Purpose: Previous studies proposed to estimate masticatory forces from electromyograms (EMG) by substituting chewing activities of a particular muscle into bite force-activity relations obtained from isometric clenching. For biomechanical reasons, this method requires that ratios between muscle forces in chewing be the same as in clenching. The purpose of this study was to investigate whether this assumption is valid. Materials and Methods: In 32 asymptomatic subjects, EMGs of bilateral temporal and masseter muscles were registered in unilateral clenching on a bite fork and in chewing. Ratios of peak activities of different muscles were compared between the two biting tasks. Results: In clenching, the working:balancing ratio of the masseters amounted to 0.92, while in chewing this ratio increased to 1.9. The working:balancing ratio of the temporalis was 1.8 in clenching and dropped to 1.3 in chewing. The temporalis:masseter ratio on the balancing side increased from 0.65 in clenching to 1.1 in chewing. Conclusion: The finding that ratios of muscle activities in chewing differed from corresponding ratios in clenching implies different combinations of muscle forces and/or different bite force-activity relations in the two biting tasks. In any of these cases, prediction of chewing force from the EMG of a particular muscle could lead to indefinite overestimations.