Referent control of anticipatory grip force during reaching in stroke: an experimental and modeling study.

To evaluate normal and impaired control of anticipatory grip force (GF) modulation, we compared GF production during horizontal arm movements in healthy and post-stroke subjects, and, based on a physiologically feasible dynamic model, determined referent control variables underlying the GF-arm motion coordination in each group. 63% of 13 healthy and 48% of 13 stroke subjects produced low sustained initial force (< 10 N) and increased GF prior to arm movement. Movement-related GF increases were higher during fast compared to self-paced arm extension movements only in the healthy group. Differences in the patterns of anticipatory GF increases before the arm movement onset between groups occurred during fast extension arm movement only. In the stroke group, longer delays between the onset of GF change and elbow motion were related to clinical upper limb deficits. Simulations showed that GFs could emerge from the difference between the actual and the referent hand aperture (Ra) specified by the CNS. Similarly, arm movement could result from changes in the referent elbow position (Re) and could be affected by the co-activation (C) command. A subgroup of stroke subjects, who increased GF before arm movement, could specify different patterns of the referent variables while reproducing the healthy typical pattern of GF-arm coordination. Stroke subjects, who increased GF after arm movement onset, also used different referent strategies than controls. Thus, altered anticipatory GF behavior in stroke subjects may be explained by deficits in referent control.

Validity of movement pattern kinematics as measures of arm motor impairment poststroke.

BACKGROUND AND PURPOSE: Upper limb motor impairment poststroke is commonly evaluated using clinical outcome measures such as the Fugl-Meyer Assessment. However, most clinical measures provide little information about motor patterns and compensations (eg, trunk displacement) used for task performance. Such information is obtained using movement quality kinematic variables (joint ranges, trunk displacement). Evaluation of movement quality may also help distinguish between levels of motor impairment severity in individuals poststroke. Our objective was to estimate concurrent and discriminant validity of movement quality kinematic variables for pointing and reach-to-grasp tasks. METHODS: A retrospective study of kinematic data (sagittal trunk displacement, shoulder flexion, shoulder horizontal adduction, elbow extension) and Fugl-Meyer Assessment scores from 86 subjects (subacute to chronic stroke) performing pointing and reaching tasks was done. Multiple and logistic regression analyses were used to estimate concurrent and discriminant validity respectively. Cutoff points for distinguishing between levels of upper limb motor impairment severity (mild, moderate to severe) were estimated using sensitivity/specificity decision plots. The criterion measure used was the Fugl-Meyer Assessment (upper limb section). RESULTS: The majority of variance in Fugl-Meyer Assessment scores was explained by a combination of trunk displacement and shoulder flexion (51%) for the pointing task and by trunk displacement alone (52%) for the reach-to-grasp task. Trunk displacement was the only variable that distinguished between levels of motor impairment severity. Cutoff points were 4.8 cm for pointing and 10.2 cm for reach-to-grasp movements. CONCLUSIONS: Movement quality kinematic variables are valid measures of arm motor impairment levels poststroke. Their use in regular clinical practice and research is justified.