That’s a good question. If the optimal workout volume at any point in time is the volume above which increases in muscle size are the same or smaller, then this optimal workout volume must be limited by some underlying biological factor or factors.
I think we can summarize those factors as follows:
IMO, the factors are: (1) limitations on the maximum post-workout muscle protein synthesis (MPS) rate, (2) the onset of CNS fatigue, (3) the amount of muscle damage that needs repairing or regenerating.
For the optimal workout volume to increase from one week to the next, the limiting factors for hypertrophy must necessarily change. Specifically, they must change such that either a greater volume (1) can stimulate a greater post-workout MPS rate, (2) can be performed with a smaller amount of CNS fatigue, or (3) can be done while causing less muscle damage.
#1. Stimulating a greater post-workout MPS rate
The post-workout rate of MPS is determined by the rate of protein synthesis per ribosome (translational efficiency) and by the amount of translational machinery in a given volume of muscle (translational capacity). Ribosomes are parts of the translational machinery that converts mRNA transcripts into protein.
Temporary increases in MPS, such as after single strength training workouts, are probably caused by a transient increase in translational efficiency. Even so, there is a cap on how large these post-workout elevations can be, due to the constraints of the translational machinery. So these will not increase from week to week.
Even so, long-term strength training is believed to increase translational capacity, which requires the production of new translational machinery through ribosome biogenesis. Such adaptations are relatively slow, so it is unlikely that increases in the maximum rate of MPS would be fast enough to allow a weekly increase in optimal workout volume.
#2. Reductions in CNS fatigue
There is some limited evidence that we can gradually reduce the amount of CNS fatigue that we experience in a workout, if we are training with lighter loads. Yet, it is not clear whether the same thing could occur with increases in moderate loads such as are common during bodybuilding.
#3. Reductions in muscle damage for a given amount of training volume
It is known that more muscle damage occurs with increasing training volume. It is also known that there is a repeated bout effect such that the second bout of training involves reduced damage for a given level of volume than the first bout. However, this effect does not continue linearly forever. In fact, the protective effect of additional bouts of exercise probably becomes exponentially smaller over time.
Therefore, to the extent that reduced muscle damage can contribute to an increasing level of optimal training volume, it will reduce exponentially over time. So we could not increase volume linearly. The largest increases would occur in the second workout, and then progressively less thereafter.
Summary
For the optimal workout volume to change from one week to the next, the underlying biological factor or factors must also change. These factors are: (1) the maximum possible post-workout MPS rate, (2) the amount of CNS fatigue that occurs at a given level of training volume, or (3) the amount of muscle damage that occurs at a given level of training volume.
The maximum possible post-workout MPS rate likely does not change very quickly from one workout to the next. We don’t know whether we can reduce the amount of CNS fatigue that occurs at a given level of training volume when using moderate loads. However, the amount of muscle damage that occurs at a given level of training volume likely does reduce, but certainly not linearly. The effect is greater from workout one to workout two, and reduces substantially thereafter.
As a result, I would not expect optimal training volume to change very much from week to week, and certainly not by so much that we could increase by >1 set per muscle group per workout every week. However, we might be able to increase optimal training volume slightly over several weeks.
