Motor impairments related to brain injury timing in early hemiparesis. Part II: abnormal upper extremity joint torque synergies

Abstract

Background: Extensive neuromotor development occurs early in human life, and the timing of brain injury may affect the resulting motor impairment. In Part I of this series, it was demonstrated that the distribution of weakness in the upper extremity depended on the timing of brain injury in individuals with childhood-onset hemiparesis.

Objective: The goal of this study was to characterize how timing of brain injury affects joint torque synergies, or losses of independent joint control.

Method: Twenty-four individuals with hemiparesis were divided into 3 groups based on the timing of their injury: before birth (PRE-natal, n = 8), around the time of birth (PERI-natal, n = 8), and after 6 months of age (POST-natal, n = 8). Individuals with hemiparesis and 8 typically developing peers participated in maximal isometric shoulder, elbow, wrist, and finger torque generation tasks while their efforts were recorded by a multiple degree-of-freedom load cell. Motor output in 4 joints of the upper extremity was concurrently measured during 8 primary torque generation tasks to quantify joint torque synergies.

Results: There were a number of significant coupling patterns identified in individuals with hemiparesis that differed from the typically developing group. POST-natal differences were most noted in the coupling of shoulder abductors with elbow, wrist, and finger flexors, while the PRE-natal group demonstrated significant distal joint coupling with elbow flexion.

Conclusion: The torque synergies measured provide indirect evidence for the use of bulbospinal pathways in the POST-natal group, while those with earlier injury may use relatively preserved ipsilateral corticospinal motor pathways.

Keywords: cerebral palsy; childhood hemiparesis; childhood hemiplegia; independent joint control; selective motor control.

Figure 1

Secondary torques associated with the primary torque direction task of shoulder abduction, with standard error bars. A significant effect of group-limb was found in the elbow (F=2.902, p=0.016) and fingers (F=3.138, p=0.010). Significant post-hoc comparisons, where p<0.05, are shown with red bars above the graph. Also shown are muscles colored to represent percentages of maximal EMG activity where differences were found between groups. These muscles include middle deltoid (Kruskal-Wallis, H=15.979, p=0.025), biceps brachii (ANOVA, F=2.201, p=0.056), brachioradialis (ANOVA, F=1.923, p=0.093) and combined wrist and finger flexors (ANOVA, F=3.097, p=0.011).

Figure 2

Secondary torques associated with the primary torque direction task of elbow flexion, with standard error bars. A significant effect of group-limb was found in the shoulder (F=6.550, p<0.001), wrist (F=4.468, p=0.001) and fingers (F=4.179, p=0.002). Significant post-hoc comparisons, where p<0.05, are shown with red bars above the graph. Also shown are muscles colored to represent percentages of maximal EMG activity where differences were found between groups. These muscles include anterior deltoid (Kruskal-Wallis, H=14.685, p=0.023), triceps brachii long head (Kruskal-Wallis, H=13.562, p=0.035), and combined wrist and finger flexors (ANOVA, F=3.192, p=0.009).

Figure 3

Secondary torques associated with the primary torque direction task of wrist extension, with standard error bars. A significant effect of group-limb was found in the shoulder (F=19.710, p=0.003) and elbow (F=14.122, p=0.026) using the Kruskal-Wallis test. Significant post-hoc comparisons, where p<0.05, are shown with red bars above the graph. Also shown are muscles colored to represent percentages of maximal EMG activity where differences were found between groups. These muscles include middle deltoid (Kruskal-Wallis, H=18.565, p=0.010), anterior deltoid (Kruskal-Wallis, H=15.797, p=0.015), triceps brachii long head (ANOVA, F=3.847, p=0.003), brachioradialis (Kruskal-Wallis, H=11.071, p=0.086), combined wrist and finger extensors (ANOVA, F=29.952, p<0.001), and combined wrist and finger flexors (ANOVA, F=3.097, p=0.011).

Figure 4

Secondary torques associated with the primary torque direction task of finger flexion, with standard error bars. A significant effect of group-limb was found in the shoulder (F=25.129, p<0.001), elbow (F=16.536, p=0.011) and wrist (F=17.466, p=0.008). Significant post-hoc comparisons, where p<0.05, are shown with red bars above the graph. Also shown are muscles colored to represent percentages of maximal EMG activity where differences were found between groups. These muscles include triceps brachii long head (Kruskal-Wallis, H=20.537, p=0.002), brachioradialis (Kruskal-Wallis, H=15.433, p=0.017), and combined wrist and finger extensors (Kruskal-Wallis, H=12.035, p=0.099).