Upper Limb Musculoskeletal Disorders and Force

Thomas J. Armstrong

The University of Michigan
Ann Arbor, MI 48109-1522

Copyright © 1998, Thomas J. Armstrong


1. Forceful Exertions

Definition

An exertion performed to overcome weight, resistance, or inertia of the body or a work object

2. Analysis Methods

2.1 Observations

2.2 Rankings

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.
Force factorExamples
Weight
  • tools
  • work objects
Resistance
  • joining parts
  • moving controls
  • moving
Reaction forces of tool or work object
  • animals
  • power tools as the start and stop
  • power tools that become caught or snagged
Friction
  • handle and work object surfaces
  • gloves
  • finger coverings
Mechanical assists
  • work station supports work object
  • jigs and fixtures support work object
  • hoists
  • articulating arms
Balance
  • hand position versus center of gravity
  • tools
  • work objects
Torque
  • handle shape, e.g., in-line, pistol, right angle
  • handle length
  • reaction bars
  • multiple spindle drivers
  • articulating arms
Rate of work
  • work standard
  • incentives
  • method
Posture
  • pinch versus grip
  • handle design
Gloves
  • hand posture
  • fit
  • material
Maintenance
  • blade
  • bits
  • molds
Quality Control
  • fit
  • re-work
Method
  • training
  • biofeedback


2.3 Worker Rating

Armstrong et al. (1989)

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.

Force (average & peak)

Low
Medium
High
0
0%MVC
2
4
6
8
10
100%MVC

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)
MaterialDryMoistCombined
(n=42)(n=42)(n=84)
Sand Paper (#320)------------0.61 (0.10)
Smooth vinyl------------0.53 (0.18)
Textured Vinyl------------0.50 (0.11)
Adhesive Tape0.41 (0.100.66 (0.14)------
Suede0.39 (0.06)0.66 (0.11)------
Aluminum------------0.38 (0.13)
Paper0.27 (0.09)0.42 (0.07)------
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

Examples:

Figure 4: Pinch force versus object weight for two different materials. (from: Frederick 1990)


2.5 Direct Force Measurement


a

b

c

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).


2.6 Electromyography


a

b
Figure 6: Electromyograms or EMGs must be calibrated by recording surface potentials over the contracting muscles, a, while exerting known forces, b.


a

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.


a

b
Figure 8: Sample EMG recordings and keyboard reaction forces associated with getting typing driven, a. Amplituded probility distribution (cumulative histogram) for alphanumeric typing.