A muscle-like actuator comprises a motor with a rotatable drive shaft and a string with a shear thickening fluid (STF) embedded therein. One end of the string is attached to the drive shaft and the other end is connected to a load to form a twisted string actuator (TSA). By controlling the speed and current of the motor, the characteristics of the actuator can be changed. Multiple strings may be located in a flexible soft tube to improve the mechanical properties of the actuator.