To dig deeper, it seems that the muscle-memory user interface (MMUI) has a few common attributes appearing in most of its manifestations.

1. Task Path – the order in which things must be done to get the desired result. Gun: Ready, Aim, Fire. Game: Find Key, Open Door, Kill Dragon. Car: Check Mirrors, Signal, Change Lanes.

2. Step State – the extant processes and conditions in effect at any point within a task path. Gun: Aiming in Motion. Game: Dodging Fireballs. Car: Lane Merge.

3. Essential Modality – the necessary conditions that must be in effect at any given step state. Gun: Have Ammunition. Game: Know Secret Code. Car: Headlights On.

Thinking more specifically about muscle-memory user interfaces for computer applications, the venerable status quo is the “Desktop/Document” UI. It was created by Xerox PARC researchers more than twenty years ago when storage, memory, and throughput were measured in KBs, not MBs, GBs, and even TBs as they are now.

Despite some valiant attempts to rethink the DDUI along the way, e.g. the IBM/Apple Pink and Taligent projects, it became and still is the norm for most Windows and Mac applications.

Menus, icons, and dialog boxes have their rightful place across computing, but that doesn’t explain why the general UI form and function in Microsoft Word and Adobe Photoshop are substantially the same. The task path for writing a document has little similarity to the one for editing a photograph, and yet both programs have a “File” menu, from which you can “Exit” the program.

In the Desktop/Document model, the function of the application largely follows the form of the UI. Those who find this OK will likely point out that it reduces the learning curve for any individual application when all applications follow the same design guidelines.

To me this sounds like the Golden Hammer (http://en.wikipedia.org/wiki/Golden_hammer).

Be that as it may, it seems that there are many task paths that go un-automated or are poorly automated because they simply cannot be made to conform to the DDUI model.

They tend to be *practical* (as opposed to cerebral or computational) task paths, e.g. music education, auto repair, land survey, or (shameless plug) photo editing.

And then, finally, as the computer itself morphs into a smart phone, where the screen small and typing and pointing functionality are limited, UI interaction is in itself a practical matter and the DDUI model is further taxed, and in a more essential way.

I hope you and readers will continue to explore this topic in the future, as I feel that there is a growth industry to be had in rethinking the UI.