Rethinking Tools and Guidelines: The Force/Repetition Interaction Sean Gallagher, Ph.D., CPE Auburn...

Rethinking Tools and Guidelines: The Force/Repetition Interaction

Sean Gallagher, Ph.D., CPEAuburn University

Industrial and Systems Engineering

MSD Risk FactorsMSD Risk Factors

• Force • Repetition• Posture• Duration

Systematic Review of Studies Systematic Review of Studies Examining Force*Repetition Examining Force*Repetition InteractionInteraction

Summary of Epi studiesSummary of Epi studies• 10 of 12 studies positive evidence for F*R interaction• Pattern of interaction very similar • F*R interactions found for:

• Low back disorders• Carpal tunnel syndrome• Hand/wrist tendinitis• Nerve conduction signal latency• Wrist discomfort• Lateral epicondylitis • Shoulder tendinitis • Shoulder discomfort• Knee discomfort• Tissue oxygenation levels

So, we see that Force and Repetition interact in terms of MSD risk in epi studies…

What happens when we repetitively load musculoskeletal tissues?

Fatigue Loading of Spine Motion Fatigue Loading of Spine Motion Segments (Brinckmann et al., 1988) Segments (Brinckmann et al., 1988)

30% UCS

40% UCS

50% UCS

60% UCS

70% UCS

Brinckmann et al. 1988 data Brinckmann et al. 1988 data

High Force(> 40 % UCS)

Low Force(< 40 % UCS)

Fatigue failure of EDL tendonsFatigue failure of EDL tendons(Schechtman and Bader, 1997)(Schechtman and Bader, 1997)

90% UTS

50% UTS

40% UTS

30% UTS

Force-repetition interactionForce-repetition interaction• Observed in epi studies (MSD risk)

• Reps with Low Force – modest incr. risk• Reps with High Force – rapid incr. risk

• Observed in biomaterials testing• Low force – MS materials last many reps • High Force – MS materials fail rapidly

• Why?

Fatigue FailureFatigue Failure

• Repetitive submaximal loading leads to area of stress concentration and failure (crack)

• % ultimate strength• % ultimate tension

• Continued loading causes crack to expand

• Damage continues to accumulate

% U

ltim

ate

Stre

ss

LFLR

HFHRHFLR

LFHR

One of the problems we have in ergonomics One of the problems we have in ergonomics is that workers often experience highly is that workers often experience highly variable load exposures (high, moderate, variable load exposures (high, moderate, low). low).

Most current tools have difficulty in Most current tools have difficulty in assessing risk with variable load exposures.assessing risk with variable load exposures.

Fatigue failure theory provides a way to Fatigue failure theory provides a way to assess MSD risk with variable loading assess MSD risk with variable loading patterns!patterns!

Fatigue Failure with Variable Fatigue Failure with Variable LoadingLoading

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Fatigue Failure with Variable Fatigue Failure with Variable LoadingLoading

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35% of Material Strength

# of cycles that would lead to failure

# of cycles experienced

Palmgren-Miner RulePalmgren-Miner Rule(Fatigue Failure with Variable (Fatigue Failure with Variable Loading )Loading )

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35% of Material Strength

# of cycles leading to failure

# of cycles experienced

40% of Material Strength

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Fatigue Failure with Variable Fatigue Failure with Variable LoadingLoading

35% of Material Strength

# of cycles leading to failure

# of cycles experienced

40% of Material Strength

45% of Material Strength

Living tissues can heal themselves!Living tissues can heal themselves!

)()()( tHtDtD S

TotalTissue Damage

Damage due to Mechanical Stress

Healing

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ntD ...)(

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1Where:

35% 40% 45% + …

0072.003.030000

390

13000

120

6000

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tD

45% 40% 35%UCS UCS UCS

Example: Worker performs 200 lifting tasks – 90 at 45% Ultimate Compressive Strength (UCS), 120 at 40% UCS, and 390 at 35% UCS. Assumes a 3% healing rate.

Positive result indicate damage exceeds repair capacity!

0072.003.030000

390

13000

120

6000

90)(

tD

45% 40% 35%UCS UCS UCS

Example: Worker performs 200 lifting tasks – 90 at 45% Ultimate Compressive Strength (UCS), 120 at 40% UCS, and 390 at 35% UCS. Assumes a 3% healing rate.

Positive result indicate damage exceeds repair capacity!

00084.003.030000

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Let’s redesign 45% UCS tasks to 40%:

40% 35%UCS UCS

Negative result indicate repair capacity not exceeded!

MSD Risk FactorsMSD Risk Factors

• Force • Repetition• Posture• Task Duration

LFLR

HFHRHFLR

LFHR

Neutral Posture

Awkward Posture

F*R interaction and Posture as an MSD risk factor

SummarySummary• Force and repetition should not be treated as

independent MSD risk factors – they appear to interact

• The interaction observed is what would be expected if MSDs result from a fatigue failure process

Summary (cont)Summary (cont)

• New perspectives from FF model:• High reps are not necessarily bad (if forces

are sufficiently low)• High force not necessarily bad, in fact may

have a training effect (if reps are limited)• However, ability to perform some High Force

tasks without injury may be deceptive• Posture may be an MSD risk factor due to

the increased force imposed on tissues in awkward postures

Summary (cont)Summary (cont)

• New perspectives from FF model:• FF model provides a rationale for

assessing MSD risk with exposure to varying levels of force/repetition

• Healing ability of biological tissues to counteract loading damage can and should be put into models

What do we need from What do we need from industry?industry?

• Partners to help develop new and better MSD assessment tools

• Funding to develop models and perform needed research in this area

• Access to plants/sites to assess jobs, perform job analyses, epi/validation studies