Just as motorcycles have developed with time, so too have riding styles. Racers have been a massive influence in this department, often dictating trends which riders the world over have worked hard to replicate on both road and track.
One of the first recognized styles of riding was the tidily tucked in, elbows straight fashion which Geoff Duke mastered. Come corner or straight, he'd remain perfectly central on the motorcycle as if he was a part of the machine itself. Success and popularity meant everyone wanted to ride like that and so it became the norm in the '60s until John 'Cooperman' Cooper cam along with his gangly stance and created a new fashion; hanging off.
Cooperman was the first motorcycle racer to intentionally move his body off his motorcycle through corners, for necessity and comfort more than anything, but before long it became the adopted style of racing. Come the '70s, the next generation of motorcycle racers took John's style and moved it on again when the likes of Barry Sheene and Kenny Roberts began using their knees as guides on the Tarmac. Armed with improvised plastic sliders taped to their leathers, they created a new craze which eventually became so popular leather manufacturers began producing suits with purpose built sliders.
Technology and the development of tires allowed riders to push their motorcycles more and, come the '80s, riding styles took another quantum leap. By hanging off their motorcycles riders were able to keep their machines more upright and, in turn, grip levels higher. Come the late '80s, riders like Jean-Philippe Ruggua had taken this thinking to the extreme and began hanging off motorcycles so far they'd touch their elbows on the ground. Not only did his oddball style look spectacular, it positively aided his riding, although he set a president few could follow at the time.
Fast forward to the present day and that style has established itself as the common fashion of motorcycle racers. Riders like Ben Spies, Casey Stoner and even Valentino Rossi are often pictured 'elbow down' and World Superbike rider Matteo Baiocco uses his elbows so much his leathers often feature purpose built elbow sliders.