Corticospinal and corticoreticulospinal projections have discrete but complementary roles in chronic motor behaviors after stroke.

After corticospinal tract (CST) stroke, several motor deficits can emerge in the upper extremity (UE), including diminished muscle strength, motor control, and muscle individuation. Both the ipsilesional CST and contralesional corticoreticulospinal tract (CReST) innervate the paretic UE, but their relationship to motor behaviors after stroke remains uncertain. In this cross-sectional study of 14 chronic stroke and 27 healthy subjects, we examined two questions: whether the ipsilesional CST and contralesional CReST differentially relate to chronic motor behaviors in the paretic arm and hand and whether the severity of motor deficits differs by proximal versus distal location. In the paretic biceps and first dorsal interosseous muscles, we used transcranial magnetic stimulation to measure the projection strengths of the ipsilesional CST and contralesional CReST. We also used quantitative testing to measure strength, motor control, and muscle individuation in each muscle. We found that stroke subjects had muscle strength comparable to healthy subjects but poorer motor control and muscle individuation. In both paretic muscles, stronger ipsilesional CST projections related to better motor control, whereas stronger contralesional CReST projections related to better muscle strength. Stronger CST projections related to better individuation in the biceps alone. The severity of motor control and individuation deficits was comparable in the arm and hand. These findings suggest that the ipsilesional CST and contralesional CReST have specialized but complementary roles in motor behaviors of the paretic arm and hand. They also suggest that deficits in motor control and muscle individuation are not segmentally biased, underscoring the functional extent and efficacy of these pathways. The corticospinal (CST) and corticoreticulospinal (CReST) tracts are two major descending motor pathways. We examined their relationships to motor behaviors in paretic arm and hand muscles in chronic stroke. Stronger ipsilesional CST projections related to better motor control, whereas stronger contralesional CReST projections related to better muscle strength. Stronger CST projections are also uniquely related to better biceps individuation. These findings support the notion of specialized but complementary contributions of these pathways to human motor function.