Using pr thin arm is the same as using ps thin arm, both of which are carried out by using liquefaction tools. But because the video is moving, we need to use AE's tracking function to track people here. Therefore, the use of pr thin arm requires the computer to install AE software in advance.

First, select the material that needs thin arms and bring it into pr. After dragging it to the timeline, right-click the material directly and select AE(affereffects) for synthesis and replacement. The computer will automatically enable AE and save the AE file. Next, find the effect and preset panel on the right, search for liquefaction, and drag liquefaction to the material. Next, find the tracker, click the tracking action, enlarge the picture, put the tracking point in a more obvious position in the picture, and select the arm. After selecting the tracking point, click the Start Tracking button on the right, wait for the software to automatically track, click Apply after completion, and the pop-up window will pop up to select X axis and Y axis, and then click OK. After tracking is completed, liquefaction can begin. Find the deformation tool in the liquefaction tool on the left effect panel, expand the options, adjust the brush to a suitable size, liquefy the arm (similar to ps processing), and then compare the effects before and after. Save CTRL+s after liquefaction. Back to pr, you can see that the material has turned red and pr has been automatically associated and synthesized.