# Validation of a Horizontally Dynamic Armrest for Joystick Controlled Mobile Equipment
## 1 Introduction  
Joysticks are used for operation of large mobile machinery in many industries, including forestry [1; 2; 3], mining [4; 5], and construction [6]. Operators in the forestry industry can be working hand operated controls for up to 95% of their working hours [3], where it has been reported that skilled forestry machine operators make as many as 20,000 movements over a 10 h work day [7]. Operators of joystick-controlled machines have been known to suffer from neck, shoulder, and arm pain [1; 2; 3; 8]. The musculoskeletal problems reported by joystick operators may be due to the repetitive nature of the task [9; 10]. Several studies have reported constant low-level loading during joystick controller use at or above 2% maximum voluntary contraction (MVC) in the upper trapezius (UT) [1; 11; 12; 13; 14]. It has been shown that the constant low-level loading during joystick use may lead to repetitive strain injuries in the shoulder and neck muscles [15]. A number of upper arm and shoulder muscles are activated during joystick controller use (Table 1). Mini-levers and pronated hand levers have been investigated as alternatives to conventional joysticks to reduce the input force and displacement associated with typical North American machine controls. Two studies found that using a mini-lever could decrease the loading in the upper trapezius, thus maintaining or improving productivity when coupled with a moveable armrest [1; 16]. Similar results were seen in another study, which also determined that mini levers decreased musculoskeletal symptoms [8]. However, mini levers have a higher precision requirement because of the smaller control form factor and throw. [PERSON] et al. [1] found that the higher precision requirements increased upper trapezius (UT) muscle activation. A study on pronated hand levers showed that operators of pronated hand levers had a higher incidence of pain in the elbow and shoulder and took more sick days due to arm pain [2]. In addition, fast, miniature, and conventional steering wheels have been investigated as an alternative to first- and second-order joysticks [17]. Joysticks were found to be ergonomically superior to steering wheel input devices; however, all five steering devices investigated exceeded joint angle or muscle activity guidelines [17]. Furthermore, some researchers have attempted to reposition controls in the cab with the ultimate goal of reducing muscle activation. However, despite reductions in forearm muscle loading, muscle activation [18] was still well above guidelines [19; 20]. Given that joysticks design alterations have not resulted in clear-cut reductions in repetitive strain injury risk, some researchers have chosen to redesign the armrest rather than the joystick. Armrests are used in joystick controlled mobile machines to support the forearm, which may reduce the need for shoulder and neck musculature to stabilize the arm. Both forearm support and shoulder posture influence muscle activation of the UT and anterior deltoid (AD), where increased muscle activation was observed with increased shoulder flexion and lack of forearm support [21]. Similarly, a neck support has been shown to reduce muscle loading in the neck during tasks with prolonged neck flexion [22]. Several studies have investigated whether armrests reduce muscle activation while completing workplace tasks; however, they have been divided in their results. [PERSON] et al. [23] showed that the use of an arm support successfully reduced neck and shoulder muscle activation when compared to no arm support. Another study found that using an armrest (among other ergonomic workplace improvements) reduced muscle activation in the UT [24]. In contrast, research by [PERSON] [25], as reported by [PERSON] [3], found no significant difference between shoulder and neck muscle activity when comparing a stationary armrest to no armrest. Although the difference was not significant, there was a reduction in UT muscle activation while participants used the armrest for all joystick movement directions except aft.