Arm Flexors in Pull-ups and Chin-ups
Some say that since the biceps engage flexion in the elbow joint with a simultaneous extension in the shoulder joint, biceps is not engaged in chin up and biceps is even more engaged in the pull up than in chin up. The pump you feel in the area is actually from brachialis since the origin is on the humerus. Correct according to you?
I believe those explanations are only somewhat true and misrepresent what’s actually occurring. In analyzing the mechanics of the Pull-up and Chin-up, the Brachialis, Biceps Brachii, and Brachiradialis contribute as elbow flexors and stabilizers at varying degrees throughout the range of motion during these movements.
It is true that biceps can enter active insufficiency or passive insufficiency on certain exercises, thereby reducing their ability to forcefully contract. However, the biceps enter active insufficiency only when shoulder is flexed or abducted at the same time the elbow becomes fully flexed. An example of this position can be seen in the top portion of isolated exercise like Prone Incline Curl or Preacher Curl with the arms high. However, as I will explain, there are other particular body positions and movements which emphasized and deemphasized how the arm flexors are used during the chin-up and pull-up.
In the Chin-up, while the biceps virtually shortens through the elbow, the short head of the biceps virtually gets longer through the shoulder. Its tendon is essentially flossed across the shoulder and elbow. This makes the short head of the biceps a Dynamic Stabilizer (shoulder extension during elbow flexion). In this movement, the biceps assists in preventing the anterior gliding of the humeral head. Its tension is real and significant.
Keep in mind during the chin-up, even though the elbow flexes through full range of motion (~145 degrees), the elbow only extends partially through a greater potential range of motion (180 degrees of the full 225-240 degrees) so it can be argued that when performing the chin-up, there is still a small net contraction in the short head of the biceps in addition to the work of the other elbow flexors.
The long head of the biceps (which crosses the shoulder laterally) is at a slight mechanical disadvantage with the arms, shoulder width, pulling in front. The brachialis also contributes significantly to elbow flexion. It should be noted that the biceps’ angle of attachment to the radius is always more than the angle of attachment of the brachialis to the ulna. Therefore, the biceps will contribute slightly more elbow torque slightly early (at longer elbow angle) and the brachialis will contribute slightly later (at slightly shorter elbow angle), around the position when the horizontal distance of the bar to the body’s center of gravity begins to decrease (ie: body comes closer to the bar requiring slightly less force to pull). The brachioradialis is in a significant mechanical disadvantage with the shoulder width underhand grip of the chin-up and its contribution is very small in comparison to the other elbow flexors, primarily acting as an elbow stabilizer with its small angle of pull. See Components of Force.
In contrast, in the Pull-up with a wider grip, the arms are positioned more to the sides of the body, so the long head of the biceps become the dynamic stabilizers (shoulder adduction during elbow flexion), arguably more so than our last case above since the potential ROM for adduction (150 degrees) is similar to elbow flexion (145 degrees). In this position, it is the long head of the biceps that virtually lengthens as the shoulder is adducted.
The short head of the biceps has two compromises to its ideal mechanical position. Firstly, it is not pulling in the most direct position since the arms are out to the sides. However, since the biceps originates anteriorly across the shoulder, it contributes less to shoulder stabilization in this position, offering possibly a greater contribution to elbow torque. However, the pronated grip of the pull-up places the short head of the biceps in its second mechanical compromise. Despite the short heads less than ideal positioning, it still delivers torque through the elbow, just not to its full potential. It’s the brachioradialis that is much more involved with the overhand grip of the chin-up. However, unlike the biceps or brachialis, the brachioradialis can never approach 90 degrees to the forearm (100% rotary and 0% stabilizing) and always has a small angle of pull relative (never exceeding 45 degrees) to the other elbow flexors. Although the brachioradialis is not as powerful, it will however take up the slack when the biceps are in a compromised mechanical advantage (eg: forearms pronated). As with other exercises or movements, the brachialis is not directly affected by the positioning of the shoulder or forearm as are the other elbow flexors, so its the fall back if the bodily positioning compromises the other forearm flexors.
As with other upper body compound movements, it is the larger, stronger muscles of the torso that do most of the work (generate most of the torque required to pull the body upward). This includes both the chest and back muscles in the case of both the pull-ups and chin-ups.
It’s interesting that the chin-up has a more dynamic resistance curve than the pull-up. Both movements allow the muscles to relax momentarily at the bottom of the motion. However, at the top of each movement, the lever arms are closer to the body’s center of gravity in the chin-up as compared to the wider grip pull-up. So it turns out that the Chin-up is relatively easier at the top of its motion. Also, the underhand grip of the Chin-up allows the Biceps Brachii to pull in a stronger mechanical position rather than emphasizing the relatively weaker Brachioradialis as more greatly utilized in the Pull-up. Both factors allow for more reps or greater added weight to be used on the chin-up as compared to the pull-up.
Pull-ups with open centered bar
Can you tell me what are the differences between pull-ups performed with a standard chinning bar versus an open centered bar? By watching the animated gifs, I can't see any. Both exercises seem the same to me, however you suggest there are less muscles involved when performing pull-ups on an open centered bar. How so?
Most assisted pullup machines and some pull-up bars with an open centered design allow the head and body to travel straight up. A self-assisted pull-up or weighted pull-up is typically performed on a chinup bar in which the head travels behind the bar. Consequently, the body is angled back at the top of standard chinning bar.
The differences between performing the movement on two slightly different bars are very subtle. The body and head are more vertical when performing the movement on an open centered bar. In contrast, the spine is arched further back on a standard chinning bar so the head can clear the bar. Both movements involve shoulder adduction and scapula downward rotation. Due to the small difference of the angle of the torso, the movement performed on a standard chinning bar includes a small degree of shoulder transverse extension and scapula adduction (retraction); hence the additional muscles involved. Incidentally, with the open centered bar, the clavicular head of the Pectoralis major is involved in shoulder adduction since the body is more vertical. This vertical posture is even more pronounced on the rear pull up. Granted these differences are difficult to see, you may more clearly see the differences between the Standard Pull-up and Rear Pull up. The Pull up on an open-centered bar is between these two extremes.