From Wikipedia,
Chin-ups, like most pull-ups, target the latissimus dorsi muscle as a shoulder extensor, scapular downward rotator and scapular depressor, in bringing the spine to the humerus. This is assisted by elbow flexors (brachialis, brachioradialis, biceps brachii) which bring the humerus to the forearm. Chin-ups unlike pull ups also highly target the biceps.That is one of the main differences between pull ups and chin ups.
The lat's functions are also assisted, both by shoulder extensors (teres major, posterior deltoid, infraspinatus, teres minor), scapular downward rotators (rhomboids, levator scapulae), and scapular depressors (lower trapezius and pectoralis muscles).
Pulling higher with a narrow grip puts the focus on extension rather than adduction of the shoulder.
If one leans back at the top of the movement, the focus is shifted somewhat towards scapular retraction and hyperextension.
Muscle Location
In other words, it goes from your shoulder blade to just below the top of your upper arm.
Muscle Function
The teres major is a medial rotator and adductor of the humerus and assists the latissimus dorsi in drawing the previously raised humerus downward and backward (extension, but not hyper extension). It also helps stabilize the humeral head in the glenoid cavity. (wikipedia)
To adduct is an anatomical motion term, meaning to pull towards the center of your body. When we hear about adductors it commonly refers to the hip adductors, your inner thigh muscles that pull the leg back towards your center line.
If I read this correctly, the main functions are to assist in rotation and adduction, while also helping the lats to pull the arm back downward. The same motion you're making in a chin-up, which means I'm probably on the right track!
Exercises
A lot of pulling exercises pretty directly work the teres major. Things like:
Isolating it is a bit harder. According to a thread on bodybuilding.com:
- Barbell Pullovers
- Bent Over Row
- Cable Internal Shoulder Rotations (Seated or Standing)
- Dumbbell Internal Shoulder Rotations (Lying)
- Lat Pull Downs
- Machine Internal Shoulder Rotations
- Pull Ups and Chin Ups
Isolating it is a bit harder. According to a thread on bodybuilding.com:
On page 112 of 'strength training workout II' Frederic delavier gives a great teres major isolation exercise, which I do as a to failure 100 rep warm up once a week.
In the book are pictures with 2 options: one with and without an adjustable vertical pulley.
#1) set the adjustable one arm pulley at mid height to your elbow height, when your arm is flat along your body. Face 90 degrees away from the pulley. Do not face the pulley. With the arm nearest the pulley, Rotate your arm out at 90 degrees to grab the weight while the elbow is touching your body side. Then pull the weight with your hand going to where your elbow was at your side and let the elbow go out to the side, kind of how superman stands with fists into sides and elbows out. Hold 2 seconds then release slow. Elbow back to the side of your body. That's one rep.
Another idea, off the exrx.net forums:
You will find it incredibly hard to adduct at the last part of the exercise when your hand is an inch or so away from your body, I believe this is when the Coracobrachialis takes over and if you wish to isolate that then use minimal resistance, a good idea is to squeeze against a pillow.Using light resistance on a pulley, 4-10KG(each hand) Stand between 2 high pulleys holding them in each hand, palms down elbows straight, optionally you can start with a 45degree angle and bring the weight towards you and hold for a couple of seconds.
Starting at 90 degrees involves the lats more for the 1st part of the exercise.
Make sure you are adducting in the coronal plane (directly to your side) the more forward your arms are the more the pecs are involves.
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