Movement Execution Impairments
As a result of the damage to the CST and other developing motor pathways described above, there are impairments in movement execution by the upper extremity. For example, the upper extremity is often weak, and the lack of selective finger movements results in the use of several fingers when fewer are required (e.g. precision grip), making movements slow and clumsy.12 The relation between type/timing of the lesion and such execution deficits is described above.
Hand motor control has been quantified for more than two decades by the examination of fingertip forces during precision grasping. Fingertip coordination in typically developing children generally approximates adult coordination by 6 to 8 years of age.13 In contrast, children with CP at this age often have force coordination resembling that of very young children, with prolonged delays between movement phases (e.g. grasp contact and subsequent load force initiation) and sequential generation of grip and load force.14Although most children with CP are capable of adjusting their fingertip forces to the object’s weight and texture, their forces are often variable and excessive, with reduced adaptation compared with typically developing children.15 Fingertip force coordination is also impaired during object release,16 which is exacerbated when speed and accuracy constraints are imposed.17 Precision grip in children with CP does improve with development18 and extended practice,19 which has helped motivate intensive rehabilitation protocols (e.g. constraint‐induced movement therapy).20, 21
Because thalamocortical somatosensory projections reach their cortical destination sites during the third trimester, typically they are undamaged by periventricular lesions, or they may circumvent the lesion to terminate in the postcentral gyrus.1 In contrast, middle cerebral artery infarctions, occurring later and often affecting the postcentral gyrus, are more likely to affect the somatosensory system.1 Thus, children with unilateral spastic CP, especially of middle cerebral artery origin, often have sensory impairments, which may further compromise fine motor skills.12, 22 Specifically, tactile perception (light touch) and discrimination, stereognosis, and proprioception are often impaired,23, 24 with the amount of impairment related to the integrity of ascending sensorimotor pathways.8 These sensory impairments may be at least partly responsible for the precision grip impairments found in CP23 as the latter impairments resemble precision grip control under digital anesthesia.25However, the relation between sensory and motor abilities is not trenchant. A relation has been established between stereognosis and motor function,23 but conflicting results are reported for other modalities.
Motor Planning Impairments
In addition to movement execution and sensory impairments, individuals with unilateral spastic CP have impairments in motor planning,26 which can also impact precision grasping. During object manipulation, the development of fingertip forces must be planned before initiation because sensory information about some object properties (e.g. weight) is not immediately available. This type of planning involves the formation and use of internal models of objects based on previous experience manipulating a given object.19 Children with unilateral spastic CP have a decreased ability to scale the amplitude of the force development in their more‐affected hand.15 However, impairments in force development scaling are reduced after extensive practice,19 providing the basis for intensive rehabilitation protocols. The observed planning deficits may at least partly reflect deficits in motor learning, involving the extraction of appropriate sensory information to form internal models, and integrating it with motor commands during subsequent actions. However, this does not mean that all planning deficits can be ameliorated with practice. One dynamic action in which children with unilateral spastic CP perform routinely yet have deficits is walking with a hand‐held object, which requires precise coupling of fingertip forces in an anticipatory manner. Because the inertial forces acting on the hand‐held object oscillate in a sinusoidal manner owing to gait‐related events (e.g. initial foot contact), the grip force must be actively timed and modulated to the resulting inertial force oscillations. This force coupling reduces the need to maintain high grasping forces (risking fatigue) by ensuring that the grip force is sufficient at critical points (initial foot contact) to prevent slips, as seen in typically developed adults.27 Figure 2 shows the fluctuations in inertial force as a result of gait and the grip force responses for a child with unilateral spastic CP. A coupling of the inertial and grip forces can be seen in the less‐affected hand, where both forces fluctuate in parallel. In contrast, the fluctuations of the grip force in the more affected hand appear to be unrelated to the gait‐induced inertial force fluctuations (Fig. 2).28
Children with unilateral spastic CP usually demonstrate ‘global planning’ impairments that are independent of the effector used.26 The relation between type/timing of the lesion and such planning deficits is unknown. Oddly, the force scaling deficits during object lifting15, 19are effector dependent: a lack of planning is observed in the more‐affected, but not less‐affected, upper extremity. A similar finding during load force perturbations was reported, with an inability to anticipate the consequences of a dynamic perturbation.29 Interestingly, after several lifts with the less‐affected hand, anticipatory planning, reflected by force rates that appropriately reflect the object’s weight, is immediately present in the more‐affected hand.30 In a subsequent study,31 we found that, despite its absence in more‐affected hand, anticipatory control may be transferred from the more‐affected to the less‐affected hand. Thus children with unilateral spastic CP achieve sufficient sensory information to form internal models for subsequent use with their less‐affected hand. This suggests that the impaired planning may be due to an inability to integrate sensory information with the motor command in the more‐affected hand. We also found that simultaneous grasp and lift of an object in each hand improved some aspects of grip performance compared with unilateral lifts with the more‐affected hand,32 although the grip–lift movements became slower, probably because of the requirement to divide attention between the two hands. These findings helped motivate bimanual training approaches (e.g. hand–arm bimanual intensive therapy).21, 33