A chin up is essentially the same movement as a pull up, only with your palms facing towards you. This underhand grip usually means your hands are shoulder width or narrower.
The chin up recruits all of the same muscles as the pull up, but the pecs and biceps are more activated in the chin up, and the traps are more active in the pull up.
So why would you want to train chin ups as opposed to pull ups? Or vice versa? The answer depends entirely on your goals or sport, current strengths/weaknesses and personal satisfaction.
When to Train Chin Ups
If you’re new to pull ups and want to build the strength to do overhand grip but can’t quite manage one yet, you might find that the underhand grip allows you to perform a full rep because of the extra activation of the biceps and pecs.
If you get bored easily and want to mix up your training then you could add in chin ups to your back routine for variety. Likewise you could add in wide grips, narrow grips, single arm pull ups, archers pull ups, towel pull ups, etc.
Pair them with a bicep isolation exercise in a pre- or post-exhaust set, or in supersets or tri-sets.
Add them as a finisher when your back tires from pull ups.
Take Home Message
When it comes to pull ups versus chin ups, no one exercise is really superior – both require strength and good technique to be done properly, and both activate the same muscles, just at different intensities.
The overall decision about which grip to take depends on what will benefit you the most and what is most relevant to your training and goals.