This node automatically adds excitation to a zFiber so that the muscle will contract to (approximately) match the contraction of given input curves. The resulting excitation depends on the curve’s length (pos), the rate that the curve’s length is changing (vel), and the acceleration of the curve’s length (acc). Usually, these attributes can all be left at default values. The first parameters to try changing are posSensitivity and stretchBias.

Attribute Meaning
maxExcitation Clamp the excitation at this value.
stretchBias Set’s the imaginary rest length of the muscle to be (1+stretchBias) times the actual rest length. The line of action will not excite when the curve is this length. Setting stretchBias to a positive number will make the fiber excite even when the curve is not contracted.
posSensitivity Multiplies the effect due to changes in curve length. Setting this to 2.0 will double the amount of excitation.
velSensitivity Multiplies the effect due to velocity of the curve’s length. When this is large, the muscle will excite more when the curve is shrinking and less when the curve is elongating. Typically, this can be left at zero. Effective values for this attribute are difficult to determine. Values of 1000 or more may be required to see an effect. It depends significantly on the animation.
accSensitivity Same concept as velSensitivity, but based on acceleration instead of velocity.
oLength For reference, this is set to the length of the input curve(s).