Weightlifting, and particularly the snatch movement, is one of the most technical competitions (2). Successful snatch lift performance is determined by the weightlifter's ability to lift the barbell overhead and keep it in that position until a confirmation signal sounds. This has to be done in no more than 3 trials. However, many trials often result in the drop of the barbell during the snatch movement. This is observed even with loads that the athlete might be able to lift in the next trial. The knowledge of the factors that can make the difference between a successful and an unsuccessful lift at a specific barbell load could increase the frequency of an athlete's successful snatch lifts. At the same time, it could reduce the number of trials needed to lift a specific weight, conserving the athlete's energy and increasing his or her possibility of lifting much higher weights.
In the past, several biomechanical parameters of snatch lift performance have been investigated during successful snatch lifts, using 2-dimensional (1,5-8,16,17) or 3-dimensional kinematic analysis techniques (3,14). Elite weightlifters have similar characteristics regarding their limb and barbell movements during the lift, independent of weight category (14), gender (12), or age (13). On the other hand, nonelite athletes show different kinematic characteristics according to their category (4) and gender (15). However, in both elite and nonelite athletes, a common movement pattern has been observed. The knees execute a characteristic extension-flexion-extension movement, where the fist knee extension is defined as the first pull, the knee flexion is defined as the transition from the first to the second pull, and the second knee extension is defined as the second pull (3,5,9,16).
For a properly performed snatch lift, the knee extension should be faster in the second pull than during the first pull, and the hip extension velocity should be greater than the corresponding velocity of the knees (3,14). Regarding the barbell's trajectory, the bar is moved toward the lifter during the first pull and the transition phase, and during the second pull it is moved away from the lifter's body. However, these anterior-posterior displacements of the barbell should be small, to avoid pointless energy consumption for the bar's horizontal displacement (3,9,14,16). Furthermore, for an effective lift, the vertical linear velocity of the barbell should be continuously increased until the end of the second pull, because the existence of 2 clear velocity peaks would demand additional energy from the lifter to overcome the negative momentum of the barbell during its velocity's decrease (2). Moreover, the mechanical work done on the bar for its vertical displacement should be greater during the first than the second pull, and the mechanical power output should be greater in the second pull (2,11,14).
Despite the detailed analysis of the snatch movement during successful lifts, there is a lack of corresponding data regarding the kinematics of the snatch movement during unsuccessful lifts, when the barbell drops onto the ground. The purpose of the present study was to determine the kinematic characteristics affecting the drop of the barbell in front of the weightlifter during the snatch movement.
The research hypothesis of the current research was that significant differences in specific kinematic characteristics would determine the final outcome of the snatch movement.
Experimental Approach to the Problem
To determine the kinematic factors that make the difference between a successful and unsuccessful lift, a large number of kinematic parameters describing both limb and the barbell movements were studied though 3-dimentional kinematic analysis.
The study sample comprised 7 high-level men weightlifters during national competitions. Two of them participated in the category of 69 kg, 2 weightlifters participated in the category of 77 kg, and 3 weightlifters participated in the category of 85 kg. All subjects were members of the adult Greek national weightlifting team and provided written consent for their participation in the study. Procedures were in accordance with the Helsinki Declaration of 1975, and institutional review board approval was obtained for this study.
Athletes' successful and unsuccessful snatch lifts with the same barbell load were recorded with 2 S-VHS camcorders (Panasonic PV-900) operating at 60 fields per second. The 2 cameras were positioned in a horizontal plane, 15 m away from the subjects. The optical axis of each camera formed an angle of 45° with the frontal plane of the subject. That arrangement allowed the movement to be viewed by each camera, both from the side and from the front.
To determine the kinematic characteristics of each lifter's body and barbell, selected points were digitized manually using the Ariel Performance Analysis System (Ariel Dynamics). These points corresponded to the great toe, ankle, knee, hip, and acromion of the right side of the body, as well as 1 point on the right edge of the barbell. The lift-off of the barbell was used for the temporal synchronization of the 2 cameras, and the 3-dimensional coordinates of the selected points were calculated using the direct linear transformation technique. For the calibration of the recorded space, a rectangular cube with 180-cm length, 90-cm breadth, 180-cm height, and 23 control points was used. The mean reconstruction errors expressed in RMS values were 3.2, 2.7, and 3.5 mm in the X, Y, and Z directions, respectively. A low-pass digital filter with a cutoff frequency of 4 Hz that was selected through residual analysis of a wide range of cutoff frequencies was used for the smoothing of the raw position-time data.
The movement was studied from the beginning of the barbell's lift-off to the point at which the lifter dropped under the barbell; this period was divided into 5 phases, according to knee angle and the height of the bar:
- The first pull: From the barbell's lift-off until the first maximum knee extension
- The transition from the first to the second pull: From the first maximum knee extension until the first maximum knee flexion
- The second pull: From the first maximum knee flexion until the second maximum extension of the knee
- The turnover under the barbell: From the second maximum extension of the knee until the achievement of the maximum height of the barbell
- The catch phase: From the achievement of the maximum height of the barbell until stabilization in the catch position with the barbell overhead in successful lifts or the drop of the barbell in front of the athlete in unsuccessful lifts
To study the movement of the body, the angular displacements (Figure 1) and angular velocities (Figure 2) of the ankle, knee, and hip joints were calculated. The barbell's movement was described by the anterior-posterior displacement of the bar, maximum height of the bar (Figure 3), vertical linear velocity, and vertical, anterior-posterior (Figure 4), and resultant linear accelerations. The external mechanical work and the mechanical power output for the vertical lift of the barbell during the first and second pulls were also calculated according to the methodology described by Garhammer (11). Furthermore, the angle θ between the resultant linear acceleration of the barbell and the vertical line passing through the position of the bar before lift-off was calculated, using the equation
where ay was the vertical component of the barbell's linear acceleration, and a was the resultant linear acceleration of the barbell.
For the statistical treatment of the data, t-tests and analyses of variance for dependent samples were used. The assumption of normally distributed samples was verified using the Kolmogorov-Smirnov test, and the level of significance was set at p ≤ 0.05.
The results reveal that there were no significant differences in the durations of the separate phases of the lift between successful and unsuccessful lifts (Table 1).
Moreover, no significant modifications were observed in the knee angle at the beginning of the lift, in the first and second maximal knee extensions, in the minimum knee angle at the end of the transition phase, or in the corresponding amount of knee flexion. The same was observed for the first and the second maximal ankle extensions and the maximal hip extension (Table 2).
Regarding the joints' angular velocities, t-tests revealed that the maximum angular velocities of the knee and the ankle joints in the first and second pulls and the maximum hip extension velocity were not significantly different between the successful and unsuccessful lifts (Table 3).
Analysis of variance for dependent samples showed no significant interaction between the “successful-unsuccessful lift” factor and the “phase of the lift” factor, regarding the maximal angular velocity of the knee's extension (F1,6 = 2.782; p = 0.146) and the maximal angular velocity of the ankle's plantar flexion (F1,6 = 0.075; p = 0.794). On the other hand, there was a significant main effect of the “phase of the lift” factor in both knee extension (F1,6 = 35.678; p < 0.05) and ankle plantar flexion (F1,6 = 128.854; p < 0.05) maximal velocities. These meant that in both successful and unsuccessful lifts, the knees were extended and the ankles were plantar-flexed significantly faster during the second pull compared with the first one. Regarding the maximal extension velocities of the hip and knee joints during the second pull, no significant interaction was observed between the “successful-unsuccessful lift” factor and the “joint” factor (F1,6 = 1.078; p = 0.339). However, there was a significant main effect of the “joint” factor (F1,6 = 23.635; p < 0.05), which meant that in both successful and unsuccessful lifts, the hip extended significantly faster than the knee joint (Table 3).
Regarding the trajectory of the barbell, no significant differences were observed between successful and unsuccessful lifts in the maximum height of the barbell or in the horizontal displacement of the barbell toward or away from the lifter, nor were any observed in the height of the barbell at the end of the first pull, the transition phase, and the second pull (Table 4).
Concerning the vertical linear velocities of the barbell, successful lifts were not found to be significantly different than unsuccessful lifts in the barbell's maximum velocity, the instant of maximum velocity achievement, the barbell's absolute velocity at the end of the first pull, the percentage of the barbell's maximum velocity at the end of the first pull, the absolute decrease of the barbell's velocity in the transition phase and its percentage in regard with the maximum velocity of the barbell, or in the velocities of the barbell at the end of the transition phase and at the end of the second pull (Table 5).
Regarding the energy characteristics of the lift, no significant differences were observed between successful and unsuccessful lifts in the work and the power generated for the barbell's vertical lift during the first and second pulls (Table 6).
Analysis of variance for dependent samples showed that the interaction between the factors “successful-unsuccessful lift” and “phase of the lift” in the mechanical work for the vertical lift of the barbell (F1,6 = 2.554; p = 0.161) was not statistically significant, but the main effect of the factor “phase of the lift” (F1,6 = 29.026; p < 0.05) was. This meant that in both successful and unsuccessful lifts, the mechanical work during the first pull was significantly greater than during the second pull. No significant interaction (F1,6 = 1.058; p = 0.343) of the previous factors was found in the mechanical power output for the lift of the barbell. Again, a significant main effect of the “phase of the lift” factor (F1,6 = 133.667; p < 0.05) was observed, but this time the power was significantly greater during the second than during the first pull (Table 6).
Furthermore, no significant differences were found between successful and unsuccessful lifts in selected parameters concerning the position of the body's limbs in reference to the barbell's position in the various phases of the lift (Table 7).
Moreover, no significant differences were found between successful and unsuccessful lifts in the mean angle of the barbell's resultant linear acceleration vector with the vertical axis Oy (Figure 5) in any of the distinct phases of the lift, except from the first pull (Table 8).
The results reveal that there were no significant differences in the kinematic characteristics of the lift between successful and unsuccessful lifts. However, the direction of the forces applied onto the bar seems to be of decisive importance. Large deviations in the line of force application on the barbell from the vertical in the first pull, along with smaller ones during the other phases of the lift, can accelerate the barbell in such a way that the lifter might not be able to control the barbell's movement and may drop it.
Regarding the durations of the separate phases of the lift, no significant differences were found between successful and unsuccessful lifts. The duration of the first pull was longer than the other phases of the lift, and the transition and second pull were particularly short in both cases. Moreover, there no significant alterations were observed in the maximal flexion and extension angles of the ankle, knee, and hip joints or in their extension velocities. In both successful and unsuccessful lifts, the knees were extended during the first pull, and, during the transition, they were flexed and moved in front under the barbell, helping the lifter to pass smoothly into the second pull. After the maximum flexion of the knees, which determines the start of the second pull, the hip, knee, and ankle joints were extended explosively and achieved their maximum extension values at the end of the second pull (3,5,9,16). Furthermore, concerning the joints' extension velocities, the same pattern was observed in both lifts. The maximal extension velocity of the knees during the second pull was always greater than the corresponding velocity during the first pull, and during the second pull the maximal extension velocity of the hip was greater than the maximal extension velocity of the knees, giving an additional acceleration onto the barbell and contributing to the execution of an explosive second pull (3,12-14).
The transition from the first to the second pull is recognized as a particularly important phase. To be effective, it should be executed rapidly and with a small bending of the knees (2). This allows the storage of elastic energy into the extensor muscles of the knees during the knees' flexion and its use during the following concentric contraction of the knees, resulting in an explosive power output during the second pull (5,9,16). It is remarkable that although the work for the barbell's lift was greater during the first than during the second pull, the power output was significantly greater during the second than during the first pull in both successful and unsuccessful lifts (10,14).
Regarding the barbell's trajectory in the successful as well as the unsuccessful lifts, the bar was moved toward the lifter during the first pull and the transition from the first to the second pull; it was moved away from the lifter during the second pull and again toward the lifter during the turnover under the barbell (9, 14). According to Isaka et al. (16), such small anterior-posterior displacement of the barbell is indispensable for an effective lift, but its magnitude should be small to avoid the loss of energy for the horizontal displacement of the barbell (3). Furthermore, a continuous increase in the barbell's vertical linear velocity until the end of the second pull of the barbell is important for an effective lift (2), whereas, according to Baumann et al. (3), the existence of 2 clear velocity peaks is an indicator of an ineffective technique, as the negative momentum of the barbell should be overcome additionally from the lifter (2). In the present study, only 2 of the subjects showed negative barbell accelerations during the transition phase, in both successful and unsuccessful lifts.
From the above, it seems that skilled weightlifters show stable movement patterns of their limbs and barbell. The failure of the lift is judged by other parameters, such as the line of force application on the barbell, which is reflected by the direction of the barbell's resultant linear acceleration vector relative to the vertical axis. In the present study, the only statistically significant difference in the angle of the barbell's resultant linear acceleration vector relative to the vertical axis between successful and unsuccessful lifts was observed during the first pull, showing the decisive importance of the appropriate beginning of the lift. Although no significant differences were found in this angle in the rest of the phases, studying the barbell's trajectory curves in combination with the barbell's acceleration vector for each subject revealed obvious differentiations in the way force was applied on the barbell (Figure 5). So, the absence of statistically significant differences in the other phases might be attributable to the great variance of the barbells' acceleration angles in these phases.
Summarizing the findings of the present study, it seems that skilled weightlifters showed a stable pattern in their limb and barbell movement, no matter what the final outcome of the lift was. Dropping of the barbell in front of an athlete during an unsuccessful lift principally seemed to occur as a result of erroneous directions of the applied force onto the barbell, beginning with the first pull, which should receive particular attention from weightlifting coaches.
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Keywords:© 2009 National Strength and Conditioning Association
kinematics; weightlifting; unsuccessful