When it comes to maximizing a workout’s effectiveness, there are many interesting suggestions put forth. Some of these relate to performing various activities during your rest intervals.
When it comes to maximzing muscle growth, as explored in the complete guide to rest intervals for building muscle article, the research suggests that longer rest intervals (2.5+ minutes of rest between sets) are superior to shorter rest intervals (1 minute or less) for compound exercises at least.
Given rest intervals typically last longer than your actual sets, the majority of your time spent in the gym would be spent passively resting. This brings up the question: could we do anything during those rest intervals to enhance muscle growth?
In another article, we explored if stretching between sets could improve muscle and strength gain (the results are pretty interesting!). But, in this article, we’ll explore if flexing between sets could help build more muscle.
For those unaware, flexing just refers to squeezing a particular muscle without any external load. For example, a front double biceps pose:
So, could flexing between sets help build more muscle?
Let’s take a look at the research.
Flexing Between Sets: The Research
This recent paper by Schoenfeld et al. aimed to find out just this.
27 men, who had at least a year of training experience, were split into either a passive rest group or flex group.
Both of the groups trained the barbell bench press, barbell overhead press, wide grip lat pull-down, seated cable row, barbell back squat, and leg press. Each exercise was carried out with 3 sets of 8-12 reps (each set to muscular failure).
This training session was repeated three times per week for 10 weeks.
The passive rest group rested 2 minutes between sets.
The flex group also rested 2 minutes between sets, however, during the first 30 seconds of this 2 minutes, they flexed.
During the rest intervals between sets of the barbell bench press as well as the barbell overhead press, the flex group flexed their triceps brachii. They held their arms by their side and aimed to extend the elbows as far as comfortable while squeezing the triceps maximally.
During the rest intervals between sets of the wide grip lat pulldown as well as the cable row, the flex group flexed their elbow flexors (biceps brachii, brachialis, brachioradialis). They held their arms by their side and aimed to flex their elbows as far as comfortable while squeezing the elbow flexors maximally.
During the rest intervals between sets of the barbell back squat as well the leg press, the flex group flexed their quadriceps. They were seated and aimed to extend their knee as far as comfortable while squeezing the quadriceps maximally.
Thickness of the elbow flexors (which included the biceps brachii and brachialis), triceps brachii, mid-thigh (which was a composite of rectus femoris and vastus intermedius), and lateral-thigh (which was a composite of the vastus lateralis and vastus intermedius again) was measured before and after the 10 weeks for both groups.
What the researchers found was that for the elbow flexors, triceps brachii, and lateral-thigh, increases in thickness was similar between the two groups.
However, for the mid-thigh, increases in thickness was slightly greater for the flex group.
Interestingly, there was an individual in the passive rest group who experienced substantial growth of the mid-thigh, while there was another individual in the flex group who experienced little growth of the mid-thigh.
Removing these two outliers meant that increases in thickness of the mid-thigh were even greater for the flex group.
Therefore, to sum up this study, flexing between sets did not seem to help build more muscle for the elbow flexors, triceps brachii, or lateral-thigh. But it did seem to help build more muscle for the mid-thigh.
These are intriguing findings, you may be wondering why in particular the mid-thigh benefited from flexing between sets.
This is probably due to the exercise selection. Both groups trained the barbell back squat and leg press, the rectus femoris (which remember is part of the mid-thigh) is poorly trained with these exercises. In our “Is the Squat Enough for Maximizing Leg Growth?” article, we detail research indicating the back squat fails to grow the rectus femoris and mention that the leg extension appears to be a particularly good exercise for growing this muscle.
The flex group was essentially performing an isometric leg extension between their sets of back squats and leg presses. Doing this likely increased tension on the rectus femoris, thus resulting in the greater mid-thigh growth.
I think there are two fair conclusions that can be taken away from this interesting study.
Given the elbow flexors, triceps, and lateral-thigh were all stimulated to a fairly good degree with the exercises used by both groups, and experienced no additional benefit from flexing between sets, it could be speculated that if both groups were performing a leg extension, the use of the isometric leg extension between sets would likely not be beneficial.
At the same time, for those pressed with time and cannot or do not wish to perform leg extensions, the use of an isometric leg extension between sets of quad exercises does seem to be an effective way of growing the mid-thigh.