In contrast to the motive literature, motivational intensity theory predicts that the implicit achievement motive (nAch) should only exert an indirect impact on effort by limiting the impact of task difficulty. To contrast these two views, sixty-eight participants with a low or high nAch performed an easy or difficult arithmetic task. Effort was assessed using cardiac pre-ejection period (PEP). Supporting motivational intensity theory's view, PEP response was low in both easy-task conditions but stronger in the high-nAch group than in the low-nAch group in the difficult task. These findings suggest that nAch exerts an indirect effect on effort investment by setting the maximally justified effort instead of directly determining the amount of effort that is invested to satisfy the motive.