Training with bouldering circuits is a very effective way to target Strength/Power, Strength/Power Endurance, or Endurance. There are two main benefits to this type of training. First of all, you don't need a partner, so if you need to get an endurance workout in, but your route climbing partner can't make it to the gym or if you're a one-man-wolf-pack...no big deal. Second, and the main reason I like training like this, is because you get to CLIMB. Climbing is important.
I like to do all three of these types of circuits in all of my training cycles. However, depending on my focus for the cycle, I will prioritize certain types of circuits more or less. Rather than use a prescription for how many circuits I do per week, I plan my training more around ratios. Whichever aspect I am looking to make the most improvement in, I will perform 2-3 of those circuits for every 1 time I perform both of the other two circuits. For example, If I was focusing mostly on developing strength and power, I would take 4-6 weeks and perform 2-3 strength/power circuits to 1 strength/power endurance circuit to 1 endurance circuit until I had done about 10-15 total strength/power circuit days, then I would take it easy for 7-10 days. To put it another way, at the end of that strength/power cycle, I would have completed a total of 10-15 strength/power circuits, 3-5 strength/power endurance circuits, and 3-5 endurance circuits. Thats a total of 16-25 training days in 30-42 days, so there is plenty of time for rest days and climbing outside.
Certainly, bouldering circuits are a great way to train for bouldering, but they have great route climbing applications too. Keeping a strength/power endurance or a strength/power circuit day in the mix every week or two can help maintain power levels for crux sections. I have also found it helpful to use the endurance framework and pick a stack of boulder problems to mimic the different sections of a project route, then climb through them with as little rest in between boulder problems as possible. Try to choose problems that are relatively similar to the sections you would climb through on the route and climb them in the order that you would encounter them on the route.
P.S. These are also workouts that can be done outside at an area with a sufficient density of problems.
P.P.S. Yes, you can add hangboarding, campusing, and supplemental strength training on circuit days, but how much you are able to handle depends on your experience and fitness level. Take it slow and when in doubt, take a rest day.
P.P.P.S. Clearly, you should be performing a warm-up and ramp-up prior to engaging in a training circuit. You can also climb after your training circuit if doing so fits the focus of your training cycle and you actually have the energy to do something else productive.
How Many Boulders:
You might not have much control when it comes to this. Ideally, you would have as many different boulders as you would perform repetitions. What I mean is, if you were going to do a set of 5 boulders with (x) amount of rest (x) amount of times, it would be nice to have 5 different boulders that all fit the style(s) you were trying to train, with the perfect amount of moves and comfortable holds. If this is an option for you, great! In most situations you will probably end up repeating at least some of the same boulders in order to complete your set within the parameters of the workout you are performing.
How Many Moves:
This of course depends on what you are training. As you approach the Strength/Power end of the spectrum, the number of moves per problem could be as little as 5-8 moves. At the Endurance end of the spectrum you could be climbing boulders that are 10-15+ moves.
This can get tricky when your terrain is limited for one reason or another. Maybe your gym/crag doesn’t have many steep boulders, or maybe there is only 1 V5 on the steep wall you have access to and everything else is too hard or too easy. You’ll have to get creative by making eliminates, coming up with your own boulders, or repeating problems multiple times. For the most part, I’d recommend hitting a broad range of terrain or focusing on one or two types of terrain that you are particularly weak at or that relate to a particular project or goal you may have.
This is essentially a limit bouldering session, but with more parameters that I think make it easier to discern whether performance is increasing. Choose 5-8 boulders that are a mix of former projects that you have sent and current projects that you are close to sending. Ideally, these boulders are no longer than 10 moves and don’t have a lot of “fluff”. Move through these boulders in an order of your choosing. You have 3 attempts per boulder. Always start the boulder from the start. If you use all 3 attempts and don’t send, move to the next boulder. If you send, move to the next boulder. The goal is to send each boulder on the first try. If you send a boulder first try 3 training days in a row, swap it out for a new, harder, boulder. If you stop making progress on a boulder 3 training days in a row, swap it out for a new, slightly easier boulder. Because of the high intensity, it is probably best to perform this workout only 1-2 times per week until you know how your body handles it and while you dial in the boulders in your circuit. People bouldering at a high level could potentially incorporate this up to 3 times per week, while being careful to monitor their overall training load and fatigue level.
Strength/Power Endurance Circuit
When choosing boulders for this workout, look for resistance boulders of about 8-12 moves that do not have very discernible cruxes or rests. These will most likely be boulders that are around your consistent on-sight level. You should send or fall near the end of each boulder, but be sending more than falling. Be sure to monitor your overall training load and fatigue level if you add this into your routine, it can be pretty brutal. This is a workout that could be performed 1-3 days per week depending on your fitness level.
The best boulders for this exercise will be about 10-15+ moves long and without discernible cruxes or rests. You’ll most likely need to pick boulders well below your consistent on-sight level. You should send every boulder, every time. You want the boulders to be relatively close together so that you will be able to get from one to the other in 30sec to 1min. Have a few alternates in mind in case someone is already climbing the boulder you were planning on doing so you do not mess up your rest interval waiting for someone else to finish. The endurance circuit workout has a relatively low intensity per boulder, but includes a massive amount of moves. This is a great workout to increase overall stamina, recovery, and efficiency. The low intensity also enables the climber to work on proper technique reinforcement while fatigued. Depending on the climber’s fitness and objectives this could be performed 1-4 days per week.