Primary movers and secondary movers are old-school terms that are useful when describing these splits. When training chest, your pecs are the primary movers. They should be doing most of the work. The triceps help out though, so they're the secondary movers. With back training, the various muscles of the back do most of the work as primary movers. The biceps would be the secondary movers.

Simple enough, but why is it important when considering your workout split? Well, as noted, the arms are secondary movers for chest and back. That gives you two options:

Option #1

  • Day 1:  Back/triceps
  • Day 2:  Chest/biceps
  • Day 3:  Legs, shoulders, abs
  • Day 4:  Off or repeat

The idea here is to keep your biceps and triceps "fresh." For example, on Day 1 the triceps will be fresh since the back needs the biceps, not the triceps, as secondary movers. You'll find that you'll feel very strong when training arms using this split and can use more weight for arm work than if you paired chest with triceps and back with biceps.

Option #2

  • Day 1:  Back/biceps
  • Day 2:  Chest/triceps
  • Day 3:  Legs, shoulders, abs
  • Day 4:  Off or repeat

With this option, you purposefully pair the secondary movers with their primary movers. After all, if you're already fatiguing the arms from training chest and back, you may as well "finish them off" with direct work.

As with most splits, we wouldn't say one is better than the other, just different. So choose one that best fits your needs or pick the option you've used the least if you need a rut-breaker.

Related:  10 Killer Training Splits

Related:  The Push-Pull Workout