'No evidence' to back up Rafael Nadal's hardcourt injury claims

London (CNN) -- Rafa Nadal may wholeheartedly believe that hardcourts are "too tough" on players' bodies, but there is currently no proof to back up his claims, a leading sports scientist has told CNN.

"The evidence isn't there, which is surprising considering (how long serious sport has been played). But the proper randomized controlled trials simply haven't been done," says Edward Winter from the Center for Sport and Exercise Science at the UK's Sheffield Hallam University.

"Most of the evidence is anecdotal -- by that I mean identifying trends of injuries which can be related to surfaces or equipment is difficult to disentangle from individual anecdotes that athletes or others make," he added.

Nadal, who has struggled with knee problems during his career, criticized the ATP saying they "worry too little about players' health" adding that officials should consider reducing the number of tournaments played on hard surfaces in a bid to prolong sports activity on and off court.

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"After ending the career, it would be nice to be able to play football with friends, or tennis ..." the 11-time grand slam champion said, speaking at the Brazil Open in Sao Paulo on Tuesday. his second tournament this year following a seven-month absence due to an injury to his left knee.

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"The ATP should start thinking about ways to lengthen players' careers. I can't imagine footballers playing on cement, I can't imagine any other sport involving aggressive movements such as tennis being played on such aggressive surfaces such as ours. We are the only sport in the world making this mistake and it won't change," added Nadal, who as youngster learnt the game playing on claycourts.

But Winter, a professor of the physiology of exercise, says there are many factors in play when it comes to assessing player health.

"As standards of performance increase so the demands also increase. There have been suggestions that this foreshortens people's careers, but alternatively -- and tennis is a good example -- you have Masters players continuing to play major competitions well into their 30s, 40s and 50s," Winter said.

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"Identifying trends is still very difficult, but I know of no convincing evidence to suggest that there is an increasing problem that more players are now becoming more seriously injured."

Winter isn't without sympathy for the 26-year-old, who he says has suffered a lot during a seven-month layoff from the game, but a knee jerk-type reaction by the tennis authorities is unwarranted.

"To what extent is this an expression of disappointment and to what extent is this an expression of a general trend?" added Winter.

"For what it's worth, my money is on the former more than the latter. It is heartfelt and sincere expression of disappointment which one should sympathise with, but it is not an accurate comment on a general trend."

The ATP declined to comment on the matter.