What is Hand Force?
A frequent concept used in ergonomics is "hand force". What is it and how can it be measured? We have been working on trying to better define the concept since 2000 when I was involved with the ACGIH Physical Agents Scientific Committee to develop a hand TLV. It was soon apparent to me that the concept of hand force was not well defined and its measurement even less well defined! We have been addressing both the conceptual issues and the measurement issues.![[IMAGE]](images/Force Concept.gif )
![[IMAGE]](images/Grasp_Taxonomy.jpg)
Using a approach from robotics, wrench, we defined a way of accounting for many prehensile activities, not just gripping. Characterizing human hand capabilities or demand created by occupational tasks has been mainly accomplished by measuring the maximum force exerted on a handgrip dynamometer or similar transducer.
If the occupational activity is not a power grip or a pinch but involves combinations of actions, such as moments and forces, how well do these measures characterize the demand on the tissues of the hand and forearm?
An approach was developed which allows a complete description of the prehensile capabilities of the human hand. The force and moment wrench ( a 6 x 1 matrix, augmented by the internal grip force) is used to describe the external forces and moments exerted. The grip force is an interface measure necessary to transmit the wrench and is dependant on the particular grip geometry and object properties.
Hand Exertion TaxonomyWells, R and Greig, M. Characterising human hand prehensile capabilities by force and moment wrench, Ergonomics, 15;44(15):1392-402, 2001.
Twenty healthy individuals (10M, 10F) with industrial manual work history, exerted wrenches on a specially developed dynamometer capable of measuring the three force and moment components as well as the pinch or power grip forces. Thirteen randomized actions, about the orthogonal axes in three grip types (power, lateral and pulp pinch), as well as combined activities, were performed at maximal and sub-maximal levels in mid pronation and a naturally adopted wrist extension posture. 13 components are required to document the cap-ability of the hand to create forces and moments. Where comparable measures exist, these data are of similar magnitude
Greig, M. and Wells R. Measurement of prehensile grasp capabilities by a force and moment wrench: Methodological development and assessment of manual workers”, Ergonomics, 2004.
Common Exposure Metric for Low Back Exposure
Wells, R., Norman, R., Neumann, P*., Andrews, D*., Frank, J., Shannon, H. and Kerr, M*. Assessment of Physical Work Load in Epidemiologic Studies: Common Measurement Metrics for Exposure. Ergonomics, 1997, 40(1): 51-62.
Mechanical Exposure: Force as an Agent
The terminology and concepts of exposure have not been consistently applied in the area of ergonomics and the epidemiology of work-related musculoskeletal disorders (WMSDs). We have defined exposure concepts, unified a variety of physical exposures and included the concept of human activity.
![[IMAGE]](images/EXPOSURE.gif)
Using force as the agent, a mechanical exposure model is developed for use in epidemiological exposure assessment and hazard assessment. We highlight the importance of the tissue response in the exposure modeling process and the creation of exposure indices. Unfortunately, the response of tissue to forces of varying amplitudes and time variation patterns are largely unknown.
Although we argue that an exposure index may be most useful at the tissue level, considerations of resources may make exposure indices based on external exposure or internal exposure preferable choices.
Time Variation Pattern of Exposure
We conceptualize mechanical exposure as being defined by three dimensions; its level or amplitude, its time variation pattern and by its duration.The risk factors for musculoskeletal disorders at an individual level are well known from a series of international reviews. Briefly, these were documented as forceful exertions, awkward postures, static postures, repetition, vibration, and cold. Some of these factors include time aspects of work, and in the ergonomic literature, time, and especially the time variation pattern of exposure appears in concepts and descriptors such as repetitive work, monotonous work, static posture, postural fixity, movement frequency, short and long cycle time, duty cycle or work/rest ratio, fatigue and recovery.The scale of time variations under consideration ranges from very short term events such as EMG gaps (0.2 sec) to those found in seasonal or contingent or seasonal work (months). Unfortunately, the quantification and conceptualization of time in ergonomics has not advanced as rapidly as those associated with postural and force amplitudes.
To better describe exposure Figure 1 presents a fictitious time history of exposure extending over a few days. Figure 1a) describes a distribution of activities over a period of 36 hours. The variety of tasks and the number and types of task may be one simple way to describe the time variation pattern of the exposure. Figure 1b) shows an internal mechanical exposure measure which could be a low back load or a trapezius muscle activation.
![[IMAGE]](images/TIME_VARIATION.gif)
The Figure illustrates that the amplitude (level) of the exposure measure varies over time. This is the time-variation pattern. It can be described using a number of approaches such as frequency or cycle time, static load, pauses, or Exposure Variation Analysis (EVA), . The duration can be described at a number of levels, shift length, time on current job etc. Figure 1c) shows an expansion of the trace seen in Figuere 1b); here we can more clearly see that this person has infrequent pauses (P).
Time Series Analysis
Epidemiological studies of work related musculoskeletal disorders have difficulty quantifying complex, time-varying mechanical exposures. Available techniques typically provide information about the relative amount of time exposed to different levels and durations of stressor. Little or no information concerning the sequence or history of exposure delivery is maintained.
We examined an approach to the quantification and use of time varying exposures in epidemiological studies. Shift-long exposure data of 68 production operators (34 cases; 34 controls) from a large epidemiological study of low back pain reporting were used to create a shift-long internal mechanical exposure profile. These were processed through a first-order system with varying time-constants to create modeled responses and two exposure indices.
The utility of these mechanical exposure indices was evaluated using low back pain reporting as the health outcome from the same epidemiological study. The modeling using the first-order system appeared to provide additional exposure information, perhaps time-history sensitive exposure information that was not captured by the unprocessed exposure profiles. Risk estimates from the peak value processed with a time constant of 2s were almost doubled compared to the risk estimates of just the peaks, and the final value when processed with a time constant of 5,000s generated higher risk estimates over the cumulative/average exposures. The approach outlined in this study provides a framework to develop exposure assessment methods for epidemiological studies or hazard assessment.
Krajcarski, S. and Wells, R. (2008) The time history of low back load as a risk factor for reporting low back pain, Theoretical Issues in Ergonomic Science, 9:1, 45–71.