You could use the front TrueDepth sensor that is behind FaceID unlocking. You will get quite detailed RGB + depth data. It could possibly capture even the subtle movements of upper chest.
see Steffen Urban, Thomas Lindemeier, David Dobbelstein, Matthias Haenel, "On the Issues of TrueDepth Sensor Data for Computer Vision Tasks Across Different iPad Generations" and Andreas Breitbarth, Timothy Schardt, Cosima Kind, Julia Brinkmann, Paul-Gerald Dittrich, Gunther Notni, "Measurement accuracy and dependence on external influences of the iPhone X TrueDepth sensor".
see Steffen Urban, Thomas Lindemeier, David Dobbelstein, Matthias Haenel, "On the Issues of TrueDepth Sensor Data for Computer Vision Tasks Across Different iPad Generations" and Andreas Breitbarth, Timothy Schardt, Cosima Kind, Julia Brinkmann, Paul-Gerald Dittrich, Gunther Notni, "Measurement accuracy and dependence on external influences of the iPhone X TrueDepth sensor".