A static pose has to be held, and holding is continuous muscle load
Aliases: isometric hold · pose endurance · static hold cost · maintained hand shape
What it is
Once a static pose counts as “currently in effect,” the person has to hold the shape. Holding is isometric: joints barely move, muscle keeps firing against gravity and tissue elasticity. The cost is not how large the pose is. It is how long it is held, and at which shoulder and elbow angles. Flashing an “OK” at a camera for half a second, and holding the same shape as a mode switch through an entire song, are not the same load. Dynamic waves cost mainly in the stroke. Poses cost mainly as a time integral.
Why it happens
With the shoulder abducted and the elbow hanging, deltoid and trapezius produce continuous isometric force. Fingers must stay extended or flexed, so forearm flexors and extensors co-contract. No surface takes the weight. Over time tremor grows, the shape starts to collapse, and joint angles slide out of the template. To stop the collapse, people press harder, and load rises again. If the recognizer requires many consecutive matching frames to keep the state, hold time is written into the protocol. Gravity direction matters: palm-up supporting is dearer than palm-forward reaching; above the shoulder is dearer than at the waist. A wave-and-drop dynamic gesture allows rest between tokens. A held pose does not.
Studying it
Factors: hold duration, shoulder–elbow angle, shape complexity (how many fingers independently extended). Measures: reported ache, percent of maximum voluntary contraction, time until the shape starts to collapse, and mismatch after collapse. Tasks need to last minutes; a single showcase misses the integral. Control: the same shape as a trigger flash (show and drop), to separate “forming the shape” from “maintaining the shape.” Loads are not comparable across shapes. An open palm and a pinch stress the forearm differently; do not average them into one “pose load.”
Where it stops holding
An elbow on a desk, chair arm, or the torso dumps most of the isometric load; raised-arm laboratory numbers do not transfer to a supported workstation. A very short pose gate (flash a shape on entry) is barely a hold problem. Using a static pose as a live mode indicator (“while this is up, we are recording”) turns the load into the function. Some shapes demand joint range that people with limited thumb opposition or finger spasm cannot form; that is not a hold failure, it is an impossible shape. Dynamic-gesture fatigue comes mainly from repeated arm raises—a different mechanism. Do not score a wave vocabulary with pose-hold ache numbers.
Applying it
- Do not use a held pose as a long-running mode switch. Use an enter edge and a leave edge so muscle can rest between them.
- If a hold is required (aiming, preview), give the elbow or wrist a support, and a visible “you may drop” signal so people do not keep the arm up out of uncertainty.
- Accept on minutes of real task. Log time to first collapse and number of arm drops, not whether someone can form the shape in a still for a poster.