# Effects of external hand force modeling on validity of inverse analysis of lifting

## TL;DR

This study shows that simplified models for estimating hand forces during lifting are inaccurate at task start and end, affecting spinal injury risk assessments.

## Contribution

The study evaluates the validity of simplified external hand force and moment models in musculoskeletal simulations for lifting tasks.

## Key findings

- Simplified models yield negligible errors during the middle phase of lifting but cause substantial errors at task start and end.
- These errors lead to inaccurate estimation of peak spinal joint reaction forces.
- Refined models that detect grip-deposit transitions are needed for accurate injury risk assessment.

## Abstract

Manual lifting tasks pose the risk of musculoskeletal injury, including damage to lumbar intervertebral discs. To assess the risk, spinal joint reaction forces are often estimated using musculoskeletal simulations. However, such analyses require information of external hand force and moment (EHF&M), which has mostly been estimated using simplified models without sufficient validation. This study evaluates the validity of two common EHF&M models by comparing their analysis outcomes to those resulting from directly measured EHF&M. Simplified models yielded negligible errors during the middle phase of symmetric lifting and lowering but caused substantial errors at the beginning and end of each task, leading to inaccurate estimation of peak spinal joint reaction forces. These findings highlight the limitations of simplified models in evaluating mechanical factors of injury risk. The measurement platform and analysis method developed in this study can also contribute to validating EHF&M modeling for other tasks and improving assessment accuracy.

•Simplified EHF&M models are accurate during the middle phase of load handling•Simplified approaches misidentify the timing and magnitude of peak joint reaction force•These errors hinder effective design and control of lifting assistive devices•Refined models that robustly detect grip-deposit transition are necessary

Simplified EHF&M models are accurate during the middle phase of load handling

Simplified approaches misidentify the timing and magnitude of peak joint reaction force

These errors hinder effective design and control of lifting assistive devices

Refined models that robustly detect grip-deposit transition are necessary

Biological sciences; Biomechanics