A snapping finger produces the highest rotational accelerations observed in humans, according to a new study.

The Marvel Cinematic Universe was dramatically altered at the end of Avengers: Infinity War with an infamous snap of Thanos’ fingers. But could the intergalactic warlord have managed to snap his fingers at all while wearing that cumbersome metal Infinity Glove? That question inspired a new paper published in the Journal of the Royal Society Interface, exploring the biomechanics of the finger snap, particularly the role of friction.

(Spoilers for Infinity War below.)

“For the past few years, I’ve been fascinated with how we can snap our fingers,” said co-author Saad Bhamla, an engineer at Georgia Tech. “It’s really an extraordinary physics puzzle right at our fingertips that hasn’t been investigated closely.”

Finger snaps are essentially a latch-like mechanism (technically, Latch-mediated spring actuation, or LaMSA), in which energy is loaded in a mass-spring system, held in place with a latch. When the latch is rapidly released, all the stored potential energy is released over a short period of time.

Several small organisms are capable of producing ultrafast moves through a similar latching mechanism: frogs’ legs and chameleons’ tongues, for instance, as well as exploding plant seeds, multiple termite species, and the mandibles of trap-jaw ants. In the case of the mantis shrimp‘s powerful punch, small structures in the muscle tendons called sclerites serve as the latch. The shrimp’s muscles pull on a saddle-shaped structure in the arm, causing it to bend and store potential energy, which is released with the swinging of the club-like claw.

A finger snap modeled as a 1D latch spring system.
Enlarge / A finger snap modeled as a 1D latch spring system.

R. Acharya et al., 2021

Bhamla and his co-authors are interested in studying these surprisingly powerful and ultrafast motions. But the impetus for the current study is partially due to Bhamla’s viewing of Infinity War in 2018. In the film, uber-villain Thanos collects all six Infinity Stones, places them into the metal Infinity Glove, and proceeds to wipe out half of all life in the universe with a single snap of his fingers. But Bhamla couldn’t dispel the nagging thought that perhaps Thanos wouldn’t have been able to produce a snap at all while wearing the metal glove.

That’s because he suspected that friction plays a much more pivotal role in the case of a finger snap than it does in other LaMSA systems. Per the authors, the arm muscles serve as the motor, which loads potential energy in the tendons of the fingers. These in turn serve as springs. Pressing the middle finger and thumb together builds up force, which is released as the middle finger slides past the thumb, generating the snap. Bhamla et al. hypothesized that the natural friction of skin between the middle finger and the thumb serves a dual role, both helping in the “latching” of the middle finger and hindering the unlatching motion. The researchers designed a series of experiments to test that hypothesis.

To measure the kinematics, they used a high-speed camera to record an array of snaps from three different people. The researchers placed circles of reflective tape on each subject’s wrist joint, the base of the fingers, and the first knuckle, second knuckle, and tip of the middle finger. They recorded five snaps apiece for each different surface they tested: a lubricated nitrile glove, latex rubber thimbles on both fingers, and a metal thimble on both fingers under the nitrile glove. (The authors note that participants were allowed to rest for one minute between snaps to avoid muscle and joint fatigue.)

Saad Bhamla demonstrates a friction-based latching mechanism with a snap of his fingers.
Enlarge / Saad Bhamla demonstrates a friction-based latching mechanism with a snap of his fingers.

Georgia Tech

The researchers’ measurements showed that the rotational acceleration of a finger snap is nearly three times faster than a professional baseball pitch, although the rotational velocity was less than that of the arms of professional baseball players. “When I first saw the data, I jumped out of my chair,” said Bhamla. “The finger snap occurs in only seven milliseconds, more than twenty times faster than the blink of an eye, which takes more than 150 milliseconds.”

But that’s only for a finger snap involving human skin, which constitutes a kind of sweet spot or “Goldilocks zone”: just the right amount of friction and compression to produce the perfect snap. The lubricated glove was so slippery, there wasn’t enough friction and thus not as much energy stored. The rubber thimbles produced too much friction, such that too much energy was lost as heat. The metal thimbles performed worst of all: there wasn’t sufficient build-up of energy to produce a snap.

The latter finding has important implications for the plot of Infinity War. “Our results suggest that Thanos could not have snapped because of his metal-armored fingers,” said co-author Raghav Acharya, an undergraduate at Georgia Tech. “So, it’s probably the Hollywood special effects, rather than actual physics, at play. Sorry for the spoiler.” That’s bad news for Thanos, but these results will likely help improve prosthetic designs and the friction dynamics of soft robots in the future.

DOI: Journal of the Royal Society Interface, 2021. 10.1098/rsif.2021.0672 (About DOIs).

Listing image by Marvel Studios