# Springs

## Watch the video tutorial on using a spring!

The spring allows you to connect two parts together within your biomechanical model using a standard damped spring that you can find described in any physics textbook. The mathematical description for this system is shown in equation 1.

 Figure 1. Standard mass-spring-damper system

K is the stiffness of the spring. Delta x is the displacement of the spring from its resting length. B is the damping coefficient, and v is the velocity of the mass. So the spring exerts a force that is proportional to the amount it is stretched or compressed. The damper produces forces proportional to how fast the mass is moving. So the faster the block tries to move, the harder the dashpot pushes back on it to resist that movement.

The spring connects to other body parts using attachments. It has two properties for the primary and secondary attachments. When you select these properties they display a drop-down list that shows all attachment points in this body for you to select the one you want. When you have set the two attachment points a colored line is drawn between the two attachment points.

Biomechanical springs can be enabled or disabled. You can set this feature directly using the Enable property. You can also do it by applying an enabler stimulus to enable and disable the spring for specific periods of time. Another way to control whether a spring is enabled is by using neural control. The default input into a spring controls whether it is enabled. Any input into a spring above 0 will cause the setting of the enabled property to be flipped. So if the spring is initially set to be disabled and you have a positive input go into the spring it will enable it while that positive input is maintained. If Enabled is initially set to true then any input into the spring will disable the spring while the input is maintained.

For more information on how the spring works please see the spring page of the mechanical tests section and watch the video tutorial.

## Spring Properties

 Figure 2. The properties of the spring.
Color
The color of the line drawn for this spring between the attachment points. This is only visible once attachment points have been selected.
Default value: Beige
Acceptable range: Any Color

Damping
The damping coefficient of the spring. This is B in equation 1 above.
Default value: 1 Kg/s
Acceptable range: 0 or greater

Enabled
Determines if this spring is enabled or not. If it is disabled then it is as if the spring does not exist in the simulation.
Default value: True
Acceptable range: True/False

Name
The name of this spring.
Default value: Blank
Acceptable range: Any string

Natural Length
The natural length of the spring. When the spring is stretched or compressed away from this length a force is produce that is proportional to the spring constant.
Default value: 1 m
Acceptable range: Any value greater than 0.

Primary Attachment
The primary attachment that connects this spring to a part of the body. When you select this item a drop-down list shows all the attachments in the body for you to choose one.

Secondary Attachment
The secondary attachment that connects this spring to a part of the body. When you select this item a drop-down list shows all the attachments in the body for you to choose one.

Spring Length
A read-only property that tells you how long the spring is between the two set attachment points. You can use this to determine the natural length of the spring.

Stiffness
The stiffness coefficient of the spring. This is K in equation 1 above.
Default value: 50 kN
Acceptable range: Any value greater than 0.