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# Understand the mechanical reasons for each key element

### This is an excerpt from Applied Sport Mechanics 4th Edition With Web Resource by Brendan Burkett.

Understanding the mechanical basis behind each key element is an important step in your sequence. The first 10 chapters laid the foundations of principles of sport mechanics. By analyzing technique we are putting this knowledge into practice - in essence, practicing applied sport mechanics.

All fundamental actions that an athlete takes within technique are founded on mechanical principles. In other words, technique is based on mechanical laws. So after you've picked out the key elements in the skill you are analyzing, you have to understand the mechanical purposes behind each element. You must be able to answer questions such as the following with responses like the ones listed here.

### Why cock and uncock the wrists during a golf drive?

Cocking and uncocking the wrists during a golf drive causes the golfer's arms and club to simulate the whiplash, or flail-like, action of the high-speed tip segments of a whip (see figure 11.4c-11.4e). When the wrists are cocked and uncocked, they act as an additional axis around which the club can rotate. The velocity developed from the swing (and length) of the golfer's arms is multiplied along the length of the club shaft (Fedorcik et al. 2012). Without the cocking and uncocking action, the arms and club would move as a fixed unit. This action would not allow the head of the club to reach optimal velocity.

### Why should a sprinter's legs and arms thrust and swing parallel to the direction of sprint during a 100 m sprint?

If a sprinter's arm swing and leg thrust (see figure 13.1) in any direction other than parallel to the direction of sprint, the forces that the sprinter applies to the earth in the direction of sprint are reduced. In reaction, the force that the earth applies against the sprinter is lessened as well. The result is that the sprinter doesn't run as fast as possible.

### Why should a freestyle swimmer pull with the hands and forearms along a line parallel to the long axis of the body rather than emphasize an S-shaped out - in - up - down pattern of pull?

Emphasizing an S-shaped out - in - up - down motion with the hands during the freestyle stroke, as shown in figure 13.10, is now thought to generate less propulsive force than pulling straight back against the water. A modified S-shaped motion still occurs during entry and exit of the swimmer's hand, but these actions occur more from body roll and the anatomy of the swimmer's body than from efforts to generate more propulsion. Pulling back against the water as far as possible parallel to the long axis of the body is now considered the correct technique. Under water, the arms flex at the elbows so that the swimmer's hands and forearms provide the major propulsive surfaces.

### Why must athletes have their center of gravity positioned behind the jumping foot as they enter a high-jump takeoff or behind both feet as they prepare to jump to block or spike in volleyball?

Positioning the takeoff foot ahead of the center of gravity gives the athlete more time to apply force with the jumping leg at takeoff (see figure 13.2). The athlete rocks forward, up, and then over the jumping foot. This large arc of movement gives the athlete time to drive down at the earth. The earth in reaction drives the athlete upward. The same principle applies to a volleyball spike, a volleyball block, a basketball layup, and a basketball block.

### Why is it important for athletes to rotate the hips and thrust them ahead of the upper body during a golf drive, shot put, and discus or javelin throw?

Rotating the hips ahead of the upper body and toward the direction of throw serves three purposes. First, it shifts the athlete's body mass in the proper direction (i.e., toward the direction in which the golf club, discus, shot, javelin, or baseball bat will be accelerated). This action extends the distance and time over which the athlete applies force. Second, the rotation of the hips acts as an important link in the sequential acceleration of the athlete's body segments. The movement of the athlete's legs and hips toward the direction of throw (or impact with the ball in golf or baseball) simulates swinging a whip handle ahead of the rest of the whip so that the tip of the whip cracks. Third, the rotation of the hips stretches the muscles of the abdomen and chest so that they pull the shoulders and throwing arm in slingshot fashion toward the direction of throw. (Notice the weight shift and hip action in the javelin throw in figure 13.4 and in the golf drive in figure 11.4.)

### Why should athletes extend the kicking leg when contacting the ball in a football punt?

By extending the kicking leg, the athlete puts the part of the foot that contacts the ball farther from the axis of rotation (i.e., the hip joint). Because of this increase in radius, the kicking foot is moving faster than any other part of the leg when it contacts the ball. The flexion of the kicking leg before contact with the ball, together with its extension at impact, simulates a whiplash action (see figure 13.11).

At a Glance

Analyzing Sport Skills

• One of the greatest challenges you'll face working in sport is watching the athlete perform and deciding which aspect of the skill needs correction (if any).
• All fundamental actions that an athlete makes within technique are founded on mechanical principles. In other words, technique is based on mechanical laws.
• After you have analyzed the performance, you need to communicate this information to the athlete. Technology can be an effective mechanism for providing feedback.

### Why must athletes extend their bodies fully at takeoff in gymnastics and diving skills?

Athletes who need to rotate quickly must apply an eccentric thrust, or an off-center force, at takeoff to initiate rotation. They must then pull the body inward from a fully extended position. The large reduction in rotary inertia caused by compacting the body mass around the axis of rotation is rewarded by a huge increase in the rate of spin (i.e., angular velocity).

All phases and all key elements in a skill are performed for specific mechanical purposes. If you know the mechanical reasons that they're performed as they are, you can confidently say to yourself, "OK, I understand what should occur in the technique of this skill, and I understand the mechanical principles behind the movements that the athlete must perform. I'm ready to watch my athlete and correct any errors that I find."

We have asked you to use elite performances as a model or reference point when working in a sport. But don't make the mistake of trying to mold a young athlete in the exact image of a high-performance athlete. When you watch a series of top performances, be sure to study the basic technique that these top athletes use - nothing more. With your knowledge of mechanics, you'll see the purpose behind these actions. As your knowledge of sport mechanics improves, you'll learn to disregard some actions that a top-class athlete uses because they are personal idiosyncrasies of no mechanical value. Accept them as something that makes an individual athlete comfortable but disregard them as a necessity for good performance.

Remember that the actions that an elite athlete performs at high velocity over a great range of movement need to be modified to be appropriate for the maturity, strength, flexibility, and endurance of a young athlete. You cannot and must not expect a young, immature athlete or a novice of any age to assume the body positions or match the explosive actions of an experienced athlete. This development comes with regular training and good coaching.

#### Application to Sport

Rowers Use Hatchet Blades to Apply More Force

High-performance rowers use oars with huge blades that look like giant meat cleavers. Called hatchet blades, these oars are shorter from the oarlock to the blade than standard oars are. The mechanical principle behind this design is that for the same effort from the rower, the hatchet blade travels more slowly through the water but applies more force. Moving slower, the blade slips less in the water but propels the shell faster. Do these blades present any problems? According to many coaches, hatchet blades, although allowable within the rules of rowing at many levels, can cause stress injuries in the lower and upper limbs because the rowers have to pull against a stiffer and less mobile resistance.

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