This tutorial describes how to use biomechanical muscle models within the AnimatLab simulation environment ([ Ссылка ]). 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.
Muscle is the primary means of producing movement for living systems. And being able to move is critical for finding food and other resources, and for finding mates. Without muscles none of that would be possible, so muscles are critical for almost all types of behaviors.
Muscles don't behave like typical man-made actuators such as the motors used in robotics applications. A motor can begin generating full torque almost instantly. It can also generate that torque regardless of the position of the limb or the load its trying to move, and motors can generate negative torques just as easily as positive ones.
Muscles are quite different. They have springs and dampers which means that they can only generate forces slowly compared to motors. The amount of force they can generate is also dependent on the current length of the muscle. A muscle that is stretched will generate less force for the same stimulus than one that is at its resting length. This means that the amount of force the muscle generates will change depending on the angle of the joint.
Finally, muscles can only pull, they can never produce pushing forces. This means that muscles must work in antagonistic pairs to produce a full range of movement. For example, your bicep muscle works to flex your arm, while your Tricep muscle works against the bicep to extend your arm.
There are two different types of models that are typically used to simulate the behavior of muscle. The first of these is the Hill model. It's a purely mechanical model built from the systems engineering perspective. The early pioneers in the field of muscle mechanics knew next to nothing about the internal structure and functioning of the muscle, so they had to treat it as a black box and attempted to map the input-output characteristics of muscle in an effort to formulate a mathematical model that could predict its overall behavior.
The second type of muscle model was formulated once the sliding filament theory was proposed by Huxley. It was built using a reductionist approach that takes into account the actual molecular structure of the muscle. Muscles work using a ratcheting mechansim between actin and Myosin molecules within the segments of the muscle. This type of model attempts to predict the developed tension by simulating the forces produced by the crossbridge attachments between the actin and Myosin molecules.
AnimatLab uses the simpler Hill type model. It provides a reasonable approximation of the behavior of muscle, while being much simpler to implement and to use. It's also runs faster, which allows you to simulate numerous muscles simultaneously. However, you must keep in mind that this is a functional model, but it does not really depict what is going on within real muscle. It only simulates the overall behavior.
It is composed of two sections, a serial elastic and a parallel elastic section. The serial elastic section has one spring with a spring constant Kse that determines the stiffness of the spring. The other section has another spring in parallel with a viscous damper and an active force generator.
All muscles resist changing their length. The damper models this by adding a resistive force that is proportional to the velocity of the muscle. All of the parts mentioned so far make up the passive properties of the muscle. But they can also actively generate tension. To do this an active force generator is in parallel with the damper. When stimulated it applies a compression force that pulls on the serial part of the muscle. The amount of tension actually applied by the muscle depends on the values of the two spring constants.
There are also three primary relationships that are important to emulate the behavior of muscle. These are the tension-length, force-velocity, and stimulus-tension curves.
