How often you perform complexes depends on what else you're doing in your routine.
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A barbell complex isn't too dissimilar to a circuit. The only differences are that in a complex, all your moves must be performed with a barbell and you mustn't put this bar down from the moment you start your first exercise until you finish the final rep of your last exercise. Complexes are a blend of strength training and cardio, meaning you need to carefully consider your training frequency before including them in your routine.
The Early Days
When you first start performing complexes, you should keep them to no more than twice per week. Muscle soreness occurs when beginners try to do too much, too soon, notes trainer Marc Perry of Built Lean, so take it slow. Even if you're not new to training or lifting weights, if you've not done complexes before, you might be surprised just how challenging they are. Leave at least 48 hours between sessions and keep each workout to no more than 20 minutes.
The Intermediate Advantage
As you become more used to barbell complexes and get fitter and stronger, consider increasing your frequency to three or four times per week. You still want to avoid performing the same exercise two days in a row though, so you might do one complex of power cleans, front squats and overhead presses one day, then Romanian deadlifts, bent-over rows and snatches or back squats the next day. No repetition in a complex should be an all-out grinder however, notes strength coach Jen Comas Keck. By avoiding these ugly reps and not going to failure, you should recover relatively quickly, meaning four complex workouts per week isn't out of the question.
On the Daily?
Provided you regulate your intensity, you may benefit from performing barbell complexes on a daily basis. Complexes are a good opportunity to practice technically advanced exercises with a light weight, according to coach John Cortese of Cortese Training Systems. Due to the full-body nature of complexes, you can use them as technique practice every day or as part of your warm-up before a strength session. Use just an empty barbell and perform two to four rounds of six to eight reps on cleans, deadlifts, overhead presses, squats and good mornings. This shouldn't be taxing, but will get you warm and limber without stressing your muscles and nervous system too much.
If you're including other forms of training in your routine, especially high-intensity strength training or weightlifting, you may only wish to perform complexes once a week. Trying to max out on the weights and testing your metal with complexes could well lead to fatigue and injury. Instead, use complexes as extra conditioning. Strength coach Jon-Erik Kawamoto recommends performing a barbell complex at the end of a strength session as a finisher. Grab a bar and perform eight reps each of bent-over rows, upright rows, overhead presses, good mornings, split squats, back squats into push presses and Romanian deadlifts. Complete five rounds with just 60 seconds rest between each.