Straight Sets (also called Sets Across) in weight training means you lift the same weight for all of your sets of a given exercise.
So if you were going to do 3 sets of 10 reps (3X10) using this method, you would do 10 reps with a weight, rest, do another set of 10 reps with the same weight, rest, and finish with 10 reps using the same weight.
The benefit of straight sets is you don’t have to worry about changing weights between sets as you would using traditional pyramid loading. But that can also create a mental barrier since you are not challenging yourself with heavier weights.
The straight set method relies more on cumulative fatigue (fatigue build up over time) rather than in-set fatigue since the weight used remains the same for all sets.
This means you will be stopping short of failure on the earlier sets but should be close to or at muscular failure on the last rep of the last set (unless you’re using long rest periods or weights that are too light).
The length of rest used depends on your goal and the target reps but could range from very short (ex. 30 seconds) if you’re trying to create metabolic stress to very long (ex. 5 minutes) if you’re more interested in neural adaptations and strength gains.
German Volume Training (GVT) where a lighter weight is used for 10 sets of 10 reps (10X10) would be an example of straight sets used for creating metabolic stress.
Something like 10 sets of 1 rep (10X1) with near maximum weight (90% or higher) and long rest periods would be more beneficial for strength and neural adaptations.
Many methods rely on straight sets including popular ones such as 5X5, 10X3, 10X10, and 10X1.
Regardless of the sets and reps chosen, the goal is to complete all of the sets and reps with the same weight. Once you can do that, to force progressive overload and gains you would either decrease rest periods or increase the weight used.
The weight you choose initially will depend on your strength endurance (ability to maintain strength levels over a period of time).
It works best to pick a weight that allows you to easily complete all sets and reps and then slowly add some weight on each subsequent workout than to pick too heavy to start and not be able to complete all sets in the first workout.
Modified Straight Sets
Depending on the rest periods used and the weight chosen, as fatigue sets it you will likely reach muscular failure before the target reps on the later sets. In this case you could modify the goal to use a rep range instead of just a single target rep number.
Using modified straight sets with the previous 3X10 example, you would use a weight that you can just complete 10 reps with, rest and do another set with the same weight. The goal now would be to complete say 8-10 reps before failure instead of just 10 reps.
You would rest and complete another set, again with the goal of completing 8-10 reps with the same weight. If you can stay in that desired rep range (8-10 reps) on all sets, you would again either decrease rest periods or add weight.
The main difference with using modified straight sets is that you complete as many reps as possible (in the rep range chosen) on every set, you do not stop short of failure as when using normal straight sets.
The benefit is you are challenging yourself on every set, but failure type training can quickly lead to plateaus usually from overtraining or even mental limitations (your mind expecting failure).