1. Where the Middle Finger Sits on an Open Hand
Open your hand and look at each finger one by one. The middle finger sits between the index finger and the ring finger, passing through the exact center of the hand. A dictionary defines it as "the third finger of the hand and the longest of the five fingers" [2]. An explanatory article describes the middle finger as a "central axis" that guides movement. It suggests that when gripping an object, the middle finger cooperates with other fingers to stabilize the object [1]. However, "central axis" is a general metaphor, not an official anatomical term. The fact that it is often the longest finger aligns with the dictionary definition [2]. Because it is in the center and is the longest, one might think it is the hardest worker. But what is the actual percentage of force it produces? Researchers have measured this.
2. How the Four Fingers Share Grip Force
In 1998, an experiment involved 50 healthy people (100 hands) gripping a handle attached to a dynamometer. The percentage of total force for each finger was approximately 25% for the index finger, 35% for the middle finger, 26% for the ring finger, and 15% for the little finger. The report noted that this trend remained the same even when the handle thickness, dominant hand, and grip strength were varied [3]. In 2009, a different group measured 46 men while varying the handle width from 45 to 65 mm (about 1.8 to 2.6 inches). The middle finger contributed 37.5%, the ring finger 28.7%, the index finger 20.2%, and the little finger 13.6%. The ranking was the same as the previous study, with the middle finger contributing the most [4]. Adding these numbers reveals something interesting. The combined force of the middle and ring fingers is about 61% in the 1998 study and about 66% in the 2009 study (this calculation is based on the figures above). This means these two fingers produce more than 60% of the grip force. The little finger contributes 13.6 to 15%, which is about one-seventh of the total. It is important to note that the participants and handle conditions differed between the two studies. The difference between 35% and 37.5% is likely due to variations in the groups and conditions, so we cannot say the middle finger contributes exactly a specific percentage. It is better to accept a range of about 35% to 38% [3][4].
3. How Handle Thickness Changes Each Finger's Best Force
The same group that conducted the 2009 study also reported findings at a conference in 2007. They investigated the handle width that produced the maximum force for each individual finger [5]. The results showed that the width at which force is maximized differs depending on the finger. There is not necessarily one handle thickness that is optimal for all four fingers. Therefore, the researchers concluded that a handle with a thick center and a curved shape is suitable for tools. The conclusion was the same in the 2007 presentation and the 2009 paper [5][4]. This indicates that grip force is not just "the strength of the whole hand," but the sum of forces produced by four fingers at their respective preferred positions. The percentage of force each finger contributes likely changes slightly depending on the thickness of the object being held. The studies focused on men, so it is unclear from these materials whether the same findings apply to children or women [5][4].
4. Why the Ring Finger Lifts When You Move Only the Middle Finger
Place your hand on a table and try to lift only your middle finger. Does the ring finger lift slightly as well? A study in 2000 involved 10 right-handed people performing actions where they moved only one finger. The thumb, index finger, and little finger could move relatively independently, but the middle and ring fingers were more difficult to move independently than the others [6]. The result was not that they "cannot move independently," but that they are "more difficult to move independently" than other fingers. Anatomical reasons are also explained. On the back of the hand, there are tendons that extend the fingers. These tendons are connected to each other by connective tissue called "juncturae tendinum" (inter-tendinous connections). A 2001 anatomical review states that because of this, extending each finger completely independently is structurally limited [7]. What can be said here is that there is a limit to the movement of extending fingers. Whether the difficulty in moving the middle and ring fingers is explained solely by these connections is not clear from the materials. The finger that produces the most grip force is connected to other fingers and is difficult to move alone. These two facts may not be contradictory. It is possible that because they are connected, the four fingers can align and grip strongly together. However, this is my speculation and not a statement made by the source materials.
5. How to Test Linked Finger Movement and Grip on Your Own Hand
You can try this today. Place your hand flat on a table and lift each finger from the thumb to the little finger, one by one. Compare which finger is difficult to lift with people around you; it may differ from person to person. Next, grip a thick pen and a thin pen, and compare which one is easier to apply force to. This connects to the idea that the optimal thickness for force varies by finger. If you want to read the source materials, many of the references for this article are English papers. You can start by looking up "middle finger" in a Japanese dictionary [2]. For the papers, the abstract of Kong et al. (2009) can be read on the CDC website [4].