Localized (Peripheral) Fatigue

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

Outline


1. Fatigue Characteristics
2.Fatigue Measurement
2. Fatigue Prediction Models: Sustained Static Exertions
2. Fatigue Prediction Models: Repetitive Work

Fatigue Characteristics

  1. Definition: Physiological and biomechanical changes that come about as a result of repeated or sustained exertions of the body.

  2. Symptoms:
    • Musculoskeletal discomfort in active body parts
    • Reduced strength
    • Reduced endurance
    • Tremor & reduced motor control
    • Electrical changes
    • Biochemical changes
    • Adaptation
    • Chronic musculoskeletal impairments

  3. Mechanisms
    • Job description, e.g., get part/control -- reach/grasp/hold/move
      • Requires muscle contraction to overcome forces of gravity and inertia
        • Increases metabolic demand to supply muscles (O2 & C6H12O6)
          Task Energy Demands, Physical Work Capacity and Fatigue
        • Causes
          • Biochemical changes, e.g., depletion of metabolic substrates & accumulation of metabolites
          • Biomechanical changes, e.g., compression and stretching of tissues, e.g., tendon/ligament strain
      • Recovery depends on:
        • Work intensity
        • recovery time
      • Insufficient recovery time can result in:
        • Pain (your body's way of telling you to take a break) -- may interfer with work
        • Tissue damage -- may become a medical issue

  4. Key terms:
    • %MVC or %ME = Exertion force or moment / Strength *100%
        (Also may be expressed as a fraction of maximum or on a scale from 0 to 10)
      %MVC = / × 100% =     
    • Work time, TW
    • Recovery time, TR
    • Total time, Tt or cycle time, Tcycle = TW + TR
    • Duty cycle = TW / Tt * 100%

Measuring fatigue

Fatigue Prediction Models -- sustained static exertions

  1. Sustained exertions
    • See Figure 3
    • Discomfort versus exhaustion -- they are not the same
    • Figure 3: Time to objectionalbe forearm pain or hand pain precedes exhaustion (from Armstrong 1976)
      Fatigue Equation (Armstrong 1976)Predicted fatigue limit for specified %MVC
      Endurance, s = 671,120 × %MVC-2.222
      Hand Pain, s = 107,920 × %MVC-2.0453
      Forearm Pain, s = 77,535 × %MVC-1.8427
      Specified %MVC =

      • Fatigue by body part -- they are not all the same
      • Figure 4: Fatigue rates vary from one body part to another. Fatigue curves compiled from many studies by Frey Law, Laura A., and Keith G. Avin. "Endurance time is joint-specific: a modelling and meta-analysis investigation." Ergonomics 53, no. 1 (2010): 109-129.

        Predicted Time to Exhaustion
        Time to Exhaustion (Frey Law, Avin 2010)%MVC
        (9-90%)
        Hand, s = 33.55*(mvc/100)-1.61
        Elbow, s = 17.98*(mvc/100)-2.21
        Shoulder, s = 14.89*(mvc/100)-1.83
        Trunk, s = 22.69*(mvc/100)-2.27
        Knee, s = 19.38*(mvc/100)-1.88
        Ankle, s = 34.71*(mvc/100)-2.06

      • Fatigue Problem 1: carrying suitcase
        • Determine the %MVC for someone with average female grip strength (50 pounds) to carry a 40 pound suitcase. (see Figure 3)
        • Predict how long someone with average female grip strength can carry the suitcase before experiencing discomfort or eshaustion
        • How far could they care the suitcase at a normal pace of 3mph (4.4 ft/s)?
        • How far could someone with Average male grip strength (100 pounds) carry the suitcase?
        • How could you modify the equipment to reduce fatigue and increase the distance the suitcase can be carried?
          (a)(b)(c)
          Figure 5: Grip strength can be measured for a given individual (a); inferred from published population data* (b). Suitcase weight can be determined by weighing the suitcase (c).
          *Example: Mathiowetz, V., Kashman, N., Volland, G., Weber, K., Dowe, M., & Rogers, S. (1985). Grip and pinch strength: normative data for adults. Arch Phys Med Rehabil, 66(2), 69-74.

Fatigue Prediction Models -- Repetitive Work

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, Circulatory and Local Muscle Responses to Static Manual Work. Ph.D. Dissertation, The University of Michigan. 1976.

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).

Frey Law LA, Avin KG. Endurance time is joint-specific: a modelling and meta-analysis investigation. Ergonomics. 2010 Jan 1;53(1):109-29.

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).