The plank is an isometric core strength exercise that involves maintaining a difficult position for extended periods of time. The most common plank is the front plank which is held in a push-up position with the body's weight borne on forearms, elbows, and toes. Many variations exist such as the side plank and the reverse plank. The plank is commonly practiced in pilates and yoga, and by those training for boxing and other sports. The plank strengthens the abdominals, back, and shoulders. Muscles involved in the front plank include: Primary muscles: erector spinae, rectus abdominis, and transverse abdominus. Secondary muscles: trapezius, rhomboids, rotator cuff, the anterior, medial, and posterior deltoid muscles, pectorals, serratus anterior, gluteus maximus, quadriceps, and gastrocnemius. Muscles involved in the side plank include: Primary: transversus abdominis muscle, gluteus medius and gluteus minimus muscles, the adductor muscles of the hip, and the external, and internal obliques. Secondary: gluteus maximus, quadriceps, and hamstrings.