Separable systems for recovery of finger strength and control after stroke

Jing Xu, Naveed Ejaz, Benjamin Hertler, Meret Branscheidt, Mario Widmer, Andreia V Faria, Michelle D Harran, Juan C Cortes, Nathan Kim, Pablo A Celnik, Tomoko Kitago, Andreas R Luft, John W Krakauer, Jörn Diedrichsen

Impaired hand function after stroke is a major cause of long-term disability. We developed a novel paradigm that quantifies two critical aspects of hand function, strength and independent control of fingers (individuation), and also removes any obligate dependence between them. Hand recovery was tracked in 54 patients with hemiparesis over the first year after stroke. Most recovery of strength and individuation occurred within the first three months. A novel time-invariant recovery function was identified: recovery of strength and individuation were tightly correlated up to a strength level of approximately 60% of estimated premorbid strength; beyond this threshold, strength improvement was not accompanied by further improvement in individuation. Any additional improvement in individuation was attributable instead to a second process that superimposed on the recovery function. We conclude that two separate systems are responsible for post-stroke hand recovery: one contributes almost all of strength and some individuation; the other contributes additional individuation.