It's performed almost exactly like a normal back extension... the lateral pulling tension is going to really hit those small stabilizer muscles of the lower back and spine and help you develop much better spinal strength and stability, while also working the bigger spinal erector muscles. (fitstep.com)
They're situated inferior to the levator scapulae, superior to the lats, deep to the middle trapezius, and superficial to the serratus posterior superior and thoracic spinal erectors. (kingofthegym.com)
They base their results on testing the muscle activity of rectus abdominis, external obliques, internal obliques and spinal erectors while the subjects performed crunches at 4 different speeds — 4 seconds, 2 seconds, 1.5 seconds and 1 second. (fitnessandpower.com)