The effect of changes in joint angle on the characteristics of physiological tremor Benoit Carignan a , Jean-François Daneault b , Christian Duval c,⇑ a Département des Sciences Biologiques, Université du Québec à Montréal, Montréal, Québec, Canada b Department of Neurology and Neurosurgery, Montreal Neurological Institute, McGill University, Montreal, Quebec, Canada c Département de Kinanthropologie, Université du Québec à Montréal, Montréal, Québec, Canada article info Article history: Received 16 January 2012 Received in revised form 14 March 2012 Accepted 20 April 2012 Available online xxxx Keywords: Physiologic Laser EMG Electromyography Muscle Muscle length Relative angle Low-frequency abstract Introduction: Physiological tremor, as a whole, can be influenced by changes in muscle activity. However, the origin of low-frequency physiological tremor oscillations has yet to be conclusively determined. It is possible that by experimentally manipulating muscular activity, a better determination of the origin of those low-frequency oscillations can be achieved. It was demonstrated that changes in joint angle modify characteristics of muscular activity. As such, we hypothesize that changes in wrist-joint angle will alter the characteristics of low-frequency physiological tremor oscillations. Objective: Assess the influence of changes in joint angle of the wrist on characteristics of physiological finger tremor. Methods: Physiological finger tremor was recorded (n = 25) using a laser displacement system while the arm and hand were supported. The relative angle between the dorsum of the hand and the forearm was altered between conditions (135°, 180°, 225° and 270°), while the hand and the finger remained parallel to the ground. EMG of the extensors and flexors were also recorded. Results: Tremor amplitude was significantly altered by changes in wrist-joint angle. This was especially the case for lower frequency oscillations. In addition, electromyography properties of forearm muscles were also significantly modified by changes in wrist-joint angles. Conclusions: This study demonstrates that changes in wrist-joint angle modify the characteristics of physiological finger tremor. This should be taken into account when interpreting tremor data as well as when developing tools to minimize tremor. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Physiological tremor (PT) was extensively studied over the past 50 years. It is a minute involuntary movement (Duval and Jones, 2005; Carignan et al., 2010) that is present in every limb of healthy individuals during a postural task (Elble and Koller, 1990). While the state of the limb examined, whether at rest or action, was shown to affect the properties of PT; one aspect that has been neglected is the influence of angles between segments of the exam- ined limb on PT characteristics. It is well known that changes in joint angles, through the changes in muscle length that they produce, can modify muscle as well as reflex properties. This has widespread implications especially when tremor is deemed to be deleterious; such as during micro-surgery where research has focused on developing tools to reduce the negative impact of tre- mor. However, the information related to limb kinematics is rarely reported in studies examining tremor even though it could impact on the interpretation of the published results. Changes in joint angle were demonstrated to alter neural activity during isometric contractions, such as motoneuron stimulation rate (Rack and Westbury, 1969; Harridge and White, 1993), afferent feedback from sensory organs within human muscles and joints (Vander Linden et al., 1991; Stein and Kearney, 1995; Suter and Herzog, 1997), and motor unit recruitment thresholds (Miles et al., 1986). This could influence PT oscillations located within the 1–8 Hz frequency range since they were proposed to stem from unfused motor unit activity (De Luca and Erim, 1994) and/or sensorimotor control processes (Morrison et al., 2006). Moreover, long-latency stretch responses were demonstrated to be dependent on joint angle (Nakazawa et al., 1997) and so were the M1 and M2 responses in the elbow flexors (Yamamoto et al., 2000). This also could influence PT oscillations located within the 1–8 Hz frequency range since another of their proposed origin is reflex activity modulated by the mechanical properties of the limb (van Buskirk et al., 1966; Yap and Boshes, 1967; Marsden et al., 1969). This indicates that several of the mechanisms possibly involved in generating PT oscil- lations, especially those postulated to generate low-frequency 1050-6411/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jelekin.2012.04.012 ⇑ Corresponding author. Address: Dept. de Kinanthropologie, Pavillon des Sciences Biologiques, 141 Avenue du Président-Kennedy, Room: SB4290, Canada H2X 1Y4. Tel.: +1 514 987 3000x4440; fax: +1 514 987 6616. E-mail address: duval.christian@uqam.ca (C. Duval). Journal of Electromyography and Kinesiology xxx (2012) xxx–xxx Contents lists available at SciVerse ScienceDirect Journal of Electromyography and Kinesiology journal homepage: www.elsevier.com/locate/jelekin Please cite this article in press as: Carignan B et al. The effect of changes in joint angle on the characteristics of physiological tremor. J Electromyogr Kinesiol (2012), http://dx.doi.org/10.1016/j.jelekin.2012.04.012