An internal focus of attention (IF) – compared with an external focus of attention (EF) – can disrupt motor performance, learning and resilience to fatigue. Although the behavioural outcomes using an IF strategy are well investigated, the underlying neural mechanisms remain poorly understood. A relatively consistent finding is that an IF increases electromyographic (EMG) activity, suggesting less economic neuromuscular output. However, the underlying brain mechanisms responsible for the increased and/or less efficient muscular activity are not known. Corticomuscular coherence (CMC) measures the synchronisation between brain (via EEG) and muscle (via EMG) electrical activity and is proposed to reflect the cortical control of motor unit firing via the direct corticospinal pathway. Research suggests that CMC between the upper-limb and contralateral motor cortex increases with increased precision and skill. Conversely, CMC is reduced under increased attentional demands and is lower in movement disorders. It is, therefore, possible that the disadvantage of an IF compared to an EF can be examined through changes in CMC and the efficiency in which cortical control of ongoing muscular activity is maintained. The aim of the present study was to determine whether the manipulation of focus of attention (external vs internal) has a direct effect on CMC during a sustained hand contraction task. We hypothesise that an IF would result in decreased force steadiness, increased EMG activity and decreased CMC compared to EF. Findings will provide novel insights on the neurophysiological mechanisms linking attentional focus with motor performance.