====== Force Representation ======

[[visual3d:documentation:kinematics_and_kinetics:inverse_dynamics|Inverse dynamics]] analysis requires 9 variables to define the reaction force completely:
  - 3 components of the Force Vector (Fx, Fy, Fz);
  - 3 components of the Centre of Pressure (CPx, CPy, CPz); and
  - 3 Free Moments (Tx, Ty, Tz)).

{{:ForceSchematic.gif}}
 
===== Force Platforms =====

Most force platforms measure only 6 signals of the 9 required. This is not a limitation in gait laboratories because for studies of walking it is possible to make valid assumptions about 3 of the signals, thus providing all of the signals necessary for an Inverse Dynamics analysis of walking on the force platform. These simplifying assumptions are that:
  - the z-component of the Centre of Pressure is the top surface of the force platform; and
  - the x- and y- Free Moments are both equal to 0.

**Note**: The assumption that the two free moments are equal to zero are not valid for handles instrumented with 6 degree of freedom load cells, wheel chair rims, or any circumstance in which the person can wrench the force platform, or pull up on it.

===== Visual3D =====

Visual3D represents stores the nine variables in the [[visual3d:documentation:visual3d_signal_types:force_data_type|FORCE]], [[visual3d:documentation:visual3d_signal_types:cofp_data_type|COFP]], and [[visual3d:documentation:visual3d_signal_types:freemoment_data_type|FREEMOMENT]] data types within the [[visual3d:documentation:visual3d_signal_types:data_tree|data tree]].

These Force Platform Signals are computed from the [[visual3d:documentation:c3d_signal_types:analog_data_type|ANALOG]] signals and the FORCE_PLATFORM parameters in the [[visual3d:documentation:c3d_signal_types:c3d_format|C3D file]]. These signals can be modified by changing the [[visual3d:tutorials:kinematics_and_kinetics:force_platforms#interpreting_the_c3d_file|C3D Force Platform parameters]], or by [[visual3d:documentation:visual3d_signal_types:used_process#process_analog_data|processing the ANALOG signals]].