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Circuit training is a type of interval training in which strength exercises are combined with endurance/aerobic exercises, thus giving you all the benefits of both a cardiovascular and strength training workout. The term circuit means that a number of carefully selected stations will be positioned around the facility consecutively, which may range from hydraulic equipment, free weights, resistance training, squat thrusts, pushups, star jumps, sit-ups or other exercises. Participants are assigned to each station, and they then rotate around the stations to the next with little or no rest performing 15 to 60 second workouts until all exercises have been completed or a certain time requirement achieved.

Made popular in the US by Curves facilities, circuit training is suitable for all levels of ability, age, or fitness level because participants can work at their own level, performing each exercise at their own speed and intensity. Beginners can develop their technique under the direction of the instructor, which can then be applied to good effect when working on their own in the gym. Advanced participants can build up to very high levels of intensity or reps while enjoying the atmosphere of a group exercise activity without having to conform to the set routines of an aerobics class.

History of circuit training and fundamentals:
Circuit training is an evolving training exercise program that was developed by R.E. Morgan and G.T. Anderson in 1953 at the University of Leeds in England.

Fundamentals:
Early in the original format of Morgan and Anderson, the circuit was made up of 9 to 12 stations. However, today, this number varies according to the design of the circuit. The program may be performed with exercise machines, hydraulic equipment, hand-held weights, elastic resistance, calisthenics or any combination. Themed circuits are possible, for example with boxing exercises (boxercise). A 15 second to 3 minute aerobics station is placed between each station, allowing this method to improve cardiorespiratory and muscle endurance during the workout.

A simpler form of the exercise has the group running round the gym with a trainer simply calling, for example, "ten push ups", "ten sit-ups" at intervals.

Studies at Baylor University and the Cooper Institute show that circuit training is the most time efficient way to enhance cardiovascular fitness and muscle endurance. Studies from Curves International has shown that circuit training helps, in their study, women to achieve their goals and maintain them longer than other forms of exercise or diet.(Curves: Permanent Results Without Permanent Dieting by Gary and Colman, Carol Heavin)

And research from Morgan and Anderson showed:
“Perhaps a most profound finding of this study, from a health perspective, is that this investigation clearly shows that performance of this circuit of exercises, at this level of intensity elicited oxygen consumption values (39% to 51.5% of VO2max) that meet established guidelines of the American College of Sports Medicine (ACSM) for the recommended intensity (40% to 85% of VO2maxR) of exercise for developing and maintaining cardiorespiratory fitness (Pollock et al, 1998). Thus, this circuit not only provides a suitable muscular fitness stimulus but also helps to meet ACSM cardiovascular guidelines and the newly published Dietary Guidelines for Americans 2005 for physical activity.”

Curves for Women:
The most popular form in the US is found at Curves facilities where 30 seconds is spent on each station. Typical circuits are anywhere from sixteen to twenty-six stations, and the workout lasting 30 minutes. The circuits are full body and are designed to firm, tone, or help women lose weight.

Advantages of circuit training:
May be easily structured to provide a whole body workout. Does not require expensive gym equipment. Participants normally work in small groups, allowing beginners to be guided by more experienced individuals, as well as benefiting from the supervision of the instructor. Can be adapted for any size workout area. Can be customized for specificity; easy to adapt to your sport. “It's the most scientifically proven exercise system. It's time efficient and incorporates strength, flexibility and cardio in the same workout. (The Cooper Institute; Dallas, TX)”

Circuit training is proven to be one of the best ways to make a resistance and strength training workout which also is challenging for your heart and lungs. This workout can range from a 20 to 60 minute workout that will take you to your maximum physical effort as well as your (maximum) target heart rate. The object of a circuit workout is not to rest in between exercises. The particular exercise will burn out the targeted muscle group as well as increase your heart rate to a fat and sugar burning zone.

Disadvantages of circuit training:
Circuit training is not necessarily the optimum choice for those wishing to build strength or muscle bulk, being better suited for firming, toning, developing aerobic endurance or fat burning. The duration of some circuit training stations can be in the region of 45 to 60 seconds, and in some cases as long as two minutes. These circuits typically mean that the number of repetitions performed on each station is relatively high, putting each exercise way out on the endurance end of the intensity continuum.

Those wishing to optimise increase in strength or muscle bulk (hypertrophy) might be better advised to reduce the number of repetitions performed and increase the weight to be lifted or increase the intensity, when hydraulics or elastics are used. On the other hand, longer station length is quite appropriate for any cardiovascular (aerobic) stations included in the circuit.

Where all the participants have an adequate level of experience, the station times could be reduced to, say, 15 or 20 seconds. This will encourage the participants to lift heavier weights, aiming to achieve overload with a smaller number of repetitions, typically in the range of 6 - 10 or 8 - 12, depending on the specific training goals. However, this would provide little time for the instructor to supervise technique, posture and form to ensure that the activity remains both safe and effective.