I have gradually swapped almost all of my standard two-handed lifts—especially cable exercises—for single-arm work.
Flyes, presses, rows, lateral raises: the workouts take longer, but the biomechanical control is worth it.
To be clear, unilateral training does not magically create more mechanical tension just because one side works at a time. If a pec moves the same load across the same range of motion, it does not know or care what the other arm is doing. The advantage is control over where that tension lands.
One cable changes the geometry
Take a single-arm cable fly. Pull both handles at once and the setup is symmetrical; the cables balance each other. Drop to one cable and the game changes.
The cable is not only pulling the arm back. It is trying to rotate the torso toward the weight stack. While the chest handles the fly, the obliques, deep abdominal muscles, glutes, and spinal stabilizers work to keep the hips and shoulders square. A chest fly becomes pec training plus anti-rotation work.
The larger benefit for me is the ability to optimize the setup. Cable resistance is dictated by geometry. Stance, pulley height, shoulder angle, torso position, and elbow path all influence where the exercise loads hardest.
For chest, I can adjust the angle until the contraction is pec rather than front delt. For back, I can alter torso lean and pull path to target the lat or upper-back division I want.
Range and asymmetry become visible
Bilateral cable flyes stop when the hands meet. Going single-arm lets the arm travel across the midline, extending the available range.
Weak links also become obvious. If the left shoulder starts internally rotating or the trap takes over late in the set, a stronger dominant side cannot mask it.
Where unilateral work loses
Single-arm training is not a replacement for everything. Heavy bilateral lifts still win on stability, and stability is valuable for pure mechanical overload.
If the abdominal muscles fail before the chest during a standing single-arm cable press, the stabilizers have capped the set. That may be useful for athletic conditioning, but it is counterproductive when the only goal is loading the pec. Bracing the free hand against a bench or upright can preserve the custom line of pull without letting stabilizer fatigue become the limit.
Single-arm work does not provide free tension. It provides more precise control over the tension already being created. It takes longer, but I would rather perform ten deliberate reps than move more weight through mechanics I did not intend.