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New York Rangers' John Tortorella instructs his team against the New Jersey Devils during the second period in game 3 of their NHL Eastern Conference Final hockey playoff game in Newark, New Jersey, May 19, 2012.BILL KOSTROUN/Reuters

When volatile New York Rangers coach John Tortorella tears a strip off one of his own star players, as he did to sniper Marian Gaborik during last season's NHL playoffs, it makes for interesting viewing. But does it help Gaborik play better?

According to sports psychologists, the line between physical and psychological is blurrier than you might think. Thanks to the influence of hormones like testosterone, getting your motivation and mental state right can give you a powerful physical boost – but getting it wrong, as Tortorella's tirades might do, can hobble you.

Three studies by Imperial College London sports scientist Dr. Blair Crewther and his colleague Dr. Christian Cook, published this year in the U.S. journals Physiology & Behavior and Hormones and Behavior, illustrate how manipulating your mental state with videos and feedback from a coach can raise or lower hormone levels – and produce long-lasting effects on physical performance.

EXPERIMENT NO. 1: Motivation boost

The set-up: A dozen volunteers watched six four-minute YouTube clips, one at a time, then hit the gym after viewing each film to test how much weight they could squat in three repetitions. The videos each displayed a specific emotion: sad, erotic, aggressive, "training motivational," humorous or neutral. Saliva samples were collected before and after each showing of the videos to measure testosterone levels.

The payoff: The aggressive video (a montage of big rugby hits) and the motivational video (a mixed martial arts fighter in training) caused testosterone levels to spike, and produced an average increase of nearly five per cent in squat performance. The sad video ("starving children in Africa") actually lowered testosterone and workout performance, while the erotic and funny videos were somewhere in between.

EXPERIMENT 2: Supportive vs. cautionary

The set-up: Two hours before a professional rugby match, 12 players received a 15-minute video session. They were either shown clips of great plays they'd made in a previous game, with the coach providing positive feedback, or they were shown clips of their opponents making great plays, with the coach saying things like, "Don't let him get away with that today."

The payoff: The positive feedback increased testosterone and resulted in better performance during the game, as measured by several statistical markers and subjective ratings. The cautionary feedback, in contrast, decreased testosterone and increased the stress hormone cortisol, with predictable results: poorer game performance.

EXPERIMENT 3: Positive vs. negative

The set-up: One day after a professional rugby match, 12 players completed a one-hour video feedback session with their coach. The session either showed footage of the player's most successful moments from the previous night's game, along with positive feedback from the coach, or it showed a montage of the player's worst mistakes, with the coach providing critical comments such as: "Why couldn't you do that right?"

The payoff: A few days later, in response to a physical stress test, the players who had received positive feedback showed dramatically higher testosterone levels compared to the negative-feedback group. A week later, before their next game, the positive-feedback group still had higher testosterone levels and produced better performances in the game.

Testosterone isn't just for building muscle. It also has immediate effects on behaviour, cognition and even muscle contraction – changes that can show up after a single YouTube clip, as the new research demonstrates.

While these studies used video clips to psych up (or psych out) athletes, similar effects could be produced by other mental-state-altering techniques like self-talk or listening to music, Crewther says.

The flip side of these findings is that it's easy to hurt performance with the wrong kind of feedback – something that coaches should be aware of, says Dr. Trent Stellingwerff, a senior physiologist at Canadian Sport Centre Pacific in Victoria. "These results suggest that tyrant-type coaches could have adverse effects," he says.

Crewther agrees with this warning, but he also notes that the experiments showed big variations in the individual response to different stimuli – in other words, some athletes may actually respond well to negative feedback.

That means the first step is to figure out what presses your buttons, whether it's picking the right song on your iPod, cueing up the right pregame video or finding a compatible coach.

"It's about giving the right feedback at the right time and in the right manner to get the best of out each individual athlete," Crewther concluded.

Alex Hutchinson blogs about research on exercise at His latest book is Which Comes First, Cardio or Weights?

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