The influence of attentional focus on movement variability
The advantage of an external focus (EF) over internal focus (IF) in performance outcomes is consistent in a large body of literature. Based on the Constrained Action Hypothesis, an external focus may promote flexibility and adaptability in the motor system which may result in higher movement variability. Limited previous evidence supports the claim that an EF promotes more functional variability. Moreover, the previous studies also suggested that task difficulty may modulate the effect of attentional focus. The purpose of the current study was to investigate the influences of attentional focus (EF & IF) and the level of task difficulty on movement variability (SD of joint angles, goal-equivalent variability [VUCM], non goal-equivalent variability [VORT]) as well as performance (COP trajectory) during a task involving standing and squatting on inflatable balancing discs. Young healthy adults (N = 36) balanced on inflatable discs while standing (low difficulty) and holding squat (high difficulty). For each level of difficulty, they completed three 10-s trials for each focus condition (baseline [no instruction provided], IF, and EF). The order of task difficulty was counterbalanced and the focus condition order was randomized. Kinematic and COP data were captured by 9 Vicon infrared cameras (250 Hz) and 2 AMTI force plates. Separate factorial MANOVAs assessed differences due to focus and difficulty for COP trajectory (SD of COP in anterior/posterior and medial/lateral directions, COPX & COPY) and movement variability as assessed by SD of joint angles and uncontrolled manifold analysis (UCM, VUCM & VORT). Sidak post-hoc tests were used for pairwise comparisons. Results showed there was a reduction of postural sway in the anterior/posterior direction (COPX) in EF compared to IF and baseline (p = .024, p < .001, respectively). An EF also decreased the SD of the ankle relative to baseline in the easier version of task (p = .003) and lowered the SD of knee and hip with reference to baseline across two level of difficulties (p = .050, p = .003, respectively). UCM measures showed no differences between an EF and IF, but there was a reduction of VUCM in the EF condition compared to baseline (p =.009). While behavioral benefits of an EF are consistent with previous research, the hypothesis that an EF promotes greater functional variability was not supported, requiring further study with an array of motor tasks to determine the veracity of the claim.