Comparison of the 1991 NIOSH lifting equation and erector spinae muscle electromyography.

This study determined whether lifts rated as acceptable by the 1991 NIOSH equation calculations elicited myoelectric amplitudes of the erector spinae musculature (ES) within acceptable muscular load limits for continuous repetitive lifting tasks. Ten male subjects had surface electrodes placed bilaterally along the spine at levels T9, L1 and L3, 4 cm, 9 cm nd 3 cm lateral to the midline, respectively. Each subject performed eight trials of five lifting conditions that were used to examine the horizontal factor (HF) and asymmetrical factor (AF) of the 1991 NIOSH equation. All lifts were ordered randomly and initiated and terminated in a standing position. The lifting motion was unconstrained and incorporated a "flatback", free-style lifting technique. EMG data were collected and linear envelopes (LE) were ensemble averaged across subject trials for each condition and normalized to a maximum voluntary contraction (MVC). Subject LE EMGs were ensemble averaged to generate condition LE EMG averages and subsequently converted to amplitude probability distribution functions (APDF). Percent MVC values from the APDF curves were compared to muscular load limits. A three-way, repeated measures, mixed model analysis of variance determined significant main effects for conditions, electrode placements and probability levels of the APDFs. There was general agreement between the 1991 NIOSH equation and muscular load limits. Bilateral T9 ES often exceeded "static" muscular loads and right L3 often exceeded "static" and "median" muscular loads. There was a significant (p 0.01) difference for each main effect. The APDF EMG analysis was more sensitive to differentiating between conditions than the 1991 NIOSH equation. Phasic and amplitude EMG analysis of the ES for occupational lifting tasks could be best represented by the musculature at L3, 3 cm lateral to the midline