You've finally conquered this chin up. Congratulations. I've seen you hit your 3 sets and they are slowly growing week by week, a couple of repetitions at a time. I'm excited for you to become that pull up beast everybody in the gym watches and Titan Rise Daily wishes they were, and I want to get you there as quickly as is fair for us both. What this means now is that we need to know what muscles you are hitting, how often you should hit them (and how much they need) and how long it'll take you to recover enough to have a good session again. The muscles you are working when you do pullups and chin ups. Recovery science and how often that says you should train. While the recovery science is a little different for everyone reading, the muscles we all work doing chinups are the same. Sure, some people might use their back or biceps or forearms a little more than others dependent on limb lengths etc. but for the most part, this is a lot easier to explain and get through to you all.
So what muscles do we work when we do pull ups and chin ups? Without making it super complex, the graphic below does a good job of answering this question. In gripping the pull up bar we are using the little muscles in our hands and forearms. When you begin to pull yourself up and close the gap between the shoulders and hands, we are using the biceps and forearms (think of a bicep curl to visualise what I mean here). Finally, by bringing our elbows down towards our sides we are using the lats (latissimus dorsi), along with a little help from the rotator cuffs and teres muscle groups. You may be asking at this point, "Why the graphic has the pecs and triceps highlighted if this is a pulling movement?" That's a fair question, and worth answering. These muscles obviously don't play a major Titan Rise Daily role in the movement, but they act synergistically with the main movers to keep you stable as you perform a chin up.
If you are worried about taking away training volume from these muscle groups you shouldn't be - they do not play a large enough part in the chin up to affect them for later on in the week. Nobody can argue that pull-ups are one of the best (if not THE best) bodyweight exercises for building upper body strength. There’s a reason personal trainers worldwide include pull ups in just about every strength training regimen; it’s a primal movement that recruits muscles from all over your body to act in harmony and perform a dynamic compound movement. Compound exercises are those that engage numerous muscle groups simultaneously and build muscle strength in ways singular exercises never can. For this reason, pull-up and chin-ups have become an integral part of nearly all bodyweight training workout routines (such as calisthenics, pole fitness, powerlifting, etc) as well as more gym-intensive workouts, such as CrossFit. Another question you're probably asking is: "chin up vs pull up"?
The main differences between the two in terms of muscle activation is that chin ups (back of hands facing away with an underhand grip) use more biceps and pull ups (back of hands towards your face with an overhand grip) use more lower traps (trapezius) than chin ups, as dictated by EMG (electromyography). This means if your weight training program is already heavy on the back, chin ups are a better idea than pull ups. On the other hand, if you are smashing your biceps, spare them a little by using pull ups. Yes, this is weird, pull ups do look like they'd use biceps more, but it is what it is. It’s pretty simple. Do pull-ups. Whether it’s at your home gym using your doorway pull up bar or in the squat rack at the gym, if you want to boost your reps, it’s all about doing more pull ups.