An unsupported raised arm fatigues in minutes

What it is

Operate with the hand hanging at chest or eye height, no desk, armrest, or torso to dump the load, and within minutes the arm aches, shakes, and drops. The nickname is gorilla arm: the arm has become a weight that has to be carried. Mid-air gestures removed the support surface. Fatigue is not misuse; it is the default posture's cost. A fingertip pinch can be tiny; the invoice is still at the shoulder. A vertical touchscreen with a hanging hand runs the same invoice. A mouse on a desk almost does not.

Why it happens

The upper limb is an open chain. Shoulder abduction and elbow flexion hold the forearm at working height with continuous isometric contraction of middle deltoid and trapezius against gravity. Moment arms grow with height: above the waist costs more than at the belly; above the shoulder costs more than at the chest. With no terminal support, the whole weight travels the joints. Ache is local ischemia and metabolites; tremor is motor units failing to rotate. Tremor grows, recognition wobbles, people press harder to “hold it still,” contraction rises again. The cycle is counted in minutes, not hours. Local wrist load and shoulder-raise load must be separated: pinch taxes intrinsics, raising taxes the shoulder girdle, and they stack.

Studying it

Factors: shoulder–elbow angle, support, working height. Measures: time to abandonment, ache, EMG, joint jitter, recognition falling over time. Tasks must last minutes, not three successes. Metrics such as consumed endurance convert present contraction as a fraction of maximum voluntary contraction into an endurance time, which compares across postures. Control: the same task with elbow on a desk or hand on the belly. Vertical touch and mid-air gestures can sit in one posture protocol, showing the invoice is support, not the “gesture” label.

Where it stops holding

A seated armrest, a counter to plant the elbow on, a headset with the hand operating naturally at the waist, and the raise invoice drops sharply. Raising a hand while lying down is a different moment arm; sitting data do not transfer. Cold brings ache sooner. Shoulder injury or rotator-cuff problems smash endurance from minutes to tens of seconds. Short bursts (click and drop) barely sample this curve; burst tasks cannot claim “not tiring.” In public people drop the arm early because the posture looks bad, not because muscle has failed—that is a different invoice.

Applying it

• Default working height below the chest, close to the torso. Above the shoulder is occasional, not the vocabulary's home.
• Give the elbow or wrist a physical support if you can. Fatigue that vanishes in posture measurements will not be erased by “smaller gestures.”
• Use minutes of continuous task for tremor and drop counts. A three-success demo is not a fatigue pass.

Related

• Same group: C4.12.2 High-frequency actions must not demand overhead or a long hover · C4.12.3 Fatigue accumulates; short tests miss it · C4.12.4 Long tasks need a supported posture or another modality
• Adjacent: C4.04 Static poses versus dynamic gestures · C4.05 Three dimensions of gesture evaluation
• Search: gorilla arm · unsupported arm · consumed endurance