Children and adolescents with cerebral palsy flexibly adapt grip control in response to variable task demands
Introduction
During active object manipulation, successful grasp requires individuals to adapt grip force (GF) to the weight and changing inertial forces of the held object (i.e., load force; LF). While initial studies of GF-LF coupling suggested that GF is continuously modulated in-phase with changing LF (Blank et al., 2001; Flanagan and Wing, 1993; Viviani and Lacquaniti, 2015), recent studies of grip control in typically developing (TD) adults indicate that GF-LF coordination patterns are flexibly organized as a function of task demands (Grover et al., 2018; Grover et al., 2019a; Grover et al., 2019b; Grover et al., 2020). Participants in those studies shifted from more continuous (i.e., strongly coordinated and responsive) to more intermittent (i.e., less responsive) GF-LF coupling when the magnitude of LFs experienced at the point of grasp decreased (Grover et al., 2018, Grover et al., 2019b) and when task requirements involved more predictable upper extremity (UE) movements and, thus, more predictable LF variations (Grover et al., 2019a). An important lesson from these studies is that it is critical to systematically vary task demands to reveal the full range of grip control strategies.
This lesson may be particularly important for efforts to characterize grip control impairments that result from pathology, like cerebral palsy (CP). Grip control in children with CP has been previously examined during the performance of sequential lifts involving a single, predictable UE trajectory (i.e., grip-lift-hold) (Duff and Gordon, 2003; Eliasson et al., 1992; Gordon and Duff, 1999). GF in the affected hands of children with CP has been characterized as excessive (i.e., greater force production) and less responsive to LF changes compared to TD peers, with a GF-LF coordination pattern resembling that of infants less than one year old (Eliasson et al., 1991). It is unknown if CP-related grip impairments occur under less predictable task conditions, which are known to promote more continuous GF-LF coupling in TD adults (Grover et al., 2019a). Additionally, previous findings of an apparent lack of GF-LF coordination in CP (e.g., Eliasson et al., 1992) may reflect differences in UE kinematics between children with CP and TD children. In that study (see their Fig. 2D, in particular the 400 g condition), children with CP produced lower UE accelerations than TD children. Because LF is proportional to acceleration, this means they experienced lower LF than TD children. The apparent non-responsiveness of GF to changing LF observed in children with CP may, therefore, be related to reduced task demands (i.e., lower LF), since lower LFs are associated with more intermittent (i.e., non-responsive) GF-LF coupling in TD adults compared to higher LFs (Grover et al., 2018, Grover et al., 2019a, Grover et al., 2019b).
The present study was designed to determine whether and how the demands imposed on UE kinematics affect grip control in children and adolescents with CP. We used a virtual reality target tracking task (cf. Grover et al., 2019a) to control UE kinematics in order to create comparable patterns of LF for participants with CP and TD peers. The virtual target moved along unpredictable trajectories (more demanding and previously associated with more continuous GF-LF coupling; Grover et al., 2019a) or predictable trajectories (associated with more intermittent GF-LF coupling). We also manipulated mass of the grasped object; lower mass leads to lower LF and was previously associated with more intermittent GF-LF coupling (Grover et al., 2018). We hypothesized that differences in grip control between children with CP and TD children would be attenuated under conditions that promote more continuous GF-LF coupling in TD adults.
Section snippets
Participants
A convenience sample of 10 children and adolescents (7 M, 3F) between 8 and 16 years with mild spastic CP and 10 age- and gender-matched TD peers participated. Grip behavior in CP reaches mature patterns by 8 years (Gordon and Forssberg, 1997). Thus, only participants >8 years were included. Participants with CP were recruited from a nonprofit academic pediatric medical center in the midwestern United States. Participants of Manual Ability Classification System (MACS) (Eliasson et al., 2006)
Tracking performance
In the final model, there was a main effect of trajectory, t(356.7) = −23.99, p < .001. There was also a significant trajectory × group interaction, t(357.0) = −5.47, p < .001. Follow-up pairwise comparisons revealed a significant difference between groups, but only in the sine condition where tracking performance was less accurate for children with CP than TD children, t(24.5) = 3.67, p < .01, 95% CI [0.01, 0.09], d = 1.08 (CP: M = 0.15, SE = 0.01; TD: M = 0.10, SE = 0.01). There was no
Discussion
This study aimed to determine whether differences in grip control between children with CP and their TD peers were modified by task demands, in particular by the amount and nature of LF variability related to UE kinematics during a target tracking task. The findings support our hypothesis: An attenuation of CP-related impairments was observed under task conditions which were previously shown to promote more continuous GF-LF coupling in TD adults (Grover et al., 2018, Grover et al., 2019a,
Conclusions
Grip control impairments in children with CP were more manifest under more predictable task contexts—contexts dominant in the literature from which current therapeutic assessments and interventions are derived. Less predictable task demands attenuate impairments and could be leveraged in interventions designed to enhance GF-LF coordination in functional tasks.
Funding acknowledgements
This work was supported in part by a Foundation for Physical Therapy Research Promotion of Doctoral Studies (PODS) I Scholarship and the University of Cincinnati Department of Psychology Frakes Graduate Student Research Award. The funding sources were not involved in the study design, collection, analysis and interpretation, or writing of this report.
Declaration of Competing Interest
The authors have no declarations of interest.
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