Embed Size (px)
Citation preview
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
k
kn
i i
i
N
n
N
n
N
n
N
n
N
nc
...3
3
2
2
1
1
1
Fatigue Failure with Variable Fatigue Failure with Variable LoadingLoading
k
kn
i i
i
N
n
N
n
N
n
N
n
N
nc
...3
3
2
2
1
1
1
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 )
k
kn
i i
i
N
n
N
n
N
n
N
n
N
nc
...3
3
2
2
1
1
1
35% of Material Strength
# of cycles leading to failure
# of cycles experienced
40% of Material Strength
k
kn
i i
i
N
n
N
n
N
n
N
n
N
nc
...3
3
2
2
1
1
1
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
k
ks N
n
N
n
N
n
N
ntD ...)(
3
3
2
2
1
1Where:
35% 40% 45% + …
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!
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
390
13000
210)(
tD
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
