Shoulder Muscle Load during Pushing and Pulling

Abstract

Substituting lifting with pushing and pulling generally reduces the disc compression in the low back. However, from a biomechanical point of view, pushing and pulling tasks are much more complex to analyse compared to lifting. Instead of being close to vertical, the force directions are virtually unknown, and may vary during different phases of the task. The relation between external force and internal load is not simple, as force direction and body posture may change accordingly. Further, focus has mainly been on the lumbar spine and not on shoulder load. Although the lumbar spine load is reduced, shoulder muscle load is not necessarily changed to the same extent during pushing and pulling compared to lifting. Therefore, shoulder load is brought into focus in a study of pushing and pulling. Seven male waste collectors pushed and pulled two-wheeled containers with up to 50 kg of waste. When performing one-handed pull behind the body, the force was up to 280 N, and the internal shoulder flexion moments were up to 80 Nm in the initial phase, close to the subject's maximum shoulder flexion moments. For the two-handed push in front of the body, the total force was up to 360 N, corresponding to approximately 180 N for each hand. Pushing caused lower shoulder moments, up to 34 Nm in the initial phase. During steady state, the total force was up to 230 N when pushing and 130 N when pulling. Shoulder moments were below 27 Nm when pulling and below 16 Nm when pushing (all results are given as mean of subjects). Regarding the relation between exposure and internal load, there was a positive relation between the magnitude of the hand force and the shoulder moment (r=0.48). For the compression in the lumbar spine there was no relation to the magnitude of the hand force (r=0.05). In conclusion, although loads on the low back during pushing and pulling are relatively small, loads on the shoulder muscles may be very high in some cases. Pushing and pulling may therefore be a risk factor for shoulder disorders.