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The sagittal plane is an imaginary line that divides the body into left and right strips, much like lines of longitude on a map. In anatomy it is used to describe the placement of body parts. The midsagittal plane, for example, divides the body exactly in half and is usually called the midline. For most people, the nose and the spine lie on the midsagittal plane.
Parasagittal planes are parallel planes to the midline. The heart usually lies on a parasagittal plane to the left of the midline. Individual body parts can also be divided into midline and parasagittal sections. When dissecting a brain, a neuroscientist might slice along a parasagittal plane in one of the hemispheres to get to a particular cluster of neurons.
Movement analysts use the sagittal plane to describe motion that occurs on that plane. Movement that occurs in this plane goes forward and backward. For example, a person usually walks in the sagittal plane when he walks forward to his destination. A crab walking sideways, however, is not moving in the sagittal plane but in the horizontal plane. A jump straight up into the air occurs in the vertical plane. Some movements happen in multiple planes. A basketball player leaping for the basket is moving both forward and up, so the player is moving both through sagittal and vertical planes.
Sagittal plane exercises include any exercise with a forward and back component such as sit-ups, arm curls, or lunges. These exercises naturally strengthen muscles running along the front and back of the body. Biceps can be strengthened through arm curls, while triceps can be built up through arm extensions to the back. Sit-ups strengthen abdominal and low-back muscles, while lunges develop the quadriceps, hamstrings, and gluteus muscles as well as the calves.
Stretches in the sagittal plane work similar muscles and tendons. To stretch the hip flexors, try a deep lunge or a split. Stretch the calves by placing one foot behind the other, straightening the rear leg and bending the front one slightly. Hamstrings can be stretched by sitting on the ground with legs extended forward. To deepen the stretch, lean forward from the waist with arms extended and hands reaching for the feet. Arm muscles are difficult to stretch properly, but twisting the spine and reaching one arm to the front and the other to the back can warm up biceps and triceps nicely.
Exercises that challenge you to cross the sagittal plane can be some of the best, both for increasing flexibility and for building muscle strength.