Introduction

Localized fatigue in the hands, elbows, and shoulders can lead to pain and disrupt normal work, daily living, and leisure time activities. It can also be an early sign of chronic musculoskeletal injuries. This site is intended to provide basic knowledge and tools for managing upper limb localized fatigue among persons performing manual work. Specific topics include 1) some historical and contemporary background about localized fatigue, 2) an overview of biomechanical and physiological fatigue factors, 3) a description of how work patterns affect load patterns on different parts of the body, 4) basic procedures for assessing load and fatigue patterns, 5) procedures for estimating fatigue limits for sustained static exertions, 6) procedures for characterizing repetitive/cyclical work patterns, 7) procedures for using the ACGIH TLV® to determine if work patterns are within recommended limits, and 8) examples to demonstrate the analysis of job and work tasks and the application of recommended limits to manage fatigue.

Print 1. Introduction

1. Some Background

1. In the beginning:

   So God blessed the seventh day and made it holy, because on it God rested from all his work that he had done in creation.

   Genesis 22 circa 6th Century BC. (English Standard Version)

2. The Romans:

   Battle scenes in Rome depict authentic Roman infantry fighting techniques including the tightly-packed "Roman Wall" of shields, gladius thrusting techniques above and below the "shield wall", and the rotation of troops on the front lines every 30–45 seconds." The rotation part makes perfect sense. Having a soldier in the first line fight until he dies and his fellows do almost nothing until it is their turn in the first line seems at best to be a waste of human life. Yet, this is how ancient fighting is generally perceived. Taking turn makes much more sense. Close combat will exhaust you quickly and no matter how skilled you are, when you are exhausted you rapidly become much slower. If you are not rotated away by when, you are pretty much dead meat.
   
   "Military Affairs of Rome," by Lt. Col. S.G. Brady, 1947” Roman infantry tactics, Wikipedia, 2023, October 28)

3. Ramazzini (1713) on writers and scribes:

   … the diseases of persons incident to this craft arise from three causes. First constant sitting, second the perpetual motion of the hand in the same manner, and thirdly the attention and application of the mind … constant writing also considerably fatigues the hand and the whole arm on account of the continual and almost tense tension of the muscle and tendons.

4. Taylor (1911):

   For a first-class shoveler there is a given shovel load at which he will do his biggest day's work. What is this shovel load? Will a first-class man do more work per day with a shovel load of 5 pounds, 10 pounds, 15 pounds, 20, 25, 30, or 40 pounds? Now this is a question which can be answered only through carefully made experiments. By first selecting two or three first-class shovelers, and paying them extra wages for doing trustworthy work, and then gradually varying the shovel load and having all the conditions accompanying the work carefully observed for several weeks by men who were used to experimenting, it was found that a first-class man would do his biggest day's work with a shovel load of about 21 pounds.
   
   This was the beginning of "Phsycophisical studies of work" that would later made famous by Snook & Irvine (1966)

5. Barcroft and Millen (1939)

   Reviewed studies of muscle bloodflow and fatigue going back to 1878. Building on this work they showed that blood flow incrased with muscle force up to 10% of maximum force. Blood flow then began to level off and became completely occluded with above 30% of maximum force. These studies helped to provide a Physiological basis for Taylor's work and provide a foundation for future work. (Humphreys & Lind 1963)

6. Earnest Hemingway (1952)

   I still need more healthy rest in order to work at my best. My health is the main capital I have and I want to administer it intelligently.

7. Monod and Sherrer (1957) and Rohmert (1960):

   Developed fatigue prediction curves for grip exertion. Endurance was an inverse fuction of force "Rohmert curve" -- see Monod and Sherrer (1965).

8. Elkus & Basmajian (1973):

   Normally, it [shoulder pain] would be thought of as "muscular fatigue," but we see now that this is incorrect. The sensation of "fatigue" that is experienced probably originates from the painful feeling of tension in the articular capsule and ligaments, not from overworked muscles. In fact, as we have seen, the muscles need not be working at all. Cain (1973) confirmed that feelings of fatigue in static contraction arise in part from structures outside the muscles. (Basmajian & De Luca 1985).

9. Byström & Fransson-Hall (1994):

   Comprehensive study of local blood flow, heart rate, blood pressure, electromyography, % maximal voluntary handgrip contraction (%MVC), and venous concentration of potassium and lactate during and up to 24 hours after intermittent handgrip exertions with the same "force+time" product, %MVC_work×T_work/(T_work+T_rest), but with various contraction-relaxation periods. Concluded: intermittent handgrip contractions at (or higher than) a mean contraction intensity of 17% MVC and continuous handgrip contractions at (or higher than) 10% MVC were considered unacceptable.

   T_total×17% ≤ T_work×%MVC_work+T_rest×%MVC_rest

10. Rashedi & Nussbaum (2015):

    Provide an up to date review of various models relevant for predicting localized fatigue for manual work.

11. Henny Youngman

    The patient says, "Doctor, it hurts when I do this." The doctor says, "Then don't do that!” goodreads

    Great advice if you can not "do that" and still do your job!

Print 1. Some Background

2. Fatigue causes & effects

A definition:

• Transient physiological and biomechanical changes, which may adversly affect motor performance and produce discomfort, that result from repeated or sustained exertions of the body.

Characteristics:

• Localized fatigue is a transient response which develops and recovers within periods of seconds, minutes or hours.
• Symptoms that persist from one day to the next may indicate a chronic musculoskeletal or neurological condition that merits medical attention.
• Symptoms may be thought of something to be tolerated and ignored
• Localized fatigue may be a precursor or harbinger of chronic musculoskeletal disorders.

Symptoms:

• Discomfort in active body parts
• Reduced strength
• Reduced endurance
• Tremor & reduced motor control
• Electrical changes
• Biochemical changes
• Circulatory changes
• Adaptation
• Chronic musculoskeletal impairments

Mechanisms

• Repetitive manual work involves repeated sequences of steps (actions or elements) to reach, grasp, hold, move, or position work objects. (see section 3: Workload Patterns). Examples of repetitive work can be found in manufacturing, food processing and services, and health care operations.
• Loads (forces & moments) --- "Dose"
  • Each work step or element produces loads (forces and moments) on various parts of the body --- "Physical Demands". Load patterns may differ from one part of the body to another, for example, an assembly line or meat processing jobs might involve sequences of brief exertions. Dental hygiene or laparoscopic surgery job, might involve sequences of brief hand exertions, but prolonged shoulder exertions.
  • Muscle contractions produce moments about various joints to support and move the body each step.
  • Muscle contractions involve and produce a cascading series of steps that require metabolism to produce energy --- "Physiological demands"
    
    Aerobic: O₂ + C₆H₁₂O₆ --> Energy + CO₂ + H₂O
    or Anaerobic: C₆H₁₂O₆ -->2CH₃CHCOOH
    
    Task Energy Demands, Physical Work Capacity and Fatigue
  • Mechanical loads casuse tissues to stretch. Stretching may be cumulative over time --- "viscoelastic"
• "Responses" occur over seconds, minutes, or hours.
• Biochemical changes, e.g., depletion of metabolic substrates & accumulation of metabolites
• Physiological changes, e.g., electrical and circulatory responses
• Biomechanical changes, e.g., compression and stretching of tissues, e.g., tendon/ligament strain
• Mental changes, e.g., e.g., effort, perceived exertion, pain
• Recovery occurs over seconds, minutes or hours. Depends on:
  • Work intensity
  • Work time
  • Recovery time
  • Occurs over minutes or hours. Symptoms should not persist from one day to the next.
• Insufficient recovery can result in:
• Pain (your body's way of telling you -- stop doing that! (take a break) -- may interfere with work and activities of daily living and liesure
• impairment -- disability

Print 3. Fatigue causes & effects

4. Workload Patterns

• Workload refers to the forces and moments exerted on various parts of the body to control the position and movements of the body and work objects that are necessary to perform each step of a complete work cycle. We focus mainly on the hand and shoulder. Forces and moments acting on the body are related to forces and moments required to grasp, manipulate, and use work objects, to the position and movements of the body, and to body size and weight. Loads may vary greatly from one part of a job to the next.
• Workloads can be expressed as absolute forces and moments or as relative percentages of maximum strength, %MVC for each part of the body, e.g., hand, wrist, elbow, or shoulder. Methods for determining strength requirements in absolute and %MVC can be found at a companion site: Strength demands & capacities
• Workload patterns can be divided into categories based on temporal and movement patterns:
• Contiunuous work: exertions that are maintained until the individual cannot continue or feels sufficient discomfort to stop.
• Reptive/intermittent/cyclical work: repeated exertions punctuated with regular periods of reduced load sufficient for some degree of recovery.
• Dynamic work: exertion of force by one or more parts of the body while moving.
• Statice work: exertion of force by one or more parts of the body without movement.
• Quasitatic work: small or slow movements that are for practical purposes treated as static work.
• Work may involve both static and dynamic elements, e.g., a worker maintains a static grip on the handle of a knife while wielding to bone a carcass.
• The following terms are used to characterize workload patterns.
• %MVC or %ME = Exertion force or moment / Strength *100%
• Hand and finger loads are computed or measured as forces acting on some part of the hand.
• Wrist, elbow, and shoulder loads are computed as moments acting about the joints.
• (Also may be expressed as a fraction of the maximum or on a scale from 0 to 10)

  Calculate %MVC = / × 100% =

• Work time, T_W
• Recovery time, T_R
• Total time, T_t or cycle time, T_cycle = T_W + T_R
• Duty cycle = T_W / T_t * 100%
• Fig 3 shows a task in which a worker exerts a sustained static exertion to carry a toolbox or suitcase. In this case there is a constant load on the hand, arm, and shoulder equal to the weight of the toolbox. There will be no recovery until the worker releases his or her grip - voluntarily or involuntarily.
• Fig 4 shows a task in which a worker exerts repeated/intermittent/cyclical forces and moments to get and pack bottles. Static hand force is exerted to grasp, move, and position the bottles.
• Dynamic moments are exerted to reach and move the bottles.
• Stretching the definition of "quasi-static to its limit, the shoulder moments are treated as "static" for assessing fatigue.
• It can be observed that no hand force is required to reach for the cartons, which results in a duty cycle of less than one for getting and packing a bottle. The moments on the shoulder only briefly approach zero, resulting in a very high duty cycle.
• Different models are used to predict and manage fatigue for prolonged static exertions than for repeated static exertions.
• Determining work patterns is the starting point for managing localized fatigue.

(a) (b) (c)

(a) Upper limb steps required to get the bottles from conveyor and place in container	(b) Moments and forces acting on the hand, wrist, elbow, and shoulder to complete task requirements	(c) The load pattern for packing one bottle of wine is repeated 6 times per hand per case.

Table 1: Force pattern for packing wine shown in Fig 3).

	MODAPTS	Hand Loc (x,y)	Time	Shoulder (moments)			Grip (force)
Step	code	mm	sec	Load, N-m Strength	Nm	%MVC	Hand force, N	Strength, Nr	%MVC
Move to Get	M5	200, 1,000	0	0.3	40.1	-1%	0	295	0%
Grasp bottle	G1	500, 1,200	0.5	7.7	43.3	33%	13	295	18%
Move to case	M5	500, 1,200	0.77	7.7	43.3	33%	13	295	18%
Position bottle over case	P2	200, 1,100	1.42	7.7	43.3	33%	13	295	18%
Put bottle into case	M3	200,1000	1.68	2.9	40.1	7%	13	295	18%
Put bottle	P0	200,1000	2.06	2.9	40.1	7%	13	295	18%

Print 4. Workload Patterns

5. Assessing fatigue

Fatigue can be assessed by quantifying its effects or various intermediate factors. There is a vast body of literature describing the use of these metrics in various settings for various purposes. The laboratory study by Byström and Fransson-Hall (1997) demonstrated the use of the most commonly used fatigue metrics to determine acceptable workloads for handwork:

• Biochemical changes -- generally involve biopsies to collect muscle samples for laboratory analysis
• Electrophysiological changes surface electrodes attached to the skin over the muscles or needle electrodes inserted into the muscles.
• Discomfort patterns -- worker reports
• Discomfort patterns are most suitable for routine field work. They are non-invasive and, if administered properly, are reasonably objective.
• Ask the worker: "Are you currently experiencing discomfort in any parts of your body?"
• "Use this picture to show me where you are experiencing discomfort." (see Fig 5b)
• "Rate the area of most discomfort on a scale of 0 to 10 where 0 is no discomfort and 10 is the worst discomfort imaginable. (see Figs 5c & 5d).
• "Now Rate the next most uncomfortable area"
• Continue until all of the areas of discomfort have been rated or the ratings fall below a level of concern.
• Compare discomfort patterns with biomechanical load patterns (Fig 5a).
• Workers may experience discomfort from non-fatigue causes or fatigue from other tasks that they previously performed.
• Changes in discomfort patterns from the beginning to the end of the task or shift, differences among workers, and biomechanical load are used to assess and interpret fatigue patterns. (see work by Saldana, et al. 1994; corlett and Bishop 1978).

(a)

(b)

(c)

(d)

Anticipate fatigue-related discomfort patterns

1. Examples of typical work tasks that involve repetitive and sustained quasi-static exertions are shown in Fig 5.
2. Where would you hypothesize the workers shown in Fig 5 would be likely to experience fatigue-induced discomfort?
3. Based on the preceding exercise, what would you consider acceptable discomfort level for working all day?
4. Design a discomfort survey to test your hypotheses.
5. What kind of interventions could be used to reduce fatigue-related discomfort in these cases?

(a)

(b)

(c)

(d)

(e)

Discussion Questions

1. How do you feel after sitting in class for 20 minutes? 40 minutes? 60 minutes? Where do you feel it? Why?
2. Reconcile discomfort patterns with biomechanical load patterns.
3. Where do you anticipate the workers shown in Fig 5 are likely to experience discomfort if the position show is repeated 1) 3x/minute and 2) 1x/hour or 3) maintained continuously? How could you test your predictions? Point at areas of the body shown in Fig 4 that are likely to be affected in each case shown in Fig 5.

Exercise: discomfort patterns

Print 5. Assessing fatigue

6. Fatigue Limits -- Sustained Static Exertions

1. Sustained (quasi-static) exertions refer to exertions that are maintained with little or no movement without regular reductions in load sufficient to provide recovery.
• Examples include carrying an object long distances, holding a tool or control for long periods, and continuously reaching to get and use work objects. (See Fig 3)
2. Discomfort versus exhaustion -- they are not the same.
   • Both are the result of physiological and biomechanical demands exceeding capacity.
   • Exhaustion -- unable to continue even if it means falling to your death -- up against a wall.
   • Discomfort ranges from "just noticeable" to "intolerable pain" or from a minor distraction to a disruption of task.
   • Fig 8 shows the relationship between time to objectionable discomfort in the hand and forearm and the time to complete exhaustion. Notice that discomfort occurs well before exhaustion.
   • The relationship between time to discomfort and time to exhaustion can be modeled as an inverse, log, log-log, or similar mathematical function (Table 3)
   • Use the calculator built into Table 3 to compare time to discomfort and exhaustion for different levels of %MVC

   

   Table 3: Time to objectionable discomfort and exhaustion equations (Armstrong 1976). Enter %MVC and click button below.

   Predicted times for specified %MVC	Time
   Endurance, s = 671,120 × %MVC-2.222
   Hand Pain, s = 107,920 × %MVC-2.0453
   Forearm Pain, s = 77,535 × %MVC-1.8427

   Enter %MVC (12-80%) =
   Click for exhaustion & discomfort predictions

   • Fatigue by body part -- they are not all the same
   • see the compilation of studies by Frey Law and Avin (2010) Fig 8 and Table 4.
3. Fatigue by body part
• The characteristics of the fatigue curves are similar for all body parts, but the fatigue rates for a given %MVC may vary from one body part to another.(Fig 9)
• Notice which body parts fatigue the fastest and the slowest.
• The calculator in Table 4 can be used to compute endurances for each body part.
• Remember that workers will experience objectionable discomfort before reaching the point of exhaustion (Fig 8)



Table 4: Time to exhaustion by body part (Frey Law, Avin 2010). Enter %MVC for each body part & click button below.

Time to exhaustion by body part	Enter %MVC (9-90%)	Predicted time
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

Click for predicted endurances

4. Fatigue Problem 1: How far are you going to carry that?

How long and how far could someone with average female grips strength (assume 50 lb) be expected to carry a 40-pound suitcase (see Fig 9) based on: 1. exhaustion and 2. unreasonable discomfort?

Assume:

• A normal walking pace of 3mph (4.4 ft/s)?
• The force required to carry a suitcase or toolbox at the side of the body using a hook grip is related to the weight of the suitcase or toolbox.
• Hook grip strength is most closely related to power grip strength that can be directly measured using a standard grip dynamometer (see Section 3, Fig 3)

• How far could someone with Average male grip strength (100 pounds) carry a 40 lb suitcase?

• How could you modify the equipment to reduce fatigue and increase the distance the suitcase can be carried?


Fig 10: Carrying a suitcase or toolbox at the side of the body using a hook grip.

Fatigue Calculations

Hand load = 40 pounds
Female hand grip strength = 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 feet<sup>1

1</sup>Based on standard walking pace of 4.4 feet per second (3mph)(1.34m/w)

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 opening of the hand has on muscle strength.

Solutions

The use of a roller bag can be used to transfer the weight of the bag from the hand to the floor (see Fig 11. The force to pull the roller bag will be related to friction and mechanical interference between the bag and the floor or ground.


(a) (b)

Fatigue Problem 2: Hanging on.

How long would you expect to hang on if you found yourself in the position shown in Fig 12?

Should you reach into your pocket for a phone to call for help?

Assume:

• Assume that the weight of the body is divided equally between the two hands.
  (see: see: "Task Hand Strength Demands and Capacities" section 3.3.; from Safety Last! (2024, January 27). In Wikipedia.

Fig 12: Hanging on.

Print 6. Fatigue Limits -- Sustained Static Exertions

7. Repetitive/intermittent Work -- characterization

• Many jobs involve alternating periods of work and rest on various body parts as workers get and manipulate work objects as described in Section 3 and Fig 4 above.
• The intermittent periods of work and rest can be simplified and characterized as a single period of work, T_work followed by a single period of rest T_rest for each work cycle, t_cycle
• Repetitive workloads are characterized as the:
• Time-weighted Average %MVC during T_work (Time when %MVC>7%)
  %MVC_avg = Σ Δt_i × %MVC_i ∕ Σ Δt_i

  where: %MVC_i > 7%^*

  Forces less than 7% are difficult to measure in most work settings and are not of practical significance.

• Duty Cycle, %DC= T_work / T_cycle × %MVC


• Workload patterns can be determined:
1. Using a stopwatch to determine the start and end time for each step or action the worker performs (event-based analysis
2. Using a clock and recording the observed worker actions fixed or random intervals (time-based analysis).
3. Constructing the steps required to perform the job and using predetermined time data to predict the times for each element.
• The observational methods analysis can be performed in real-time, but it is much easier to use a video recording that can be stopped, moved ahead, or moved back as desired
• It is important to work with workers and supervisors to:
• Obtain all necessary permissions.
• Obtain recordings that are representative of normal work or parts of the job of particular interest.
• The are a number of suitable video players that provide the time between the first and current video frame and have slider bars or other controls that enable the user to quickly move forward or backward to find the beginning or end of a job action.
1. Apple "Quick Time Player©"
2. Microsoft "Windows Media Player©"
3. Multimedia Video Task Analysis™ (MVTA™)(Must be purchased)
4. Video Methods Analysis App
• Estimate or measure the load and other relevant attributes affecting the loads on each part of the body. For example, to determine the load moments acting on the shoulder, it is necessary to determine the forces acting on the hand or arm and the location of those forces with respect to the shoulder.
• Various methods for estimating and measuring loads are described in: "Task Hand Strength Demands and Capacities"
• Example: Case packing job (see Fig 13)
• Standard work method: Gets/erects cases - 2 at a time; close flaps; aside to tapping machine (Fig 13)
• Work standard 12 boxes; 3 cases per minute>
• The starting time and estimated force were determined for each step from a representative video recording. The estimated force pattern for the right and left hands are shown in Fig 14/
• The load patterns for the right and left hands and arms must be examined separately
• The time during which MVC>5%, T_work, and the time-weighted average force during this time, %MVC_work along with T_cycle, and Duty Cycle (%) calculated for right and left hands (Fig 14)

7. Repeated/cycle exertions

8. ACGIH® TLV® for Localized Fatigue

• Applies to involving repetitive exertions of a hand, elbow, or shoulder work performed for 2 or more hours
• Quasi-static exertions
• Recommends maximum %MVC for a given duty cycle (or maximum duty cycle for given %MVC) to minimize localized fatigue to acceptable levels
• TLV® applied separately to each task performed 2 or more hours.
• Single exertions should not exceed 20 minutes
• The TLV® specifies a maximum recommended %MVC for a given Duty Cycle or a maximum recommended duty cycle for a given %MVC (see Fig 13).
• TLV®s are advisory. They are not enforced standards.
• References: ACGIH® (2022, 2024)

Basis of ACGIH® TLV® for Localized Fatigue

Enter %DC (5-90): Max recommended %MVC	Recommended max %MVC versus %DC (ACGIH 2024) Duty Cycle
Enter %MVC (10-90%):

Fig 16: %MVC versus %DC (ACGIH® 2022, 2024)

Application of the TLV®

• Determine the maximum recommended average %MVC (work) for the observed duty cycle.
• Determine the maximum recommended duty cycle (work) for the observed %MVC (work).

Calculate additional recovery time required for the recommended duty cyle.

• Calculating additional recovery time required for a recommended %MVC, T_work, and T_rest
• Duty cycle = T_w / T_total and T_total =T_work + T_rest
• Recommended: T_rest = T_w × ((1 - DC) / DC)
  1. Enter %MVC:
  2. Enter T_work:  
  3. Enter T_rest:    

  Calculate:

• Duty Cycle (Observed):
• Duty Cycle (Recommended):
• Total Recovery Time (Recommended):
• Additional recovery time (recommended):
• Additional recovery time does not mean idle time.

Examples:

1. Given duty cycles of 25%, 50%, and 75% for the hand, what are the maximum recommended average %MVC during work (excluding rest) for each case?
2. Given average %MVCs during work of 15%, 30%, 45% and 60% for the hand, what are the maximum recommended duty cycles for each case?
3. Based on the example in Section 6, it was found: average %MVC_Work = 33%; T_Cycle = 19.5s, and DC = 42%.
   1. What is the maximum recommended duty cycle for this job?
   2. How much total recovery time is recommended for the observed work time?
   3. How much additional recovery time (excluding the observed) is recommended?
4. Given an observed %mvc work for the shoulder of 10%, what is the maximum recommended duty cycle?

Print 8. ACGIH® TLV® for Localized Fatigue

9. Examples

Example 1: Application of ACGIH TLV® to case packing job

• Use of time study methods and biomechanics to determine hand force profile & application of TLV®

Example 2: Application of ACGIH TLV® to turkey thigh boning job

• Use of surface EMG to determine hand force profile & appliation of TLV®

Example 3: Application of ACGIH TLV® to application of binder clips to 3"x5" cards

• Methods and biomechanical analysis for for determining job force paterns and application of TLV®

Print 9. Examples

10. Selected References