The rate at which people can learn to juggle shows the level of difficulty of juggling different numbers of balls. While three balls can be mastered in a few hours, it may take months to learn to juggle four, and years for five. This shows that the skill level necessary escalates out of proportion to the number of balls juggled.
There are also different difficulty levels with juggling different objects. The world records for the greatest number of balls, rings and clubs juggled are 11, 12 and eight respectively; these figures are owing to the skills of holding and throwing them accurately.
It has been observed that the more skilled jugglers tend to juggle with a lower throw than those with less experience. Juggling with lower throws means that mistakes are less likely to be made, but it leaves less scope for correction.
The rhythm used by jugglers shows two points of interest. The limbs of the body tend to move with the same frequency. The ratio of time the ball spends in the hand to the time between two throws or catches is called the dwell ratio.
For juggling three balls it varies between 0.5 and 0.8, but tends towards simple fractions such as .75 and .625, showing that there is a tendency in humans to solve physical tasks with rhythmic solutions. If the dwell ratio is small there is more scope for collisions, since the balls spend more time in the air proportional to the time spent in the hand.
More experienced jugglers had less need of seeing the balls than the novices
When juggling three or more balls, it is impossible to watch all three at once, so the juggler tends to look only at the top of the throw. Scientific experiments have been conducted to find out how much of the throw need be seen for the juggling to continue without error, by placing a screen with a slit in the top between the juggler and the balls. It was discovered that only the top inch of the throw need be seen.
Another test was carried out by putting liquid-crystal glasses on the juggler, and opening and closing them at intervals. Sometimes the juggler adjusted the rhythm of the juggling to match the rhythm of opening and closing of the glasses. In these cases the juggler watched the ball immediately after it reached the highest point of the throw, showing that this is the point that must be seen for the juggling to continue.
It was also observed from these experiments that the more experienced jugglers had less need of seeing the balls than the novices. They were able to judge the point at which the ball would land from the direction and force with which they had thrown it.
Juggling is complex. Errors in the throw of each ball must be anticipated in the catch, while also concentrating on the other balls. If a robot could be built that would juggle in a human fashion, its basic properties would be useful in other fields. The skills needed for juggling are reflected in driving a car on a busy road or walking in a cluttered room.
So far, no realistic robot juggler has been built. A "bounce-juggling" robot, which bounces the balls rather than throwing them up, was constructed in the Seventies. This is easier than normal juggling, since the balls are caught at their lowest speed, rather than their highest. Several two-dimensional robots have been designed to juggle, but no one has succeeded in making any robot juggle in even a passably human way.
However, the scientific connections now established with juggling were hardly thought of 20 years ago and the rate of advancement has been very fast; so perhaps the world of robots that juggle is not as far off as it may seem.