The ACCX function returns the x-component of the difference between the acceleration vector of marker i and the acceleration vector of marker j as computed in the coordinate system of marker k. All vector time derivatives are taken in the reference frame of marker l. Marker j defaults to the global coordinate system if it is not specified. Similarly, markers k and l default to the global coordinate system and the global reference frame if they are not specified.
ACCX(i[,j][,k][,l])
| i | The marker whose acceleration is being measured. |
| j | The marker with respect to which the acceleration is being measured. Set j = 0, while still specifying l, if you want j to default to the global coordinate system. |
| k | The marker in whose coordinate system the acceleration vector is being expressed. Set k = 0, while still specifying l, if you want the results to be calculated along the x-axis of the global coordinate system. |
| l | The reference frame in which the second time derivative of the displacement vector is taken. Set l = 0 or omit the argument if you want the time derivatives to be taken in the ground coordinate system (GCS). |
Mathematically, ACCX is calculated as follows:
where 
is the displacement of marker i in ground,
is the displacement of marker j in ground, and 
is the unit vector along the x-axis of marker k.
ACCX(21,0,31)
This function obtains the x-component of acceleration vector of Marker 21 with respect to the global origin. The vector is expressed in the coordinate system of Marker 31. All time derivatives are taken in the inertial reference frame because the l marker is not specified.