The primary factors that affect the performance of weightlifters in the snatch event of weightlifting competitions are the explosive power output required to lift a heavy weight and the skill required to lift a barbell efficiently. The average power output generated during a snatch lift ranges from 1,300 to 4,000 W among elite male lifters (5), and weightlifters use the elastic energy produced during the stretch-shortening cycle by flexing their knee between the first pull and the second pull (7). In light of these facts, many studies have analyzed the lifting motion of the barbells during weightlifting competitions in terms of different factors such as the kinematic characteristics of the barbell (1–3,10,12,15) and the strength and power of lifters (11,16). However, it is reported that the barbell path suggested to be correct is different in each region (15). Because the lifting technique is affected by coaching, it is important to reveal the characteristics of the lifting technique in consideration of the methods of coaching in each country or each region.
With regard to lifting techniques (2,8,9,13), Baumann et al. (1) demonstrated that the hip extensor moment plays a dominant role in weightlifting and that the impact of the extensor moments of the knee and ankle on the weightlifter's performance is relatively small. Furthermore, the position of the knee joint with regard to the direction of the ground reaction force is an important technical factor that should be considered during analysis. Okada et al. (13) reported that the time interval between the peak angular velocity of the hip joint and peak vertical velocity of the barbell for Japanese weightlifters was longer than that for international weightlifters. Because these technical factors seem to considerably affect the kinetics of lifting, kinematic analysis of elite weightlifters provides useful information for improving the weightlifting performance.
With regard to the barbell kinematics, the barbell displacement, barbell velocity, barbell acceleration, and angle of the resultant acceleration of the barbell are used as parameters for technical evaluation. Stone et al. (15) reported that the amount of forward or backward displacement from the liftoff position greatly influenced the success of a lift; by comparing successful and unsuccessful attempts, they found that the horizontal displacement during successful attempts was small. Although studies on the lifting patterns of world-class weightlifters suggested that top weightlifters pull the barbell backward during the first pull and the transition phase (1,10), the study of the lifting patterns of college weightlifters suggested that no significant relationship existed between the horizontal displacement of the barbell and the number of successful attempts (14).
With regard to the barbell velocity and acceleration in the vertical direction, it is reported that the velocity curves for the best lifters (BLs) seldom show any notable dip (1,7,10) and that the maximum barbell velocity for the BLs was smaller than that for other lifters (1). In a study conducted for elite Asian weightlifters, Isaka et al. (10) observed 3 vertical acceleration peaks of the barbell during the pull movement. However, no studies have been conducted that determine the relationship between the horizontal component and the lifting motion.
Although many studies have researched the lifting motion and the barbell kinematics, it is unclear as to why a big difference is seen in weightlifting performance because the motion analysis and the barbell analysis have been disconnected. We hypothesized that international weightlifters show rational motion and barbell trajectory during the first pull and the transition phase to exert the explosive force during the second pull. To consider the factors affecting the lifting ability, it is important to recognize the characteristics of the lifting technique and the coaching method of lifting in the region. Thus, this study had 3 major purposes: (a) to describe the characteristics of the barbell kinematics and kinetics for international weightlifters and Japanese weightlifters; (b) to describe the characteristics of the lifting motion for international weightlifters and Japanese weightlifters; (c) to look at the rational motion to exert the explosive force during the second pull in the snatch lift.
Experimental Approach to the problem
To examine whether there are any differences in the snatch technique between international and Japanese female weightlifters, the lifting motion and the kinematics and the kinetics of the barbell were quantified by analysis. The data for this study were collected at the 2008 Asian Weightlifting Championships held in the city of Kanazawa, Japan. Permission for filming was granted by the Japan Weightlifting Association.
The snatch techniques of 10 female weightlifters were recorded under competitive conditions during the 2008 Asian Weightlifting Championships held in the city of Kanazawa, Japan. The heaviest successful snatch attempts of the 5 BLs and Japanese lifters (JLs) in 5 weight categories (i.e., 48, 53, 63, 69, and 75 kg) were analyzed. The weight category, nationality, age, height, body mass, and barbell mass of each subject are listed in Table 1.
Two high-speed cameras (HSV-500C3, Nac, Tokyo, Japan) operating at 250 Hz were used to record the movements of a lift as in previous research studies (1,2); the cameras were placed approximately 30 m from the platform. The shutter speed was set at 1/1,000 seconds. Three-dimensional data were obtained using the direct linear transformation method. A calibration system (3 × 3 × 2.9 m) was positioned on the platform. Barbell trajectories during the snatch were recorded using another camera (Sony Inc, Tokyo, Japan) that was placed perpendicularly to the platform (Figure 1) and the lifter's sagittal plane (9,10,14,15). The sampling frequency was 60 Hz, and the shutter speed was set at 1/1,000 seconds.
A digitizing system (DKH Inc., Tokyo, Japan) was used to manually digitize 25 points on the body and the barbell. The coordinate values were filtered digitally using a Butterworth-type fourth-order low-pass filter. The cutoff frequencies for the 3-dimensional coordinates and the barbell were 4 Hz (4). Because the barbell position on the platform was different for each attempt during the competition, the location of the X3 dots parallel to the x-axis was calculated using 4 control points (C1–C4) and the y-coordinate of the barbell was calculated after considering the depth of the video image (Figure 1). The coordinate values of the barbell were interpolated at 1/250-second interval by cubic spline interpolation.
The analysis focused on the part of the snatch technique beginning from the barbell liftoff to the lowest position of the barbell during the catch phase. The movement was divided into 5 phases based on the height of the barbell and the change in the angle in which the knee moved (Figure 2). (a) The first pull: from the barbell liftoff to the first maximum knee extension. (b) The transition from the first to the second pull: from the first maximum knee extension to the first maximum knee flexion. (c) The second pull: from the first maximum knee flexion to the second maximum knee extension. (d) The turnover under the barbell: from the second maximum knee extension to the maximum height position of the barbell. (e) The catch phase: from the maximum height position of the barbell to the stabilization in the catch position.
To study the movement of the body, the angular displacements (Figure 2) and angular velocities of the knee and hip joints in the sagittal plane were calculated to compare the movements of the BLs and JLs.
The barbell's movement was described by the displacement (Table 2), the velocity (Figure 3), and acceleration (Figure 4) in the vertical and horizontal directions. The parameters used for analyzing the barbell kinematics were based on the horizontal and vertical barbell velocities during the snatch. Regarding the velocities and acceleration of the barbell, positive values represent the movement of the barbell toward the lifter, and negative values represent movement away from the lifter. In this study, the angle between the direction of the resultant acceleration vector of the barbell and the horizontal line was calculated by a method similar to that reported by Isaka et al. (10) (Figure 4). The forces applied to the barbell in the vertical and the horizontal direction were calculated by the following equation:
Values for each parameter were recorded as mean ± SD for each of the BL and JL groups. An unpaired t-test was performed to test the difference in the mean values. The correlation coefficient between 2 variables was calculated and tested for significance. The level of statistical significance was set at 5%.
Kinematics and Kinetics of the Barbell
Barbell trajectories for the BLs and JLs are shown in Figure 5. The trajectories for the BLs did not cross a vertical reference line projected upward from the initial position of the bar. For JLs, except those in the 53-kg category, the trajectories of the barbell crossed the vertical reference line. Compared with the trajectories of the BLs, the forward barbell displacement (Dx3) of the JLs between the most backward position and the most forward position was greater than that of the BLs. With regard to the relative vertical displacement, the height of the maximum position (Dy1/BH) and heights of the most forward position (Dy5/BH) and catch position (Dy2/BH) for the BLs were significantly lower than those for the JLs (Table 3).
The vertical and horizontal velocity curves of the barbell are shown in Figure 6. As for the peak value of the barbell velocities, no difference was observed in the pvV value, but the phVf values of the JLs were greater than those of the BLs (Table 4). There was a significant negative correlation between phVf and Dx2 (r = −0.881, p < 0.001) and Dx3 (r = −0.961, p < 0.001).
The vertical and horizontal acceleration curves of the barbell are shown in Figure 7. Three peaks of vertical acceleration were observed during the pull movement for the BLs and the JLs. Negative values of vertical acceleration were observed for 6 of the 10 lifters during the transition phase. With regard to horizontal acceleration, no significant difference was observed in the pAf value between the BLs and the JLs (p = 0.081), but the values of pAf value for the JLs were greater than those for the BLs in each category. As for the ARA of the barbell for the second pulls, there was no significant difference between the BLs and JLs. Because the timing of pAf did not necessarily correspond to the timing of pAv, the interval time between pAv and pAf affected the values of ARA (Table 4).
Figure 8 shows the curves of the angular displacement of the knee and hip joints for BLs and JLs. No significant differences were observed in the angles of the knee joint and the hip joint at the end of the first pull (Table 5). All the weightlifters flexed their knees during the transition phase. The mean values of the knee flexion angle were 9.7 ± 4.3° for the BLs and 6.7 ± 4.3° for the JLs. With regard to the joint angle at each phase, the hip joint angles of the BLs at the barbell liftoff (BLs: 40.5 ± 3.8°, JLs: 33.6 ± 4.5°) and at the maximum vertical acceleration (VAmax) (BLs: 166.5 ± 8.9°, JLs: 145.1 ± 10.7°) were significantly greater than those of the JLs. In addition, the trunk angles to horizontal plane of the BLs at VAmax (BLs: 92.3 ± 10.9°, JLs: 75.5 ± 4.9°) and at the end of the second pull (BLs: 115.4 ± 2.8°, JLs: 110.2 ± 3.0°) were significantly greater than those of the JLs. Furthermore, there were major differences between the BLs and JLs in terms of the hip joint angle at the end of the transition phase (p = 0.053) (BLs: 123.9 ± 3.9°, JLs: 116.4 ± 6.2°), knee joint angle at VAmax (p = 0.053) (BLs: 136.8 ± 7.7°, JLs: 126.9 ± 6.0°), thigh angle at VAmax (p = 0.079) (BLs: 67.6 ± 3.8°, JLs: 62.2 ± 4.5°), hip joint angle at the end of the second pull (p = 0.060) (BLs: 202.4 ± 4.6°, JLs: 193.2 ± 8.2°), and maximum hip angle (p = 0.053) (BLs: 203.3 ± 4.4°, JLs: 193.6 ± 8.5°) (Table 4). With regard to the angular displacement (Table 6), no significant differences were observed between the BLs and JLs in the first pull, transition phase, second pull, and turnover. However, the displacements of the knee joint (BLs: 15.1 ± 4.8°, JLs: 5.6 ± 4.3°) and thigh angle (BLs: 13.5 ± 2.5°, JLs: 8.9 ± 3.4°) from the end of the transition phase to VAmax for the BL were significantly greater than those for JLs. There was also a major difference in the displacement of the hip joint (BLs: 42.7 ± 11.7°, JLs: 28.7 ± 8.7°) from the end of the transition phase to VAmax (p = 0.064). There were no significant differences in the angular velocities, but the angular velocity of hip flexion for BLs was greater than that for JLs (p = 0.087) (BLs: −12.68 ± 2.10 rad·s-1, JLs: −10.42 ± 1.51 rad·s-1).
The trajectory of the barbell is decided by the force applied to it by the lifter. Stone et al. (15) reported that the bar path suggested as “correct” in many coaching articles published in the U.S.A. is similar to those in European, Asian, and Canadian lifters in some regions of the bar trajectory, although being quite different in other regions. In the Japanese coaching method for the snatch, the lifters are directed to pull the barbell backward to hit the barbell to the lifter's pubic bone during the second pull (12). For achieving “efficient lifting,” it is important to closely investigate the barbell trajectory by analyzing the displacement, velocity, and acceleration of the barbell based while considering the coaching method in the region. In particular, the horizontal parameters seem to be of considerable significance for the snatch (1,9,15). Baumann et al. (1) reported that the horizontal displacement of the barbell for the BLs was smaller than that for the poorest lifts in each weight category. In this study, no significant difference was observed between the Dx2 and DxL values of BLs and JLs. However, the forward displacement of the barbell (Dx3) after the second pull for JLs was significantly greater than that for the BLs (Table 3) and the trajectories of the barbell for JLs in the 63-, 69-, and 75-kg classes crossed the vertical reference line projected upward from the initial position of the barbell. In contrast, the trajectories of the barbell for the BLs did not cross the vertical reference line, similar to world-class male weightlifters (1,10). Allowing these points, we can say that a greater Dx3, which results in a trajectory crossing the vertical reference line, is one of the most significant features of Japanese female weightlifters.
The relative maximum height (Dy1/BH), relative catch position (Dy2/BH), and height of the barbell's most forward position (Dy5/BH) for the BLs were significantly lower than those for the JLs. According to Okada et al. (13), the maximum heights of the barbell for Japanese female weightlifters and international female weightlifters in the 2006 Women's Junior World Weightlifting Championships were 0.63 and 0.64, respectively, that is, there was no significant difference between these groups. In this study, the maximum height of the barbell for the BLs was lower than these values, whereas that for JLs was almost the same as these values. These results indicated that the distinctive feature of the barbell displacement for BLs was a small forward displacement after the second pull and low height of the barbell at the maximum vertical position. This implies that the lifting performance of JLs could be improved by adopting a catching technique with a lower lifting position than the position used currently.
Regarding the vertical velocity and the vertical acceleration, there was no major difference between BLs and JLs. The values of the maximum vertical velocities were almost the same as those for the junior female weightlifters (13) and Greek female weightlifters (6). The vertical acceleration curves of the barbell are required to evaluate the different aspects of the lifting technique, such as the timing, amplitude of the acceleration during the second pull, and the direction of the applied force (6,10). Isaka et al. (10) reported that 3 peaks that corresponded to the first pull, transition phase, and the second pull were clearly observed in the vertical acceleration curves and that the appearance of the peak during the transition phase could be used as a criterion for judging the lifting skill. In this study, peaks were observed during the transition phase for both the BLs and JLs. However, a negative vertical acceleration of the barbell was also observed during the transition phase for some BLs and JLs. This result may indicate that there is a technical problem associated with the transition phase for female weightlifters.
It appears that the horizontal velocity and acceleration of the barbell are important criteria for assessing the lifting technique. However, no studies have been conducted that determine the relationship between lifting ability and the horizontal velocity and acceleration of the barbell. With regard to the relationship between horizontal velocity and displacement, Ikeda et al. (9) reported that the values of phVf for Japanese male weightlifters in the 2007 All Japan Weightlifting Championships were negatively correlated to the values of Dx3 (r = −0.942, p < 0.001) and Dx2 (r = −0.709, p < 0.001). In this study, the same results with respect to the relationship between phVf and Dx2 and Dx3 were obtained, and the values of phVf, pAf, and Dx3 for the BLs were smaller than those of the JLs. To lift the barbell efficiently, applying a large forward force during the second pull is a positive disadvantage. In consideration of Japanese technical instruction during the second pull, it is likely that the way of applying the force to the barbell during the second pull for the JLs affects the barbell kinematics. These results suggested that Japanese weightlifters and coaches would be better off reconsidering the technique of the second pull.
There was no significant difference between the BLs and JLs in terms of the ARA of the barbell at second pulls, whereas a major difference was observed in the phVf during the second pull. This result in terms of the ARA can be attributed to the fact that the value of the peak vertical acceleration for the JLs during the second pull did not correspond to the value of the peak forward acceleration (Table 4) and that the JLs accelerated the barbell vertically in proportion to the forward acceleration. According to Isaka et al. (10), the average ARA for Asian weightlifters was 140°, with a range of 110–160° in the second pull. The ARA values obtained in this study were slightly smaller than the value of Isaka et al. (10). Thus, female weightlifters in this study accelerated the barbells more anteriorly compared with male Asian weightlifters.
With regard to the patterns of leg and trunk movement, almost the same pattern was observed for the BLs and the JLs (Figures 8 and 9). The knee joint angle reached a first maximum angle in the first pull and then decreased in the transition phase and reached maximum extension in the second pull for both lifters. Baumann et al. (1) suggested that the position of the knee joint with regard to the direction of the ground reaction force appears to be an important technical factor in the transition phase. By comparing the lifting techniques of male and female weightlifters, Gourgoulis et al. (6) found that female lifters flexed their knees significantly less and slower than male lifters during the transition phase. The values of knee flexion displacement and angular velocity obtained in this study were almost the same as those obtained in a previous study (6). As for the function of the knee flexion during the transition phase, some studies suggested that the usage of the stored elastic energy permits the lifters to exert the explosive force during the second pull (6,10), but another function must be the lifting technique. That is, lifters do not need to move the barbell toward the lifter by using the knee flexion such that the knees are pushed toward the barbell during the transition phase. Moreover, it is possible for lifters to exert vertical force rather than horizontal force at the beginning of the second pull phase. The technique of the transition phase remains as a matter to be discussed further.
The results revealed that the timing of extending the knee and hip joints was different for the 2 groups. The angle of the hip joint of the BLs at the end of the transition phase was greater than that of the JLs (p = 0.053), and the angular displacements of the knee and hip joints of the BLs from the end of the transition phase to VAmax were greater than those of the JLs (Tables 5 and 6). Okada et al. (13) suggested that the interval between the peak velocity of the hip joint and peak vertical velocity of the barbell for international female weightlifters was greater than that for Japanese female weightlifters. Judging from the results of this study and that of Okada et al. (13), the way of applying the force to the barbell by hip and knee extensors during the second pull differs between Japanese female weightlifters and international weightlifters. This difference might be attributable to the recognition of the second pull technique in JLs that the barbell is pulled toward themselves and is intentionally hit to the part of the hip to accelerate the barbell.
The results show that the maximum hip flexion velocity during the turnover for the BLs was greater than that for the JLs. Little attention has been paid to the difference in terms of hip flexion velocity during the turnover in previous studies. However, because the barbell can be caught at a low height by maintaining a high velocity of hip flexion, the fast flexing movement of the hip joint is an extremely important technique for improving the performance of weightlifters.
In this study, we examined the joint movements and the barbell trajectory of Japanese and international weightlifters during the snatch event of the 2008 Asian weightlifting Championships. We found that the peak horizontal velocity in the forward direction (phVf) and the forward displacement during the second pull for the Japanese weightlifters were significantly greater than those of the best weightlifters and that the best weightlifters accelerated the barbell in the vertical direction earlier than the Japanese weightlifters did. Judging from this study, Japanese weightlifters may need to reconsider the way of applying the force to the barbell during the second pull. In addition, it is possible that performance of the female weightlifters could be greatly improved with the greater and faster flexion of the knee during the transition phase, as in the male lifting technique.
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