Two high-speed cameras (Mega-speed MS1000, sampling rate = 120 Hz) were set on the left side of the lifter to film the trajectory of the left barbell. The angle between the optical axes of the 2 cameras was about 90° (Figure 2). A calibration cubic volume, 148 cm long, 100 cm wide, and 175 cm high, with 24 control points was used in this study. The 3-dimensional spatial coordinates of the selected points were calculated using the direct linear transformation procedure of the Kwon3D motion analysis software. The raw data were smoothed using a fourth-order Butterworth low-pass filter with a cut frequency of 6 Hz. When the lifter was ready to start to lift the barbell off the platform, an experienced experimenter pushed a button triggering 2 synchronized cameras to film the snatch motion.
This analysis focuses on the snatch movement from the beginning of the barbell lift-off to the instant when the lifter catches the barbell overhead. In previous studies, the snatch was divided into 5 phases: the first pull, the transition, the second pull, the turnover under the barbell, and the catch phase. These phases are primarily determined by changes in the lifter's knee angle. However, because the athletes lift the barbell as close to their bodies as possible to decrease the torque produced by the barbells' weight, 5 new events were chosen for this study. To easily describe the barbell's trajectory, the 5 events are defined as the barbell lifting off the floor (LO), the barbell clearing the knee of the lifter (CK), the lifter extending his hip joints to push the bar away from his body (PB), the barbell reaching its maximum vertical height (MH), and the lifter catching the bar overhead (CB). To compare between lifters, the vertical positions are normalized by the height of the lowest position of the barbell during each catch phase.
Six horizontal variables, including the horizontal position of the barbell at CK (XCK) and PB (XPB), the horizontal travel range (HTR, the horizontal displacement measured starting from the barbell being pushed away from the body to its most forward position), the projectile range (PR, the horizontal displacement from the barbell's most forward position to the catch position), the horizontal velocity of the barbell at its highest position (MHVh), and the maximum horizontal velocity after the lifter pushes the barbell away (MaxVh), and 4 vertical variables, including the maximum height (MH), the normalized maximum height (N-MH, the maximum height normalized by the height of the catch position), the vertical travel range (VTR, the vertical displacement from the maximum height to the catch position), and the maximum vertical velocity (MaxVV) of the barbell, are defined and measured for each lift.
Two experienced experimenters digitized the coordinates that the barbell to make sure of the data are reliable. The test-retest reliabilities are determined using an intraclass correlation coefficient (ICC). A coefficient below 0.40 is considered poor, 0.40-0.59 fair, 0.60-0.74 good, and 0.75-1.00 excellent (2). This 1-way ANOVA is used to evaluate the differences in the 10 barbell kinematical variables during the snatch for each of the 3 groups. The LSD method is used for post hoc comparisons and the statistical significance level is set at p ≤ 0.05. All statistical calculations were performed using the SPSS version 13.0 for Windows (SPSS, Inc., Chicago, IL, USA).
Table 2 shows the ICCs for all the dependent variables. The high coefficients (0.72-0.99) indicate that the test-retest reliabilities are good-to-excellent for all the variables measured in this study.
The very small displacement of the barbell in the media-lateral horizontal direction shows that the barbell movement is almost exclusively in the sagittal plane for all the lifters. The 3 horizontal and the 3 vertical variables are significantly different in all of the 3 groups (Table 3). XCK is significantly greater in the middle and better groups as compared with the worse group. A smaller PR and a slower MHVh are observed for the better group as compared with the worse group. Although a smaller MH is shown for the middle and better groups, a greater normalized MH is found for the better group as opposed to the other 2 groups. The better group also has a greater vertical travel range than the middle and worse groups.
The comparisons in the barbell trajectories of the 4 best lifts (RBM > 1.91) and the 4 worst lifts (RBM < 1.21) are illustrated in Figure 3. The paths of the barbells are similar until the barbell achieves a height of about 50 cm (before the PB event) for T1 (RBM = 2.09), T2 (RBM = 1.99), and T4 (RBM = 1.91). And with the exception of T3 (RBM = 1.97), the paths of the other 3 lifts stay to the right of the vertical reference line. So, compared with the 4 best lifts, the paths of the 4 worst lifts all cross the vertical reference line.
Phase diagrams showing displacement-velocity relationships are used in this study to express the movement patterns of the barbell (Figure 4). The results of the movement patterns show that there is greater variation in the horizontal movement of the barbell than in the vertical movement. The greater horizontal displacement and velocity of the barbell is shown for the 4 worst lifts and than for the 4 best lifts. The movements in the vertical direction show that a greater normalized maximum height and a greater vertical travel range occur in the 4 best lifts. The 4 best lifts seem to show the same pattern for the first 60% in height starting from the lowest position of the barbell during the catch phase.
The purpose of this study is to describe the movement of the barbell during the snatch phase using a 3-dimensional kinematic approach. A very small displacement of the barbell in the media-lateral horizontal direction was found in this study. This shows that 2-dimensional kinematics in the sagittal plane is appropriate for describing a barbell's movement in weightlifting. This is not true, however, if the initial position of the barbell is different for each lift in real competition conditions, for example. Therefore, in this study a 3-dimensional kinematic analysis is necessary to decrease the measurement errors that will occur in the 2-dimensional approach.
In the study of Thé and Ploutz-Snyder (10), a prediction equation for weightlifting performance was derived from cross-validation analyses dependent on the age and body mass of the lifter as follows: ABS (kg) = 259.854 + (−3.147*age [yr]) + (1.286*body mass [kg]), where the ABS is the barbell mass summation in the snatch and clear and jerk. In the present study, the average of the actual achievements in the better group is significantly greater than their respective predicted accomplishments by about 28.2 ± 14.6 kg. It is worth to note that the mass summation of subject 1, the lifter performing at level T1 in this competition, is greater than his predicted value by about 45.6 kg. This champion lifter has won gold medals in international weightlifting competitions in his 62 kg weight class. The RBM indexes of the lifts for the better group, more than 1.63, in this study are similar to the indexes for elite Asian lifters (5,7). This indicates that the better group in this study which is determined by their RBM is representative of elite lifters.
The horizontal variables of the barbell have shown greater variation between lifters than the vertical variables (5). Also, less variation in the horizontal movement of barbell occurs more often for the best lifts than for the poorest lifts (1). In this study, although only XCK, PR, and MHVH are significantly different between groups, even from the worse group to the better group all the mean horizontal variables systematically increased for XCK and XPB or decreased as is the case for HTR, PR, MaxVH, and MHVH. The lack of a statistically significant difference is perhaps because of smaller statistical power values rejecting the null hypotheses. The results indicate that, when just starting to lift the barbell off the platform, a barbell pulled more toward the lifter will result in a better lift. After this, a barbell from a better lift moves with less horizontal displacement and velocity even after being pushed away from a lifter. As shown in Figure 3, the barbell trajectories of the best lifts, except for T3, do not cross the vertical reference line because of the first pull motion being toward the lifter and horizontal travel ranges being less than XPB. This barbell trajectory has been used by elite male Asian lifters for the snatch, and is considered to be a new and widely used technique (1,7).
The vertical variables of a barbell have often been used to evaluate the techniques of lifers. Elite lifters have been characterized by steady increases in the velocity of the barbell and by the formation of a single peak in the vertical velocity curve (1,5,7). In this study, no notable dip was observed in the best or worst lifts, except for T23 (Figure 4). The maximum height of the barbell is higher for the worse group because of higher lifters in the worse group. The lifters' heights are 165, 170, and 175 cm for the better, middle, and worse groups, respectively. Interestingly, a greater mean normalized maximum height and vertical travel range is found in the better group. Minimization of the vertical travel range of the barbell had been considered to be important for an effective technique (7). However, a vertical travel range is necessary to permit the lifter to drop under the bar. In this study, the skillful lifters pull the bar to a higher position to attain a greater vertical travel range. This allows for a gradual slowing down of a barbell's drop velocity because of gravity. With this catching technique, a lifter can catch heavier barbells successfully with the same amount of muscle strength.
In this study, the mean vertical travel range of the barbell for the better group 19.5 ± 1.9 cm is greater than the results of male weightlifters in previous studies which show only a range of about 10-14 cm (1,5,7). Similar ranges of about 19-21 cm (4,6) have been found for female weightlifters. For male weightlifters, the vertical travel range is increased as the barbell mass is increased (1,7). From the above results, the lifters seem to increase their vertical drop displacement of a barbell to catch the barbell under weaker strength or heavier barbell mass conditions. The best lifters in this study are good at using this cushioning technique to catch a heavier barbell successfully.
There has not been any kinematic variables of a barbell that clearly show separate successful lifts from unsuccessful lifts by statistical methods (9). Variability also occurs in the horizontal variables in this study, but there are no significant differences between the variables of successful and unsuccessful lifts. Therefore, after surveying the movement patterns of a barbell it is better to evaluate the snatch technique rather than just to capture the kinematic variables. The displacement-velocity relationship curve forms a circle from the time the barbell clears the knee until it reaches a maximum positive velocity in the horizontal plane. To minimize the area of the indicated circle, it is an effective lifting technique that the barbell should move nearly parallel along the vertical reference line in order to reduce horizontal work (7), for example, in the curve of T1 (Figure 4). When looking at the vertical component, the same pattern has been observed for the 4 best and worst lifts, with the exception of a smaller vertical displacement from the maximum height to the catch position for the worst lifts which we discussed in detail above.
From the results of this study, we conclude that elite Taiwanese weightlifters pull the barbell more toward their bodies from the time the barbell lifts off the platform until it is pushed away from the lifters with complete extension of the hip and knee joints. Then they jump backwards to catch the barbell leading to a greater vertical travel range. The horizontal movement patterns of the barbell show that elite Taiwanese weightlifters perform with less displacement and velocity in the horizontal direction during the pull phase. Although the movement pattern in the vertical direction is similar, there is no standard trajectory for each weightlifter because of the variations in their horizontal movement patterns.
Asian male weightlifters usually perform well in world weightlifting competitions. In this study, elite Taiwan male lifters perform the same barbell trajectories as other Asian male lifters, except that they have an increased vertical travel range. This catching technique reveals a higher maximum height for the barbell and a backwards jump to catch the barbell in a deeper squat position. The weightlifters seem to have been instructed to increase the vertical travel range of the barbell in order to catch it under weaker strength (e.g. in female lifters) or heavier barbell mass conditions. The results of this study suggest that coaches can film the trajectory of a barbell during a lifter's snatch and replay it immediately to give real time feedback to the lifter and instruct him or her how to find a more correct and successful barbell trajectory.
The authors would like to thank the National Science Council in Taiwan for their financial support of this project (NSC-95-2413-H-006-014). We would also like to thank the Chinese Taipei Weightlifting Association for its approval of this experiment.
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Keywords:Copyright © 2010 by the National Strength & Conditioning Association.
barbell trajectory; phase diagram; relative barbell mass; biomechanics