Using electromyography (EMG), the neural strategies of muscle activation can be measured.[4] Ramp-force threshold refers to an index of motor neuron size in order to test the size principle. This is tested by determining the recruitment threshold of a motor unit during isometric contraction in which the force is gradually increased. Motor units recruited at low force (low-threshold units) tend to be small motor units, while high-threshold units are recruited when higher forces are needed and involve larger motor neurons.[5] These tend to have shorter contraction times than the smaller units.