The force of muscle contraction can be increased by all of the following except:

Action potential follows an all or none law. When sufficient amplitude is reached to produce a motor response, increasing the amplitude fuher will not have any effect on the motor response. Ref: Review of Medical Physiology by William F Ganong, 19th Edition Page 70 ; Biomechanics and Biology of Movement By Benno Maurus Nig, Page 275.