Aim: mirror movements are a transient phenomenon during childhood, which decrease in intensity with motor development. An increasing inhibitory competence resulting in the ability of movement lateralization is thought to be the underlying mechanism. We aimed to quantify unintended mirror movements systematically across the lifespan and to investigate the influences of age, sex, handedness, and task frequency.
Method: a total of 236 participants (127 females, 109 males; 216 right-handed, 20 left-handed; age range 3-96y, median 25y 8mo) first performed four clinical routine tests while mirror movements were rated by the observer. They were then asked to hold a force transducer in each hand between the thumb and index finger and to perform oscillatory grip force changes in one hand, while the other hand had to prevent the force transducer from dropping.
Results: age showed a strong nonlinear effect on the mirror-movement ratio (the amplitude ratio of the mirror and active hand, adjusted by the respective maximum grip force). Initially, there was a steep decline in the mirror-movement ratio during childhood and adolescence, followed by a gradual rise during adulthood. Males had lower mirror-movement ratios than females. The high-frequency condition triggered lower mirror-movement ratios. No significant differences of mirror movements between dominant and non-dominant hand, or left- and right-handed participants, were found.
Interpretation: this study provides, for the first time to our knowledge, normative values of mirror movements across the lifespan that can aid differentiation between physiological and pathological mirror movements.