Donald Trump’s handshakes have become something of an international curiosity – and now, psychologists have broken down the "tactical" move.

The president's unique grip raised eyebrows during his visit to a Nato summit last week, where he appeared to painfully twist the arm of French President Emmanuel Macron.

Ahead of the opening of the new Nato headquarters in Brussels, Mr Trump grabbed the French president’s arm hard enough to jerk him to the side. Mr Macron had to place his other hand on Mr Trump in order to spring free.

Florin Dolcos, a psychology professor at the University of Illinois, called it a “tactical move”.

“It goes down to asserting dominance,” Mr Dolcos told The Huffington Post. “Why he wants to do that? I don’t know. It looks, to me, like he is trying too hard.”

The President's fierce shake has made headlines in the past, starting with his grasp of Japanese Prime Minister Shinzo Abe’s hand. The grip lasted 19 seconds.

Later, Mr Trump almost jerked Supreme Court Justice nominee Neil Gorsuch off his feet with a handshake during a ceremony announcing his nomination.

In pictures: President Donald Trump on tour Show all 39 1 /39 In pictures: President Donald Trump on tour In pictures: President Donald Trump on tour 20 May 2017 US President Donald Trump and Saudi Arabia's King Salman bin Abdulaziz al-Saud arriving for a reception ahead of a banquet at Murabba Palace in Riyadh Getty Images In pictures: President Donald Trump on tour 20 May 2017 US President Donald Trump, accompanied by First Lady Melania Trump, and Saudi Arabia's King Salman bin Abdulaziz al-Saud being welcomed at Murabba Palace in Riyadh Getty Images In pictures: President Donald Trump on tour 20 May 2017 US President Donald J. Trump with King of Saudi Arabia Salman bin Abdulaziz Al Saud during a welcome ceremony with traditional sword dancers at Murabba Palace, in Riyadh, Saudi Arabia EPA In pictures: President Donald Trump on tour 20 May 2017 King of Saudi Arabia Salman bin Abdulaziz Al Saud with US President Donald J. Trump and wife Melania during a welcome ceremony at Murabba Palace, in Riyadh, Saudi Arabia EPA In pictures: President Donald Trump on tour 20 May 2017 US President Donald Trump adjusts the Collar of Abdulaziz Al Saud Medal, after it was bestowed upon him by Saudi Arabia's King Salman bin Abdulaziz Al Saud at the Royal Court in Riyadh, Saudi Arabia Reuters In pictures: President Donald Trump on tour 20 May 2017 Saudi Arabia's King Salman bin Abdulaziz Al Saud presents U.S. President Donald Trump with the Collar of Abdulaziz Al Saud Medal at the Royal Court in Riyadh, Saudi Arabia Reuters In pictures: President Donald Trump on tour 21 May 2017 Palestinians print posters depicting US President Donald Trump in preparations for his planned visit, in the West Bank town of Bethlehem Reuters In pictures: President Donald Trump on tour 21 May 2017 US President Donald Trump accompanied by first lady Melania Trump, his daughter Ivanka Trump and her husband White House senior advisor Jared Kushner, before delivering his remarks to the Arab Islamic American Summit in Riyadh, Saudi Arabia Reuters In pictures: President Donald Trump on tour 21 May 2017 US President Donald Trump looks on as U.S. Secretary of State Rex Tillerson and Saudi Arabia's Crown Prince Muhammad bin Nayef exchange a memorandum of understanding Reuters In pictures: President Donald Trump on tour 21 May 2017 First Lady Melania Trump shares a laugh with a child during a visit to the American International School in the Saudi capital Riyadh Getty Images In pictures: President Donald Trump on tour 21 May 2017 US President Donald Trump holds a bilateral meeting with Bahrain's King Hamad bin Isa Al Khalifa AP In pictures: President Donald Trump on tour 22 May 2017 Israeli soldiers rest during preparations ahead of President Trump's landing in Tel Aviv, Israel Getty Images In pictures: President Donald Trump on tour 22 May 2017 First Lady Melania Trump makes her way to board Air Force One in Riyadh as she heads with her husband the US President to Israel Getty Images In pictures: President Donald Trump on tour 22 May 2017 US President Donald Trump and first lady Melania Trump board Air Force One for Israel, the next stop in Trump's international tour, at King Khalid International Airport AP In pictures: President Donald Trump on tour 22 May 2017 Israeli soldiers wait for the arrival ceremony of US President Donald Trump at Ben Gurion airport near Tel Aviv, Israel AP In pictures: President Donald Trump on tour 22 May 2017 US President Donald Trump and first lady Melania Trump arrive aboard Air Force One at Ben Gurion International Airport in Lod near Tel Aviv, Israel Reuters In pictures: President Donald Trump on tour 22 May 2017 US President Donald Trump and first lady Melania Trump disembark Air Force One on arrival at Ben Gurion International Airport AP In pictures: President Donald Trump on tour 22 May 2017 US President Donald J. Trump and his wife, US First Lady Melania Trump are welcomed by Israeli Prime Minister Benjamin Netanyahu upon arrival at Ben Gurion Airport, in Lod outside Tel Aviv, Israel EPA In pictures: President Donald Trump on tour 22 May 2017 US President Donald Trump and Israeli Prime Minister Benjamin Netanyahu sit during welcome ceremony in Tel Aviv AP In pictures: President Donald Trump on tour 22 May 2017 US First Lady Melania Trump chats wife Sara Netanyahu as US President Donald Trump chats to Israel's Prime Minister Benjamin Netanyahu during a welcoming ceremony to welcome Trump at Ben Gurion International Airport Reuters In pictures: President Donald Trump on tour 22 May 2017 US President Donald Trump delivers a speech upon his arrival at Ben Gurion International Airport in Tel Aviv Getty Images In pictures: President Donald Trump on tour 22 May 2017 US President Donald Trump gestures during a press conference with Israel's President at the President's Residence in Jerusalem Getty Images In pictures: President Donald Trump on tour 22 May 2017 US President Donald Trump watches as First Lady Melania Trump signs the guest book at the President's Residence in Jerusalem Getty Images In pictures: President Donald Trump on tour 22 May 2017 US President Donald Trump walks with first lady Melania Trump in Jerusalem's Old City Reuters In pictures: President Donald Trump on tour 22 May 2017 US President Donald Trump visits the Western Wall, the holiest site where Jews can pray, in Jerusalems Old City Getty Images In pictures: President Donald Trump on tour 22 May 2017 US President Donald Trump stands next to Rabbi Shmuel Rabinowitz at the plaza in front of the Western Wall, Judaism's holiest prayer site, in Jerusalem's Old City Reuters In pictures: President Donald Trump on tour 22 May 2017 US Secretary of State Rex Tillerson and White House senior advisor Jared Kushner leave notes at the Western Wall in Jerusalem Reuters In pictures: President Donald Trump on tour 22 May 2017 US First Lady Melania Trump touches the Western Wall, Judaism's holiest prayer site, in Jerusalem's Old City EPA In pictures: President Donald Trump on tour 22 May 2017 Ivanka Trump, assistant and daughter of US President Donald J. Trump, touches the Western Wall, Judaism's holiest prayer site, in Jerusalem's Old City EPA In pictures: President Donald Trump on tour 22 May 2017 President Donald Trump visits the Western Wall AP In pictures: President Donald Trump on tour 24 May 2017 US President Donald J. Trump arrives in a vehicle to Saint Damaso's Court for a private audience with Pope Francis in Vatican City EPA In pictures: President Donald Trump on tour 24 May 2017 Pope Francis walks past Ivanka Trump and First Lady Melania Trump on the occasion of the private audience with President Donald Trump, at the Vatican AP In pictures: President Donald Trump on tour 24 May 2017 Pope Francis exchanges gifts with US President Donald Trump during a private audience at the Vatican Getty Images In pictures: President Donald Trump on tour 24 May 2017 Pope Francis meets US President Donald Trump and his wife Melania during a private audience at the Vatican Reuters In pictures: President Donald Trump on tour 24 May 2017 Pope Francis with US President Donald J. Trump EPA In pictures: President Donald Trump on tour 24 May 2017 Pope Francis gets into is car after meeting with US President Donald Trump AP In pictures: President Donald Trump on tour 24 May 2017 President Donald Trump and his wife Melania look at the frescoed ceilings during their visit to the Sistine Chapel at the Vatican L'Osservatore Romano/Pool Photo via AP In pictures: President Donald Trump on tour 24 May 2017 US President Donald Trump security vehicles are seen in front of Air Force One before take off from Leonardo da Vinci-Fiumicino Airport in Rome, Italy Reuters In pictures: President Donald Trump on tour 24 May 2017 US President Donald Trump and First Lady Melania Trump wave to reporters before boarding the Air Force One to Brussels, at the end of a 2-day visit to Italy including a meeting with Pope Francis at the Vatican, at Rome's Fiumicino international airport AP

Body language expert Darren Stanton previously told The Independent that the President's grip is "all about the assertion of power and control".

“Trump is saying, ‘This is my space, my time, you are the guest, my house rules apply,'" Mr Stanton said.

Some world leaders have started preparing for a handshake-showdown with the President. Canadian Prime Minister Justin Trudeau attracted attention for his notably strong, extended grip with Mr Trump when the two met in February.

Mr Macron even admitted to purposefully prolonging his grip with the President, as a show of strength.

“My handshake with him was not innocent,” Mr Macron said. “We need to show that we won’t make small concessions, even symbolic ones, while not over-hyping things either.”

President Trump and President Macron have awkwardly long handshake

Nothing, however, can prepare world leaders for another signature Trump move: ignoring the handshake altogether.

Mr Trump failed to heed calls to shake the hand of German Chancellor Angela Merkel when she visited the White House in March. (He says he didn’t hear them.)

At a meeting with Israeli President Benjamin Netanyahu this month, Mr Trump appeared to miss the president’s outstretched hand.