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Objective: Spontaneous recovery is an important determinant of upper extremity recovery after stroke and has been described by the 70% proportional recovery rule for the Fugl–Meyer motor upper extremity (FM-UE) scale. However, this rule is criticized for overestimating the predictability of FM-UE recovery. Our objectives were to develop a longitudinal mixture model of FM-UE recovery, identify FM-UE recovery subgroups, and internally validate the model predictions. Methods: We developed an exponential recovery function with the following parameters: subgroup assignment probability, proportional recovery coefficient r k, time constant in weeks τ k, and distribution of the initial FM-UE scores. We fitted the model to FM-UE measurements of 412 first-ever ischemic stroke patients and cross-validated endpoint predictions and FM-UE recovery cluster assignment. Results: The model distinguished 5 subgroups with different recovery parameters (r 1 = 0.09, τ 1 = 5.3, r 2 = 0.46, τ 2 = 10.1, r 3 = 0.86, τ 3 = 9.8, r 4 = 0.89, τ 4 = 2.7, r 5 = 0.93, τ 5 = 1.2). Endpoint FM-UE was predicted with a median absolute error of 4.8 (interquartile range [IQR] = 1.3–12.8) at 1 week poststroke and 4.2 (IQR = 1.3–9.8) at 2 weeks. Overall accuracy of assignment to the poor (subgroup 1), moderate (subgroups 2 and 3), and good (subgroups 4 and 5) FM-UE recovery clusters was 0.79 (95% equal-tailed interval [ETI] = 0.78–0.80) at 1 week poststroke and 0.81 (95% ETI = 0.80–0.82) at 2 weeks. Interpretation: FM-UE recovery reflects different subgroups, each with its own recovery profile. Cross-validation indicates that FM-UE endpoints and FM-UE recovery clusters can be well predicted. Results will contribute to the understanding of upper limb recovery patterns in the first 6 months after stroke. ANN NEUROL 2020;87:383–393 Ann Neurol 2020;87:383–393.

Constraint-induced movement therapy (CIMT) is a commonly used rehabilitation intervention to improve upper limb function after stroke. CIMT was originally developed for patients with a chronic upper limb paresis. Although there are indications that exercise interventions should start as early as possible after stroke, only a few randomized controlled trials have been published on either CIMT or modified forms of CIMT (mCIMT) during the acute phase after stroke. The implementation of (m)CIMT in published studies is very heterogeneous in terms of content, timing and intensity of therapy. Moreover, mCIMT studies often fail to provide a detailed description of the protocol applied. The purpose of the present paper is therefore to describe the essential elements of the mCIMT protocol as developed for the EXplaining PLastICITy after stroke (EXPLICIT-stroke) study. The EXPLICIT-stroke mCIMT protocol emphasizes restoring body functions, while preventing the development of compensatory movement strategies. More specifically, the intervention aims to improve active wrist -and finger extension, which is assumed to be a key factor for upper limb function. The intervention starts within 2 weeks after stroke onset. The protocol retains two of the three key elements of the original CIMT protocol, that is, repetitive training and the constraining element. Repetitive task training is applied for 1 hour per working day, and the patients wear a mitt for at least 3 hours per day for three consecutive weeks.