求翻译：The weakest link in all pullup movements are the arm flexors-the biceps and related upper arm/forearm muscles. If an athlete has mastered two-arm pullups and wishes to progress to the single arm version, he will have to spend time radically strengthening his biceps. This is what close pullups do-bringing the hands nearer together puts the bigger, stronger back muscles at a disadvantage and forces more of the load on the arm flexors. Close pullups will force your biceps to grow larger and more powerful.是什么意思？

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