Since no commercial models of the human hand exist, and
there is a need to understand discomfort whilst undertaking
hand-intensive tasks such as packaging access, the authors (Yoxall
et al., 2007, 2008) developed a Finite Element model (using LSDyna)
of a human hand. The types of grips used to open packaging
such as jars and child-resistant closures, were photographed,
categorised and drawn schematically (as shown in
Fig. 6). Finite element models of all these grip types were then
analysed (see Fig. 7aef) with outputs for compressive and shear
stresses on the joints produced for all grip types and at different
grip spacings. The spherical grip model and typical output are
shown in Fig. 8a and b. This was used to assess the effects of choice
of grip type and hand dimensions on joint stresses during gripping
tasks. Whilst understanding pain and comfort is a difficult
task, assessment of stress in joints should allow for some
comparative understanding.
Briefly, it was found that certain grip types and larger hand sizes
result in lower joint stresses (see Fig. 9). This was supported by
parallel ethnography studies which demonstrated that consumers
will tend to use these grip types if the size of the packaging allows
(Rowson and Yoxall, 2011).
In these previous studies, the model was used to purely assess
one action; that of gripping, whether a lateral, box or spherical grip.
The results showed that hand size affected the stress levels in the
hand and that differing grip types produced lower stresses in the
hand and therefore were in effect more comfortable than others.
From the McConell survey (2004) whilst jars ranked highly in
terms of difficulty to access the highest ranked packaging product
was bleach bottles with a child-resistant closure (CRC). A typical
bottle and closure that uses a squeeze and turn combination are
shown in Fig. 10.
Little or no previous work has been done on the ergonomic
assessment of accessibility of squeeze and turn bottles (work has
been undertaken however by de La Fuente and Bix (2010) on
medical packaging, predominantly push and twist). Hence it was
decided to use the techniques described earlier to undertake an
assessment of this pack type. Therefore a series of studies are either
undertaken or being developed to understand the issue in more
detail, namely:
a device to measure with grip force and twist concurrently is
currently under development
ethnographic studies of users accessing packaging of this type
analytical modelling of closure performance
numerical modelling of the squeeze and turn action.
This paper outlines the work undertaken in a small ethnographic
study looking at users accessing this sort of packaging and be noted that the grip type recorded is that which was used to
perform the initial movement. In virtually every case it was
observed that once the child-resistant mechanism had been overcome,
the user released the closure and switched to a different
technique to remove it from the remainder of the thread.