When testing the nerves in the knee with a reflex hammer, what is different when the subject is holding a book?

This is part of my homework, except I don't have a reflex hammer or someone to test it on. However, I do know that according to the way nerves work that the knee will jerk upwards starting with touch receptors in the knee, carried by the sensory receptors to the interneuron which carries two signals to the brain and spine and then one impulse travels back to the motor neuron which controls the effector and hence the knee-jerk reaction. But how does holding a book change all of this?