Even as a complete amateur, I probably practise juggling several hundred hours a year. Although I juggle solely for fun, I've often wondered if there were some ways of making the most of my available practise time. Is one way of practising more beneficial than another? With this in mind, I contacted Professor Buckolz of the physical education department at the University of Western Ontario.
It turns out that there's been a tremendous amount of research in this area, much of which is highly applicable to juggling. In fact, the reference book on Motor Learning Research  recommended by Professor Buckolz features a juggler doing five balls on the front cover!
Almost everyone who has tried to learn a difficult trick has encountered a performance plateau. During this time it seems that your skill level is not improving at all, or even that you're getting worse. Academics have some good news for jugglers here. As you will see, it's important to make the distinction between learning and performance.
Experiments have shown that while an individual's performance may level out or even decrease over the short term, the long-term rate of improvement is unaffected. Magill  states that "Plateaus may appear during the course of practise, but it appears that learning is still going on; performance has plateaued, but learning continues." This suggests that it is still worthwhile to practise on those "off days" when none of your hard tricks work - your tricks will have improved on the next good day.
Researchers have also looked at the effect of fatigue on the learning of motor skills. Near the end of a long practise session, you're having a rough time with difficult tricks and dropping a lot because you're tired. Personally, I've worried that this might reinforce poor juggling technique. Again the news is encouraging. Experiments indicate that mild to moderate muscle fatigue causes poorer practise performance, but does not affect learning . However, extreme muscle fatigue does impair learning. So, even though you're not performing at your peak, juggling while slightly tired will still improve your skill.
Perhaps the most exciting research finding that I came across has to do with something called "Transfer of Learning." Transfer of Learning is the influence of a previously practised trick on the learning of a new trick. This effect can be negative or positive. For example, absolute beginners usually encounter a negative Transfer of Learning effect when trying to learn three ball juggling. Often they have previously practised passing two balls in a shower and it's a hard habit to break when trying to learn the cascade pattern.
As an experienced juggler, however, you can use Transfer of Learning to advantage when trying to develop equal skill with a trick on both sides of your body. For example, equal skill is needed in both hands when juggling even numbers of objects in columns. It is well supported by experiments that "Transfer of Learning can be expected to occur between the same limbs when only one limb has been actively involved in practise ."
Simply put, your left hand will improve by practicing only with your right hand! This effect is undisputed among researchers. Also, experiments show that a skill is transferred more quickly from one side to the other if you first develop a fair level of skill on your favourite side . So, equal amounts of practise with both sides in a particular trick is not the fastest way to develop equal skill. Jugglers may find this surprising since it seems to be a common conception that it's best to practise both sides at once when first learning a trick.
From my own experience, I feel that I learned the head roll faster with the above approach. Contrary to the advice in Dick Franco's book, first I learned how to roll the ball up and down only on one side of my head. Once I had the technique figured out on one side, the other side came much more quickly.
There's an enormous amount of research into the best ways to practise,
and I've only touched on a few of the important findings for jugglers. In a
future article I'll discuss other important questions: Is it best to practise
in one long session or several shorter sessions? While practising, should you
focus on one trick or various tricks? Should you always practise in the same
place or change your setting? Stay tuned!
David Naylor is presently doing a Ph.D. in mechanical engineering at the U. of Western Ontario, London, Ontario, Canada. David has been juggling since 1983 and as yet, has not dropped a prop on the cat.