A surgical instrument assembly disclosed herein can comprise an end effector configured to capture tissue, an electric motor couplable with a power supply, and a rotatable drive shaft operably coupled with the electric motor, wherein the rotatable drive shaft is configured to drive a firing drive during a firing stroke. The surgical instrument assembly can further comprise a sensing system configured to detect the load applied to the firing drive at the initiation of the firing stroke. The surgical instrument assembly can also comprise a lockout circuit configured to sense the current through the electric motor and to interrupt the current through the electric motor based on the sensed current at the initiation of the firing stroke.