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Factors affecting stability

This is an excerpt from Applied Sport Mechanics 5th Edition With HKPropel Access by Brendan Burkett.

The conditions that give the gymnast minimal stability and the wrestler a considerable amount of stability are clues to the mechanical princi­ples that determine dif­fer­ent levels of stability. ­These princi­ples occur in ­every sport skill. Let’s look at them one at a time to see their effect on how a ­human being ­will move or function.

Increase the Size of Base of Support

The bigger an athlete’s base of support is, the greater the athlete’s stability is. Base of support commonly refers to the area on the ground enclosed by the points of contact with the athlete’s body. The first impression of this requirement is that the base of support is only under­neath the athlete’s body, but the base of support is not always below the athlete. Anything that provides ­resistance against forces exerted by the athlete can become a base.

• If a student in an aerobics class leans against the wall in a calf-­stretching exercise, their base includes the wall and the ground.
• A gymnast hanging from one of the uneven bars has a bar for a base, and it happens to be overhead.

What do we mean by area when we talk about the base of support? Imagine a gymnast who is facing along the length of the beam and is on one foot. In this situation, she uses the area of her foot as a base of support. If she places her other foot on the beam, her base of support now stretches from one foot to the other. The gymnast is now more stable than when she is on one foot alone. As you can guess, with two feet on the beam, her stability is better forward and backward than it is from the side. Shoving the gymnast off the beam is more difficult if you push her from the front or the back than if you push her from the side. In figure 9.4, the defending wrestler’s base of support stretches from the single hand on the mat to where he contacts his opponent. ­Because his opponent is trying to turn him over onto his shoulders for a pin, the opponent can hardly be considered part of a stable base of support!

You can see from ­these examples that stability is directly related to the size of the supporting base. An athlete who makes their base as big as pos­si­ble is more stable. A gymnast in a one-­handed handstand has a base that is solely the area of one hand. If the gymnast moved to a headstand, the base would become the triangular shape that runs from the head to the hands and back again (see figure 9.6).

FIGURE 9.6 In a headstand, the supporting base is the triangular area from the head to the hands.

When a wrestler lies facedown and flat on the mat and spreads out his legs and arms as wide as pos­si­ble, he covers a huge area that runs from his fingertips to his feet. As far as the size of his base is concerned, the wrestler has made himself maximally stable. Wrestlers make use of this position when both defending and attacking. If a wrestler is attacking and wants to hold the opponent in a par­tic­u­lar position and stop him from moving, he maximizes his stability just as he would if he ­were defending. Maximizing stability makes it as difficult as pos­si­ble for the opponent to mount a counterattack.

Centralize Line of Gravity Within Base of Support

An athlete’s balance is maintained as long as a vertical line passing through their center of gravity falls inside the perimeter of the base of support, as shown in figure 9.5. The closer to the center of the base this line of gravity (i.e., a vertical line from the athlete’s center of gravity) falls, the more stable the athlete becomes. Conversely, the closer to the edge of the base the line of gravity falls, the more unstable the athlete becomes. The larger the base is, the easier it is for an athlete to make sure that this vertical line falls well within the base.

Unlike inanimate objects, ­human beings can maneuver and shift position, and in this way keep their line of gravity within the perimeter of their supporting base. If you are balancing above a tiny base and allow your center of gravity to shift the slightest distance, then ­you’ve given gravity or an opponent a chance to apply a torque that you may not be able to counteract. For that reason, a gymnast’s one-­handed handstand is a phenomenal feat that requires tremendous strength and control. The base of support is solely the area covered by the gymnast’s supporting hand. The center of gravity cannot be allowed to shift from directly above the supporting hand, so the muscles are put to work to maintain this position.

In total contrast to the gymnast’s one-­handed handstand is the wrestler’s spread-­eagle defensive position. Assuming that the wrestler’s center of gravity is close to his navel, the line of his center of gravity would have to be shifted a meter (roughly 3 ft) in any direction before it got anywhere near the perimeter of his base. Not surprisingly, wrestlers often use this position.

You must not think that athletes always want to have their line of gravity within their base, however. When you walk or run, you take a series of steps forward. With each leg movement, your center of gravity shifts outside your base, and you shift from a stable position to an unstable position. When you put your foot down at the end of the step, your center of gravity moves back between your feet, and you are in a stable position again (see figure 9.7).

FIGURE 9.7 Taking a step forward, an athlete becomes unstable before restabilizing when both feet again contact the ground.

• When athletes sprint, they go from one unstable position to another as they drive themselves along the track.
• One moment their line of gravity is outside their supporting base, and the next instant it is back inside again. This sequence happens all the way down the track.
• The same situation occurs when hockey players skate along the ice.

When runners, speedskaters, cyclists, and slalom skiers go around a curve, they all lean into the turn to maintain stability. This position is an example of dynamic stability (i.e., stability while on the move). If they came to a sudden stop in the ­middle of the curve, they would fall. As they lean into the curve, their line of gravity no longer falls within their supporting base. The faster they move and the tighter the curve is, the more they lean inward and the farther their center of gravity (and their line of gravity) shifts into the curve. The correct amount of lean balances the forces acting into the curve with ­those acting in the opposite direction (see figure 9.8).

FIGURE 9.8 An athlete rounding a curve is in a state of dynamic balance. Forces acting into the curve counteract ­those acting outward.

­We’ve been talking at length about maximizing stability. Do athletes perform some skills in which they purposely minimize their stability? Yes. Examples occur during the explosive acceleration of basketball or hockey players during a fast break and in swimmers and sprinters at the start of their races. In a sprint start, sprinters aim to get out of the blocks as fast as pos­si­ble. On the “set” command, they shift their line of gravity forward so that it is close to their hand positions on the track (see figure 9.9). This highly unstable position satisfies two requirements:

• First, it extends the sprinters’ legs into a power­ful thrusting stance.
• Second, before the start, it moves the athletes as far as pos­si­ble in the desired direction ­toward the finish line.

FIGURE 9.9 In a sprint start, the athlete’s line of gravity is shifted close to the forward edge of the supporting base.

In sport skills that involve sudden changes of direction, athletes want to be able to shift quickly in any direction. The set position in a sprint start is excellent for sudden, fast movement ­toward the finish in a 100 m race. But you’d be highly amused if you saw tennis players using a sprinter’s set position when ­they’re waiting to receive a serve. The set position might be good for a sudden move in one direction, but it is useless for moving quickly in other directions.

Volleyball players receiving a serve and soccer goalkeepers defending the goal all need to be quick off the mark in any direction. They cannot foresee the direction, velocity, and spin of the ball, and they certainly ­don’t want to commit themselves too early. Consequently, ­these athletes use a small base with their line of gravity centralized. In this way their stability is reduced so they need no more than a split second to shift in the direction they want to move. They can react quickly and move fast in any direction.

Lower Center of Gravity

Athletes with a center of gravity that is elevated high off their base of support tend to be less stable than athletes whose center of gravity is lower. For this reason, ­great ­running backs who can twist and turn and suddenly cut one way and then the other tend to be shorter than other football players. They run low to the ground so they are more stable and maintain their balance better than taller athletes can.

To understand how the degree of stability relates to the height of an athlete’s center of gravity, let’s have an athlete using the same-­size base first stand erect and then crouch down. In both positions ­we’ll tip the athlete sideways the same ­angle. Notice in figure 9.10 that for the same ­angle of tilt, the line of the athlete’s center of gravity shifts beyond the edge of the supporting base when the athlete is standing erect. When the athlete is crouching down, as shown in figure 9.11, the line of gravity is still within the base. If other stability variables remain the same, the lower the athlete’s center of gravity is, the more stable the athlete becomes.

FIGURE 9.10 For an athlete who is standing erect, the line of the athlete’s center of gravity shifts beyond the edge of the supporting base when the athlete is tilted.

FIGURE 9.11 For an athlete who is crouching, the line of the athlete’s center of gravity remains within the base when the athlete is tilted.

The princi­ple of lowering the center of gravity to increase stability is one of the reasons athletes crouch or lie flat on the mat when they are defending in combat sports. Athletes in wrestling and judo not only lower their center of gravity but also widen their base of support. This practice increases their stability twofold, making it considerably more difficult for an opponent to destabilize them.

Ski jumpers know that they lose style points if they stagger on landing. Lowering their center of gravity by using a telemark landing in which the legs are flexed helps them maintain their balance (Chardonnens et al. 2014). Kayakers who are tall and who have most of their weight in their upper bodies have a high center of gravity. They are less stable than shorter athletes or athletes who have most of their weight in the area of their hips and seats (taller kayakers can counteract this prob­lem by using kayaks that have a wider beam, which widens their base of support).

Weightlifting gives us many ­great examples of reduction in stability when the center of gravity is elevated. The more weight a weightlifter hoists and the longer the athlete’s arms are, the higher is the combined center of gravity of athlete and barbell. The line of this common center of gravity must stay centralized above a relatively long but narrow base formed by the weightlifter’s feet. This base of support can vary if the weightlifter’s feet are aligned parallel or in a staggered formation. In addition, the barbell and the athlete’s extended arms tend to rotate at the axes of the shoulder joints.

If an athlete allows the barbell to shift the slightest distance out of line, he must immediately reposition his base and fight with his shoulder muscles to bring the barbell back in line so that it is again centralized above his base. Figure 9.12 shows the final position in the jerk phase of a clean and jerk.

FIGURE 9.12 In a clean and jerk, the common center of gravity of the weightlifter and bar must be centralized above the supporting base. The barbell also must be positioned directly above the shoulder joints.

Stability in this position is better forward and backward ­because the base is large in that direction. But the base is narrow from side to side. Even small movements by the athlete in a sideways direction can shift the common line of gravity of athlete and barbell outside the periphery of the base. From the final leg-­split position of the jerk, the weightlifter must bring his feet together, stand erect, and control the barbell for 3 s. Obviously, controlling a heavy barbell in this position is difficult. The slightest shift of the barbell out of line can cause loss of control and failure of the lift.

Increase Body Mass

This princi­ple simply says that if all other ­factors relating to stability remain unchanged, a heavier and more massive athlete is more stable than one with less body mass. For that reason, combat sports establish weight divisions to help ensure a fair competition between athletes within the same weight division. What hope would a featherweight wrestler have trying to lift and rotate an athlete in the super heavyweight division?

The value of body mass in combat sports is duplicated in American football, in which huge linemen are given the duty of maintaining their position in the face of ­whatever is thrown against them. The heavier they are, the more force (and torque) it takes to throw them off balance and knock them out of position (Brock et al. 2014). Therefore, they weigh close to or more than 136 kg (300 lb). But football has no weight divisions as ­there are in Olympic wrestling!

In all sport skills, heavier athletes who get out of control and lose their balance must exert more muscular force to regain their balance than lighter athletes do. If heavier athletes ­don’t have the muscular strength to control their actions, their extra body mass becomes a ­great disadvantage and a severe liability.

In judo, athletes always try to make use of their opponent’s body weight, and if their opponents are on the move, they try to make full use of their opponents’ momentum. Trying to halt an opponent’s push or thrust and then drive the athlete in the opposing direction is inefficient and exhausting. Better to use the opponent’s movement and have him rotate around an axis (such as your hip or leg) and then add your force to that of gravity as your opponent rotates ­toward the mat.

Smaller sumo wrestlers who try to overcome heavier opponents also attempt to carry out ­these maneuvers. Obviously, shifting out of the way when an athlete weighing 181 kg (400 lb) is charging at you is a good idea. If ­you’re a lighter opponent, you could try to destabilize him (by quickly stepping aside and si­mul­ta­neously tripping him) as his huge mass rushes by. You then add your own force to that of gravity to drive him to the floor or, more satisfyingly, to heave him among the spectators!

Extend Base in the Direction of an Oncoming Force

Irrespective of the type of base, stability increases if an athlete’s supporting base is enlarged in the direction in which force is being received or applied. For example, a force can come from an opponent who is trying to block or tackle you, or you can apply a force in a par­tic­u­lar direction when you are throwing, hitting (e.g., in baseball), or lunging (as in fencing). When a ­running back wants to keep his stability and continue ­running when hit by an opponent, he must consider not only the force of the hit but also the direction it’s coming from.

• To maintain stability and stay upright, the ­running back must widen his base in the direction of the applied force.
• If the hit is coming from the front, he widens his base from front to back.
• If the hit comes from the side, he widens his base in that direction.
• Naturally, he is ­going to lean into the hit as well. The mechanical princi­ples of leaning into a hit are explained ­later in this chapter.

The second application of this princi­ple involves situations in which athletes apply force in a par­tic­u­lar direction. If you watch baseball players, you’ll notice that they widen their base in the direction in which they are applying force, as shown in figure 9.13. They do this so that they have a good stable base that allows them to apply force over a considerable distance without losing balance. If they ­didn’t do this, ­they’d be thrown in the opposite direction. ­Whether they are blasting a home run, hurling a fastball, or just having fun in a pickup softball game, the same princi­ples apply.

FIGURE 9.13 A baseball pitcher lengthens his supporting base in the direction in which force is applied (i.e., in the direction of the pitch).

The ­actual size of the base an athlete should use depends on how much force is applied. Imagine tossing a superlight ­table tennis ball a few meters. You could do this without any trou­ble while balancing on one foot. The ­table tennis ball has ­little mass and in this situation has hardly any velocity. Now try throwing a huge medicine ball as far as pos­si­ble while balancing on one foot. You’ll find that as the medicine ball goes in one direction, you get pushed in the opposite direction, highlighting the importance of stability.

Let’s now compare catching a ­table tennis ball that is lobbed ­gently ­toward you with catching a heavy object (like the medicine ball) traveling at a much higher velocity. You could easily catch the ­table tennis ball while balancing on one foot. Its momentum would be minimal, and you’d also be able to maintain your balance on one foot without any trou­ble. The medicine ball pre­sents a dif­fer­ent situation. If you want to maintain your stability and not have the medicine ball knock you over, you must assume a wide stance with both feet well planted on the ground. Reach ­toward the ball and extend your stance in the direction from which the medicine ball is approaching. The more massive the ball is and the greater its velocity is, the more stable your stance must be.

Shift Line of Gravity ­Toward an Oncoming Force

The princi­ple of shifting your line of gravity (and your center of gravity) ­toward an oncoming force is directly related to the princi­ple stating that you should widen your base ­toward an oncoming force.

• ­Running backs purposely lean into tackles.
• Hockey players lean into opposing players who are trying to body-­check them.
• Wrestlers lean into their opponents when they grapple.
• Baseball batters lean into the oncoming pitch.

An in­ter­est­ing aspect of this princi­ple is that athletes are temporarily destabilizing themselves by moving their line of gravity closer to the perimeter of their supporting base. This position requires the attacking athletes to apply considerable force (and momentum) to drive the defenders back beyond the rear perimeter of their supporting base. If the attackers are successful, then the defenders are destabilized and ­will fall over. But the time taken by the attackers in thrusting the defenders from the forward perimeter of their base to beyond the rear perimeter is frequently sufficient for the defenders to spin away from the attack and to turn defense into attack.

The princi­ple of shifting your line of gravity ­toward an oncoming force applies not only in tackles and checks, but also in catching a heavy medicine ball and many other sport applications. You widen your base and shift your center of gravity ­toward the oncoming medicine ball. In this way, the momentum of the medicine ball pushes you back into a stable position.

Equally impor­tant, you give yourself plenty of time to apply force to slow down the medicine ball. Do you recognize that “plenty of time to apply force” is an expression of impulse (see chapter 6) used for stopping (i.e., a small force applied over a long period)? A small force applied over a long period rather than a huge force applied over a short time allows you to bring the heavy medicine ball comfortably and painlessly to a stop.

An impor­tant difference exists when you are an athlete applying force. In this case, you lengthen your base in the direction in which you are applying force. But you position your center of gravity to the rear of your base, in many cases temporarily outside the rear of your supporting base. All good javelin throwers start their throws from a position where they are leaning backward with their center of gravity outside the rear of their base of support; then in the follow-­through, they finish with their center of gravity beyond the front edge of their base (see figure 9.14). In this way, they apply ­great force over the longest pos­si­ble distance and the longest pos­si­ble time.

FIGURE 9.14 In the javelin throw, the athlete’s center of gravity begins to the rear of the supporting base and ends in front of the base.

In combat sports like wrestling and judo, athletes must be aware of the danger of shifting their center of gravity too close to the perimeter of their base. Imagine that ­you’re in a judo competition. ­You’re pushing and leaning into your opponent, and the opponent is ­doing the same to you. Your center of gravity is close to the front edge of your base. Suddenly your opponent stops pushing and instead pulls hard. Now the force of your own push has suddenly been increased by the addition of your opponent’s pull. You find that your center of gravity is suddenly outside your base and that ­you’re unstable. ­You’re about to be thrown!

If you sense that a push is about to change into a pull, immediately shift your center of gravity in the opposing direction and widen your supporting base in the same direction. Attack–­counterattack sequences involving the positioning of the athletes’ center of gravity are the essence of judo. One instant you can be stable, but if you are not careful, in the next instant you can be rotating ­toward the floor around an axis set up by a leg sweep from your opponent.

Remember that all the ­factors that control rotary stability are closely interrelated. Making one maneuver to increase stability is in­effec­tive if all the other ­factors controlling stability ­aren’t satisfied as well. For example, if an athlete widens their base of support, they must also move it in the direction of an applied force. A soccer player can widen their base ­toward a tackle coming from the right, but if the line of gravity stays close to the left edge of their base, they ­will easily be knocked over and beaten by the tackle.

A wrestler in the super heavyweight division is more stable than one in a lower weight division, but extra body weight is of ­little use if this super heavyweight uses a narrow base and stands with his center of gravity high off the ground and close to the edge of his base. Likewise, an athlete who lowers their center of gravity improves their stability, but this action is a valuable maneuver only if they keep their line of gravity well within his base. All the princi­ples of stability are related, and each depends on the ­others. Obeying only one princi­ple of stability ­isn’t good enough.

More Excerpts From Applied Sport Mechanics 5th Edition With HKPropel Access

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