The Washington State SHARP Approach to Exposure Assessment - Fundamentals and Assessment Tools for Occupational Ergonomics - page 825

Several researchers have evaluated electrogoniometers and the introduction of significant cross-talk
in flexion extension measurements and deviation measurements from extreme forearm rotation

(Armstrong et al., 1993; Smutz et al., 1994; Buchholz and Wellman, 1997; Roberts, 1997).
Researchers have developed procedures that may reduce errors caused by cross talk (e.g., Smutz et al.,
1994; Buchholz and Wellman, 1997; Roberts, 1997). These results show promise for the use of an electro-
goniometer and electrotorsiometer in tandem to measure motions and allow for error correction.
Researchers appear to agree that an electrotorsiometer and electrogoniometer can be used in a telemetric
system to perform angle measurements with errors less than 5

/

8 (Armstrong et al., 1993; Smutz et al.,

1994; Buchholz and Wellman, 1997).
Force exerted by muscle groups is commonly measured by force transducers placed in the line of action
or by the use of surface electromyography (EMG) (Armstrong and Chaffin, 1979). Force transducers may
provide accurate information if specific conditions exist where their placement does not affect the work.
EMG has become relatively easy to perform in the field with the use of disposable surface electrodes
and portable measurement devices (Winkel and Gard, 1988; H¨gg et al., 1997). Typically, EMG data may
be measured and analyzed for either physical signs of fatigue or for comparison of static force levels
(NIOSH, 1992). Electrodes transmit motor unit action potentials from the underlying muscles. These
signals, when root-mean-square (RMS) transformed or integrated, have shown a linear or exponential
relationship (r 2

0.90) to developed static force (NIOSH, 1992).
Direct estimation, an alternative to EMG for calculation of hand force, can be classified as a direct
measurement method. Field practitioners and consultants commonly use this method to obtain job
force requirements. Direct estimation can be done simply by measuring the force requirements of a
tool or piece of equipment with a force gauge. When this is not possible, estimation of force can also
be accomplished through the reproduction of the exertion on a force gauge in the same orientation
and type of grip as performed by a worker. Kingdon and Wells (2000) conducted a laboratory study
on the accuracy of matching a manual gripping force using a hand dynamometer. Initial findings
have shown that force matching may be relatively accurate and consistent at lower force levels. A
study published by the Safety & Health Assessment & Research for Prevention (SHARP) program on
113 government workers showed that with the use of a hand dynamometer, the force matching
method can be quite accurate and consistent in estimations of power grip force and pinch grip force
(Bao and Silverstein, 2005). These results support the use of this method as an alternative to more
time-consuming and expensive instrumentation techniques for quantifying hand force levels in large
epidemiological studies.
In conclusion, no method is perfect and different methods may be used in different situations for
different purposes. Direct measurement using current techniques represent the most accurate and
reliable exposure assessment method. Work by Spielholz et al. (1999) comparing self-reports, video
observation, and direct measurement showed that video observation may have approximately 30%
more error than direct measurement in some risk factor measurements. Direct measurement would
be the preferred method given unlimited resources; however, modern video observation techniques
have the advantage of providing larger numbers of evaluated participants due to less time-consuming
data collection and analysis.
Due to the population size requirement of most epidemiological studies, direct measurement of all
participants would require resources well beyond what is available from a granting agency. Video-
based observational assessment in combination with direct measurement and estimation of forces is
the only method that would allow measurement of all participants with an acceptable level of accuracy
and reliability. Additionally, discrimination calculations by Spielholz et al. (1999) show that the estimated
tenfold increase in number of measured participants (100 to 1000) possible with video-based techniques
over direct measurement will give a more accurate exposure assessment at the group level despite the
increased measurement error. For these reasons, video-based observation and direct force estimation
techniques used previously by the SHARP (1999) program were chosen as the primary exposure asses-
sment method in a large prospective study of upper extremity musculoskeletal disorders conducted by
SHARP (referred to as the SHARP Study in the subsequent text).

.