Interaction design needs an energy budget
Aliases: session energy ledger · pose power budget · input has a battery
What it is
Every mid-air pose has a wattage. Every session has a length of local endurance that will be spent. Interaction design needs an energy budget: schedule the session as an energy ledger, not as an infinite aerial workstation. Seconds spent in a horizontal raise, a locked elbow, a continuous gesture must have a source, and rest that can credit the ledger back.
A budget is not “tell users to pace themselves.” That hands the ledger to the individual. A budget is the design deciding first how much shoulder endurance this experience intends to spend, then which actions are worth that spend.
Why it happens
Local endurance can be treated as a pool that leaks and slowly refills. Holding an abducted pose leaks fast under models such as consumed endurance; hands hanging at the sides nearly stop the leak and refill begins. A session is a sequence of poses; total spend is the integral of the sequence, not the difficulty of one button. Line high-spend poses up continuously and the pool hits empty while the content is still going — empty shows up as changed movement, dropped functions, or the headset coming off, rarely as a silent continuation of the raise.
Desktop interaction almost never writes this ledger, because the desk takes gravity and the pool barely leaks. Spatial input gives gravity back to the person, and the ledger reappears. Not writing a budget is assuming an infinite pool. That assumption holds for the five minutes of a demo.
Studying it
Make a pose timetable for a full-length experience: shoulder–elbow angles of each stretch, one hand or two, whether a drop is allowed. Estimate relative spend per stretch with consumed endurance or integrated EMG, sum the session, and compare against mid-session quits and end-of-session effort ratings.
Independent variables: total duration of high-spend poses, whether they are bunched or broken by rest, the session length the product claims. Dependent variables: completion rate, number of voluntary drops, end effort rating, whether function use decays in the second half.
Split “content duration” from “high-spend duration.” A ninety-minute narrative with eight minutes of raise may fit the budget; a twelve-minute tool with eleven minutes of raise will blow it. Labs like to measure time-to-failure of a single pose — that is unit price. A budget wants the session integral. Without the integral, an accurate unit price still cannot schedule an experience.
Where it stops holding
Experiences in which people insert their own rest (talking, watching, putting controllers down) integrate below the design integral and the budget looks loose — do not write that looseness as “so they can hold forever.” Stretches the content locks into a continuous raise (a timed ritual, a weapon that must stay up) have no refill window; the budget must be drawn against the worst continuous block, not the average. Pool size differs across people; scheduling for those who last longest excludes everyone else from the second half. An energy budget governs endurance spend, not headset mass, heat, or visual fatigue; those are other pools and cannot be credited against the shoulder ledger.
Applying it
- Write a pose timetable for the experience. Mark any continuous raise longer than a minute or two, and insert a drop, a move to near-body, or a channel that does not raise the arms.
- Spend high-cost motion only on rare, high-value acts. Do not write frequent confirms into a raise.
- Claim session length against the integral, not against the content clock. Content still playing with an empty pool is a budget that was never made.
- How to check: run the claimed duration and count voluntary drops, switches to substitutes, and early doffs. A rise in all three in the second half means the integral was overspent. Move or split the high-spend stretches and run again; the three should fall — otherwise the change was copy, not budget.