Journal of Electromyography and Kinesiology 11 (2001) 373–380 www.elsevier.com/locate/jelekin Influence of memory demand and contra lateral activity on muscle activity L. Finsen * , K. Søgaard , H. Christensen Department of Research in Visual Display Unit Work, National Institute of Occupational Health, Lersø Parkalle ´ 105, Copenhagen DK 2100, Denmark Abstract Computer mouse work often includes memory demands and contra lateral activity. This study simulated video display unit (VDU) mouse-work and the focus was on forearm muscle activity as a result of standardised postural loads, memory demands and contra lateral activity. Surface and intramuscular electromyography (EMG) were recorded from the right forearm muscles during finger elevation and rest with and without memory demands and with and without contra lateral activity i.e. activity of the left hand. In most situations, memory demand increased activity in the m. extensor carpi radialis brevis and m. flexor digitalis superficialis. Also contra lateral activity increased activity in situations with and without memory demands. While surface EMG level of the m. extensor digitorum communis did not increase during memory demands, intramuscular EMG level increased when memory demands and contra lateral activity was combined. Influence of memory demands and contra lateral activity were most pronounced, in situations where activity levels were small. We presume that it is not only prolonged time of active computer mouse use that is a risk for development of musculoskeletal disorders, but also the time when people interact with the computer mentally or with the ‘non-mouse hand’, while resting their ‘mouse-hand’ on the mouse.  2001 Elsevier Science Ltd. All rights reserved. Keywords: Mental demand; Contra lateral activity; Muscle activity; Forearm 1. Introduction The number of computer mouse users and the amount of time that users spent working with Video Display Units (VDU) and computer mouse has increased dra- matically during the last ten years [1]. Previous studies have shown that during computer mouse work static and median muscle activity levels in the forearm muscles are about 1.5% EMGmax and 4.5% EMGmax, respectively [11,13]. Despite the low muscle activity level many employees develop musculoskeletal disorders [17,18,24]. Carpal tunnel syndrome (CTS) and tendonitis are the two most frequently studied disorders among VDU workers, however, also more specific symptoms as pain, stiffness, aching etc. have often been reported [24]. Repetitive motion as computer mouse use may result in continuously activation of low threshold motor * Corresponding author. Tel.: +45-39-16-52-00; fax: +45-39-16- 52-01. E-mail addresses: lf@ami.dk (L. Finsen), ks@ami.dk (K. Søgaard), hc@ami.dk (H. Christensen). 1050-6411/01/$ - see front matter  2001 Elsevier Science Ltd. All rights reserved. PII:S1050-6411(01)00015-3 units (MUs) eliciting muscle tension and muscle tissue pressure, which may cause muscle injuries [21]. There- fore, a large increase in the number of computer oper- ators developing disorders may be expected as a conse- quence of the dramatic increase in information technology. This is presumably one of the reasons for the many studies, which have dealt with identification of risk factors related to VDU work [2,3,6], and the present study supplements these studies. The postural load on the forearm and hand operating the computer mouse is relatively low [4,11,14], since the hand rests on the mouse and only limited force is needed to operate it. This indicates that there must be other fac- tors such as mental demands contributing to develop- ment of musculoskeletal disorders. Jensen et al. have shown that mental demand such as remembering infor- mation during both work and interruptions are present during computer mouse work [11]. Moreover, it has been shown that mental demands increase the activity of the muscles, and that this increase is highest in the muscles of the upper part of the body and decreases in a caudal direction except for the extensor muscles of the hand