Upper Extremity Muscle Activity During In-Phase and Anti-Phase Continuous Pushing Tasks

To determine the effect of anti-phase, in-phase bimanual and unimanual simulated industrial pushing tasks and frequency on upper extremity muscle activity.

Research investigating symmetrical (in-phase) and asymmetrical (anti-phase) pushing exertions is limited despite a high prevalence in industry.

Fifteen female participants completed five pushing tasks using a dual handle apparatus at three frequencies: 15 cycles per minute (cpm), 30 cpm, and self-selected. Tasks included two bimanual symmetrical pushes (constrained and unconstrained), two bimanual asymmetrical pushes (reciprocating and continuous), and one right unimanual push. Surface electromyography (EMG) from the right anterior, middle, and posterior deltoid (AD, MD, and PD); right and left trapezius (RT and LT); right pectoralis major (PM); and right and left external obliques (REO and LEO) was collected and normalized to maximum voluntary effort.

There was a task by frequency interaction in the AD, MD, PD, and RT ( p < .005), where activity in AD, MD, and PD was highest in the continuous task at 15 cpm, but activity was similar across task in 30 cpm and self-selected. Muscle activity coefficient of variation was lowest during continuous task across all frequencies.

Continuous, anti-phase pushes and constrained, in-phase pushes had the highest muscle activity demands and the least amount of variability in muscle activity and therefore may present the greatest risk of injury.

Anti-phase pushing is known to have a greater cognitive demand, and this study demonstrated that it also has a greater physical demand when performed continuously.