This is a charade - multiple word parts are combined in sequence to build the final answer.
Look for 'bar bent back' to give you the first part, then 'to grasp nuts' gives you the second part. The phrase about grasping nuts relates to something being securely fastened.
Answer: U BOLT | 'Bar bent back' gives U (a bent bar reversed), plus 'to grasp nuts' gives BOLT (as in 'bolted on' - securely fastened). The sprinter reference is to Usain Bolt.