© 2020 SPIE. Fishing-line artificial muscles can exhibit various motions. Twisted Polymer Fiber (TPF) actuators, which are a class of fishing-line artificial muscle actuators, generate torsional motion by applying heat. In general, untwisted fiber contracts by heating. However, measuring the blocking thermal tensile force of a TPF, we have discovered that the tensile force decreased and fluctuated depending on the initial number of twists in the TPF. This suggests that a TPF expands depending on the initial number of twists. Furthermore, the tensile force does not decrease monotonically but fluctuates during heating.