Muscular Exposures During Standardized Two-Handed Maximal Pushing and Pulling Tasks

Abstract

OCCUPATIONAL APPLICATIONS We found markedly different muscular exposures and external hand force-generating muscle efficiencies when individuals produce maximal pushing and pulling forces in several experimental conditions (e.g., 100 cm versus 150 cm handle height, horizontal verss vertical grip orientation). For the same task characteristics, females produced higher levels of muscle activation and higher ratios of normalized muscle activity to force, which were likely the result of differing anthropometry between sexes. Females thus appear to be at higher risk for muscular overload during push/pull tasks. Pulling at waist height and pushing at shoulder height are recommended when possible, for similar task conditions to those studied. The high prevalence of pushing and pulling tasks in occupational settings enables use of these findings, in concert with traditional strength-based ergonomics assessments, to help protect both males and females from potential musculoskeletal overload.TECHNICAL ABSTRACT Rationale: Pushing and pulling tasks are highly prevalent across occupational settings. Despite increased availability of push and pull strength data, scarce information exists regarding specific muscle-based exposures when performing these tasks in common work configurations, and how these exposures differ between males and females. Purpose: We evaluated the influence of sex and several workplace factors on surface electromyography (EMG)-based exposure estimates during standardized, symmetric, two-handed maximal voluntary isometric push and pull tasks. Methods: Individual muscle and mean EMG activations (i.e., bilaterally and total), as well as weighted EMG to normalized force ratios (wEMG/nForce), were evaluated for seven bilateral shoulder and trunk muscles, in eight experimental conditions. Workplace factors manipulated were exertion direction (push and pull), handle height (100 cm and 150 cm) and handle orientation (vertical and horizontal). Results: For the muscles studied, pushing resulted in nearly twice (∼1.82×) the muscle activation than pulling. Across individual muscles, the largest difference between heights existed bilaterally for the pectoralis major, with pushing at 100 cm resulting in approximately twice the EMG amplitude. When pulling, only the right triceps activity differed between heights (i.e., 40% higher when exerting at 100 cm). Females produced higher normalized muscle activations (i.e., 23% higher across all muscles), and were generally less efficient than males, as indicted by a wEMG/nForce ratio approximately 26% higher than males across all tasks. The wEMG/nForce ratio was always higher when pulling at 150 cm compared to pulling at 100 cm, but handle height did not appear to influence wEMG/nForce when pushing. Conclusions: Females appear to be at a higher risk for muscular overload in the pushing and pulling tasks examined here. Adjusting the height of handles according to the task conditions can be a useful intervention to lower muscular demands.