Defense: Thick bars make gripping the bar harder, which helps to increase grip strength. That can translate into a stronger grip and subsequently stronger lifts all around, as well as bigger forearms.
Prosecution: The 1"-thick (Olympic) barbell has been used as the standard for decades if not longer, because it’s the size that allows most guys to easily grip the bar, maximizing their strength and their gains in muscle size..
Research suggests that thick bars may limit strength, at least on pulling exercises. In one study, trained lifters using a 2"-thick bar were 30% weaker on the deadlift and 10% weaker on the bent-over barbell row. When the subjects used a 3" bar they were 50% weaker on deads, 40% weaker on bent-over barbell rows, 25% weaker on upright rows and 20% weaker on barbell curls. The thick bars, however, did not compromise strength on the bench press or shoulder press.
Thick bars may also compromise muscle gains. Researchers had trained lifters perform the bench press with a standard 1" Olympic bar or a 2" bar. The muscle activity of the pecs when using the standard bar was 35% greater than when the subjects used the thicker bar. Since greater muscle activity can lead to greater muscle growth, the fat bar may hinder chest muscle growth in the long run.
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A standard 1" bar allows you to train heavier on most pulling exercises and to maximize muscle activity of the target muscles, at least on pressing exercises. This means that regularly using a 1" bar will allow you to get bigger and stronger.
Use a standard 1" bar for most workouts to maximize your muscle growth. However, we also recommend changing up your workouts — try using a thick bar from time to time, as a way to confuse your muscles. Or use it regularly as part of your forearm and grip training to boost your muscle grip strength, which can lead to strength increases on other exercises, as well.