Our body moves in three planes–the frontal (or coronal) plane, the sagittal plane, and the transverse plane. The frontal plane includes movements that go out to the sides (for example, the jumping jack). The sagittal plane includes movements that go forward and backward (for example, walking and running). The transverse or horizontal plane includes twisting or rotational movements (for example, throwing a medicine ball from waist height).
To encourage range of motion, a nicely varied workout should include movements in all three planes, and ideally some movements that include more than one plane (for example, forward lunge with medicine ball twist).
Of these three planes, the most neglected is the frontal plane. Most exercises and most daily activities happen in the sagittal plane. This makes sense–we move forward and we work on things that are in front of us; we don’t often shuffle and bend to the side. Likewise, movements in the transverse plane are fairly common–throwing a frisbee, hitting a tennis ball.
The simplest exercises in the frontal plane can make little alarm signals go off in our body. Exercises like the lateral shoulder raise, the side press, and the side plank are surprisingly hard.
To repeat my main point: mixing it up in the resistance phase of a workout doesn’t just mean mixing up upper, lower, front and back; it also means these three planes of movement.