Anthropometrics: Important Aspects of Users’ Bodies - Foundations for Designing User-Centered Systems - page 59

that involve the use of upper limbs. It focuses on the common risk factors that
contribute to the development of upper limb disorders in situations that involve
repetitive work. The tool is freely available from the Health and Safety Execu-
tive's website ( http://www.hse.gov.uk/msd/art-tool.htm ) .
3.2.2 Load Bearing
More and more people are using portable and mobile devices (phones, tablets, and
so on). With these devices the user often has to support the weight of the display
(and hence the interface) during interaction. Normally the user will have to carry
the device around with them too. The net result is that the user must be able to
support both the weight of the device when using it and the weight of the device
whilst carrying it around for extended periods of time.
At this point, all we can encourage you to do is to note what the device weighs,
to study existing standards and similar devices, and to observe and talk to users.
Simply asking users to complete a survey about the device is unlikely to generate
much useful data, because they may not be able to judge weights abstractly or to
consider their desire to carry an object without knowing and understanding its
functionality. It probably makes more sense to give the devices to the users and
observe how they explore and use them. If the device is for regular, routine use,
you should observe them over a realistic period of time that includes the variety of
environments in which the device will be used. The technology and functionality is
rapidly changing in this area, which may change what users think is an acceptable
weight to carry.
3.3 Interacting with Haptic Devices
Touch is usually regarded as the third most important sense after vision and
hearing (which are both covered in the next chapter). What most people refer to as
the sense of touch, however, is really a combination of two senses: the skin (or
cutaneous sense) and kinesthesis (Loomis and Lederman 1986 ). The cutaneous
sense is responsible for creating the feeling that the outer surface of the human
body has been stimulated, which it does using receptors in the skin together with
the associated nervous system. Kinesthesis (or the kinesthetic sense) generates an
awareness of static and dynamic body posture based on information coming from
the muscles, joints, and skin (afferent information), along with a copy of the signal
sent to the motor system (an efferent copy). The efference copy is used to help
distinguish between information coming from within the body and information
coming from outside the body, and helps to explain why you do not laugh when
you tickle yourself (there is a match between the afferent information and the
efference copy)!