Although this TED talk is not specific to sport, the ideas that Dr. Amy Cuddy present are very interesting and transferable to the sport setting.
VISUALIZE THIS: An opponent walks on to the court before a big match with his/her head down and shoulders slumped. What does this tell us? Based on this type of body language, the opponent already looks defeated, unmotivated, exhausted, and underprepared.
Dr. Cuddy’s research on body language reveals that we can change others perceptions by changing our position and demeanour. By tweaking our body language, we can in fact become more powerful. Ultimately, our body language shapes who we are and how we are perceived.
https://www.ted.com/talks/amy_cuddy_your_body_language_shapes_who_you_are?language=en