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:
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
%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%
Discomfort patterns (Figure 1)
Figure 1: Localized fatigue is realted to moments produced about the joints by body weight and exertions to complete a task (a).
Discomfort patterns are associated with load patterns and can be recorded using a body map.
Discomfort can be quantified by rating each affected area from 0 to 10. (Adapted from: Corlett, E. & Bishop, R.:
The ergonomics of spot welders. Applied Ergonomics, 9:23-32, 1978.)
Hold your arm outstretched in front of your body.
How does your shoulder feel after 0.5 minutes? 1 minute? 2 minutes? 3 minutes. etc.
Rate your level of discomfort on a scale from 0 to 10 after each minute.
Plot your results & compare with fatigue predictions below.
Should you expect someone to work in this posture?
How would you compute the shoulder load? (See Figure 1a)
How do you feel after sitting in class for 20 minutes? 40 minutes? 60 minutes? Where do you feel it? Why?
Predict the expected discomfort patterns for someone working in the following postures. (see Figure 1a)
Figure 2: Predict the discomfort pattern if each of the above postures is
repeated 1) 3x/minute and 2) 1x/hour
Discomfort versus exhaustion -- they are not the same
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
Time to Exhaustion (Frey Law, Avin 2010)
Predicted Time to Exhaustion
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?
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.
Hand Force = 40 pounds
Female Hand Strenth = 50 pounds
%MVC = 40 / 50 × 100% = 80%
Time to hand exhaustion = 671,120 × 80-2.222 = 40s
Distance to hand exhaustion = 4.4 × 40s = 176 feet1
Time to hand pain = 107,920 × 80-2.0453 = 14s
Distance to hand pain = = 4.4 × 14s = 62 feet1
Forearm Pain = 77,535 × 80-1.8427 = 24s
Distance to hand exhaustion = 4.4 × 24s = 106 feet1
1based on standard walking pase of 4.4 feet per second (3mph)
Note: As a practical matter, most people can carry the suitcase further than predicted.
This is due to friction between the hand and the handle and to the effect that openning of
the hand has on muscle strength.
Fatigue Prediction Models -- Repetitive Work
Many jobs involve alternating periods of work and rest on various body parts as workers get and maniuplate work objects, e.g., Figure 5
Figure 6: The inermittent periods of work and rest for case packing (a)
can be collasped into a single period of work and a single period of reset for each work cycle, (b).
Duty cycle, DC=29%
Figure 7: Force pattern for sample case packing job was determined by
examining a video recording at fixed intervals and estimating the %MVC for the right hand (a).
The periods in which %MVC>7% were collapsed into a single period, TW, and
the periods in which %MVC≤7%, were collapsed into a single period, TR for
each complete work cycle, (b). The time-weighted force is based on force values in which %MVC>7%.
ACGIH TLV® for preventing localized fatigue for the hand, arm and shoulder
Figure 8: The TLV can be used to determine an acceptable duty cycle for a given %MVC (a) or a given %MVC for a given duty cycle (b).
from: American Conference of Governmental Industrial Hygienists (ACGIH®): Upper Limb Localized Fatigue TLV®.
2019 Threshold Limit Values and Biological Exposure Indices, pp. 209-211 (2019).
Enter Duty Cycle: %MVC =
What are some ways that you could lower %MVC to the recommended limit?
Acceptable %Duty Cycle (for given %MVC) = 100%*e(0.066-(%MVC/100))/0.143)
Enter %MVC: %DC =
How much additional recovery time is required for the recommended duty cyle?
Caculate the additional recovery time required for the recommended duty cycle based on the recommended duty cycle and Tw:.
Duty cycle = Tw / Ttotal and Ttotal =Twork + Trest
Recommended: Trest = Tw×(1-DC)/DC
Duty Cycle (Observed):
Duty Cyle (Recommended):
Total Recovery Time (Recommended):
Additional recovery time (recommended):
What are some ways that you could utilize this additional recovery time?
Example: For the case packing job shown in Figures 6 and 7 determine:
Determine the maximum recommended %Duty Cycle for the given %MVC (Figure 7).
Compare the observed %Duty Cycle with the maximum recommended %Duty Cycle.
How much should the recovery time be increased or the average %MVC be decreased?
Observed Average Hand Force = 29%MVC
Observed Duty Cycle, %DC = 29%
Using Figure 8a, find that the maximum recommended duty cycle for the observed %MVC = 21%
The accpetable duty cycle must be decrease to provide sufficient recovery time.