Trunk muscle response to lifting unbalanced loads with and without knowledge of centre of mass

To examine the effects of lifting a bin with a variable centre of mass on muscle activity, with and without knowledge of the centre of mass.

Numerous parameters related to lifting have been examined yet the effects of changing the load centre of mass in two dimensions, with or without knowledge, has not been examined.

Participants lifted a 6 kg industrial tote bin with a 7 and 11 kg mass randomly placed in each of nine compartments, into which the interior of the bin was partitioned. Participants were not restricted in lifting style other than using the handles, which were equipped with force gauges. Two series (9 lifts per series) were completed using the 7 kg load without knowledge of the load placement and one series with knowledge of the load placement. One series was completed using an 11 kg mass without knowledge of load placement. Electromyographic activity of the upper and lower erector spinae, latissimus dorsi and the external obliques were collected bilaterally.

Left and right muscle pairs demonstrated mirror images for all muscles with lowest activity levels when the load was placed nearer the lifter in the sagittal plane. Peak electromyographic activity of the upper erector spinae and latissimus dorsi increased with the weight in the compartments nearest the body and/or the ipsilateral handle. Conversely, peak electromyographic activity of the lower erector spinae and the obliques increased when the weight was on the contralateral side. Peak upper erector electromyographic activity reached up to 41% of maximum and the lower erectors reached 50% of maximum, while the obliques and latissimus dorsi were below 5% and 7%, respectively. No electromyographic activity differences were found between the known and unknown load placements.

A segmental control strategy appears to exist during lifting that works from the upper to lower torso based on peak electromyographic activity activity. When lifting a bin with a varied centre of mass, highest peak electromyographic activity for the upper and lower erector spinae occurred when the load is closest to the body, regardless of load knowledge. Based on our findings with asymmetrical loads, we conclude that the moments acting on the wrist play an important role in spinal loading and must be included in future studies.

Asymmetrical loads are often encountered in daily life. Regardless of the lifter's knowledge of the balance of the load, the differential and asymmetrical loading of the muscles of the back play a role in the development of low back pain.