Cross-Education is characterized as the improvement in strength of the contralateral homologous muscle after unilateral practice. One potential mechanism to explain this phenomenon is that a unilateral maximal voluntary contraction (MVC) induces an involuntary irradiation of the contralateral homologous muscles. The purpose of the present two studies was to determine if contralateral irradiation is a potential mechanism to explain effects of cross-education. Contralateral irradiation was measured as the EMG co-activation in the homologous unpracticed arm. In Study 1 a unilateral dynamic extension/flexion arm movement was used to activate the triceps. The results indicated that unilateral practice induced a contralateral co-activation on the unpracticed arm. The same result can be reported for an isometric contraction (Study 2). The two studies provided empirical evidence that unilateral MVC did induce an involuntary muscle co-activity on the contralateral homologous muscles with increasing practice time in one testing session.