The plank strengthens the abdominals, back, and shoulders. Muscles involved in the front plank include: 1  Primary muscles: erector spinae, rectus abdominis (abs), and transverse abdominus.he plank strengthens the abdominals, back, and shoulders. Muscles involved in the front plank include: 1  Primary muscles: erector spinae, rectus abdominis (abs), and transverse abdominus.