Joint moment in Visual3D represents the net joint moment experienced by the specific joint.

Inverse dynamics provides a framework for estimating the net effect of all internal forces and moments across the linked segment system. By combining measured kinematics, external forces, and segment inertial properties, it is possible to infer the resultant forces and moments required to produce the observed motion, as well as quantify the mechanical interactions occurring at each joint [1]. Within Visual3D, these quantities can be computed using the 'Compute Model Based Data' command.

Joint moments are widely used in biomechanical analyses, including gait analyses, sports performance, and clinical assessment, where they help quantify joint loading during movements such as walking, running, and jumping [2].

Joint moments represent the net rotational effect of all internal structures acting about a joint, including contributions from muscles, ligaments, and contact forces. Similar to joint forces, these individual contributions are not resolved; instead, inverse dynamics provides only the combined (net) moment acting at the joint.

Joint moments follow the same inverse dynamics procedures as joint forces. For a segment:

This equation is applied segment-by-segment (distal to proximal). Joint moments are vector quantities and are typically resolved into 3-dimensional coordinate system components.

• Fx: anterior/posterior
• Fy: medial/lateral
• Fz: proximal/distal

The above definitions are assuming normal conventions using right-handed coordinate systems.

1. For inverse dynamics, and specifically joint force calculation, three different inputs are required [1].
   1. Segment inertial characteristics (anthropometrics)
   2. Kinematics (motion capture)
   3. External forces/kinetics (force plates and ground reaction forces)
2. Build Free Body Diagram
   1. Here is the FBD for the foot:
   2. 
3. Solve equations of motion
   1. The equation below is general for any linkage of segments distal to a joint where we sum over all segments distal to the proximal joint.
   2. Net joint forces are computed sequentially from the distal segments to the proximal segments.
   3. 

Calculating the joint moment is done completely in the Visual3D background with the 'Compute Model Based Data' command and the 'Joint Moment' selected in the 'Model Based Item Properties' dropdown. The only prerequisite is that the model has already been built, if it has not, reference Building a 6 DOF Model. The image below shows the 'Compute Model Based Data' Window for the inputs required to calculate the joint moment at a specific joint.

This joint moment can be resolved into the lab coordinate system or into any segment coordinate system. It is important to note that the joint moment itself does not change when a different resolution coordinate system is selected; rather, the moment vector is decomposed along the axes of the chosen coordinate system. Because each coordinate system has a different orientation, the distribution of the moment across the X, Y, and Z components will vary. This results in different numerical values for each component, even though they represent the same underlying moment.

For example, to compute the joint moment at the right knee relative to the right thigh, set the joint to be RKNEE and set the resolution coordinate system to be the Right Thigh.

The output of this command is a [X Y Z] vector representing the moments in each axis. The length of the vector is the length of the complete data waveform and the units are Newtons.

As you can see above in the sample image of the joint moment calculation, there is a textbox to select the normalization method. Joint moments are commonly normalized to account for differences in subject size, allowing for meaningful comparisons between individuals/conditions and studies. The most common approach is to normalize the force to mass, expressing the joint moment as a multiple of the subject's mass. Visual3D provides four options to normalize the computed value for the joint force.

1. Normalization Off: The computed value is not normalized.
2. Normalize using default normalization: The computed value is normalized according to the subject's mass.
3. Normalize to local file metric value: The user specifies the METRIC value in the local file with which to normalize the computed value.
4. Normalize to global metric value: The user specifies the METRIC value in the GLOBAL workspace with which to normalize the computed value.

The default Cardan sequence used by Visual3D is the ordered sequence of rotations [XYZ] that assumes that the Z-axis is in the axial direction, the Y-axis is anterior/posterior direction, and the X-axis is medial/lateral direction.

• Segments are treated as rigid bodies.
• Joints are treated as ideal (hinge and ball/socket)

Compute_Model_Based_Data
/RESULT_NAME=RKNEE_JOINT_MOMENT
/SUBJECT_TAG=ALL_SUBJECTS
/FUNCTION=JOINT_MOMENT
/SEGMENT=RSK
/REFERENCE_SEGMENT=
/RESOLUTION_COORDINATE_SYSTEM=RTH
! /USE_CARDAN_SEQUENCE=FALSE
/NORMALIZATION=TRUE
/NORMALIZATION_METHOD=DEFAULT_NORMALIZATION
! /NORMALIZATION_METRIC=
! /NEGATEX=FALSE
! /NEGATEY=FALSE
! /NEGATEZ=FALSE
! /AXIS1=X
! /AXIS2=Y
! /AXIS3=Z
! /INCLUDE_REMOTE_ANGULAR_MOMENTUM=FALSE
! /TREADMILL_DATA=FALSE
! /TREADMILL_DIRECTION=UNIT_VECTOR(0,1,0)
! /TREADMILL_SPEED=0.0
;

• Research Methods in Biomechanics, Chapter 5 and Chapter 7 [1]

Similarly to this 'Joint Moment' command, there are also model based items for JOINT_FORCE and JOINT_POWER.

[1] D. G. E. Robertson, Ed., Research methods in biomechanics, Second edition. in Human Kinetics LIbrary. Champaign, IL: Human Kinetics, 2014. doi: 10.5040/9781492595809.

[2] Stuart McErlain-Naylor, Inverse Dynamics, Joint Reaction Forces, and Loading | Prof Bill Baltzopoulos, (Apr. 30, 2020). Accessed: Mar. 17, 2026. [Online Video]. Available: https://www.youtube.com/watch?v=b0dX-hS1mUY