This tutorial describes how to use biomechanical muscle stretch receptor within the AnimatLab simulation environment (http://animatlab.com). AnimatLab is a neuromechanical simulation system that allows you to build a physically accurate, biomechanical model of the body an organism. Hill muscle models within that body can be controlled using biologically realistic neural networks to reproduce behaviors found in the real animals. The muscle tutorial discussed the hill type muscle model, what is parameters are, and how to use them to produce muscle contractions using a neural network. Just as its important for animals to be able to use muscles to move, its also important for them to get sensory feedback on how they are moving so they can tell if the movement they were attempting to make was completed successfully, and to know where their limbs are located. If you don't have any idea where your limbs located, then there is no way for you to generate a motor plan to move your arm to reach a food source or strike a predator. Proprioceptors provide this type of sensory feedback information. Stretch receptors are dynamic sensors that allow an organsim to detect the length and velocity of change in the muscle. It can use this to compare predicted movements to what is actually happening to detect errors. These sensors are dynamic in the sense that the nervous system can adjust them to cancel the predicted movements. Stretch receptors are fibers that interspersed in with normal muscle fiber, but since there are so few of them they produce very little tension. This means that when the muscle's length changes the overall length of the stretch receptors will change by the same amount. Two types of signals come from these receptors. Type Ia fiber signals send back information on the velocity of the muscle, and type II fiber signals send back information on the length of the muscle. Stretch receptors are also simulated using a hill model in animatlab. In fact, this is really the exact same model that was used for regular extrafusal muscle fiber. Stretch receptors are split into two sections. A nuclear bag region and a pole region. The nuclear bag region does not contract, while the pole region can contract. When the pole region is made to contract this changes the sensitivity of the receptor to changes in length. The bag region is where the type Ia signal originates that measures changes in the velocity of the muscle, while the pole region is where the type II signal originates that measures length changes. The model assumes that the length of the Se and Pe sections of the muscle model are linearly proportional to the firing rates of the type Ia and type II fibers.