Holding a chosen finger combination costs extra, locally, in the hand

What it is

Showing “three” or “four” is not opening the whole hand. Some fingers extend, some flex, and that split has to be held. Isolated fingers cost more than a fist or a fully open palm: interossei and lumbricals fight tendon coupling, ring and little fingers especially. The bill is inside the hand. Supporting the shoulder and elbow does not cancel it. Using a count as a mode switch (keep showing three for channel three) asks that split to last the whole task. The general hold cost of a static pose sits at the shoulder. This is an extra invoice from the intrinsics.

Why it happens

Fingers are not five independent servos. Middle, ring, and little share extensors and some flexors; extend the middle and flex the ring and you are pulling on a common tendon. Young adults in a lab can hold for seconds. Fatigue or arthritis drops the ring finger first, so four becomes three or three becomes two. To stop the collapse, people cock the wrist and abduct the little finger, sending the local load into wrist and forearm. If the recognizer wants many frames of canonical gap width, that isometric stretch is written into the protocol. One and five are relatively cheap, close to the rest shapes fist and open palm. Middle values are expensive, and products love them as options.

Studying it

Have people hold specified combinations (2, 3, 4, and a non-adjacent “index plus little”) against a clock. Log time to collapse, local ache, and compensatory wrist angle. Controls: fist and open palm. Duration at least tens of seconds. Stratify by age and hand conditions. EMG on interossei is possible; subjective time-to-collapse is more directly useful for design. Separate “forming the shape” from “maintaining the shape”: flash a three and drop it versus hold a three through a song are not the same load.

Where it stops holding

A flash as a one-shot choice makes the split-finger bill negligible. A persistent indicator does not. Musicians and signers have higher isolation endurance; their data cannot excuse a public vocabulary. Some combinations are insults or gang signs in a given culture; people refuse before load appears. A prosthesis that cannot independently flex a digit is not a load problem; that combination is unexecutable. Shoulder raise fatigue is a different ledger. Supporting the shoulder does not mean finger isolation is solved.

Applying it

• Do not hold 3, 4, or non-adjacent combinations as high-frequency or long-running state. Use an edge (flash three to jump to item three, then drop).
• Prefer 1, 2, 5, fist, and open palm in the vocabulary. Middle values only as short choices.
• In acceptance, have people hold a middle-value combination for at least thirty seconds. Log collapse and compensation. If they cannot last, do not make that combination a standing mode.

Related

• Same group: C4.09.1 Extended-finger count can express a choice or a quantity · C4.09.2 Palm size and shape change which canonical forms are recognized · C4.09.4 Recognition and bodily feel must be logged by hand morphology
• Adjacent: C4.04 Static poses versus dynamic gestures · C4.05 Three dimensions of gesture evaluation
• Search: isolated finger extension · intrinsic hand muscles · finger independence