First a story of broken bones
A while back I slipped, flew, landed on my left hand and managed to sustain fractures of my radius and ulna. Living with my left (non-dominant) hand out of action for 6 weeks got me thinking a lot about the holding and stabilizing function of the left hand.
What was particularly interesting was how when I used other body parts accomplish a task, I naturally adjusted the direction of amount of force to suit the task. When I used my teeth to hold the end of a sleeve so that I could pull a sleeve off my arm I adjusted my tooth grip and neck action in exactly the right direction to accomplish the task.
Opening jars is done be gripping the jar between my thighs while grasping and turning the lid with my right arm. Try this action and you will notice how your adjust the thigh grip to counteract the direction of the force developed by your right hand as it turns the lid.
Another task was that has been tricky is mixing stiff batter in a bowl which needs the left hand to stabilize the bowl. On one occasion I asked my 6 year old granddaughter to hold the bowl so that I could stir. This did not work at all because she was not able to adjust the forces she applied to the bowl as the direction of my movements changed.
The reason I am able to use just right stabilizing forces when using my thighs and teeth instead of my left hand to stabilize an object is that my "movement brain" has a representation of the underlying structure of the task which is used to assemble the motor plan for the task. This plan includes anticipating the forces that will be generated by the actions of my right hand and producing equal and opposite forces at just the right time to oppose them.
From an early age infants start to understand the structure of two handed tasks and are ability to generalize from one situation to another similar situation allows them to learn a new task with at least a rudimentary plan in place.
However efficient use of the two hands to perform a task dependents on task experience and task specific practice which is goal oriented and makes use of repetition to adapt the and refine the motor plan.
Bilateral coordination is task specific
Infants start to coordinate the actions of their hands to grasp, feel and manipulate objects from the first months. The desire to reach and grasp successfully provides a powerful stimulus for developing anticipatory control of the trunk and head to counteract the destabilizing effects of arm actions. Given the right opportunities for practice infants as young as 3 months
From an early age infants start to understand the structure of two handed tasks and are ability to generalize from one situation to another similar situation allows them to learn a new task with at least a rudimentary plan in place. However efficient use of the two hands to perform a task dependents on task specific experience and practice which is goal oriented and makes use of repetition to adapt the and refine the motor plan.
The video clips below show how young infants who have had a lot of experience in handling objects have developed a sophisticated ability to integrate arm, hand, head and trunk movements to achieve their goals.
I would also argue that the task dependent nature of bilaterally coordinated hand function means improving a child's grrasp amd manipulation skills requires task specific training and will not be improved by a general set of bilateral activities as is often advocated in process oriented therapies. Examples can be found in popular books such as the Out of Sync Child (2005) p 239 and on many websites. Here are two examples that popped up right at the top of Google search: Therapy Street for Kids and OT Mom Learning Activities.
Playing with a favorite flower
This first clip shows Will at four months reaching for and grasping a cloth flower as it is brought into easy reach.
Moving a crate
In the clip Roan at 11 months is learning all about the forces that are generated and need to be controlled when moving a biggish object.
She has learned to anticipate the the destabilizing forces that will arise from moving the crate and makes effective postural adjustments to keep the head and trunk stable.
She moves the crate quite rapidly and the resulting momentum needs to be controlled as she reaches the end of the desired trajectory.
Throwing blocks and fitting a lid
This clip shows Toesies using the left and right arm to throw a block and then quickly shifts to using two hands together to pick up and position a lid.
Notice how adept he is at moving the two hands from different starting positions to reach the lid at the same time.
Posting blocks into a long tube
This short clip beautifully illustrates how Toesies at 19 months has learned to coordinate the actions of the two hands as he grasps and positions a tube in readiness for posting a block into the tubes one end.
The movements of the two arms are closely coordinated in time and space so that the block reaches the aperture just as it is moved into a suitable position for posting.