Grip Strength

push the pen up to the line. hold it there.

a pen on a spring writes on rolling paper: push it into the band and hold, level by level, then hold it by feel with the pen hidden

Holding one exact level is a skill of its own, separate from strength: labs call it force steadiness. Your hand does it by watching, and with the feedback taken away a held finger force drifts, usually down, without the person noticing (Latash and colleagues, from 2015). The card reads what this phone can measure and says which: touch force where the screen reports a changing pressure, fingertip spread where it reports a contact patch, and otherwise how far you push your thumb. Force and spread are scaled to your own hardest press, not to newtons.

Concepts: strength, proprioception.

An interactive you play with one thumb, on Wundr.