Published on March 5, 2008
Chapter 21: Chapter 21 Temporal Ergonomics Reducing Fatigue: Reducing Fatigue Maintains or increases work productivity Provides “optimal” stress Consider and optimize both goals. Kinds and Locations of Fatigue: Kinds and Locations of Fatigue General body fatigue Muscular fatigue Mental fatigue Factors in Fatigue: Factors in Fatigue Physical exertion Physical discomfort Lack of energy Lack of motivation Sleepiness Factors in Recovery Value of Rest: Factors in Recovery Value of Rest How fatigued the muscle is when the rest begins The length of the rest What happens to the muscle during rest Axioms: Axioms Most jobs have peaks and valleys of demand. Fatigue increases exponentially with time. Rest is more beneficial when it occurs prior to “too much” fatigue. The value of rest decreases exponentially with time. Different parts of the body have different recovery rates. Active rest and working rest are alternatives to passive rest. There is “output” during work and “no output” during rest. Work Hours: Work Hours Most countries have a workweek of 40 hours or less. The number of hours worked per year also has declined. The hours worked per lifetime has declined. Daily hours have changed. Firms use overtime instead of inventories. Firms use part-time workers. A compressed workweek is becoming more common. Rest Hours (Allowances): Rest Hours (Allowances) Normal time is developed from time studies. Normal time is adjusted with allowances to yield standard time. Allowances are based on an 8-h day and 5-day week. Allowances consider only duration of rest. Each minute of rest is assumed to be equally valuable. Cardiovascular System: Cardiovascular System Fatigue occurs during heavy work (e.g. manual handling). Percent of capacity depends on work duration. NIOSH Lifting Guidelines consider short/moderate/long categories of lifting duration. Musculoskeletal System: Musculoskeletal System Static work Dynamic work VDT work Muscle Fatigue: Muscle Fatigue Types of muscles: Slow-twitch Fast-twitch Static work is more fatiguing than dynamic work. Muscle Recovery Time: Muscle Recovery Time Effect of Load: Effect of Load VDT Work: VDT Work Time before a break Break length Microbreaks Active/working/passive rest Brain: Brain Optimum stimulation decreases fatigue. Lack of motivation increases fatigue. Overload and underload predict fatigue. Concentrationand Attention: Concentration and Attention Are vulnerable to overload and underload. Regulatory agencies provide guidelines for work hours and rest. Changes that reduce boredom are beneficial. Sleep: Sleep Restores the functions of the brain. Sleep duration follows a normal distribution. Many people have mild, chronic sleep deprivation. Sleep deprivation of 24–48 h affects motivation. Longer sleep deprivation affects ability to perform. Circadian Rhythm: Circadian Rhythm Various physiological functions vary in a daily pattern. The period is about 25 h without external clues. The main zietgeber is outdoor light and darkness. Alertness peaks in the later afternoon and bottoms during the night. “Morning people” tend to be less tolerant to shiftwork. Countermeasures to Sleep Deprivation: Countermeasures to Sleep Deprivation Rest Naps usually take place in the afternoon. “Investment” naps may aid alertness. Nighttime naps are of some benefit. Sleep inertia may result from night naps. Drugs/Food Caffeine and amphetamines are effective. Short-acting sedatives may aid daytime sleep. Alcohol and older antihistamines increase sleepiness. Effects of some compounds are affected by circadian rhythm. Food with carbohydrates may relieve fatigue. Countermeasures to Sleep Deprivation (cont.): Countermeasures to Sleep Deprivation (cont.) Environmental Stimulation Bright lights Ventilation Auditory stimulation Conversation Physical activity Shiftwork: Shiftwork Shiftwork means not starting work between 7 am and 9 am 20–30% of workers are on some kind of shiftwork. Expensive capital encourages shiftwork. Social need may require shiftwork. Shiftwork Problems: Shiftwork Problems Economic: higher pay Social: Scheduling social activities Child care Health/safety Shift System Design Recommendations: Shift System Design Recommendations Avoid permanent night work for the majority of shiftworkers. Consider advantages and disadvantages of 12-h shifts. Avoid an early start for the morning shift. Consider distribution of leisure time. Rotate forward. Make the schedule simple and predictable Shiftwork Planning: Shiftwork Planning Decide on total hours/day of operation. Decide on 5-day or 7-day plan. Consider having workers in different time zones. Decide on fixed, alternating, or rotating shifts. Guideline 1: Guideline 1 Have a Work Scheduling Policy Problem is insufficient rest. Avoid too many hours. Avoid work hours at the “wrong time.” Guideline 2: Guideline 2 Optimize Stimulation During Work Problem is too much or too little stimulation. For too much stimulation, reduce environmental stimulation. For too little stimulation: Add physical activity. Add task variety. Add environmental stimulation. Guideline 3: Guideline 3 Minimize the Fatigue Dose Problem is that fatigue may become too great. Reduce high stress levels. Permit rest before fatigue becomes excessive. Schedule a break. Use part-time workers. Guideline 4: Guideline 4 Use Work Breaks Problem is that there is no productivity during break. Work with a different part of the body to rest the fatigued part. Rest during the automatic part of a machine cycle. Consider job rotation. Guideline 5: Guideline 5 Give Frequent Short Breaks Problem is how to divide break time. Remember that fatigue recovery is exponential. Give breaks in small segments, during the work period. Permit operator-controlled breaks if possible. Guideline 6: Guideline 6 Maximize the Recovery Rate Problem is to recover as quickly as possible. Reduce contact with environmental stressors. Provide good blood circulation for muscle recovery. Take active rest. Consider working rest. Guideline 7: Guideline 7 Increase the Recovery/Work Ratio The problem is insufficient time to recover. Increase the recovery time or decrease the work time. Moonlighting and 12-h shifts can cause problems. Encourage rest on holidays, weekends, and vacations.