Failure to properly consider tendon and applied forces in designing a hand tool can have harmful effects on users. Previous study has indicated that flexor digitorum profundus (FDP) and flexor digitorum super-ficialis (FDS) tendon forces can be up to 3.7 times the external forces. These values are indirect estimates derived from biomechanical models developed for the hand. However, these include many assumptions and may not be accurate. Therefore, direct measurement of tendon forces using a cadaver model provided novel insights into measuring internal impact of flexor tendons generated by power grip motion and then determining optimal handle size reducing internal tendon loads. In the result, there was a negative relationship between handle diameter and grip force, which showed that the grip force decreased from 38.3 to 23.0 N, as the cylindrical handle diameter increased from 30 to 60 mm. Thus, the highest grip force was generated on the smallest handle size (30mm), and the lowest grip force was on the largest diameter handle (60mm). In terms of the ratio of the internal tendon force to the external grip force, the internal tendon load on the smallest handle size (30mm) was, for external grip force F, 4.2F and the largest handle showed 7.0F (i.e. seven times the applied external force). These relationships should be useful for the design of handles that require power grip motion. Consequently, this study provided novel insights into the direct measurement of internal impact of flexor tendons generated by power grip motion with handles.