A systematic performance analysis of the judo competitions during the 2016 Summer Olympic Games in Rio de Janeiro revealed a more offensive combat behavior and a high action density as well as an increased effectiveness of individual techniques (Heinisch et al., 2017). In this regard, it has been shown that defeated judo players displayed less proficient throwing techniques compared with the winners. This finding was previously substantiated by a deficient judo-specific pulling movement during the onset of the throwing technique in defeated judo players who were not able to sufficiently perturb balance of their opponent (uke) (Heinisch et al., 2012). Of note, the preparatory phase (i.e., kuzushi) of a judo maneuver has been deemed critical to perform a successful judo throwing technique (e.g., morote-seoi-nage, tai-otoshi) (Blais et al., 2007a; Gutierrez et al., 2009; Imamura et al., 2006). More specifically, kuzushi represents the first and critical phase of a throwing technique with the goal to perturb an opponent's balance (Gomes et al., 2017). In this regard, kuzushi is a typical movement that is performed several times during judo-specific training to increase the effectiveness of judo throwing techniques in competition (Franchini et al., 2013; 2014).
In terms of kuzushi performance, high levels of muscle strength and particularly muscle power are important determinants for the successful performance of throwing techniques (Callister et al., 1991; Drid et al., 2015). Furthermore, powerful kuzushi movements have the potential to limit uke's time to initiate a defensive maneuver and to counteract balance-threatening situations (Imamura et al., 2007; Imamura et al., 2006). Indeed, studies revealed that maximal force and isokinetic torque production of upper limb muscles (e.g., elbow flexors and extensors) were significantly associated with judo-specific performance measures and/or success during judo competitions (Callister et al., 1991; Drid et al., 2015). However, there are only a few tools and devices available for the standardized assessment of sport-specific kinetics during kuzushi movements.
In this regard, Blais and colleagues (Blais et al., 2007b) introduced a judo-specific apparatus to assess kuzushi performance during dynamic change of position (tsukuri). Ecological validity was examined using force sensors for the lifting and pulling arm. Significantly different pulling forces were found between the two test exercises (judo-specific training machine vs. uke). The same authors explained this finding with differences in the resistive load when working with the apparatus compared to the opponent. Of note, the judo-specific apparatus in the studies of Blais et al. (2006; 2007a; 2007b) is a stationary device and only pulling masses can be executed during judo-throwing techniques (e.g., morote-seoi-nage).
A new judo ergometer system (JERGo[c], Institut fur Forschung und Entwicklung von Sportgeraten, Berlin, Germany) has been introduced as an alternative approach for the assessment of judo-specific kinetics. The JERGo[c] system immediately provides independent knowledge of result (kinetic parameter) and performance (force displacement characteristics) during kuzushi with and without tsukuri for the pulling and the lifting arm, respectively. Further, the JERGo[c] system is a mobile system and easy-to-administer and install in regular judo gyms (dojos). Moreover, the apparatus' resistance can be individually adjusted according to each athlete's weight category. Thus, kuzushi performance with and without tsukuri cannot only be tested but also trained using the JERGo[c] system (Helm et al., 2018).
An important pre-requisite for the application of the JERGo[c] system during testing and training is that it provides valid and reliable data regarding the performance level (discriminative validity). Thus, two studies using the JERGo[c] system were designed to determine discriminative validity and test-retest reliability as well as ecological validity of judo-specific performance measures in male elite and sub-elite judo athletes. With reference to the study of Blais et al. (2007b) and because of the technical possibility to adjust the loads in accordance with the athletes' weight category, we expected acceptable discriminative validity and test-retest reliability as well as ecological validity of judo-specific pulling kinetics using the JERGo[c] system.
The main participant's characteristics are summarized in Table 1. In both experiments (study 1 and 2), at least two athletes from each weight category (-60 kg, -66 kg, -73 kg, -81 kg, -90 kg, -100 kg, +100 kg) were tested. Local ethical permission was provided and both studies were conducted in accordance with the latest version of the Declaration of Helsinki.
Our experimental approach included two studies to determine discriminative validity and test-retest reliability (study 1) as well as ecological validity (study 2) of a judo ergometer (JERGo[c]) system while performing judo-specific movements. Both experiments used a standardized general warm-up comprising 60 seconds rope-skipping and a judo-specific warm-up consisting of 10 submaximal and 3 maximal kuzushi movements with and without tsukuri using the JERGo[c] system and uke. A 3 seconds rest was provided between trials and 5 minutes were considered between each test condition. Discriminative validity and test-retest reliability of sport-specific parameters (mechanical work, maximal force, power) were assessed during 10 maximal kuzushi movements with (Figure 1a) and without (Figure 1b) tsukuri using the JERGo[c] system. The first and the last trial were removed and the best out of 8 trials (2nd to 9th) was used for further analysis. To examine test-retest reliability, measurements were repeated within a one-week interval (five-to-seven days).
Ecological validity of trunk and upper limb muscle activity was determined during 6 maximal kuzushi movements without tsukuri using either the JERGo[c] system (Figure 2a) or uke (Figure 2b). The first trial was removed and the average of 5 trials was used for further analysis. The testing of the two conditions (study 1: kuzushi with or without tsukuri, study 2: JERGo[c] system or uke) were carried out in randomized order. Rest between trials was 3 seconds and rest between two test conditions amounted to 5 minutes. All athletes were measured while performing the standardized judo technique morote-seoi-nage.
Testing with the JERGo[c] system
The testing apparatus consists of a wall bracket, two mobile JERGo[c] systems (lifting and pulling arm) and a combat judo mat of four square meters (Figure 3). The rotor of the eddy current brake is connected to a winding drum via a shaft. The pulling cable is rolled up on the shaft and contains judo-specific grips (see Figure 3). The shaft is rigidly connected to the rotor of the eddy current brake and thus the force transmission takes place in only one direction of rotation. During a pulling movement, forces are transmitted through a free wheel of the shaft onto the rotor of the eddy current brake. For the JERGo[c] system, lifting and pulling arm grips were manufactured according to a judo kimono (Adidas company, see Figure 3). This allows the athletes to perform a judo-specific sleeve-reverse grip. The JERGo[c] software (JERGo2000 V 5.1) was developed using LabView 8.6. Data transfer from the sensor is controlled by a microcontroller (AT Mega 128). In addition, the controller adopts the pulse width control for the eddy current brake and the communication with the PC (via USB). Data (e.g., athlete, testing place, testing date) and results were recorded using a custom-made software. Before the JERGo[c] system is ready for testing, a calibration is carried out for zero-point transfer. The individual adjustment of the eddy current brakes (height of lifting and pulling arm, brake resistance) is interlocked by the athletes' weight category and body height as well as the preferred judo technique. Judo-specific kinetics (mechanical work [the amount of energy transferred by a force], maximal force [peak force of the time-force curve], power [the rate of doing mechanical work]) as well as force displacement characteristics for the pulling and lifting arm were analyzed and displayed on a laptop. Relative values (normalized to body mass) were used to determine discriminative validity and absolute values were used to calculate test-retest reliability. Figure 3 shows the JERGo[c] system with an online recording screenshot for kuzushi without tsukuri. Resistance of the two eddy current brakes was regulated according to each athlete's weight category using seven brake levels (-60 kg = 500 N, -66 kg = 600 N, -73 kg = 700 N, 81 kg = 800 N, -90 kg = 900 N, -100 kg = 1,000 N, +100 kg = 1,000 N). In addition, a grading of the brake load was conducted at pull-out length (waypoint [WP] 1: 100% [[greater than or equal to] 0 cm], WP2: 80% [[greater than or equal to] 20 cm], WP3: 50% [[greater than or equal to] 40 cm], return [RT]: 50%). One-hundred percent resistance at WP1 was defined by the respective weight category (i.e., -100 kg = 1,000 N and -60 kg = 500 N at WP1). The optimal resistance by weight category and at pull-out length was determined in pilot studies with the male German judo national squad. The height of the eddy current brake was adjusted for the lifting hand at athletes'...