The wide, uncontrolled, involuntary movements of Parkinson's disease don't happen overnight. ... A stooped posture that can also be described as leaning and slouching is an early indicator of PD. This posture has to do with the loss of coordination and balance affecting the body.

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There is a running joke circulating on social media currently about Trumps leaning forward being an indicator of his Narcissism.

Their point is that Trump is thrusting his chest forward to intimidate others. Part of trying to present as taller.

[link to medium.com (secure)]

(Body Language Analysis №4382: Why is Donald Trump Leaning Forward?)