Synergistic or Non-Synergistic Split Training - Is Intl. Chest + Biceps Monday Dead? Quite the Opposite, Study Says

Yes, we are talking about a study in trained individuals, bros :-)

This is not the first SuppVersity article to address the usefulness and efficacy of different split routines (learn more). What makes the study at hand special, though, is that it addressed the issue in previously resistance trained subjects with at least 2 years of regular training under their weight lifting belts.

The study that has been published ahead of print in the Journal of Strength and Conditioning Research (Castanheira. 2015) does, therefore, join the ranks of the few scientific studies that have not been conducted in rookies, for whom literally "everything works".
After initial testing that involved height and body mass assessment, familiarization with the procedures of the study, and 10 maximum repetitions (RM) loads assessment, the subjects randomly performed three split training routines on subsequent visits:

• pull-pull exercise (synergist routine, SN): 6 sets of 10RM seated row exercise followed by 4 sets of 10 repetitions of preacher biceps curl exercise using an isokinetic dynamometer, 
• push-pull exercise (non-synergist routine, NS): 6 sets of 10RM of bench press exercise followed by 4 sets of 10RM of preacher biceps curl exercise using an isokinetic dynamometer, and 
• control: 4 sets of 10RM of preacher biceps curl exercise using an isokinetic dynamometer. 

In the context of the study, a "synergist muscle" was considered a group of muscles that assist the agonist muscle in producing the same joint movement - for the biceps that's almost every back exercise, or the triceps it would be almost every upper body (shoulder or chest) press movement.

Figure 1: Muscle activity of the biceps, measured by EMG during the three training conditions (Castanheira. 2016).

The scientists' hypothesis before doing the study was "[...] that there will be no differences in elbow flexor neuromuscular performance between the synergist and non-synergist split training routines" (Castanheira. 2016).

Figure 2: Total work (J); control, non-synergistic (NS) and synergistic (SN | Castanheira. 2016)

A minor problem with the design was that the biceps curl exercise was performed using an isokinetic dynamometer instead of the isoinertial free weight exercise. Thus, the authors wanted to "allow more precise measurements of decreases in torque, work, fatigue, muscle activation, and performance within each repetition and throughout the sets" (Castanheira. 2016), but obviously also made their training routine less realistic.

Figure 3: Peak torque (N/m); control, non-synergistic (NS) and synergistic (SN | Castanheira. 2016)

As Castanheira et al. point out, "[t]he main purpose of the split training system is to maximize training volume within a training session and to allow appropriate muscle recovery" (Castanheira. 2016). In this regard, their experiment yielded unambiguous results: the force production, volume, and recovery were greater in the non-synergistic (NS) than the synergist (SN) condition. What the experiment cannot tell us, however, is whether a real-world approach with three workouts per week would yield additional gains. After all, the increased volume comes at the cost of reduced recovery times between workouts, because the biceps would be hammered directly on a chest + biceps day and afterward indirectly during a back workout on another day.
References:

• Castanheira, RPM et al. "Effects Of Synergist Vs. Non-Synergist Split Resistance Training Routines On Acute Neuromuscular Performance In Resistance Trained Men." Journal of Strength and Conditioning Research (2016): ahead of print | doi: 10.1519/JSC.0000000000001762