Parametric control of fingertip forces during precision grip lifts in children with DCD (developmental coordination disorder) and DAMP (deficits in attention motor control and perception)

Abstract

Twenty boys with developmental coordination disorder (DCD), 11 of whom had associated attention deficit disorder (ADD), were compared with an age-matched control group of 12 boys to examine mechanisms that adapt the grip force at the digit–object interface in a precision grip task. An experimental grip object equipped with pressure transducers registered the grip forces (normal to the surface) and the load force (tangential to the surface) generated by the fingertips. The surface of the object was changed to vary the frictional properties. Both study groups exhibited disturbances of the basic coordination of forces in the initial phase of the movement, manifested by longer time latencies and higher force levels than the control group. All subjects were able to adapt the force output in response to the friction at the digit–object interface. Higher grip forces and safety margins were documented for the DCD group in comparison to the controls. Furthermore, there was greater variation in the parametric control of the grip force in the DCD group. The results suggest that the control of the grip force is similar in children with DCD, regardless of whether they have associated ADD or not, but it is impaired in comparison to that of controls.

Introduction

‘Clumsy’ children with relatively mild motor problems are diagnosed as having developmental coordination disorder (DCD) according to the criteria of the American Psychiatric Association [3]. In Scandinavian countries, they are diagnosed as having motor perceptual dysfunction (MPD). When attention deficits is an associated factor, a diagnosis of deficits in attention, motor control and perception (DAMP) is made on the basis of the classification of Gillberg et al. [15], [16], [17]. Cerebral palsy and mental retardation (IQ<70) are exclusion criteria for the diagnosis of DAMP.

The prevalence of DCD is estimated to be around 5% of the child population [32]. A common assumption is that motor difficulties in DCD are the result of deficient sensory motor integration. Henderson et al. [19] found prolonged reaction times when pointing towards small targets in children with DCD and suggested specific deficits in motor planning. Other studies on DCD have suggested impaired motor function [30] and dysfunction in the perceptive system [28], [42] and disturbed visual-kinesthetic matching [19], [27], [29].

Motor problems are frequently described in association with attention deficit disorder (ADD) or attention deficit hyperactivity disorder (ADHD) and it has been estimated that 50% of all ADHD children have some type of motor dysfunction [4]. It is not known whether the motor problems have a similar pathophysiology in children with DAMP, ADHD and DCD. Motor impairments in ADHD might be related to disturbed executive functions, in which the processing of sensory information could be affected [5]. Children with ADHD also perform less well in a study on kinesthetic acuity [41]. A hypothesis was also made that deficits involving the output rather than the input stages of information processing underlie the symptoms of ADHD [36]. A defect of the translation process from perception to action has been hypothesized by Smyth [37]. In addition, deficient dopaminergic frontal-striatal pathways have been suggested as a possible background for ADHD [38].

Hand skills seem to be particularly affected in children with DCD and ADHD [32], [42]. The control of the isometric fingertip forces applied to an object is important for proper execution of various hand tasks as it contributes to grasp stability and precision movements of the fingers, e.g. picking a ripe raspberry without crushing it between the fingertips. The function is dependent on tactile information from the finger–object interface, and adapts the grip force level just above the required level to avoid the object slipping out of the grip. Developmental studies have shown that small children use excessive grip force and a large safety margin (i.e. the difference between the force applied and the force needed to avoid the object from slipping is large) [11], [13]. These functions develop gradually during the first years of life and become similar to the adult capacity between 5 and 10 years of age. Children with spastic diplegic or hemiplegic cerebral palsy, however, usually do not develop these control mechanisms, but continue to use excessive grip force [8], [10].

Previously the paradigm of Johansson et al. [21], [39] was used to study the neural control of the precision grip lift in a group of children with ADHD [31]. Half of these children had additional motor problems, which would have given them a DAMP diagnosis. Interestingly, this subgroup differed from the others in several specific control functions. The disturbances mainly affected the control of the grip force rather than the load force. The most clear-cut findings were related to the parametric control of the finger tip forces. This control depends on information about the objects’ physical properties, obtained during earlier lifting and handling of the object and stored in memory representations [21], [22], [23], [39]. Furthermore, the force control, which is dependent on long term storage of information from familiar objects [18] was disturbed.

In the present study one address the question of whether the previously identified impairment of the parametric control of the forces in some children with ADHD+(DAMP), is caused by disturbances to the mechanisms adapting the grip force to the digit–object interface. The question of whether the motor problems identified in children with DCD and DAMP are caused by similar or different mechanisms was also addressed.