Teachers, therapists and website posts talk quite a lot about poor midline crossing.  The  body is said to have an imaginary line that divides the workspace in front of the body into a left and right hand side. Crossing the midline refers to reaching with the preferred (dominant) hand across this midline to pick up and move objects. 

It is also assumed that poor midline crossing is a brain based problem and has something to do with poor connectivity between the left and right side of the brain.

Will reaching with the left hand for a beaker in the left hand workspace and then reaching across the midline to add it to a stack in the right hand workspace.

Will drawing a circle on A3 paper

At 4 1/2 years Will has good control of the shoulder movements for drawing a large circle and long lines across a sheet of A3 paper keeping the trunk fairly steady. The confidence and ability to move the hand smoothly from left to right and right to left over the paper is the result of many hours of drawing practice, starting with toddler scribbling. 

Will easily, and with confidence moves his hand across the midline of the paper because his ability to keep the trunk relatively steady when moving the arm. 

It is important to take into account the size of the paper relative to the length of the child's arm. In order to reach right across the paper Will emplys some lateral shift of the trunk to increase his reach to his left. Also notice that his combination of shoulder/elbow movements is strongly influenced by his grip. 

Research has shown that in simple pick-up-and-move tasks, right handed children and adults will reach with the left hand for objects placed far to the left of the workspace, and will choose the right hand for objects positioned closer to the midline. However, if the task requires more complex manipulation, right hand subjects are more likely to reach across the midline with the right hand. This tendency to reach across the midline is less marked in left handed subjects. 

In research tasks the tendency to reach across the midline with the preferred hand for more complex tasks increases with age.  3-4 year old children are more likely to use the hand closest to the object than older children. However it should be remembered that the choice of hand for a task depends on the task  as well as whatever else is happening at the same time. 

Observation of young children also shows research findings using simplified tasks probably underestimates midline crossing in young children. They reach across the body from an early age depending on the tasks and context.  

In the images below you see a right handed 6-year-old packing up cups using her left hand, and even reaching far across into the right hand workspace to retrieve the last cup. She was not too keen on doing the task my way, and this is reflected in the position of her right hand on the mouth. With the right hand occupied, she had to use the left hand to complete the task with an awkward reach to the right. 

The research into midline crossing shows that reaching for an object in the most efficient manner depends on good motor planning based on the goal of the task and the most effective way to achieve the goal. Young children become more adept at planning their actions with more experience and increasing levels of motor control as they get older.   

The two images below show a child lifting and pouring water from a flask into a beaker. The handle of the flask is placed to the right. Notice how at the same time she reaches for the handle of the flask, she also reaches for the beaker with the left hand and then moves it closer to the midline for a more comfortable pouring position. The movements that make up this task are carefully preplanned for easier pouring from the flask. 

Below is another example of motor planning for effective use of two hands together to achieve a goal. This time the flask is placed on the left with the handle on the right to facilitate a right handed grip. What the child does next is very sensible in terms of getting ready to pour water from the flask into the beaker on the right. She reaches across from the right to turn the flask so that the handle faces the left, picks up the flask with her left hand and uses the right hand to move the beaker closer to the midline. 

In most everyday tasks the hand does not need to reach far across the body midline. 

The tasks used in formal assessments to test midline crossing use a set up that is designed to evoke certain kinds of behaviour and do not necessarily reflect everyday function. In fact if you take a look at everyday actions that require a child or an adult to reach across the body, the hand seldom reaches very far past the midline of the body. What mostly happens is that the trunk is turned to allow the hand to reach the object without reaching across the body midline.  

And when we are using two hands to manipulate an object we naturally move the object so that it is positioned more or less in front of the body with the head in the midline. This ability to move objects into a more comfortable position manipulation requires effective motor planning. 

In all of these pictures you can see that the trunk and head are held steady and the arms are moved in different directions. What is important about these actions is that the child has learned to keep the head steady in space. The head and eyes have become the frame of reference for planning postural adjustments and arm movements. In other words  postural adjustments and arm movements are planned and performed in such a way as to keep the head steady. 

If an object is placed to one side of the body, the natural tendency is to turn the trunk so that the object can be reached, lifted and manipulation in the the space that lies directly in front of the trunk. 

Young children tend to move the head, trunk and arms as a unit

This ability to keep the trunk and head steady when moving the arms only develops after the age of about four years. Younger children tend to move the head, trunk and arm as a single unit.

The pictures below show a child who is just four drawing on a large sheet of paper. The paper is positioned to his left, but despite this he keeps his trunk more or less steady and is able to move his hand away from his body to draw on the right hand side of the paper. This despite his unconventional pencil grip. 

Return to Development of reach-to-grasp and shoulder control for drawing 
 

Bibliography 
Bryden, P. J., Mayer, M., & Roy, E. a. (2011). Influences of task complexity, object location, and object type on hand selection in reaching in left and right-handed children and adults. Developmental Psychobiology, 53(1), 47–58.

Fisher AG, Murray EA, Bundy AC (1991) Sensory Integration: Theory and Practice. 

Lynch, a, Lee, H. M., Bhat, a, & Galloway, J. C. (2008). No stable arm preference during the pre-reaching period: a comparison of right and left hand kinematics with and without a toy present. Developmental Psychobiology, 50(4), 390–8.

Stilwell, J. M. (1987). The development of manual midline crossing in 2- to 6-year-old children. The American Journal of Occupational Therapy : Official Publication of the American Occupational Therapy Association, 41(12), 783–9.