Complex sentence: Ramp-force threshold refers to an index of motor neuron size to test the size principle by determining the recruitment threshold of a motor unit during isometric contraction in which the force is gradually increased.
Split into simpler sentences: 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.