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

MSD Risk Factors Force Repetition Posture Duration

Systematic Review of Studies Examining Force*Repetition Interaction

Summary 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 Segments (Brinckmann et al., 1988) 30% UCS 40% UCS 50% UCS 60% UCS 70% UCS

Brinckmann et al data High Force (> 40 % UCS) Low Force (< 40 % UCS)

Fatigue failure of EDL tendons (Schechtman and Bader, 1997) 90% UTS 50% UTS 40% UTS 30% UTS

Force-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 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

% Ultimate Stress LFLR HFHRHFLR LFHR

One of the problems we have in ergonomics is that workers often experience highly variable load exposures (high, moderate, low). Most current tools have difficulty in assessing risk with variable load exposures. Fatigue failure theory provides a way to assess MSD risk with variable loading patterns!

Fatigue Failure with Variable Loading

35% of Material Strength # of cycles that would lead to failure # of cycles experienced

Palmgren-Miner Rule (Fatigue Failure with Variable Loading ) 35% of Material Strength # of cycles leading to failure # of cycles experienced 40% of Material Strength

Fatigue Failure with Variable Loading 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! Total Tissue Damage Damage due to Mechanical Stress Healing Where: 35% 40% 45% + …

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!

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! Let’s redesign 45% UCS tasks to 40%: 40% 35% UCS Negative result indicate repair capacity not exceeded!

MSD Risk Factors Force Repetition Posture Task Duration

LFLR HFHRHFLR LFHR Neutral Posture Awkward Posture F*R interaction and Posture as an MSD risk factor

Summary 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) 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) 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 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