Fatigue changes movement amplitude and precision
Aliases: tired reaches come up short · fatigue costs accuracy · endurance rewrites kinematics
What it is
A button that was in reach at the start often sits a fraction short twenty minutes later; the click also starts to land beside it. Fatigue changes movement amplitude and precision: the same command is no longer the same movement on the way out. People are not loafing. They are operating with kinematics that endurance has already rewritten.
This is not “the air is shakier than the desk.” The precision gap between air and desk is a different claim. This one is time: the same mid-air action, once tired, becomes shorter, slower, and less accurate.
Why it happens
When local endurance drops, the centre changes strategy to keep what can still be finished. Amplitude is cut first: less shoulder abduction, an earlier endpoint, a target that sat on the edge of comfort becoming unreachable. Speed next: to spend less maintenance power, the movement is broken into shorter segments with drop-and-raise in between. Precision follows, because a fatigued muscle fires more synchronously and more shakily, and the endpoint cloud grows; if people still aim at the opening hit zone, misses rise.
These changes enter the score sheet before the subjective “I am tired.” Performance breaks first; the report arrives later. A session that only asks “are you tired” will miss the already-happening shortfalls and near-misses. People also rewrite the task themselves: they use far functions less, batch confirms into one raise, substitute a head point for a hand point. On the surface the functions remain; the movement has been swapped.
Studying it
Repeat the same pointing or grabbing in the early, middle, and late parts of a session, measuring amplitude and error. Do not measure once at the start.
Independent variables: elapsed time, whether a drop-and-rest is inserted, target distance from the shoulder. Dependent variables: decay of actual arrival distance from the opening baseline, endpoint scatter, movement time, rate at which far targets are abandoned.
Log “subjective fatigue” and “kinematic change” separately. If scatter and amplitude have already moved while the Borg score is still low, the design cannot wait for the user to call tired before switching layout. A rest condition tests recovery: after a few tens of seconds of hands down, does amplitude return to baseline. If it does not, the session’s budget is already overspent, not a local ache.
Where it stops holding
When the task itself is getting harder (targets shrinking, the scene getting busier), a precision drop cannot all be billed to fatigue; task difficulty must be matched across time. Operations that never raise the arm — wrist turns, pinches only — barely move the amplitude term; the precision term may still drop from whole-body tiredness and attention, a mechanism that is not only shoulder endurance. Training shifts the whole curve and does not cancel “worse later than earlier.” Lab protocols that insert a maximum voluntary contraction every five minutes measure fatigue clearly and also drive people into fatigue sooner than real use — the measurement spends endurance. This leaf does not discuss parking the arm on a desk to buy precision back at once; that is support versus precision, not fatigue rewriting movement over time.
Applying it
- Do not assume, in the second half, that an arrival distance measured at the start still holds. Far actions that need a locked elbow should move in, or be allowed on another channel.
- Widen hit zones in the second half, or offer snap, to catch the larger endpoint cloud.
- Give a legal short rest: hands down for tens of seconds, state preserved, drop not treated as exit.
- How to check: hit the same target ten times at minute 2 and ten times at minute 25, and overlay endpoints. A later cloud that is clearly shorter and more scattered means fatigue has rewritten the movement. A product that ran one pointing test at the start did not measure the hand people will actually use.