Summary of Reaching Development

Age
Skills/Behavior
Newborn period
Arm movements appear to be purposeful and spatiotemporally structured. Reaching is visually triggered.
Hand is typically open during foward extension of the arm.
7 weeks
Rate of reaching briefly decreases. Infants seem more interested in looking at the object.
Hand posture is more likely to be fisted.
12 weeks
Frequency of reaching increases. Hand prepared for reaching.
Infants acquire ability to visually determine realistic reaching distances.
12 to 18 weeks
Acquire skill in aiming reaches. Infants can contact a moving object in as much as 90% of trials.
Between 15 and 18 weeks, contact shifts from just touching to catching the moving object.
19 weeks
Reaches include most movement units that are seen in adults. Reaching path is not straight.
31 weeks
First movement unit is longer, and functions as the transport unit similar to adults. Reaching path is straighter.
5-9 months
Variability in reaching path and movement units while reaching for a stationary object.
9-13 months
Development of controlled, fractionated finger movements producing more difficult grasping. ex. pincer grasp

Reaching

Intrinsic Factors Influencing Reaching in Infancry
Internal factors that ultimately affect quality and accuracy in reaching include age, experience, and posture control. Infants from 1 to 19 weeks will display interest in an object when it is slowly presented to them. However in periods when neonatal reaching changes to functional reaching, it appears that the role of vision changes, and there are certain times when this reaching is "less readily observed". These declines tend to be seen between 8-16 postnatal days and also around the seventh postnatal week. Vision helps to elicit reaching during the transition from neonatal to functional reaching, however it does not guide hand trajectory or direct the motion toward the object before initial contact during this transition of skill level. It appears that infants to not require vision of their hands to reach, contact, and grasp of an object.

Extrinsic Factors Influencing Reaching in Infancy
In addition to age, reaching experience, posture, and developmental delay, task demands/extrinsic factors of a task could cause changes in reaching strategies. Infants from 5 to 9 months use the adult pattern of control when seated and reaching to grasp a stationary object that is placed on a flat surface. Then from 5 to 9 months, infants start to use visual information at the end of the reach as the hand approaches the target to correct for errors in hand path trajectory. If vision of the infant's hand is blocked, reaching performance becomes disrupted. During the first postal year, the coordination of the arms and hands during reaching is variable. The degree of coupling between arms depends on whether the infant has learned to differentially control the other arm, and the complexity of the bimanual cooperation that is required to perform the task.

Reaching Strategies in School-Age Children
Research has shown that children change very little in regard to reaching strategies from 9 months until approximately 7 years old. Research findings suggest that 5 year olds use ballistic strategies where as 7 year olds are able to monitor and change their movements with a closed loop strategy. Around age 7, the child then goes through a transitional period where they mature towards an adult reaching strategy. In addition, research supports that there is a relationship between postural control and reaching. Altered timing with postural muscles have an affect on the quality and speed of the reaching movements.


Grasping

Rudimentary Perceptual Control of Grasping
Infants demonstrate early grasping behaviors from 1 to 5 months of age. Opening and closing of infants’ hands first occurs when objects are placed near the hands. From 2 to 4 months of age, infants begin to demonstrate preprecision and precision grasp rather than fisting. By 4 months old, infants are developing the control of self-directed grasping. It is thought that the early phases of grasping help mold children’s ability to anticipate grasping after reaching as they age. Grasping in infants does not mirror reaching in the ballistic or open-loop execution processes. The hand does open during reaching around 3 months of age, but only when a visual target is in place. By 5 to 7 months, visually planned grasp emerges over tactile feed-back grasp. Infants are able to adjust grip based on object size at this point. Reach to-grasp movement patterns can be performed at this time, and become reliable at 9 months of age.

This video gives a good visual for a childs ability to grasp (and also reach) at 6 months of age.
Emergence of Anticipatory Control
When infants first start to functionally reach for objects, they typically do not reach for objects that are farther away than they can reach. At 5 months old, infants begin to orient their hand in the direction of an object before or during reach, as well as begin to shape their hand in anticipation of grasping the object. In order to successfully grasp an object, infants at this age rely mostly on contact with the object to shape their hand correctly. Throughout the next three months, infants begin to rely more on vision of the object to shape their hand in anticipation of grasp. By the age of 9-13 months, infants display anticipation of object size similar to that of adults. In adults, the distance between the thumb and index finger is appropriate for the object size by the onset of reaching. Still, immature reach/grasp characteristics are noted in infants of this age.

Maturation of Precision Grip and Load Force Control
Adults and infants vary in force production during a gripping activity. Adults are able to grasp an object and determine the appropriate amount of force needed to lift it as well as to prevent the object from slipping out of the hand at the same time. Adults have more practice in estimating the force needed to move an object by looking at it before they grasp it. Infants on the other hand will grip and object and then, once the hand is around the object, figure out the amount of force to pick it up. They also tend to apply a negative force to the object pressing it down into the surface before changing the direction of force to pick the object up. Infants usually use more force than is necessary to lift an object but then become more efficient as they develop.


Emergence of Object Exploration
During the first year, infants tend to use objects for mouthing, shaking, or banging. These activities decrease as the child progresses through his or her first year, and then object exploration and rotation begins to increase. Between 14 and 16 months, the child is typically able to use size and shape of objects as indicators of how much force will be required to lift the object. After one year of age, the child begins to develop skills that require more precision in movement, demonstrating that he or she is developing coordinated movement patterns and reaching/manipulation behaviors. For example, between 13 and 15 months, the child can stack two cubes on top of each other, and at 18 months, the child will be able to stack three cubes, up until around 2 years of age when the child is able to stack six cubes.

<iframe width="560" height="315" src="http://www.youtube.com/embed/yhl6O6wi6j4" frameborder="0" allowfullscreen></iframe>

Object Manipulation

Development of fine motor function has 2 major features:
1. Control of the hand as the terminal device for reaching and grasping
2. Object manipulation and release

Stages

Stage
Age
Skill
Description
1
2 months
Rotation of held objects
Infant learns that objects can be held and appearance transformed by moving it
2
3 months
Translation of grasped objects
Infant learns to move object to look at or mouth; and it is not possible to reach farther than arm's length
3
4 months
Vibration (shaking) of held objects
Infant learns that noise can be made by rapidly flexing/extending arm while grasping object
4
4.5 months
Bilateral hold of 2 objects
Infant learns it is possible to do 2 things at once
5
4.5 months
2-handed hold of a single object
Infant learns that 2 hands are better at holding an object and able to hold larger objects
6
4.5 - 6 months
Hand-to-hand transfer of an object
Infant learns that what can be done in one hand can be done in the other
7
5 - 6.5 months
Coordinated action with a single object when one hand holds the object while the other manipulates (bangs) it
Infant learns that 2 hands can accomplish more than 1; still reach with only one hand
8
6 - 8.5 months
Coordinated action with 2 objects (ex: striking 2 blocks together)
Infant able to move one object towards another
9
7 - 8.5 months
Deformation of objects
Infant learns it can change how something looks by bending, squeezing, or pulling apart
10
7.5 - 9.5 months
Instrumental sequential actions
Infant learns that coordinated use of the hands can lead to desired outcomes

References

1. Campbell, S., Vander Linden, D., & Palisano, R. (2006). Physical Therapy for Children (3rd Ed.). P 35-36, 39, 107-108. St. Louis: Saunders Elsevier
2. Campbell, S., Palisano, R., and Orlin, M. (2012). Physical Therapy for Children (4th Ed.). P 111-115, 118. St. Louis: Saunders Elsevier.
3. Campbell, S., Palisano, R., and Orlin, M. (2012). Physical Therapy for Children (4th Ed.). P 60-61. St. Louis: Saunders Elsevier.