Joystick stiffness, movement speed and direction effects on upper limb muscular loading

Abstract

The manipulation of joysticks to control heavy machinery requires repetitive wrist and upper limb movements which can increase operator susceptibility to repetitive strain injuries. The purpose of this study was to analyse muscle activation using surface electromyography (EMG) on eight muscles of the upper limb during joystick manipulation. Experiments (n=8 subjects) involved a series of 4 motion types (forward, backwards, inwards, outwards) at 2 speeds (fast, slow) using 3 identical joysticks with different stiffnesses (light, regular, heavy). Results showed that all experimental conditions required at least a constant low level (between 2–5% Task Maximal Voluntary Contraction) activation for all muscles. The joystick utilized in this study maintains the wrist in a more neutral posture, however, Integrated EMG (iEMG) and peakEMG results suggest that the muscle strain is transferred from the wrist to the shoulder. EMG results also suggest that shoulder strain is further exacerbated by the armrest as it forces the operator to elevate the shoulder while pulling the controller backwards and inadequately supporting the forearm while moving it in the forward direction. Muscles involved as prime movers had higher activation levels when joystick stiffness was increased, however, muscles that provided directional, positional or postural support to the prime movers were relatively unaffected by joystick stiffness. Muscle activation was increased for all muscles when the joystick was moved quickly. This finding may be important for work environments using joysticks which require increased precision and fine movements coupled with short, highly repetitious cycle times.