A

-330 ms

-190 ms

0 ms

-300

-200

-100

0

A

\

-300

-200

-100

-300

-200

-100

2000

Figure 4.6a,b. Approximate positions of the body segments of a vertical jumper 330, 190, 60 and 0 ms before toe-off (a). The mean velocity and the mean acceleration of the center of mass of the athlete and the ground force acting on him as a function of time before the takeoff (b).

The vertical ground force acting on the athlete is approximately three times his weight at 0.2 s into the progression of the jump.

Bobbert and van Ingren Schenau (1988) measured the time course of the ground force, electrical activity of leg muscles, and the angle of rotation of body segments during the course of repeated vertical jumps for a group of 10 proficient male volleyball players. Figure 4.6 reproduces some of the experimental data obtained by these authors. The results indicate that during the period before the takeoff the vertical ground force is at first smaller then the weight of the subject, then larger than the weight by about twofold, and finally decreases toward zero. At the takeoff, the ground force must be equal to zero.

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Getting Started With Dumbbells

Getting Started With Dumbbells

The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.

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