Most athletes think of their "core" as their six-pack abdominal muscles. However, it is also important to incorporate training for the glutes and lower back when you're working on your core. Grete Eliassen does the Back Extension with Dumbbell Row exercise in the preseason to target her multifidus, glutes and lower back. This exercise trains the entire posterior chain.
Start with a back extension to target your lower back and glutes. Next, squeeze your shoulder blades and bring the dumbbells up to your sides and then lowers them in a controlled motion. Start with three sets of six repetitions with medium to heavy dumbbells for this exercise.