This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
The force of muscle contraction can be increased by all of the following except : 
Select and explain. 
A. Increasing the frequency of activation of motor units, B. Increasing the number of motor units activated, C. Increasing the amplitude of action potentials in the motor neurons, D. Recruiting larger motor units
C: Increasing the amplitude of action potentials in the motor neurons
Answer: C: Increasing the amplitude of action potentials in the motor neurons 
Explanation: C i.e. Increasing the amplitude of action potential in the motor neurons.Action potential follows an all or none law. If sufficient amplitude is reached to produces a response, increasing the amplitude fuher will not have any effect on the function to follow.Factors res onsible for gradingof muscular activityNumber of motor unitsWith minimal voluntary activity only a few motor units discharge. With increasing voluntary(Recruitment)effect more and more units are brought into play - 'Recruitment phenomenon'. With increasing motor units, force of muscle contraction increasesFrequency of ContractionFrequency of discharge in the individual fibre play a role. Tension developed during a tetanic(Summation)contraction (with rapidly repeated stimulation) is greater than during individual twitchesLarge motor unitsForce of contraction depends on length of muscle - starling's law