CRITICAL FEATURE 1 (LOWERING THE CENTER OF MASS BEFORE THE TURN)
The relatively erect torso and minimal squat of the participant employing the FMS in particular do not allow for much force generation against the ground compared with a deeper squat. By lowering down into a deep squat, the leg muscles are preloaded and as a result are able to produce greater vertical and horizontal force into the ground, creating a larger ground reaction force in the intended direction of travel at takeoff.
Although the initial step backward of the FSP may appear to be ineffective, it does allow for effective use of the stretch-shortening cycle. By preloading the muscles of the trail leg with potential elastic energy, a greater amount of force may potentially be produced over a greater amount of time (greater impulse). Given the relationship between impulse (force [f] × time [t]) and momentum (mass [m] × velocity [v]), this strategy could result in greater movement velocity, which could arguably make up for the increased time taken by the initial step backward (6).
CRITICAL FEATURE 2 (MOVING THE CENTER OF MASS INTO THE SPRINTING DIRECTION)
As soon as the downward motion is initiated, the body begins to transfer weight into the new direction (to the left). Force is applied horizontally, and body parts are aligned in the desired movement direction.
CRITICAL FEATURE 3 (ARMS AND LEGS CLOSE TO THE BODY WHEN TURNING)
The body's rotational inertia (I) (resistance to turn) is primarily dependent upon the distribution of the body's mass around the axis of rotation (I = mr2). Increased rotational inertia (arms wide) increases the stability of the rotating body but results in a decreased turning effect. By bringing the arms (mass, m) closer to the body (axis of rotation, r) during the turn, a faster rotation will occur while still maintaining stability through the squat and postural adjustments already being employed (2,9).
CRITICAL FEATURE 4 (CENTER OF MASS AHEAD OF THE TAKEOFF FOOT)
The distance of the center of mass in relation to the takeoff foot when it leaves the ground is known as the takeoff distance. The larger this distance is, the greater the step length will be, resulting in increased takeoff velocity and a faster sprint (assuming that the frequency of each step is maintained) (10). The takeoff distance of the first foot take off is clearly larger for the participants using the FSP and PC strategies than the FMS. In both the pivoting strategies, the participants begin with a wider stance, which allows for a greater takeoff distance once the pivot has been completed. In contrast, the FMS participant begins with a narrower stance and the takeoff is completed before any foot adjustments (pivot, false start, etc). If a wider base of support was employed by this participant, then the takeoff distance would not be increased as the takeoff foot will always be the foot closest to the new direction, as opposed to the rear foot in the pivoting strategies. Additionally, by creating a simultaneous or near simultaneous takeoff and touchdown of contralateral legs, the flight phase is minimized or possibly eliminated, thereby increasing the ground contact time (GCT). Because propulsive force can only be produced when in contact with the ground, the increased GCT may allow for greater impulse to be generated than might occur if the flight phase was increased. An increase in generated impulse would likely result in a faster sprint time (f × t = m × v) (5,9).
Once the player has rotated into the new direction (second foot takeoff), the takeoff distance is similar across all the 3 strategies. However, at this point, the participant employing a FMS strategy uses a lateral takeoff, whereas the pivoting participants are able to potentially generate more force in the direction of travel at takeoff through a foot placement parallel to the direction of travel (9).
CRITICAL FEATURE 5 (FULL LATERAL EXTENSION OF THE TAKEOFF LEG)
There are conflicting reports as to whether superior sprinting performances use a full triple joint extension (ankle, knee, and hip). By applying force into the ground over a longer time as the leg extends fully, a greater velocity can be attained (f × t = m × v) (5,9). However, it may be an abbreviated range of motion at these joints that is more beneficial for tasks that require quick adjustments to their direction and speed (3,10,13). Because minimizing the amount of time taken to complete a directional change is the goal of this movement, a full extension at the ankle, knee, and hip may not be essential.
The perpendicular position of the trail leg at takeoff in relation to the rest of the body, as well as the movement direction, may not be as effective at producing the large propulsive forces as a foot positioned in the intended direction of motion. When placed parallel to the intended direction, the foot is able to produce potentially greater amounts of force into the ground through plantarflexion as opposed to eversion with a perpendicular (lateral) foot placement (4).
CRITICAL FEATURE 6 (INTENSE DRIVING ACTION OF THE ARMS)
As the athlete reaches the final portion of the turn, a rapid elbow extension occurs. This movement increases the rotational inertia, causing the body to slow (or stop) the turning effect (9). This movement may be more noticeable in the PC strategy but is present to some extent in all 3 strategies. The more rapid this movement is performed, the faster the rotation will cease and the sooner the player can continue on in the sprinting direction. The intense driving action of the arms once the body has completed the turn can assist in the takeoff velocity when accelerating (1,8,10), although it is important to note that this driving action must be performed in line with the body, as opposed to lifting the arms away from the sides, which would create a tendency to rotate.
SUMMARY AND CONCLUSIONS
The COD movement strategies that athletes commonly employ and the technical cues to improve activity and/or sport-specific COD have received little attention and provide an exciting area for research. Of the 3 COD movement strategies discussed, the fastest COD time through both the first and the second steps in the new direction likely occurs with the PC. The slowest of the 3 strategies is likely the FMS (Table 2). It appears that 2 technical characteristics may be critical features to a superior 90° COD movement performance when using the PC: aggressive driving arm action through the turn and a limited forward lean (both of which are critical features of effective sprinting). Differences using a static start compared with a dynamic situation need further investigation.
Several factors (i.e., individual anthropometric measures, physical coordination, situation-dependent requirements, etc) may contribute to the ability to execute these strategies with a superior performance. A greater distribution of body mass from the axis of rotation will increase the rotational inertia that the player must overcome when turning. Therefore, certain adaptations or adjustments to the COD movement strategy may be needed to overcome this factor. Likewise, an athlete who is less proficient at completing rapid movements, those involving proprioceptive awareness or gross/fine motor skills, may not be as successful at the same COD movement strategy as a more proficient athlete. However, this aspect has the potential to be improved with practice.
Finally, the sporting task or situation that the player is responding to may have specific postural characteristics. For example, a netball player must remain relatively erect to read player movements and catch or intercept a pass. In contrast, an ice hockey player adopts a lower center of mass as a result of where the puck is played (on the ice as opposed to in the air) as well as to increase the length of reach and protect the puck when in possession. Although both players may have similar body types and coordination, the demands of the sport may determine which COD movement strategy is most likely to result in a superior COD movement performance.
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Keywords:© 2010 National Strength and Conditioning Association
change of direction; technique; 90° turn; agility; kinematics; critical features