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.
Increasing the frequency of activation of motor units
Increasing the number of motor units activated
Increasing the amplitude of action potentials in the motor neurons
Recruiting larger motor units