The attentional focus emphasised in verbal instruction influences movement and muscle recruitment characteristics, with an external focus (onto movement effects) typically benefiting performance. However, contrasting findings suggest either a selective isolation or spreading activation effect on associated muscles as a result of internally focused instruction (movement characteristics). In the present experiment, participants completed maximal isokinetic concentric leg extension exercise using internally (muscle specific: vastus medialis oblique) or externally (outcome specific) focused instructions. Integrated Electromyography (iEMG) of the vastus lateralis, vastus medialis oblique and rectus femoris muscles was obtained in addition to knee extensor torque. There were no differences in torque production between conditions. Externally focused instruction produced significantly lower iEMG magnitude across muscles, whereas an internal focus produced the greatest activity but with no evidence of a selective isolation effect of the vastus medialis oblique. The muscle-specific internal focus of attention resulted in a spreading activation effect, such that activity is elevated in muscles not within the focus of attention. Whilst an external focus did not improve performance, force was produced with lower muscular activity reflecting increased efficiency. The resultant noise in the motor system associated with an internal focus inhibits movement economy and attempts at selective activation.