operation underwent tests using the ERIK. The subject was a professional soccer player in his twenties. The testing method was the same as that described in the VM previous section. Gmed The results are shown in Figs. 22, 23 and 24. In the Gmax isotonic mode, the Gmed was loaded with a particu- larly large force from the opening of the exercise. Compared with the previous section, Gmax showed a slightly smaller but stable EMG during the period. In addition, the VM generally exhibited large EMG values. In the isokinetic mode, large forces are loaded on three muscles from the opening to the middle in spite of the moderate force loaded from the middle. In addition, we conducted comparative tests using a conventional training device with a rubber band. Because the conventional device generates 80 N in the fully tensioned position of the platform, the results 0 0.5 11.5 in the isokinetic mode of 80 N by the ERIK were also examined. Time [s] The results are shown in Figs. 25 and 26. Loads using Fig. 20 RMS EMG during isotonic exercise. The target force was 120 N the conventional device are shown as the terminal RMS EMG [ µV] RMS EMG [µ V] RMS EMG [µ V] Koyanagi et al. Robomech J (2018) 5:15 Page 10 of 12 VM VM Gmed Gmed Gmax Gmax 200 200 0 0 0 0.5 1 01 2 Time [s] Time [s] Fig. 24 RMS EMG of an ACL reconstructed subject during isokinetic Fig. 22 RMS EMG of an ACL reconstructed subject during isotonic exercise. The target speed was 0.25 m/s exercise. The target force was 0 N VM VM Gmed Gmed Gmax Gmax 01 2 01 2 Time [s] Time [s] Fig. 25 RMS EMG of a subject with a reconstructed ACL during Fig. 23 RMS EMG of an ACL reconstructed subject during isotonic isotonic exercise. The target force was 80 N exercise. The target force was 120 N Discussions resistance mode. The EMG of the VM had a greater Excessive external knee valgus moment is a major factor value from the middle compared with EMG during in noncontact ACL injury [17, 18]. Gluteal muscle activ- exercise with the ERIK. In contrast, Gmed exhibited ity is important for preventing this, as well as excessive moderate EMG signals when opening, followed by low hip adduction . Quadriceps femoris weakness is a values. Little force was loaded on Gmax during this common problem after injury and reconstruction of ACL period. [20, 21]. Proper isotonic mode and isokinetic mode exercise using the ERIK increased the load of Gmed and Gmax RMS EMG [µ V] RMS EMG [ µ V] RMS EMG [µ V] RMS EMG [µ V] Koyanagi et al. Robomech J (2018) 5:15 Page 11 of 12 Authors’ contributions KK contributed to the conception and design of the ERIK, data collection, VM analysis and interpretation, and drafting the article. YK contributed to concep- tion of the work and EMG data collection, analysis and interpretation. MK Gmed contributed to conception of the work and EMG data collection. AI designed Gmax and built the ERIK, including the ER brake. TT, KS, TM, HM and TO contributed to design of the measurement system and critical revision of the article. All authors read and approved the final manuscript. Author details Faculty of Engineering, Toyama Prefectural University, 5180 Kurokawa, Imizu, Toyama 939-0398, Japan. Department of Rehabilitation, Osaka University Hospital, 2-15, Yamadaoka, Suita, Osaka 565-0871, Japan. Faculty of Biomedi- cal Engineering, Osaka Electro-Communication University, 1130-70, Kiyotaki, Shijonawate, Osaka 575-0063, Japan. ER-tec Co., 2-1-31, Sakuragaoka, Minoh, Osaka 562-0046, Japan. Acknowledgements This work was supported by JSPS KAKENHI Grant Numbers 22500574, 25282192 and 17H02135. The authors wish to thank Mr. Y. Yamamoto, a former graduate student at Toyama Prefectural University, for help conducting the experiments. 01 2 Time [s] Competing interests A. Inoue is the CEO of ER-tec Co., Japan, a developer of the ER fluid. Fig. 26 RMS EMG of a subject with a reconstructed ACL using conventional equipment Ethics approval and consent to participate Experiments with subjects in this paper were approved by the research ethics committee of each organization ( Toyama Prefectural University: No. H27-4, Osaka University Hospital: No.11527). All participants gave fully informed consent prior to testing. without reducing the load of VM, suggesting that these exercise modes may be useful for preventing ACL injury Publisher’s Note and recovering after ACL reconstruction. The results also Springer Nature remains neutral with regard to jurisdictional claims in pub- lished maps and institutional affiliations. indicate the ERIK was able to produce a more effective load on the gluteal muscles than the conventional device. Received: 30 January 2018 Accepted: 29 May 2018 Because ACL injuries often occur at shallow knee bend- ing, the ability to apply specific loads on gluteal muscles from the opening of training is important for the preven- tion of ACL injury. These findings suggest that the ERIK References provides an effective device for this type of training. 1. Japanese Orthopaedic Association (ed) (2012) Clinical practice guideline on the management of anterior cruciate ligament injury of the knee, pp 5–6. 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Published: Jun 4, 2018
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