SAI (Simulation and Active Interfaces) is a simulation environment designed to perform interactive task-level simulation of complex robotic systems and, more recently, biomechanical systems.

SAI is written in C++ for the Windows OS. It is comprised of a set of libraries. At the core of SAI is a multi-body dynamics library for simulating kinematic chains. Additionally, collisions and contact between objects are simulated. A haptics interface library allows for interaction with the system using a force feedback device. Outside of the core dynamics and interaction capabilities most of SAI's infrastructure is devoted to algorithms for task-level control.

The core SAI environment has been extended to incorporate the simulation of muscle-driven systems through the inclusion of a muscle library. Systems can be specified with a set of Hill-type musculotendon actuators. The environment allows feed-forward operation in which control inputs are sent to each of the muscles, as well as an implementation of the task-level control methodology.