When I just started to workout, I followed a fitness routine with a structure that emphasized the number of reps per set. For example, my triceps workout would include ten reps of overhead dumbbell triceps extensions as a set. There is nothing wrong with following a workout with this type of structure and for a few years, that is exactly what I did. It was not until I was introduced to the idea of a maximum (max) set that my idea of a set was changed.
First, let me provide a brief explanation of a max set. A max set is essentially a one rep set where you try to resist the force exerted by the weight on your muscles over a duration of time. Take for example the barbell bicep curl. A max set performed with the bicep curl involves starting with the barbell about shoulder level and resisting the force exerted by the barbell as the barbell drops towards the ground.
The max set provided a new perspective on how I defined a set. Instead of defining a set as a finite number of reps performed without any rest between each rep, the definition can be extended to include a duration of time where my muscles were under tension, which I refer to as the time under tension. With this new perspective on the set, I started to think about how I can maximize the efficiency, effectiveness and intensity of a workout incorporating a muscle's time under tension.
Luckily, my friend who introduced the idea of a max set to me also explained to me how to structure a workout to include sets that are represented by a duration of time instead of a finite number of reps. Basically, the contractions of all muscles are fueled by breaking down carbohydrates, fats and proteins. There are two mechanisms that your body uses to turn these nutrients into sources of energy, the aerobic metabolism and the anaerobic metabolism. The term "aerobic" means with oxygen and the term "anaerobic" means without oxygen. The aerobic metabolism is used to fuel "low" intensity workouts (such as jogging) and the anaerobic metabolism is used to fuel "high" intensity workouts (such as lifting weights...well usually).
My friend continues on to tell me that since one of my goals when lifting weights is to increase the size and strength of my muscles, then the contractions of my muscles need to be of high intensity, which will be fueled by the anaerobic metabolism. This anaerobic metabolism can only fuel my muscle's high intensity contractions over a window of time between a few seconds to a few minutes. My friend concludes that from experience, a time interval ranging from 30 to 90 seconds works well.
Given the above information, when I perform sets that are defined by an interval of time, I try to keep the duration of these sets from 30 to 90 seconds. Here is an example of a macro cycle I perform for the max set of the concentration curl:
Week 1: 30 secs
Week 2: 40 secs
Week 3: 50 secs
Week 4: 60 secs
Week 5: 70 secs
Week 6: 80 secs
Week 7: 90 secs
Week 8: 90 secs
I start with a time interval of 30 seconds and by the end of the macro cycle, I should be working with a time interval of 90 seconds. Each week, I increase this time interval by 10 seconds.