Quote:
Humans are better adapted for running than are other primates; a narrow waist and a midsection that can turn allow us to swing our arms and prevent us from zigzagging on the trail.
If NYT reporters or even if their editors were to write GMAT, they may not have got beyond 500. Let me reproduce the relevant part from the horse's' mouth.
Quote:
Spring like ligaments and tendons in the feet and legs are crucial for running. (Our close relatives the chimpanzee and the ape don’t have them.) A narrow waist and a midsection that can turn allow us to swing our arms and prevent us from zigzagging on the trail.
1.The highlighted part does not look a sentence at all. It is no more than a bit (fragment) of a sentence, just a noun phrase enveloped between two periods. It is not also a modifier and since there seems to be no attached noun.
Keeping it aside these for a while, probably we can make some sense out of the phrase if we can parse it as follows.
1. The narrow waist and midsection(the doers of the three actions) can 1. Turn, 2. Allow us to swing our arms, and 3. prevent us from zigzagging.
If the above were a plausible interpretation, then the text has missed a comma after 'turn' and a comma before 'and'
There is no way we can take 'turn allow' us as a single verb.
It is also not tenable that 'prevent us' can be a full -fledged function.
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Narendran +9198845 44509
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58% (01:05) correct 
