Fielding drills can cause injury and hold back improvements in technique if players are not given enough rest. It's rest times that can make or break a good fielding drill session.
It's a delicate balance. Too much rest and waiting between drills can quickly make the activity boring. On the other hand, too little rest leads to fatigue building up and sloppy technique creeping in.
Where do you find the perfect balance?
The answer depends on the goals of your session. Not all drills are created equal.
Warm up drills
The most common drills for adult players are those used in the pre-match warm up. Too much here is certainly not a good idea as you may enter the game feeling tired before you have started.
For this reason the tactic is to have a few short drills done at a high intensity. This gets you ready for the match intensity and gets your body warm to reduce the risk of injury. However it will not tire you out completely.
A good rule of thumb is to be able to comfortably talk at the end of the fielding drill session but still be a bit out of breath.
During the session keep rest times to a minimum by going in several small groups. Too much rest here causes you too cool back down again. You can still take a break but don't do your drills then sit down for 20 minutes before the start of play.
Skill development drills
Younger players will spend a lot of time learning the techniques of the game: Picking up, throwing, stopping and catching. Drills of this nature are hard on the nervous system but easy on the heart and lungs. In other words, the effects are hidden.
When coaching younger players in groups purely to develop skill it's important to not spend too long on technical work. It gets hard quickly for kids as they only have a limited capacity to concentrate on coordinating their bodies correctly.
That said, you don't want to keep them sitting around resting too long either as that is boring. Keep the skill based drills short and focused then move on to something else. Remember they don't need to be out of breath to be mentally fatigued.
Conditioning based drills
A lot of people equate 'fitness' with doing fielding drills that make you out of breath. This is certainly not the be all and end all of fitness but it is an important part. Fielding drills can build highly specific endurance and work capacity and be more fun for the participants than interval training.
To get a training effect the drill will involve skills the participants are already skilled in. The time doing the drill will be longer (longer chases for example) and the rest time between performing the drills will be short. Small groups are best for this.
Too much waiting for your turn will negate the point of these drills. As a rule of thumb no one should stand around for more than 60 seconds. Less is better. Work time (that's when they are doing the drill) should be on a 1:2, 1:1 or 2:1 basis depending on fitness levels. So a 1:1 ratio is 30 seconds work, 30 seconds rest. 1:2 is 30 seconds work with 60 seconds rest.
The intensity of the drill will be slightly lower as we don't want to burn out the nervous system by combining high intensity work with endurance work. You can do one or the other but not both. However, lower intensity does not mean easy. Players should be gasping for air at the end of a session like this. I would recommend 10-30 minutes total training time to get a a good effect.
Speed/agility based drills
Drills designed to improve all out speed and agility are often overlooked but may be more important than endurance/conditioning type work. I would always pick a fast agile fielder over one who doesn't get tired (although ideally we want all fielders to have both skills).
Speed work requires an all out effort followed by a full recovery. As a general guideline each 10 yards of full speed sprinting requires 1 minute of rest. To put that in cricketing terms, if you are practicing speed between the wickets each player will need around 2 minutes recovery per run.
Any less recovery starts to turn the drill into conditioning work and detracts from the idea of developing speed and/or agility.
Remember we are trying to improve the techniques of speed and not get the players out of breath. Players should feel they are not tired at all between sprints. You can't combine both without risking injury.
Also, avoid doing these drills more than once or twice a week (and never 2 days in a row) as the recovery time is high.
Choose your poison
If you are responsible for the drills in your team, think carefully about what the goal is. Getting it wrong can make things dull, have no effect on performance and perhaps even injure your best player! Get it right and you will have fast, agile and skilful fielders who can chase the ball all day.