It is a common observation that exertion of effort is associated with a specific facial expression. However, this facial expression has never been quantified during physical tasks and its relationship with effort is unknown. The aims of the present study were to measure frowning muscle activity during a physical task with electromyography (EMG) and to investigate the relationship between facial EMG and effort. Effort was experimentally manipulated by increasing task difficulty and inducing muscle weakness. Twenty men performed leg extensions with four relative workloads. The fatigue group (n = 10) repeated the leg extensions after fatiguing eccentric exercise, and the control group repeated just the leg extensions. Facial EMG amplitude, ratings of perceived effort (RPE), and leg EMG amplitude increased significantly with increasing task difficulty and with muscle fatigue. Facial EMG, RPE, and leg EMG all correlated significantly. The results suggest that frowning muscle activity reflects effort during physical tasks.