Tendon slack lengths are used in a dynamic muscle and limb model to animate the upper extremity based on user supplied activation levels. This paper provides tendon slack length estimates for actuators crossing the elbow and inserting on the radius or ulna. These values were not previously available as a complete see for the upper extremity. Muscle parameters used in making tendon slack length estimates, including the joint configuration in which peak isometric muscle force develops, were taken from the literature or were measured in vivo. The sensitivity of tendon slack length estimates to errors in the joint configuration in which peak isometric muscle force develops is examined.