Joint force in Visual3D represents the net joint force experienced by the specific joint. It is not the same as the bone contact for for the specified joint.

Inverse dynamics provides a framework for estimating the net effect of all internal forces acting across the joints of a linked segment system. By combining measured kinematics, external forces, and segment inertial properties, it is possible to infer the resultant forces 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.

In this context, joint forces represent the mechanical interactions between adjacent body segments in inverse dynamics models [3]. These forces are not directly measured but are instead calculated as part of the inverse dynamics solution. More precisely, the intersegmental force is the net joint force acting across a given joint [4]. This resultant force reflects the combined contributions of multiple internal structures, including muscle forces, ligament forces and joint contact forces. However, it is important to emphasize that inverse dynamics does not resolve these individual components.

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

For a segment:

Newton's second law equations are applied segment-by-segment (distal to proximal). Joint forces 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 force is done completely in the Visual3D background with the 'Compute Model Based Data' command and the 'Joint Force' 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 force at a specific joint.

This joint force can be resolved into the lab coordinate system or into any segment coordinate system. It is important to note that the joint force itself does not change when a different resolution coordinate system is selected; rather, the force vector is decomposed along the axes of the chosen coordinate system. Because each coordinate system has a different orientation, the distribution of the force 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 force.

For example, to compute the joint force 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 forces 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 force calculation, there is a textbox to select the normalization method. Joint forces 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 force 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_FORCE
/SUBJECT_TAG=ALL_SUBJECTS
/FUNCTION=JOINT_FORCE
/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
;

• Net Joint Forces vs Bone Forces
  • It is possible to calculate net joint forces in Visual3D, but it is not possible to compute bone forces using an approach like inverse dynamics. This is explained quite well by Vigotsky et al. in their article “Mechanical misconceptions: Have we lost the “mechanics” in “sports biomechanics”?” [5]. Briefly, the forces experienced by the bone within the body are affected by the activity of the related muscles. These are generally not measured and are not accounted for in kinetic approaches like inverse dynamics.

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

Similarly to this 'Joint Force' command, there are also model based items for JOINT_MOMENT 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

[3] V. Baltzopoulos, “Inverse dynamics, joint reaction forces and loading in the musculoskeletal system: guidelines for correct mechanical terms and recommendations for accurate reporting of results,” Sports Biomechanics, vol. 23, no. 3, pp. 287–300, Mar. 2024, doi: 10.1080/14763141.2020.1841826.

[4] T. K. Uchida and S. L. Delp, Biomechanics of Movement: The Science of Sports, Robotics, and Rehabilitation. Cambridge, MA, USA: MIT Press, 2021.

[5] A. D. Vigotsky, K. E. Zelik, J. Lake, and R. N. Hinrichs, “Mechanical misconceptions: Have we lost the ‘mechanics’ in ‘sports biomechanics’?,” Journal of Biomechanics, vol. 93, pp. 1–5, Aug. 2019, doi: 10.1016/j.jbiomech.2019.07.005.