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Philip Goodman

BIOMECHANICS OF THE SNATCH: TOWARDS AH G HE RT R AN NG EFFICIENCY Klaus E. Bartonletz, PhD Olympic Training Center Rhineland-Palatinate/Saarland, Schifferstadt, Germany Athletes in strength and power sports such as American football, weightlifting, and track & field events use various types of power and speed/strength exercises in their training. The snatch and its variations are useful training exercises for these athletes. There are several variations of the snatch, for example: ▯ squat snatch (used in weightlifting competition). ▯ power snatch. ▯ split snatch. ▯ snatch from kneeling position (bending and extending hip joints and putting one leg In front). ▯ snatch from hang. ▯ snatch from blocks (using different heights). ▯ one-legged snatch. ▯ various pulling movements (e.g. with weights or dumbbells). It is Important to understand the biomechanics of the snatch. Biomechanical and technical knowledge of both competition and training exercises can help one select the appropriate exercises to train for various sports.V ariations in the snatch (which differ from the squat snatch used in competition) can alter movement patterns and barbell velocity, resulting in different specific adaptations. For example, many sports require a high power output from the hip can be advantageous in that It forces the hip and knee extensorsg position to work in unison. The coach is fundamentally concerned with guiding the adaptation process toward the demands ofpee, or competition. Because the specificity of the exercises used in competition dictate the structure of the training process, a good understanding of biomechanics will help the coach guide the athlete toward greater training efficiency. The external movement pattern of the snatch is described by Derwin (5), Garhammer and Takano (6), and Takano (11). Figure 1 shows the movement path of the barbell with the corresponding body positions at different points in time for two lifters. In the first pull (see Positions 1-3) the knees and hips are extended, ankles are plantar-flexed, feet are in complete contact with the floor, and the trunk is held almost constant to make for an effective force transition. The working area for the knee and hip extensors can differ depending on the size of the athlete. Taller athletes start with more bending of the knees, as shown in Position 1 of the bottom example in Figure 1. The shorter athlete begins the first pull from a knee angle of 80° whereas the taller athlete bendsthekneestoan angle of 47°. At the end of the first pull the bar reaches about 80% of its maximum velocity. In the transition from the first to the second pull, shown in Positions 3 and 4 of Figure 1, the knees are pushed toward the barand the knee angle is decreased about 20°. Byhep ligtheilter easei nto the second pulling phase while continuing to extend the hips, this action, when performed correctly, results in a barbell movement without any decrease in velocity. The second pull, shown in Positions 4-6ofF iure1, follows with a smooth transition to the powerful ankle plantar-flexion (raising the heels), knee, and hip extension. Ankle work can contribute up to 10% of maximum bar velocity (12). At the end of the extention phase of these joints, the weight reaches maximum velocity. The slight lifting from the floor initiates a repositioning of the body. Positions 6▯8 of Figure 1 show the turnover and catch. During the upward movement and beginn▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯t move his or her body downward and, with feet completely in contact with the floor once again, decelerate the downward movement of the barbell. The arms should be locked at the conclusion of the catch phase. The differences in the trajectory in Figure 1 are the result of differences in barbell velocity. The shorter lifter shifts the bar (vertical displacement) about 72% oft he distance utilized by the taller lifter. Therefore differences in the necessary bar velocity are inevitable. Athletes of the 52-kg category need 1.50-1.60 m/s. By comparison, athletes of the super-heavyweight category need 1.90-2.00 m/s (9). Movement of the Barbell A perfect coordination of the separate movements ensures that the trajectory of the weight remains behind the vertical line at all times, as shown in Position 8 in Figure 1 and also in Figure 2b. By training the muscle snatch (catching the bar overhead while only slightly bending the knees) or high pull exercises, the lifter will, in the second phase, move the weight slightly in front of the vertical line. Barbell trajectory will be ineffective if hip extension is too rapid in the first pull or if arm / trunk angle is too great in the second pull. The velocity of the bar should increase continuously. A short velocity plateau between the first and second pull is acceptable. Figure 2b shows a flowing transition from the first to the second pull. A movement coordination that results in a continuously increasing bar velocity is mechanically effective because the lifter only transfers a minimum of physical work to reach a given velocity. There is often a dip in the velocity vs. time relationship between the first and second pull (see Figure 2a and Figure 3). The short decrease in velocity requires ahiherlvelo fai forth e second pull. For the snatch performance shown in Figure 3, a▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯al decelerating impetus of - 55.5N ▯▯▯▯▯▯85 kg x -0.3 m/s), or about 16% of the total impetus (185 kg x 1.9 m/s = 351.5 N ▯▯s). Too fast of a first pull hinders the transition phase. The after-effect will be a noted decrease in velocity. Research by Martjanov and Popov (10) has revealed there is great variability in velocity changes for successful lifts, as can be noted in the area between the dashed lines in Figure 3. It appears that a prerequisite for movement stability during weightlifting competition is a corresponding stable movement execution of the main training exercises (e.g. velocity vs. time. characteristics). But such zones hardly contain feedback about the effectiveness of the movement pattern (compare th▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯M▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯tpattern requiring less energy that of Athlete in Figure 3, is more effective. Note that a slight decrease in velocity during the transition phase, as a result of knee movement toward the bar, hypothetically can create higher muscular pretension of the quadriceps muscle group for the second pull. This is illustrated inFiure1i the decrease of knee angle between Positions 3 and 4 of both lifters. Therefore a final assessment of the lifting technique must consider the general mechanical and physiological effects along with the ▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯ abilities. Maximum velocity of the barbell is an important factor in ensuring that the training load is applicable to the weight used. The product of maximum velocity (V max ) and weight (m ▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯ (measured on the barbell) physical power component for the vertical barbell lift at this critical time. This parameter is called speed-strength power (P ): s-s mxgxv max =P s-s (kg x m/s xm/s=W) Note that the total power contains a component for barbell acceleration as well. The value of this component is fairly low compared to the lift component. Because power is the work (energy) performed in a given time period, the relationship between the work required to accelerate and to hit the barbell expresses therealinsip between the power components: Kinetic energy 2 m/2 x V max Potential energy m x g x h where m = barbell mass; V max = maximum barbell velocity; g = 9.81m/s 2:h = path of vertical barbell lift. The following example illustrates this relationship (barbell mass = 100 kg, maximum velocity = 2.0 m/s, vertical lift path = 1.25 m): Kinetic energy 2 100 kg/2 x 2 m/s = 200 N ▯ m Potential energy 2 100 kg x 9.81 m/s x1.25m=1226.3 N ▯ m The lift component of the work is about six times greater than the acceleration component. The relationship between the power components are comparable to the work relationship. In several European countries, the parameter of speed- strength power is used late in the strength training of weightlifters, throwers, and sprinters (1). Real data points to the importance of speed-strength power inplanning the training regimen. The example shown in Figure 4s i takenfromBitzand Borkeloh (3). The athlete, who was Olympic champion in 1992 and world champion in 1995, snatched 180 kg with a maximum velocity of 1.93 m/s (P s-s= 3.40 kW) and 190 kg with a maximum velocity of 1.83 m/s (P = 3.42 kW).H i personal target velocity was 1.83 m/s. He could snatch a weight >190 kg if he could accelerate the 190 kg up to the 1.83 m/s level. A training load of 170 kg must be lifted with a velocity of 2.04 m/s (170 kg ▯▯▯▯▯▯▯▯/s ▯▯▯▯▯▯▯▯/s = 3.40 kW) for a maximum training intensity. In 1988 this athlete snatched a personal best of 200.0 kg., and at the 1992 Olympics he snatched 192.5 kg. The goal of training must be higher power values, for example using high pulls. The relationship of velocity vs. barbell mass for high pulls, demonstrated in Figure 4, shows that speed-strength power is greater than the 4-kW level over the whole tested interval (192.5 - 222.5 kg. A competition result of 207.5 kg would require aP s-slevel of 3.77 kW. This power level could be developed with high pulls because they make greater demands on power. In addition to the four ways of estimating exercise intensity - magnitude of resistance, number of repetitions per set, number of repetitions with maximal weight, workout density (13) - the product of weight and velocity is another important factor. It appears that young athletes should stress movement velocity rather than weight, along with proper technique. Only in the later stages of athletic development do weight increases become the most important factor. Some training experiments have shown that power abilities (e.g. for throwers) can be developed through weight training as well as through speed of movement execution (4.7). Once the maximum velocity of the barbell is reached and there is no further accelerating force on the bar, the remaining path of the weight,which is the distance s, will be determined by the following equation: 2 s=v max/2g For vmax = 1.6 m/s, distance would be a barbell path result of 0.13 m; for V max= 1.9 m/s. distance would be a barbell path result of 0.18 m. It is not enough to turn over and catch the barbell. The intensive shrug of arms and shoulders must act against the deceleration of the barbell. The lifter needs to extend the upward path of the bar. It is possible to lengthen the path interval of about 0.10 - 0.15 m during the non-support phase in this way. This is the result of ▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯ since the total impetus of▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯▯ system cannot be changed during the non-support phase. Catching the barbell overhead requires a fast turno
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