The centre of mass has been referred to several times. The terms centre of mass and centre of gravity often are used interchangeably in sport to represent the unique point in an object or system (in this case, the human body) that can be used to describe the body’s response to external forces and torques. Although a solid object has a fixed centre of mass that cannot be altered, the human body is a shape that is not only complicated but also continuously altering as it changes positions.
As the position of the limbs relative to the trunk changes (and the mass of any implements or load that is being carried is considered), the position of the centre of mass changes as well. As figure 4.9a shows, in a standing human the centre of mass typically is at the level of the E5649/Brewer/fig 04.07/542738/mh-R3 Vertical Horizontal Time F orce Action force Reaction force COM stays same height
upper third of the sacrum, slightly towards the front of the body. But as the positioning of the trunk changes relative to the positioning of the limbs, the centre of mass changes. Coaches and athletes can use this reality to manipulate the selection and execution of techniques to achieve a given skill challenge.
For example, a tall athlete who has rela- tively long femurs or a long spine may strug- gle to achieve a mechanically efficient squat position (detailed in chapters 6 and 10), but placing the bar overhead instead of behind the neck raises the centre of mass relative to the hip (figure 4.9b). This technique enables the athlete to maintain a much more upright trunk position.
The centre of mass can also move outside the body to achieve a technical advantage. The best-known example of this is the Fos- bury flop high-jump technique (figure 4.9c). Gravity influences every segment of the body, but in terms of total-body movement, gravity acts through the centre of mass. Vertical jump
height is therefore determined by how high the athlete can raise the centre of mass. After take-off, height cannot be achieved by exerting more force.
In the flop technique, the athlete’s body is supine; the body is 90 degrees to the bar, and the head and shoulders cross the bar before the trunk and legs do, giving the flop its char- acteristic ‘backwards over the bar’ appearance. While in flight the athlete can progressively arch the shoulders, back and legs in a rolling motion, keeping as much of the body below the bar as possible. As figure 4.9c illustrates, this technique positions the centre of mass outside the body and indeed at a point below the bar, allowing the body to travel higher over the bar. The athlete who executes this technique well may clear the bar while the body’s centre of mass remains as much as 20 centimetres below it.
The centre of mass of an object, whether a human body, a discus, or a kicked ball, follows a parabolic curve flight trajectory following
Figure 4.8 Applying force into the ground through different parts of the foot: (a) double-foot stance; (b)
single-foot plant.
E5649/Brewer/fig 04.09/547882/mh-R2
Figure 4.9 Position of the centre of mass changes with body position: (a) normal standing; (b) different
bar positions in a squat exercise; (c) Fosbury flop high-jump technique.
release or take-off. Gravity acts on the object, forcing it towards the floor, because air resis- tance slows the speed of the object. The larger the object is, the more influential air resis- tance will be. How far the object travels is primarily influenced by the magnitude of the force imparted into the object (the product of force and velocity) and the angle of take-off or release, which determines the direction of the force.
In some athletic endeavours, such as throw- ing, the height of the centre of mass at release is also an important consideration. For that reason, elite throwers typically are tall; a tall height at release means that the thrown object has farther to fall to the ground, extending its parabolic flight path.
When the athlete is in contact with the ground, the position of his or her centre of mass determines how stable the body is at any
time. Indeed, as long as the centre of mass falls within the base of support, an object will retain its equilibrium. Ergo, the more stable the body (or any object) is, the harder it is to accelerate the body. In performance, therefore, athletes need to be able to adjust their base of support (relative to their centre of mass) according to whether they need to be anchored and stable (figure 4.10a) or mobile and able to change direction rapidly (figure 4.10b).
Although the size of the base of support (figure 4.11) is a primary influence on the sta- bility of the body, a number of biomechanical principles come into play (table 4.1). Note that these principles apply to making an athlete not only more stable but also more unstable. Stability makes movement easier to achieve; an inverse relationship operates between stability and mobility when referring to the movement of the body as a whole.
a b
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Figure 4.10 Altering the base of support influences the stability of the movement: (a and b) defensive
positions of stability and mobility; (c and d) vertical strength balances.
D av id D ow /N BA E v ia G et ty I m ag es b d