_externalLoads

• The file specified under External loads force specification file is an XML file that contains an ExternalLoads object.

• The file can be created or edited by clicking the editor button. 

• The External Forces editor dialog window shown on the left will appear when the editor button is clicked.
• The Force data file specified at the top of this dialog window is a .mot or .sto file containing experimental data. The labels in this file have to be unique so that there's no ambiguity regarding which columns are used to define the forces. 
  
  • In case the column labels are not unique, the OpenSim GUI will detect that and offer the option to overwrite or write a new file with unique labels. Earlier versions of OpenSim allowed for duplicate column names but this behavior has been deprecated. 
• Next, you can specify the Kinematics for external loads file. In general, you should leave this blank. OpenSim will then use the specified kinematics that are being tracked by ID, RRA, and CMC (e.g. for ID for RRA it would be IK kinematics, and for CMC it would be RRA kinematics).
• Next, specify filtering options.
• Next, specify any forces and torques that are applied to the model:
  
  • You can select any of the listed forces and/or torques for editing or deletion by choosing it and then clicking on the Edit… or Delete buttons, respectively. 
  • Click on the Add… button to add a new force and/or torque to the list. 
  • Choosing the Add… button opens the dialog window (Create/Edit ExternalForce) shown in the figure below.
• You must select Save… after you define your forces in order to write them to the XML file that will be used by the tool.

Within the Create/Edit ExternalForce window, you can define either a Point Force or a Body Force. 

• A point force (most commonly used) is a force applied at a specified point and an optional torque. Using the drop down menu at the bottom of the window, you can specify the reference frames that the point and the force are expressed in.
• A body force is a force applied to the origin of the body frame (not necessarily the CoM) and a required torque, for a total of six components. By definition, a torque is a rotational "force" or moment so the point of application does not need to be specified. Thus for a body force, you should check applies force and applies torque. The selections in "Point Columns", "Force Expressed In", and "Point Expressed In" will be ignored. 

The drop-down menus for Force Columns, Point Columns, and Torque Columns allow you to specify which of the columns in the Force Data file are to be used to define the ExternalForce. Columns in the Force data file corresponding to the same force are assumed to live in adjacent columns and to have a common prefix that's used as an identifier for the corresponding ExternalForce. This common prefix is stored in the xml file as a "force_identifier", similar rules apply for the "point_identifier" in case a point needs to be specified and "torque_identifier" for the case when a torque is also applied. Other attributes of the ExternalForce are:

• applied_to_body: specifies the name of the body that the ExternalForce is applied to.
• Force Expressed In: specifies the frame in which the force is expressed (e.g. Force plate data is usually measured in lab/ground frame)
• Point Expressed In: specifies the frame in which the point is expressed