Sustained moderate stimulation can also cause harm

What it is

A stimulus well below the acute pain threshold can still cause discomfort, or real harm, if it is sustained long enough — safety isn't just about whether peak intensity crosses a line, it's about whether intensity combined with duration crosses one, because moderate force or moderate heat held for a long time can produce effects similar to a brief intense stimulus, just via a different curve.

Why it happens

This is a "duration axis" independent of the instantaneous peak ceiling: sustained moderate pressure compresses local blood vessels and impairs capillary perfusion, and tissue starved of adequate blood flow will sustain real damage given enough time, regardless of whether the pressure itself ever reached the acute pain threshold — that's a separate question from pain onset. Similarly, a moderate temperature that would never cause a burn on brief contact can still produce a low-grade burn if maintained long enough, because tissue damage depends on cumulative thermal dose, not just peak temperature — the lower the temperature, the longer it can safely be sustained, but there is still a limit. What makes this worse is that exactly this kind of "moderate, sustained" stimulation is the kind adaptation mechanisms dull fastest, so a user may never become aware the stimulus is still present before enough damage has accumulated to cause noticeable discomfort.

Studying it

Time-temperature burn threshold curves come mainly from burn medicine and ergonomics research and clinical data on contact burns (the contact duration needed to cause a burn rises non-linearly as temperature decreases). Ischemic injury from sustained pressure draws more from prosthetic-interface and pressure-injury research, measuring how long a given pressure can be sustained before objective markers of tissue stress appear. Some studies specifically test whether the point at which users subjectively report discomfort comes before or after tissue already shows objective stress markers, to judge whether subjective report alone can serve as a safety criterion.

Where it stops holding

The cumulative-damage relationship is non-linear and highly dependent on the specific site and tissue type — skin directly over bone behaves differently from skin over well-padded muscle, and areas with poorer circulation (slower blood-flow recovery after compression) are more vulnerable to sustained moderate pressure. Existing evidence indicates that tissue stress typically begins before users subjectively report discomfort, meaning that relying purely on "the user says it's uncomfortable, so stop" is not a reliable safety net for long-duration contact — precisely because this kind of moderate stimulation is the kind most easily masked by adaptation.

Applying it

• For long-duration-contact tactile or thermal output (the contact surface of a wearable, a handle held for extended periods), don't design only against "instantaneous intensity stays under threshold" — set a separate time limit for "how long before it must auto-downgrade or pause," enforced by software rather than left to the user to switch off.
• That time limit should be set against published time-intensity injury curves, not decided by "the tester said it was still fine, so it's fine."
• How to check: run a long-duration exposure test in the lab using the same contact area, material, and pressure or temperature as real wear, objectively measuring changes in local blood flow or skin response, rather than relying on participants' verbal comfort reports to judge safety.

Related

• Same group: A4.07.1 Force feedback and thermal feedback both have a safety ceiling · A4.07.3 Safety ceilings must be set for the most sensitive population
• Nearby: A4.08 Tactile adaptation and dulling · A8.20 Static load versus dynamic load
• Search terms: cumulative thermal dose · pressure ischemia · contact burn threshold · sustained load injury