Interface stability influences torso muscle recruitment and spinal load during pushing tasks

Abstract

Handle or interface design can influence torso muscle recruitment and spinal load during pushing tasks. The objective of the study was to provide insight into the role of interface stability with regard to torso muscle recruitment and biomechanical loads on the spine. Fourteen subjects generated voluntary isometric trunk flexion force against a rigid interface and similar flexion exertions against an unstable interface, which simulated handle design in a cart pushing task. Normalized electromyographic (EMG) activity in the rectus abdominus, external oblique and internal oblique muscles increased with exertion effort. When using the unstable interface, EMG activity in the internal and external oblique muscle groups was greater than when using the rigid interface. Results agreed with trends from a biomechanical model implemented to predict the muscle activation necessary to generate isometric pushing forces and maintain spinal stability when using the two different interface designs. The co-contraction contributed to increased spinal load when using the unstable interface. It was concluded that handle or interface design and stability may influence spinal load and associated risk of musculoskeletal injury during manual materials tasks that involve pushing exertions.

Figure 1

Experimental arrangement.

Figure 2

Mechanical schematic of the T-bar interface viewed from above, including the horizontal push force generated by the subject. Potential energy in the system can be expressed as ${\mathrm{V}}_{\mathrm{Interface}}=\{-{\mathrm{F}}_{\mathrm{Push}}\mathrm{L}\cos {\varphi }_1+1∕2{\mathrm{K}}_1{\varphi }_1\}+\{{\mathrm{M}}_{\mathrm{Z}}{\varphi }_2+1∕2{\mathrm{K}}_2{\varphi }_2\}$ where F_Push is push force, L is the distance from the load cell to the subject, M_Z is the moment applied about φ₂, and K₁,K₂ represent elastic resistance to rotation about the interface axes of φ₁ and φ₂.

Figure 3

Theoretical prediction of muscle activity in the internal oblique (black lines) and external oblique (grey lines) muscles for the stable (filled symbols) and unstable (open symbols) interfaces. MVE = maximum isometric voluntary exertion; IO = internal oblique; EO = external oblique.

Figure 4

Theoretical predictions of spinal compression with the stable and unstable interfaces.

Figure 5

Measured electromyographic (EMG) activity with the rigid interface and the unstable interface (with values averaged across exertion levels). ^*denotes statistically significant differences at p50.05. MVE = maximum isometric voluntary exertion; RA rectus abdominus; IO = internal oblique; LP = lumbar paraspinal; EO = external oblique.