Introduction. Intermittent Tasks. Endurance as a Function of Effort

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1 1 11 9 7 3 1 3 7 9 1 1 13 1 1 1 17 1 3 7 9 MALE FEMALE Introduction Preventing Fatigue During

Department of Kinesiology Physical ergonomists must determine acceptable demands for a variety of

Data exists for maximum efforts many strength studies available However, less data exists for

as a Function of Effort Intermittent Tasks 1 11 9 Maximum Holding Time (min) 7 3 Manenica (19)

Duration Maximum Holding Time (min) Maximum Moment (N m) Joint Strength Angle [K] (in degrees)

1 MALE FEMALE Introduction Preventing Fatigue During Repetitive Tasks: Predicting Maximal Acceptable Efforts Using Duty Cycle Jim Potvin, PhD Department of Kinesiology Physical ergonomists must determine acceptable demands for a variety of tasks. Data exists for maximum efforts many strength studies available However, less data exists for prolonged and/or fatiguing tasks prolonged static intermittent complex time-histories Endurance as a Function of Effort Intermittent Tasks Maximum Holding Time (min) 7 3 Manenica (19) Rohmert (1973) Rohmert Rest Allowance Variable Demand Capacity Joint Moment Strength Holding Time Duration Maximum Holding Time (min) Maximum Moment (N m) Joint Strength Angle [K] (in degrees) Endurance Time Effort as a Percentage of Maximum Frey Law & Avin () Rest Allowance = 1! Holding Time 1. Holding Time Endurance Time Effort as a Percentage of Maximum Strength Demand Maximum Strength -.1

Intermittent Tasks Rohmert Rest Allowance Rohmert Rest Allowance % MVC Limitations Task must always be On/Off 3 3 1 3% MVC 3% % Assumes <1% MVC can be held

Time (min) 1 11 9 7 3 Time (s) 1 3 7 9 Effort as a Percentage of Maximum Intermittent Tasks Psychophysics in Ergonomics Psychophysics has shown promise in

relationship between sensations and their physical stimuli raised to some power these powers vary for different stimuli.

2 Intermittent Tasks Rohmert Rest Allowance Rohmert Rest Allowance % MVC Limitations Task must always be On/Off % MVC 3% % Assumes <1% MVC can be held indefinitely physiological data to the contrary even for 1 hour Maximum Holding Time (min) Time (s) Effort as a Percentage of Maximum Intermittent Tasks Psychophysics in Ergonomics Psychophysics has shown promise in Ergonomics Manual Materials Handling Tasks Liberty Mutual Texas Tech Snook and Ciriello Ayoub The power law in psychophysics Stevens (197) exponential relationship between sensations and their physical stimuli raised to some power these powers vary for different stimuli. Snook & Irvine (197) First published psychophysical study in physical ergonomics determine acceptable demands for repetitive manual materials handling tasks Snook et al (199) began psychophysical studies of the upper limbs

Psychophysical Methodology Example: Ciriello et al () Snook, Ciriello et al Subject

duration Procedure repetition rate controlled task requirements - Screw driving - Ulnar

the amount of resistance by moving an unmarked dial resistance changed every 1 minutes

incentive basis, getting paid for the amount of work they performed.

the hands, wrists, or forearms symptoms were monitored Subject Training - days ramping

Electrical Connections - Wrist Extension - Simulated in-line screw running Description

3 Psychophysical Methodology Example: Ciriello et al () Snook, Ciriello et al Subject controls maximal acceptable value typically resistance could also be frequency, rest, duration Procedure repetition rate controlled task requirements - Screw driving - Ulnar deviation - Handgrip gripping, screwdriver, wrist ulnar deviation etc subject controls the amount of resistance by moving an unmarked dial resistance changed every 1 minutes randomly selected direction and amount subjects instructed to work as if they were on an incentive basis, getting paid for the amount of work they performed. They were asked to work as hard as they could without developing unusual discomfort in the hands, wrists, or forearms symptoms were monitored Subject Training - days ramping from to 7 hours/day testing over another 3 weeks Moore & Wells () Potvin et al () Manual Electrical Connections - Wrist Extension - Simulated in-line screw running Description mating of electrical connectors in automotive assembly Study Biomechanics and Psychophysics 1 hours of training 9 hours of collection

Methods: Electrical Connectors Methods: Simulation Device Subjects females to 9 years Spring Independent Variables 3 Frequencies, 7, 1/min 3 Grips

Impulse (N s) Maximum Voluntary Contraction (MVC) Tension Adjusting Dial Removable Connector Fixture Force Transducer Travel Distance of 13 mm

Estimating Duration from Impulse and Peak Force Peak Force = N Average Peak Force. ±.7 lbs (.

4 Methods: Electrical Connectors Methods: Simulation Device Subjects females to 9 years Spring Independent Variables 3 Frequencies, 7, 1/min 3 Grips Oblique Grasp, Pulp Pinch, Finger Press 3 Wrist postures Neutral, Extended, Ulnar Deviated Dependent Variables Maximal Acceptable: Peak Force (N) Force Impulse (N s) Maximum Voluntary Contraction (MVC) Tension Adjusting Dial Removable Connector Fixture Force Transducer Travel Distance of 13 mm Frictionless Frictionless Gliding Sliding Blocks Blocks Results: Sample Data Methods: Duration 1 Sample Data 7/min, Finger Press, Neutral Wrist Estimating Duration from Impulse and Peak Force Peak Force = N Average Peak Force. ±.7 lbs (.% CV ) 3 Force (lbs) Average Force Impulse (Area under Curve).97 ±. lbs s (7.% CV ) Force (N) 3 1 Rectangular Estimate of Duration = Impulse / Peak Force =. / =.11 s Mean duration =.1 s Time (s) Impulse =. Ns Time (s)

Methods: Peak Force Time-History Andrews et al (7) 1 1 1 Sample Force Time History (S3, OG, Ulnar Dev, 1/min) Hose

lbs CV = % Independent Variables 3 Frequencies 1, 3, /min Postures 1 3 Time (min) Dependent Variables Maximal Acceptable

5 Methods: Peak Force Time-History Andrews et al (7) Sample Force Time History (S3, OG, Ulnar Dev, 1/min) Hose Insertions Subjects 1 females 1 to 9 years Peak Force (lbs) 1 Mean = 1. lbs StDev = 1. lbs CV = % Independent Variables 3 Frequencies 1, 3, /min Postures 1 3 Time (min) Dependent Variables Maximal Acceptable Peak force (N) Force Impulse (N s) MVC Integration of Psychophysical Results Duty Cycle Frequency = /min 3 Duty Cycle = 3 s / s =. 3 1 s (x /min) s Duty Cycle = Work Time. Work + Rest Time Frequency = 1/min Duty Cycle = 3 s / s =. 1 1s 1s (x 1/min)

6 7 Studies Used Duty Cycle = Freq x Duration Time n = 9 Snook et al (199) Ciriello et al () Moore & Wells () Study Subjects Freq (/min) Duration (s) Duty Cycle (s/min) Duty Cycle (%) Tasks. 1..7% - Wrist flexion: power grip...% - Wrist flexion: pinch grip days/week % - Wrist extension: power grip (n = Total of 1 tasks) % % days/week % - Wrist flexion: power grip (n = 1) Snook et al (1997) Snook et al (1999) Ulnar Deviation Pronation & Supination Duty Cycle = % Duty Cycle = % Duty Cycle = 3.3% Potvin et al () Andrews et al () % % % %. 1..3% % % % 1...% % 1.1..% 1.1..% % % % %. 3..% % % %.. 3.3%.. 3.3%.. 3.3%.1.3.% % % % % % - Ulnar deviation: power grip (n = 3) - Wrist extension: pinch grip (n = 3) - Ulnar deviation: power grip (n = 3) - Supination: 31 mm screwdriver - Supination: mm screwdriver - Supination: 39 mm yoke - Pronation: 31 mm screwdriver (n = 1) - Wrist extension: in-line powered srcrewdriver (n = 11) - Push: pulp pinch - Push: finger tip (n = ) - Hose insertions: hand locations (n = 1) Maximum Acceptable Effort (%)! 7%" %" %" %" 3%" %" %" %" 1 Frequency (Efforts per Minute)! Snook et al (199) D - Wrist Flexion (Power Grip)" Snook et al (199) D - Wrist Flexion (Pinch Grip)" Snook et al (199) D - Wrist Extension (Power Grip)" Snook et al (199) D - Wrist Flexion (Power Grip)" Snook et al (1997) Ulnar Deviation" Snook et al (1999) Wrist Extension (Pinch Grip)" Ciriello et al () Supination (31 mm handle)" Ciriello et al () Supination ( mm handle)" Ciriello et al () Supination (39 mm yoke)" Ciriello et al () Pronation (31 mm handle)" Ciriello et al () Ulnar Deviation" Moore & Wells () Wrist Extension - DS =." Moore & Wells () Wrist Extension - DS =." Moore & Wells () Wrist Extension - DS =.3" Potvin et al () Pulp Pinch Push" Potvin et al () Finger Push" Andrews et al (7) Hose Insertion - (Medial Far)" Andrews et al (7) Hose Insertion - (Medial Near)" Andrews et al (7) Hose Insertion - (Push Forward)" Andrews et al (7) Hose Insertion - (Pull Back)" Andrews et al (7) Hose Insertion - (Push Down)" Snook et al (199) D - Wrist Flexion (Power Grip)" 7%" Snook et al (199) D - Wrist Flexion (Pinch Grip)" Snook et al (199) D - Wrist Extension (Power Grip)" 9 Upper Extremity Psychophysical Data Maximum Acceptable Effort (%)! %" %" %" 3%" %" Snook et al (199) D - Wrist Flexion (Power Grip)" Snook et al (1997) Ulnar Deviation" Snook et al (1999) Wrist Extension (Pinch Grip)" Ciriello et al () Supination (31 mm handle)" Ciriello et al () Supination ( mm handle)" Ciriello et al () Supination (39 mm yoke)" Ciriello et al () Pronation (31 mm handle)" Ciriello et al () Ulnar Deviation" Moore & Wells () Wrist Extension - 3/min" Moore & Wells () Wrist Extension - /min" Moore & Wells () Wrist Extension - /min" Moore & Wells () Wrist Extension - /min" Potvin et al () Pulp Pinch Push" Potvin et al () Finger Push" Andrews et al (7) Hose Insertion - (Medial Far)" Maximum Acceptable Effort (%) 7 3 r =.7 RMS Error = 7.% MVE Equation MADC = (1 Effort).17 1/, %" %" %" %" %" 3%" %" %" %" 7%" %" 9%"! Andrews et al (7) Hose Insertion - (Medial Near)" Andrews et al (7) Hose Insertion - (Push Forward)" Andrews et al (7) Hose Insertion - (Pull Back)" Andrews et al (7) Hose Insertion - (Push Down)" DC =.1 (~7 s/min) Effort =. (% MVC) % % % 3% % % % 7% % 9% %

Maximum Acceptable Effort (%) 9 7 3 Equation Rohmert (1973) - rest allowance equation for 3 s cycle Rohmert (1973) - rest allowance equation for 3 s cycle Jonsson (19) - recommended for hours

7 Maximum Acceptable Effort (%) Equation Rohmert (1973) - rest allowance equation for 3 s cycle Rohmert (1973) - rest allowance equation for 3 s cycle Jonsson (19) - recommended for hours Bjorksten & Jonsson (1977) - fatigue within 1 hour Sjogaard et al (19) - fatigue within 1 hour Jorgensen et al (19) - fatigue within 1 hour Bystrom & Kilbom (199) - fatigue within 1 hour Bystrom & Fransson-Hall (199) - fatigue within 1 hour Frequency Effects: NIOSH Lifting Equation % % % 3% % % % 7% % 9% % Frequency Effects: ACGIH TLV Lifting & Lowering Curves Tolerance Limit Value (TLV) Effort (9th percentile) Hand Activity Level (HAL) TLV (Controls Needed) Action Limit (General Controls) Max Acceptable Weight of Lift/Lower (% of 1 per day) 9 7 Lift (Floor to Knuckle) Lift (Knuckle to Shoulder) Lift (Shoulder to Reach) Lower (Knuckle to Floor) Lower (Shoulder to Knuckle) Lower (Reach to Shoulder) 3 Snook & Ciriello (1991) % 1% % 3% % % % 7% % 9% %.

Lifting & Lowering Fit with Equation Example: Maximum Acceptable Effort Maximum Acceptable Load (%MVC) 9 7 3. best coefficient for Lifting & Lowering RMS Diff =.% MAE r-squared =.

8 Lifting & Lowering Fit with Equation Example: Maximum Acceptable Effort Maximum Acceptable Load (%MVC) best coefficient for Lifting & Lowering RMS Diff =.% MAE r-squared =.1 MAL% % 1% % 3% % % % 7% % 9% % % % % 3% % % % 7% % 9% % 9 7 Upper Extremity Psychophysical Data Snook & Ciriello (199) Lift & Lower MAWLs Fitted Equation Greig & Wells () Assume:.3 efforts/min. s/effort DC = (.3 x.) / DC =.31 (3.1%) MAE = 1 (.31 1/,). = 1.33 =.7 (.7%) th Female Power Grip mean = 1.3 N MAF = 1.3 x MAE = 1.3 x.7 = 9. N th Female Lateral Pinch mean =. N th =. (13. x.7) =. N MAF =. x.7 = 31.7 N Limitation of MAE Equation: Maximum Acceptable Effort = 7.%

Future Directions: Yung & Wells (1) More research needed

time-histories combined tasks Isometric Sinusoidal On/Off

Sonne (1) MinMax 1 Percent Wells et al (11) Wells et al

% 1% % % % 9% % 7% % Force produced after plateaue (%MVC)

1% % % % % -% -% Change in MVC Force Production Between

% 1 3 1 17 1 199 1 3 3 3 3 37 3 Time at End of Each

9 Future Directions: Yung & Wells (1) More research needed intermittent tasks non On/Off tasks complex time-histories combined tasks Isometric Sinusoidal On/Off Most Fatiguing Least Fatiguing Fatigue Response Continuum Sonne (1) MinMax 1 Percent Wells et al (11) Wells et al (11) % % % 1% Plateau Intensity Level (%MVC) % % 3% 3% % % 1% % % % 9% % 7% % Force produced after plateaue (%MVC) Plateau Intensity Level (Blue Bars) (%MVC) % % 3% 3% % % 1% % % % % -% -% Change in MVC Force Production Between Current and Previous Force Plateau (Coloured Bars) (%MVC) % Time at End of Each Plateau (s) % % Time at End of Each Plateau (s) -1%

Mike Sonne PhD Thesis Mike Sonne PhD Thesis High duty

in complex tasks flexor pollicis longus complex

Upper Extremity Psychophysical Data Equation Maximum

development motor unit model 3 % % % 3% % % % 7% % 9% %

Most tasks are repetitive, but most of the research

Psychophysics can provide valuable data related to

Duty cycle can be used to estimate maximum acceptable

10 Mike Sonne PhD Thesis Mike Sonne PhD Thesis High duty cycle psychophysics study Peripheral vs central fatigue in complex tasks flexor pollicis longus complex intermittent task voluntary vs stimulation maximums 9 Upper Extremity Psychophysical Data Equation Maximum Acceptable Effort (%) 7 Xia & Frey-Law () Model further development motor unit model 3 % % % 3% % % % 7% % 9% % Take Home Message Acknowledgements 1. Most tasks are repetitive, but most of the research data relates to single effort strengths. Psychophysics can provide valuable data related to acceptable forces and torques during repetitive tasks 3. Duty cycle can be used to estimate maximum acceptable efforts for repetitive, intermittent tasks with no associated psychophysical data. Rohmert Rest Allowances appear to overestimate acceptable demands. More research is needed for tasks that are: intermittent complex combined

11 Contact Information Jim Potvin Associate Professor Department of Kinesiology, McMaster University 1 Main Street West, Hamilton, Ontario, LS K1 phone: (9) potvinj@mcmaster.ca website:

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