According to the Constrained Action Hypothesis, motor learning is believed to be more efficient when an external focus (EF) of motor control is given to the performer instead of an internal focus (IF) of motor control. This systematic review investigated whether findings of studies focusing on the Constrained Action Hypothesis may be transferred to rehabilitation settings by assessing the methodological quality and risk of bias (ROB) of available randomized controlled trials (RCTs). Of the 18 selected reports representing 20 RCTs, the methodological quality was rather low, and the majority of the reports appeared to have a high ROB. The 18 reports included 68 patients tested in a rehabilitation setting and 725 healthy participants. The time scale of the motor learning processes presented in the selected articles was heterogenic. The results of this systematic review indicate that the assumption that an external focus of control is to be preferred during motor learning processes is not sufficiently substantiated. The level of available evidence is not large enough to warrant transfer to patient populations (including children and the elderly) and raises doubts about research with healthy individuals. This implies that based on the methodology used so far, there seems to be insufficient evidence for the superiority of an external focus of control, neither in healthy individuals nor in clinical populations. The relationship between EF instructions and motor learning research and its effect in both patient rehabilitation settings and healthy populations requires further exploration. Future adequately powered studies with low ROB and with rehabilitation populations that are followed over extended time periods should, therefore, be performed to substantiate or refute the assumption of the superiority of an EF in motor learning.
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