Interfering effects of multitasking on muscle activity in the upper extremity Alvin K. Au, Peter J. Keir * ,1 School of Kinesiology and Health Science, York University, 4700 Keele Street, Toronto, Ont., Canada M3J 1P3 Received 8 December 2005; received in revised form 26 May 2006; accepted 7 June 2006 Abstract Multitasking, where workers are required to perform multiple physical tasks with various levels of cognitive load is common in today’s workplace. Simultaneous physical and mental demands are thought to cause task interference and likely increase muscle activity. To test the interfering effects of multitasking, 16 healthy participants performed hand and shoulder exertions with combinations of four grip conditions (no grip, 30% grip with low precision, 30% grip with high precision, and maximal grip) and three shoulder conditions at 90° abduction (maintaining posture, 40% force-controlled moment, 40% posture-controlled moment), with and without the Stroop test while surface EMG was recorded from eight upper extremity muscles. Both 40% MVC shoulder moments increased extrinsic forearm muscle activity by 2–4% MVE (p < 0.01). Grip exertion at 30% MVC reduced anterior and middle deltoid activity by 2% MVE (p < 0.01). Exerting a constant force against the transducer (force-controlled) required 3–4% MVE greater middle and posterior deltoid activity (p < 0.001) compared to supporting an equivalent inertial load at the same shoulder angle (posture-controlled). Performing the mental task (Stroop test) concurrently with either 40% MVC shoulder moments significantly increased trapezius activity by nearly 2% MVE (p < 0.05). Interestingly, the Stroop test also reduced all deltoid activity by 1% MVE (p < 0.05). The addition of both the Stroop test and force-control shoulder exertion independently reduced maximal grip force by 7% and 10% MVC, respectively. These results sug- gest that more complex workplace tasks may act to increase muscle load or interfere with task performance. These small but significant findings may play a role in the development of long-term musculoskeletal disorders in the workplace. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: EMG; Grip force; Shoulder; Stroop test; Multitasking; Motor control 1. Introduction ‘‘Multitasking’’ has become central in today’s work- place. Workers are often required to perform multiple tasks simultaneously which may reduce performance and likely contributes to elevated muscle activity. Hand gripping or grasping is common in industrial tasks and often occurs in conjunction with forceful shoulder or elbow exertions. Repeated gripping has been implicated as a risk factor for carpal tunnel syndrome and tendinitis (Silverstein et al., 1986). In addition to physical factors such as posture and increased external forces acting on the arm, several other factors have also been shown to increase muscle activity in the upper extremity. These include the method of controlling a mechanical load (Hunter et al., 2002; Sjø- gaard et al., 2000), the need for precision (Laursen et al., 1998; Visser et al., 2004) and the level of mental processing required (Davis et al., 2002; Finsen et al., 2001). Further examination of these factors and their effects on muscle activity is needed to fully assess muscular workload. While the realm of variables and their interactions involved in multitasking may be too great to have been fully evaluated together, several somewhat isolated aspects 1050-6411/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jelekin.2006.06.005 * Corresponding author. Tel.: +1 416 736 2100x40085; fax: +1 416 736 5774. E-mail addresses: pjkeir@yorku.ca, pjkeir@mcmaster.ca (P.J. Keir). 1 As of July 1, 2006 contact information will change to: Department of Kinesiology, McMaster University, 1280 Main St. West, Hamilton, Ont., Canada L8S 4K1. Tel.: +1 905 525 9140; fax: +1 905 523 4025. Journal of Electromyography and Kinesiology 17 (2007) 578–586 www.elsevier.com/locate/jelekin