While We’re on the Subject of Human Factors


The reality of designing for functional reach in a complex interface (Martin Marietta Dyna Soar mockup)

People are complex, and not just their behaviour…

A recurring human factors design task is laying out the controls at a seated workstation so that they are reachable by the operator. Should be simple right? Wrong.

We’ve all seen the basic reach curves in human factors text books, or those generated by your friendly CAD packages ‘ergo man’. Just a simple set of circular curves around the left and right shoulder, like the example below, right?

Wrong again, unfortunately these sort of simplified representations significantly dumb down the actual complexity of the functional reach envelope whose envelope varies as a factor of:

  • the height of the reach plane,
  • whether an active or passive grip is being used (derived from control and task design),
  • the type of grasp used (palm, thumb or finger tip requirements, again control design derived),
  • whether safety restraints or protective clothing (e.g. a pressure suit) are worn,
  • the seat backs incline angle, or
  • the operators posture (e.g. whether you are leaning forward or back in the seat).

Even more constraints can be introduced if the operator is required to exert significant force on a control or if there is an interaction between the visibility of a control and it’s use, for example an illuminated pushbutton control/indicator.

A somewhat naieve (and incomplete) representation of seated human functional reach curves. For example at hip level you cannot reach through your thigh!

Then there are the skeleto-muscular constraints that mean the reach envelope is a complex function of different joint mobilities whose flexibility and range of motion are in turn influenced by others. For example if the elbow is flexed at 90 degrees and the humerus is rotated at 90 degrees about its own axis the approximate outward rotation is limited to 25 degrees.

What you end up with is a curve that looks like the figure below. To generate this plot I used the data derived from a USAF study (quoted in Woodson and Tillman (1992)) combined it with reach data from NASA’s human factors 3000 series standard and developed an excel spreadsheet functional reach tool to plot the resultant reach envelopes.

A somewhat more accurate set of functional reach curves

I developed this tool as a ‘quick calculator’ aid for conceptual design studies after having a long discussion with a customer over why they couldn’t just simplistically assign controls and displays to ‘functional zones’. Feel free to download it!


1. Woodson, E.W.(Ed.), Tillman, B., & Tillman, P., Human Factors design Handbook, 2nd ed., 1992.

2. NASA, NASA-STD-3000, Volume 1 NASA Man System Integration Standard.

One response to While We’re on the Subject of Human Factors


    Nice reminder. NASA has a posting on the “man rating” of the Atlas V heavy and on the top is Orbital Space Plane, which was withdrawn from the proposal phase in the first round of acquiring what is now Orion (Crew Exploration Vehicle). At the same time NASA decided to build their own launch vehical – Ares – which of course was a complete bust, splashing in the Atlantic on a one-way trip for testing. Now ULA is trying to man rate Atlas 5 years late.

    We’d likely be back on schedule for Orion sitting on top of a ULA vehicle had not NASA been focused on employment in Houston.