Upper Limb Musculoskeletal Disorders and Force
Thomas J. Armstrong
The University of Michigan
Ann Arbor, MI 48109-1522
Thomas J. Armstrong
1. Forceful Exertions
An exertion performed to overcome weight, resistance, or inertia of the body or a work object
2. Analysis Methods
- Direct measurements
- Electromyography, EMG
- It is possible to identify forceful exertions by observing the job or examining the work methods analysis.
- Each step of the method is inspected to identify exertions with the upper limb.
- Work elements in which forceful exertions commonly occur:
- Resist reaction forces
- Example: Notebook Packing (see Figure 1). It can be seen that a force must be exerted with the shoulder to
"reach," "grasp," and "move" the bottle. A similar analysis can be used to identify the forces and moments on the
elbow, wrist and hand. Factors affecting the force of exertion are listed in Table 1.
Often it is possible to rank exertion force based on an analysis of task attributes. For example, it can also be seen that greater force is required to "move" the bottle than to "reach" and "grasp" the bottle. Similarly it can be anticipated that transfer of a 12oz bottle requires less force than a 1 gal jug. Factors affecting force of exertion are listed in Table 6.6.
Table 1: Factors relating to force of exertion for manual tasks.
- joining parts
- moving controls
|Reaction forces of tool or work object|
- power tools as the start and stop
- power tools that become caught or snagged
- handle and work object surfaces
- finger coverings
- work station supports work object
- jigs and fixtures support work object
- articulating arms
- hand position versus center of gravity
- work objects
- handle shape, e.g., in-line, pistol, right angle
- handle length
- reaction bars
- multiple spindle drivers
- articulating arms
|Rate of work|
- work standard
- pinch versus grip
- handle design
- hand posture
2.3 Worker Rating
Armstrong et al. (1989)
- used force ratings to determine acceptable weights for tools used in an automobile trim department.
- workers were asked to rate their tools on a visual analog scale where
- 0 = Too Light
- 5 = Just Right
- 10 = Too Heavy
- The results are shown in Figure 1.
- Nearly all of the tools with masses less than 1.5 Kg were rated as "Just Right," while nearly all tools with masses greater than 2.25 Kg were rated as "Too Heavy."
Figure 1: Ratings of tool weight by 23 subjects using 32 hand tools in automobile trim plant. (from: Armstrong, et al., 1989).
Similar techniques were used by Drury (1980) and Pheasant and O'Neil (1975) to evaluate handle sizes.
2.3 Observer Rating
Force is an index of the effort exterted get, hold, or use a work object or to support the weight of the body.
Both averate and peak forces should be assessed. 15% is considered the maximum force that can be exerted
for a prolonged period without exhaustion. Higher forces may be exerted for shorter
periods of time. Both average and peak forces should be rated. Force
can be assess from observations of the worker
and from consideration of task factors.
- Smooth controlled exertions indicate low force
- Jerky motions, bulging muscles indicate high forces
- weight, resistance and reaction forces
- size, shape and surface friction of the grip object
- glove fit, stiffness, bulk, and friction
Force (average & peak)
Figure 2: Visual analog scale for rating average and peak force as fraction of maximum possible.
(%MVC= Percentage of Maximum Voluntary Contraction)
2.4 Force Calculations
Enough force must be exerted to keep objects from slipping from the fingers. Pinch force, Fp, required to overcome gravity is related to weight and the coefficient of friction, m, as shown in Figure 6.9 (Armstrong 1985; Buchholz, Frederick and Armstrong, 1988; Bobjer, Johansson and Piguet 1993):
Fp > Weight / (2 x m)
--- see Table 6.7 for coefficient of friction, m.
Some people exert considerably more than the minimum force to keep objects from slipping from their hand.
Figure 3: The pinch force, Fp, required to support an object, W, is related to both the weight and friction, µ, of the object.
Table 2: Coefficients of friction, µ, (average (standard deviation)) for human palmar skin against various materials, n=7 subjects (from Buchholz, Frederick and Armstrong, 1987)
Also see: Bobjer O, Johansson S, Pigue S: Friction between hand and handle.
Effects of oil and lard on textured and non-textured surfaces; perception of discomfort
Applied Ergonomics 24:190-202, 1993
|Sand Paper (#320)||------||------||0.61 (0.10)|
|Smooth vinyl||------||------||0.53 (0.18)|
|Textured Vinyl||------||------||0.50 (0.11)|
|Adhesive Tape||0.41 (0.10||0.66 (0.14)||------|
|Suede||0.39 (0.06)||0.66 (0.11)||------|
|Paper||0.27 (0.09)||0.42 (0.07)||------|
- Calculate the pinch force required to hold a 5 pound file as shown above
(see Table 2 for coefficients of friction, µ)
- Moist skin: Fp > 5 / (2 x .42) = 6.0 pounds
- Dry skin: Fp > 5 / (2 x .27) = 9.2 pounds
- The required pinch force increases as the skin dries!
- Johansson and Westling (1984) and Frederick (1990) demonstrated that the force exerted to
lift an object with the fingers is related to weight and friction, but there is significant inter-subject variability (Figure 3).
Figure 4: Pinch force versus object weight for two different materials. (from: Frederick 1990)
2.5 Direct Force Measurement
- Exertion forces sometimes can be measured by placing the work on a force gage or attaching force sensitive
materials to the work object.
- Instrumentation for measuring force may require expensive equipment and considerable expertise.
- Armstrong et al. (1991) utilized force transducers under keyboards to estimate finger forces exerted
by keyboard operators as shown in Figure 4a.
- Figure 4b. shows the average force exerted on the "e" key by ten subjects while typing. It can be seen
that the subjects exert more force on Kb3 than Kb2.
Figure 5: Finger forces on keys are estimated from transducers that measure reaction forces under the keyboard, a.
Key force-displacement curves for three keyboards, b.
Average recordings for multiple "e" keystrokes by ten subjects are shown in c. (from: Armstronget al 1994).
- Electromyography is the measurement of electrical potentials produced by contracting muscles.
- Electromyograms or EMGs are generally expressed as a root mean square, RMS.
- The EMG can be calibrated by recording the surface EMGs over the forearm finger flexor muscles
for corresponding grip forces (see Figure 5a and b). The force exerted during a work task can then
be estimated by recording the EMG as the subject works. The system must be calibrated each time
for each subject and each hand posture.
- Use of Electromyography to estimate hand forces was demonstrated by Armstrong et al. (1982) --- see Figure 5a and b.
- Electromyography was well suited for this type of job because of technical problems involved in
calculating forces or instrumentation of knife handles for force measurements. EMGs are also well
suited for comparing the effort required to use two or more tools (see Figure 6 for a carton packing job)
- Job force requirements can be characterized as a frequency distribution of the normalized EMG recording (% maximum
- Jonsson B. Quantitative electromyographic evaluation of muscular load during work.
Scand. J. Rehab. Med. 1978; Supplement 6: 69-74.
- Jonsson B. The static load component in muscle work. Europ. J. Appl. Phys. 1988; 57: 305-310.
Figure 6: Electromyograms or EMGs must be calibrated by recording surface potentials over the
contracting muscles, a, while exerting known forces, b.
Figure 7: Sample EMG recordings associated with getting and erecting cases, packing cases, and closing and asiding cases, a.
Amplitude probability distribution (cumulative histogram) for EMG data from case packing job, b.
Figure 8: Sample EMG recordings and keyboard reaction forces associated with getting typing driven, a.
Amplituded probility distribution (cumulative histogram) for alphanumeric typing.