Are back and leg muscle strengths determinants of lifting motion strategy? Insight from studying the effects of simulated leg muscle weakness

Abstract

This paper describes the first of a series of studies aimed at elucidating whether back and leg muscle strengths are the intrinsic determinants of lifting motion strategy. The study started with an experiment in which six female subjects performed lifting tasks under three conditions: one unconstrained condition in which subjects used a self-selected lifting strategy and two simulated muscle weakness conditions in which they used a real-time EMG-based biofeedback technique to control either gastrocnemius or rectus femoris contraction level at 45% of the pre-calibrated maximum strength. The lifting trials were measured and then kinematically modeled to derive index values that quantified the motion strategies. The effects on initial posture and peak joint angular velocity were also examined. Results showed that both simulated muscle weakness conditions had a marked effect on the lifting strategy. While subjects preferred a strategy that predominantly or significantly relied on the legs under unconstrained condition, a general trend of alteration towards a back-lift strategy was observed when simulated leg muscle weakness was administered. Implications of a suggested causal relationship between muscular strength and lifting motion strategy are discussed. Relevance to industry This work seeks to identify and quantify a causal relationship between degraded back or leg muscular strength and lifting motion alteration. Such a relationship can contribute towards the development of a motion-based strength evaluation system for applications including return-to-work assessment and rehabilitation progress monitoring. It can also lead to practical guidelines and computer simulation models for designing consumer products or workplaces that better accommodate special populations with degraded strength capabilities.