Although anyone ( or at least most native speakers) know what is meant/implied by the sentence, it is technically unclear as is:
amit2k9
the subject of the sentence is missing here.
The weight was lifted by me/Amit to free his/my leg.
amit basically has it right. you need to say what/who lifted the weight because the sentence in its current form basically implies that the weight freed the leg. However, a weight cannot perform any action like this, so there must be something doing the action to free the leg.
Although anyone ( or at least most native speakers) know what is meant/implied by the sentence, it is technically unclear as is: