Task Strength Demands and Capacities

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

Contents

1. Task hand strength demands
2. Determining hand strength demands
  2.1 Worker ratings
    2.1.1 Verbal ratings
    2.1.2 Visual analog scales
    2.1.3 Borg Scale
    2.1.4 Force matching
  2.2 Observer ratings
  2.3 Instrumentation
  2.4 Biomechanical analysis of task hand force demands
  2.5 Extrapolate task hand force demands from previous studies
3. Hand strength
  3.1 Factors affecting hand strength capacities
  3.2 Hand strength measurement
  3.3 Published hand strength data
  3.4 Biomechanical models

1. Task hand strength demands

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The tool box with tools weighs 50 pounds(222 Newtons)
(a)(b)(c)
Figure 1: Determine the strength demand for holding a tool box at the side of the body (a). Action on the body - the weight of the tool box is supported by the fingers, which produces traction forces on the wrist, elbow and shoulder(b). An outstretched are produces moments about the wrist, elow and shoulder (c).

Percent capable (the lower percentiles)

Percentile1%2.5%5%10%25%50%75%90%95%97.5%99%
z-2.326-1.96-1.645-1.282-0.67500.6751.2821.6451.962.36
Figure 2: Normal distributions used to estimate strength percentiles. Use log normal distributions when acurate estimates are required -- especially for very low or hig percentiles.

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

The tool box with tools weighs 50 pounds Grip strength capacities for a sample of 30-34 year old females and males were found to be 121.8∓22.4 pounds and 78.7∓19.2 pounds respectively. (Mathiowetz et al. 1985)Standard normal values for z=(x-average)/ std dev
(a)(b)(c)
Figure 2: Percent capable calculation

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

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2. Ways to estimato task hand strength demands

2.1 Worker Ratings

2.1.1 Verbal 0-10

  1. The worker/user is asked "Rate the force/effort to perform a specified step of a task on a scale of 0 to 10 where 0 is no force and 10 is the greatest that you can imagine."
  2. The question should focus on a specific too, part or body part.
  3. Performing a maximum exertion in the same way that the task is performed can increase the accuracy of verbal ratings.

Examples:

  • "Please rate the force to join those connectors on a scale of 0 to 10 where 0 is no force and 10 is the greatest that you can imagine."
  • "Please rate the force to trim this branch with these loppers on a scale of 0 to 10 where 0 is no force and 10 is the greatest that you can imagine. Figure 3a and 3b."

Figure 3a: Simple pivot action (fiskars.com)

Figure 3b: Extended handles with compounded linkage to amplify hand force (fiskars.com)
Figure 3: Simple oral ratings on a 10 point scale can be used for nominal force assesments for comparing a tool with a simple pivot action (a) with that of a tool with a compound action (b).

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2.1.2 Visual analogue scale (anchor points ⚓)

Figure 4: The visual analog scale, vas, used to rate perceived exertions is typically a 10cm line with at least two verbal achor points. Multiply by 10 to convert to %MVC.

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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2.1.3 Borg RPE scale

  • Based on geometric relationship between perceived effort and physical exertion
  • see Figure 5.
Figure 5: Borg scale used by worker to rate their perceived exertion for a give task. Multiply by 10 to convert to %MVC.
  • Borg, GAV. 1982. Psychophysical bases of perceived exertion. Medicine and Science in Sports and Exercise, 14: pp. 377 – 381

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2.1.4 Force matching

   
(a)(b)
Figure 6: grip (a) and pinch (b) strength gauges used for force matching. (https://www.alimed.com/)

  • 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

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    2.3 Instrumentation







    Instrumentation for determining the effects of friction on break away force Use of EMG (electromyography) for indirect measurement of hand fore) Use of instrumented force plate and EMG to compare finger forces for different keyboards Instrumented flange and EMG combined with motion tracking to measure complex hand movements and forces for installing flexible hoses.
    Fgrip = Weight-Friction Fgrip = k × Opening Fgrip> = FChuck × r1 / r2 Fpinch = w / (2 × CoF)
    CoF = Coefficient of Friction
    (a)(b)(c)(d)
    Figure 7: Instrumental methods used for estimateing force demands
    • Armstrong T, Foulke J, Joseph B, Goldstein S. Investigation of cumulative trauma disorders in a poultry processing plant. Am Ind Hyg Assoc J 43(2):103-116, 1982.
    • Armstrong T, Foulke J, MartinB, Gerson J, Rempel D. Investigation of applied forces in alphanumeric keyboard work. Am Ind Hyg Assoc J 55(1):30-35, 1994.
    • Grieshaber D, Armstrong T. Insertion loads and forearm muscle activity during flexible hose insertion tasks. Human Factors 49(5): 786-796, 2007.
    • Seo NJ, Armstrong TJ, Ashton-Miller JA, Chaffin DB. The effect of torque direction and cylindrical handle diameter on the coupling between the hand and a cylindrical handle. Journal of biomechanics. 2007 Jan 1;40(14):3236-43.

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    2.4 Biomechanical analysis of task hand force demands

       

    Grip force = Object weight (neglecting friction) Grip force = Grip force is a function of clamp or clip opening Grip force a function of tool chuck force + possible over exertion force Pinch force related to weight of object and friction Finger and thumb forces related to the weight of object and finger & thumb locations.
    Fgrip = Weight Fgrip = k × Opening Fgrip> = FChuck × r1 / r2 Fpinch ≥ w / (2 × CoF)
    CoF = Coefficient of Friction
    Ffingers = (W × a) / b
    Fthumb = W + Ffigners
    (a)(b)(c)(d)(e)
    Figure 8: Biomechanical analyses can be used to estimate minimum force Requirements based on characteristics of the work object. In some cases, people will exert more force than the necessary "Safety margin."
    • Westling G, Johansson RS. Factors influencing the force control during precision grip. Experimental brain research. 1984 Jan;53(2):277-84.
    • FREDERICK LJ, ARMSTRONG TJ. Effect of friction and load on pinch force in a hand transfer task. Ergonomics. 1995 Dec 1;38(12):2447-54.

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    2.5 Extrapolate task hand force demands from previous studies

       
    Figure 9: Estimates of hand/finger forces to perform selected automotive assembly tasks based on woker ratings(0=no force to 10 greatest force imaginable). Multiply ratings by 10 to estimate %MVC (from Ebersole and Armstrong 2004)

    Ebersole ML, Armstrong TJ. An analysis of task-based worker self-assessments of force. In Proceedings of the Human Factors and Ergonomics Society Annual Meeting 2004 Sep (Vol. 48, No. 12, pp. 1300-1304). Sage CA: Los Angeles, CA: SAGE Publications.

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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

     
    (a)

    (b)
    Figure 10: Traditional grip (a) and pinch (b) strength measurements (https://www.alimed.com/) Population strength norms are often approximated using a normal distribution. Sample male and female hand strength distributions (c) and (d). Data based on Mathiowetz et al. 1985)

    (Note: strength tends to be bounded on the lower end. and skewed towards the higher end. The use of a normal approximation can lead to significant errors for estimates of extreme high or low percentiles.)

    See: 3.3 Published hand strength data

    Exercise: http://www-personal.umich.edu/~tja//handStrengthExercise.html

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    3.3 Published hand strength data

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    3.4 Biomechanical models

     
    (a)
    >
    Figure 11: Kinematic and biomechanical models are used to determing the forces that can be exerted at a given location with a given part of the body.

    Chaffin, D.B., 2006. Occupational biomechanics. Wiley Interscience.

    Armstrong TI, Choi I, Ahuja V. Development of hand models for ergonomic applications. Handbook of Digital Human Modeling: Research for Applied Ergonomics and Human Factors Engineering. 2008 Nov 20;12:1.

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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).