Acknowledgments
Photo credits: Naomi Polatsek
Introduction
Hand motor skill development begins early in utero with spontaneous movements and is fully completed during adolescence with the mastery of fine motor skills and coordination. During the first few years of a child’s life, the hand motor skills undergo rapid and remarkable developmental change to allow for exploration and use of objects. Early spontaneous movements give way to nonspecific, generalized movements. The infant uses the generalized movements to explore the surrounding area and gather information from the environment. These critical sensory experiences are fundamental to the development of voluntary controlled hand movements. Nonspecific movements further evolve into exploratory movements that begin with rudimentary grasp patterns and develop into precise movements that allow for dexterous manipulation of objects. The skills required for functional hand use encompass multiple discreet sequential components: recognition of an object, accurate reach for the object, proper hand orientation, proper calibration of aperture (hand opening) as it approaches the object, proper calibration of forces to grasp the object, and finally, grasp and manipulation of the object with one or both hands for the intended use. Timing and coordination for each of these subskills develop throughout early childhood, as the reach-to-grasp movements develop from an immature feedback mechanism to a mature feedforward, anticipatory approach. In late childhood and early adolescence, these skills continue to develop for maximum control of speed, accuracy, and coordination. Fig. 2.1 represents a timeline of key components of hand motor skill development from the prenatal period through adolescence.
The development of hand skills integral for hand use and function emerges from a complex interaction of multiple systems and maturation of the central nervous system that is further influenced by many contributing factors such as postural control, cognitive and perceptual abilities, vision, sensory experiences, and interaction with the environment. The developmental process parallels the typical sequential progression of neuromotor maturation yet varies in timing of onset among children. Typical patterns of hand skill development emerge at specific stages and age ranges with a wide variation in individual development that corresponds to maturation of the central nervous system. Influences of the environment and sensory experience on development of hand skills are believed to be largely responsible for this variation. Visual, tactile, and proprioceptive sensory experiences play an important role in the development of hand function. Sensory and visual stimulation provide essential feedback for ongoing development of skills and can be manipulated by caregivers and clinicians to enrich the environment and promote and encourage development in typically and atypically developing children.
The purpose of this chapter is to provide clinicians with an understanding of how hand skills develop from basic crude movements to precise patterns that enable skilled manipulation of objects for functional use. This chapter will review the motor development of the range of skills necessary for functional use of the hand for each of these skills: the reach-to-grasp movement, object release, object manipulation, and bimanual coordination. The first section describes the characteristics of spontaneous movements, the second section covers the development of unimanual skills and function, and the third section describes the development of bimanual hand skills and function. Each section explicates the specific characteristics and skills that emerge and mature throughout the developmental continuum and the important factors that influence and shape development. The components and characteristics of key hand motor skills with the corresponding developmental age ranges are listed in Table 2.1 .
| Category | Movement | Developmental Age Range | Characteristics/Examples |
|---|---|---|---|
| Spontaneous movements | Generalized movements | 8–36 weeks prenatal | Spontaneous, large variability, reach to mouth |
| Writhing movements | 36 weeks prenatal–8 weeks postnatal | Proximal, slow-to-moderate speed, small-to-moderate amplitude | |
| Fidgety movements | 2–4 months postnatal | Distal, smaller amplitude, low speed, varied acceleration | |
| Reach | Spontaneous reach | Birth to 2 months | Gross, symmetrical, swipe for objects, predominantly bilateral |
| Voluntary unilateral reach | 4–5 months | Irregular trajectory, variable movement patterns | |
| Voluntary bilateral reach | 4 months | Attempted bilateral movements | |
| Mature reach | 1–2 years | Straighter, smoother movement path, consistent trajectory | |
| Grasp | Reflexive | 1–4 months | Grasp objects placed in hand, nonpurposeful |
| Instinctive/Squeeze | 5 months | Touching, feeling, raking, beginning anticipatory grasp | |
| Ulnar palmar grasp | 6 months | Four finger grasp, no thumb | |
| Radial palmar grasp (superior palmar grasp) | 7 months | Radial fingers and thumb press object into palm | |
| Scissors grasp | 9 months | Small objects picked up with thumb and lateral border of finger | |
| Inferior pincer | 10 months | Objects held between pads of thumb and fingertip | |
| Superior pincer | 12 months | Objects held between tips of thumb and fingertip | |
| Deft and precise grasp | 15 months | Adjustments for weight and size, varies grip | |
| Dexterity and manipulation | 18 months | Increased control for speed and precision, manipulates utensils | |
| Release | Reflexive | 1–4 months | Response to tactile stimulation of extensors |
| Accidental release | 5 months | Object falls out of hand | |
| Forced withdrawal | 6 months | Pulling | |
| Release to table or surface | 7 months | Variable, and clumsy | |
| Intentional dropping | 10–11 months | Throwing/dropping food | |
| Precision release | 12 months | Graded release, stacking blocks | |
| Controlled release | 18 months–2 years | Accurate, precise release, puzzle pieces | |
| Manipulation | Premanipulation | 1–4 months | Object exploration, fingering, raking, shaking objects |
| Transfer objects | 4–6 months | Transfer from hand to hand, banging, wave and rotate objects | |
| Finger differentiation | 9–10 months | Able to isolate finger movements, pointing | |
| In-hand manipulation | 1–2 years | Move items from palm to fingers |
Spontaneous Movements
The earliest movements exhibited by infants are nonspecific, nonpurposeful, and seemingly random. These movements, called as generalized movements, are not isolated to the upper extremity and occur in all four limbs. They are described as spontaneous movements because they are not elicited by any cause. In the upper limb, general movements have been identified as early as 8 weeks of gestation and continue through early infancy until purposeful and directed reach emerges by approximately 16 weeks. Early fetal preterm general movements show large variability until 36–38 weeks of gestation. These movements are replaced by writhing movements that are observed between 36 weeks of gestation to 2 months postterm. Writhing movements are proximal, and characterized by a slow-to-moderate speed and small-to-moderate amplitude. Fidgety movements appear next between 2 and 5 months old, and are more distal, have smaller amplitude, lower speed, and varied acceleration. General movements of the arms are thought to be precursors to reaching and grasping movement and are described in the literature as prereaching behavior because they allow infants to explore their own bodies and surrounding surfaces to gather information critical for future development of reach and grasp. Absent, abnormal, and erratic spontaneous movements (especially fidgety movements between 2 and 5 months) are highly predictive of risk for later neurologic dysfunction and conversely, normal movements are predictive of normal development. When impairment is suspected, the environment can be manipulated by caregivers and clinicians to provide opportunities for enriched sensory experiences to allow for early intervention and further stimulate development.
The general movement assessment is the most commonly used early screening tool to assess abnormal motor activity and identify infants at risk for developmental disorders. It is the most reliable clinical assessment currently available and considered highly predictive for cerebral palsy. Motion capture techniques also provide the opportunity to detect, assess, and track hand and arm movements in infants over the course of development. These techniques are not as well developed but are becoming more available and feasible for use. An extensive review of the advantages and disadvantages of various movement recognition technology (video cameras, 3D motion capture, and direct sensing) in assessing spontaneous general movements was recently published by Marcroft et al. Regardless of the method used, early detection of infants at risk allows for early intervention with a focus on encouraging and facilitating motor control of the hand and arm by providing opportunities that encourage increased use.
Unimanual Function
Unimanual Reaching
Voluntary, goal-directed, functional movements begin to emerge by 3 months old and replace spontaneous movements. These changes are associated with changes in neuromotor developmental processes and a shift from subcortical to cortical processing.
The reach movement observed in early infancy broadly encompasses two types of reach: reaching toward an object, a precursor for a reach-to-grasp movement, and reaching toward the self that is described as bringing the hand to the face (usually mouth and eye). Both movements have a directed arm movement in common. The movements differ in the sensory determination of the target. The reach-to-self target is determined by proprioceptive input, whereas the reach-to-object target is determined by visual input. The earliest observation of reaching to a target has been documented prenatally at 22 weeks of gestation in the context of reaching toward the mouth.
Reach to grasp
Reach
The phases of a mature reach-to-grasp movement include reach, grasp, transport, and object release. The reach component refers to the movement of the arm toward a target and occurs in parallel with the hand opening and shaping in preparation to grasp the object. Although these movements occur simultaneously in mature reaching, each component develops sequentially during infancy with voluntary goal-directed reaching preceding grasp formation. Successful reaching toward a target requires integration of visual and proprioceptive information. A mature reach is characterized by a smooth trajectory with a consistently continuous straight path to the target, although the speed and trajectory of reach vary depending on the size and location of the target and the intended action. The velocity of a mature reach has a defined acceleration and deceleration phase as the arm approaches the target and concludes with a grasp phase as the hand prepares to grasp the object. These distinct phases are indicative of motor planning and reflect the ability for anticipatory control ( Fig. 2.2 ).
In infants, goal-directed reaching that is characterized as anticipatory and visually guided emerges by the age of 4 months. Before this, infants may attempt to occasionally swipe at objects within their visual field with occasional success. Over time with repeated exposure and experience, hand and arm movements become more purposeful and directed toward a target. However, this early reach is characterized by an irregular trajectory that lacks smoothness and consistency and is highly variable. During early reaching, infants must learn to coordinate the movements of the shoulder, arm, and hand. They begin by using the shoulder and torso to move the hand to the target, while keeping the elbow stiff in an attempt to control the degrees of freedom. Movement trajectories of reach begin to stabilize at the age of 1 year with straighter movement paths, and stereotypical patterns by the age of 2–3 years.
Grasp
Successful manipulation of objects is achieved through a combination of several discreet components of hand motor skills: controlled grasp and release, the ability to transfer an object from one hand to another, and individuation of the fingers. Mature grasp is characterized by an anticipatory mechanism that evolves during the reach movement with the hand simultaneously opening and shaping to match the approximate size and shape of the object. The timing of hand opening and closing is critical for smooth and coordinated grasp. Closing the hand too early or too late results in unsuccessful or awkward grasp. During mature grasp, the hand opening (aperture) reaches the maximum opening at about 75% of the reach movement and begins to close as it nears the object.
In infants, purposeful grasp is preceded by spontaneous opening and closing of the fingers. These nonpurposeful “pregrasp” hand movements have been called as “vacuous hand babbling.” Reflexive grasp exists in infancy from birth to 4 months and is observed as infants curl their fingers around an object in response to stimulation of the palm. Purposeful grasp control develops between 4 and 6 months through exposure to tactile and verbal stimulation. During this time, infants begin to integrate visual information to prepare the hand in anticipation of grasping an object. The combination of tactile and visual stimulation is critical for the development of the ability to grasp, orient, and adjust the hand to objects for purposeful grasp. Grasp patterns emerge over time from experience and interaction with a variety of object shapes and sizes. At 5 months, a child will touch and feel an object. Voluntary grasp develops through these experiences and is at first accidental. Subsequently, a range of grasp patterns develop through stimulation and exposure to various objects and toys. The characteristics and development of the repertoire of grasp patterns is discussed and detailed later. The earliest anticipatory grasp ability is seen in 5–6 month olds as they open the hand in preparation for grasp and start to close the hand before making contact with the object.
Preshaping the hand to match the object size begins to develop by 8 months old and continues over the next year. Young children, up to age 6, overshoot when reaching for objects and open their hands wider than necessary ( Fig. 2.3 ). Older children demonstrate accurate grip formation to the size and shape of the object with normalization of hand shaping occurring by age 6–8. The ability to adjust and orient the grip orientation to the object begins to develop at 6 months and continues to become more accurate up to the age of 15 months. Calibration of force control is another critical component of precision grasp that allows for objects to be held without being dropped or crushed. The amount of force generated during grasp is dependent on the size, friction, weight, and texture of the object. The ability to anticipate force control develops gradually beginning in the second year. Before that, infants control force development via a feedback mechanism. Fingertip force control continues to develop throughout early childhood and reaches adult levels by age 6–9.
