## Task Strength Demands and Capacities

T. Armstrong
U of Michigan
http://www-personal.umich.edu/~tja/
5-7-2021

### 1. Task hand strength demands

• Strength demands refer to the forces or moments that must be exerted to position or move the body and to gain and maintain control over a worker object and to all of the factors affecting those forces and moments.
• Task demand should not be confused with worker or user strength capacities.
• Factors affecting task force deands and user capacities are described in sections:
2.4 Biomechanical analysis of task hand force demands
3.1 Factors affecting hand strength capacities

• #### Describing hand strength Demands

• Absolute force (pounds, Kgf, Newtons) or moment (inch-pounds, N-m) vectors
• Percent capable- Fraction of percentage of selected population strength
• Relative force: Percent of Maximum voluntary contraction, %mvc, or Percent of Maximum exertion, %ME
• %MVC or %ME = [(strength required) / (strength)] × 100%

• #### Estimate hand strength demand

• Determine 1) external force vector, 2) where it is exerted on the body, 3) posture, 4) action
1. Example: Tool toolbox (see Figure 1)
2. Fdemand = weight of tool box = 50 pounds
3. Location: palmar side of fingers
4. Posture: Hook grip (power grip)
5. Action (Figure 1)
• Hand: forces hand opening
• Wrist, elbow, shoulder: traction forces stretch joints, no torque
• Back: lateral rotation & compression

#### Percent capable (the lower percentiles)

• The percent of population (male, female or combined) with sufficient strength to maintain control over the work object.
• i.e., % of population where %MVC for give step is < 100%
• z = (FDemand-Faverage)/Standard Deviation
• Look up percentile (cumulative probability, α) for the calcuated z value (Table 1)
or use online percentile calculator

Example: Estimate the percent of workers with sufficient strength to lift and carry the tool box shown in Figure 1.

• Start with female strength as on average it is less than male strength.
• Use right-hand strength in the 30-39 year range (Strength and most biological functions tend to max out in this age range). Strength estimates can then be adjusted up or down based on other considerations.
• See links to published studies below: 3.3 Published hand strength data
• 30-39 year old right-hand female strength: 78.7±19.2 pounds
• Calculate standard normal:
• Percentile calculations
• Normal probability calculations
• z = (FDemand-Faverage)/Standard Deviation
• z= (50-76.4)/15.7 = -1.682
• from Figure 2 it can be seen that over 95% of Females should have sufficient hand strength to "briefly: hold the tool box.
• It can be seen that most females should be able to hold the tool box, but those on the lower end of the strength range (Figure 2b) will be exerting nearly 100% of their maximum strength. They will
• only to be able to hold it for a short time
• have only a small safety margin to resist the inertia during lifting or other forc perturbations.

Example: Determine the percent maximum voluntary contraction, %MVC, for someone with average female grip strength to hold the toolbox as shown in Figure 2a:

• %MVC = 50 pounds / 78.7 pounds x 100% = 64%.
• 64%MVC can be maintained for only about 60s with significant discomfort.
• Determine estimate how long someone with average female strength could maintain this exertions. see: Localized Fatigue

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#### 2.1.2 Visual analogue scale (anchor points ⚓)

• 10cm horizontal line with verbal anchor points at each end describing extremes (additional anchor points may be used)
• User/worker indicates perceived force by drawing a vertical line through the scale
• The score is determined by measuring the distance between lowest extreme and vertical line in centimeters.
• See Figure 4.
• Precision can be improved if subjects exert maximum force in same position.

Marshall, M., T. Armstrong and M. Ebersole (2004). "Verbal Estimation of Peak Exertion Intensity." Human Factors and Ergonomics 46(4): 697-710

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#### 2.1.4 Force matching

• Subject exerts an equivelant amount of fore on a calibrated force gauge for the task of interest.
• Device used to demonstrate force must be matched to tasks (see Figure 6)
 (a) (b)

• Bao S, Silverstein B. Estimation of hand force in ergonomic job evaluations. Ergonomics. 2005 Feb 22;48(3):288-301.

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• #### 2.2 Observer ratings

• Visual analogue scales -- similar to that used for worker self ratings
Also, observers may simply rate on scale of 0 to 10 (see Ebersole et al. 2006)
• Experience/training necessary
• Benchmarks or anchor points ⚓ can improve consistency
• Individual v. group ratings
• see: Ebersole ML, Armstrong TJ. Analysis of an observational rating scale for repetition, posture, and force in selected manufacturing settings. Human factors. 2006 Sep;48(3):487-98.

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#### 2.3 Instrumentation

• Direct force measurements -- work object is instrumented to measure contact forces between the body and the work object (see Figure 7a)
• Indirect force measurements -- Electrical activity of muscles can be measured and calibrated to estimate exertion forces(see Figures 7 b, c and d)
• EMG must be calibrated for same posture in which task is performed
• Many sources of variability -- movement, individual anatomy & body composition
• Statistical variability can be quite high.

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#### 2.4 Biomechanical analysis of task hand force demands

• Basic laws of physics tell us that all of the forces and moments acting on a work object must add up to zero
• You must exert an upward force equal to the downward force of gravity to prevent a tool box from falling out of your hand (Figure 8a)
• You must exert a force greater than that exerted by the spring to open the clamps in Figures 8b)
• The forces required to support a work object may be amplified by the linkages in the handle of a tool (Figure 8c)
• The forces required to support a work object may relay on friction (see figure 8d)
• Work objects may be balanced on the fingertips with the weight of the object on one side and the fore of the thumb on the other side (Figure 8e)

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#### 2.5 Extrapolate task hand force demands from previous studies

• Studies of forces required to perform a given task in one setting, sometimes may be used to estimate force required for a similar task in another setting.
• Studies of keyboard work show Ffingers = 1-3 × key activation forces
• Studies of auto assembly (see Figure 9)

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### 3. Hand Strength

• One of the primary reasons for measuring strength is that it can be compared with the strength of the person or persons who do or might perform the task.
• It is important to consider all of the factors that can affect strength demands and strength capacities.

### 3.1 Factors affecting hand strength capacities

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### 3.2 Hand strength measurement

• Strength is very sensitive to body posture if which it is measured (see Figure 10 below)
• It is important that measurements of strength be based on postures and conditions that match as closely as possible those in which the task is performed.

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### 3.3 Published hand strength data

• Bohannon, R.W., Peolsson, A., Massy-Westropp, N., Desrosiers, J. and Bear-Lehman, J., (2006). Reference values for adult grip strength measured with a Jamar dynamometer: a descriptive meta-analysis. Physiotherapy, 92(1), pp.11-15.
• Bohannon, R.W., Peolsson, A., Massy-Westropp, N., Desrosiers, J. and Bear-Lehman, J., 2006. Reference values for adult grip strength measured with a Jamar dynamometer: a descriptive meta-analysis. Physiotherapy, 92(1), pp.11-15. https://www.sciencedirect.com/science/article/pii/S0031940605000878
• Barter JT, Frey EI, Truett B (1956). Anthropometry and Biomechanics of the Hand. unpub. ms., Wright-Patterson Air Force Development Center Aero. Med. Lab., Ohio. (Cited in: Damon, A., H. w. Stoudt and R. A. McFarland (1966). The Human Body in Equipment Design. Cambridge, MA, Harvard University Press, p. 221.
Finger press
• Dempsey PG, Ayoub MM. The influence of gender, grasp type, pinch width and wrist position on sustained pinch strength. International Journal of Industrial Ergonomics. 1996 Mar 1;17(3):259-73.
Pinch Data
• Dodds RM, Syddall HE, Cooper R, et al. (2016). Global variation in grip strength: a systematic review and meta-analysis of normative data. Age Ageing. 45:209–16. https://www.sciencedirect.com/science/article/pii/S0363502308000129
• Hertzberg HTE (1955). Some Contributions of Applied Physical Anthropologyto Human Engineering. Annals of the New York Academy of Sciences. 63:616- 629.
grip Strength with and without gloves
• Kroemer KHE, Gienapp EM. Hand-held device to measure finger (thumb) strength. Journal of Applied Physiology 1970;29(4):526-527
Grip and thumb strength
• Mathiowetz, V, Kashman, N, Volland, G, Weber, K, Dowe, M and Roger, S. 1985. Grip and pinch strength: Normative data for adults. Archives of Physical Medicine and Rehabilitation, 66: pp. 69 – 76
• Nilsen, T., Hermann, M., Eriksen, C.S., Dagfinrud, H., Mowinckel, P. and Kjeken, I., 2012. Grip force and pinch grip in an adult population: reference values and factors associated with grip force. Scandinavian journal of occupational therapy, 19(3), pp.288-296 https://www.tandfonline.com/doi/pdf/10.3109/11038128.2011.553687?needAccess=true

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### 3.4 Biomechanical models

• biomechanical models can be used to compute forces and moments about the major joints for a given person performing a given task in a given way. (Figure 11)
• To determine strength, it is necessary to first determine the spacial relationship between the hand and the work object.
• Strength can then be estimated based on underlying muscle strength or maximum joint torques

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American Conference of Governmental Industrial Hygienists (ACGIH®): Lifting TLV®. 2019 Threshold Limit Values and Biological Exposure Indices, pp. 198-201 (2019). www.acgih.org

American Conference of Governmental Industrial Hygienists (ACGIH®): Upper Limb Localized Fatigue TLV®. 2019 Threshold Limit Values and Biological Exposure Indices, pp. 209-211 (2019). www.acgih.org

Armstrong, T., Foulke, J., Joseph, B. and Goldstein, S. Investigation of cumulative trauma disorders in a poultry processing plant. American Industrial Hygiene Association Journal, 43(2), pp.103-116 (1982).

Ebersole, M. and Armstrong, T. Analysis of an observational rating scale for repetition, posture, and force in selected manufacturing settings. Human factors, 48(3), pp.487-498 (2006).

Mathiowetz, V., Kashman, N., Volland, G., Weber, K., Dowe, M., Rogers, S. Grip and pinch strength: normative data for adults. Arch Phys Med Rehabil, 66(2), 69-74 (1985).

Rohmert W. Problems in determining rest allowances: part 1: use of modern methods to evaluate stress and strain in static muscular work. Applied ergonomics. 1;4(2):91-5 (1973).