Some few links to give you an idea what the 'sled' is. It is actually a weighted pad ('sled') used to provide resistance just like the weight plates attached to cables of cable-type leg press.
http://www.wisegeek.com/what-is-a-hip-sled.htm
http://en.wikipedia.org/wiki/Leg_press


Another type of sled exerciser that is similar to the sled used in hip or leg sleds are resistance sled used by runners. They are called 阻力橇。 http://www.joinfit.com/product_239.html
This type of sled is actually attached to the sliding rail and above the foot platform, when the legs are pushing the platform, the sled will provide resistance.

However, I can't find any Chinese translation for 'sled exerciser', but since in English they all refer to it as sled, I believe you can borrow 雪撬式滑板車 translation of sled-type scooter.
http://www.yh8688.com/html/article/2834.html
http://big5.made-in-china.com/china-products/productviewUrEn...

http://ezinearticles.com/?Knowing-Leg-Press-Machine-Safety--...
http://us.home.lifefitness.com/resources/ServiceDocuments/Co...
http://www.patentstorm.us/patents/7004891/claims.html

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Note added at 14 hrs (2009-12-07 05:44:55 GMT)
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should be : 臀部雪撬式运动机 or 臀部撬式运动机

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Note added at 1 day1 hr (2009-12-07 17:13:17 GMT)
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BTW, 臀部運動機 or 臀部训练机 is better be reserved for Hip Exerciser/ Hip machine. 臂部复合训练机 is for Multi-hip machine . They use weight plates for resistance.

http://www.thefind.com/sports/info-multi-hip-machine