Journal of Electromyography and Kinesiology 12 (2002) 295–304 www.elsevier.com/locate/jelekin Inﬂuence of interelectrode distance and force level on the spectral parameters of surface electromyographic recordings from the lumbar muscles Britt Elfving a,c,* , David Liljequist b , Eva Mattsson a , Gunnar Ne ´meth c a Department of Physical Therapy, Karolinska Institutet, Stockholm, Sweden b Department of Physics, Stockholm University, Stockholm, Sweden c Department of Orthopedic Surgery, Karolinska Hospital, Stockholm, Sweden Accepted 17 December 2001 Abstract In order to study the inﬂuence of interelectrode distance and force level on the electromyographic (EMG) spectral parameters and on their reliability, bipolar surface EMG measurements were performed on the lumbar muscles of 15 subjects. Two test contrac- tions (45 s) at 40% of maximal voluntary contraction (MVC) were performed, one with 2 cm interelectrode distance and the other with 4 cm, followed by two contractions at 80% MVC with the same change in interelectrode distance. Increasing the interelectrode distance from 2 to 4 cm caused a signiﬁcant mean decrease (about 8%) in the initial median frequency. It is shown that this shift is of an order of magnitude that may be expected from the bipolar electrode ﬁlter factor, and we further conclude that the observed individual variations in the shift are likely to be connected to ﬂuctuations in the shape of the power spectrum and to variations in conduction velocity. No signiﬁcant change was found for the median frequency slope when changing the interelectrode distance. Increasing the force (from 40 to 80% MVC) also caused a signiﬁcant mean decrease (about 10%) in the initial median frequency. The median frequency slope became signiﬁcantly more negative by more than 200%. We conclude, however, that torque ﬂuctuations during the fatigue contractions should have had only minor inﬂuence on the standard error of measurement of the initial median frequency and of the median frequency slope.  2002 Elsevier Science Ltd. All rights reserved Keywords: Electromyography; Bipolar electrodes; Interelectrode distance; Force level; Fatigue; Erector spinae; Reliability 1. Introduction Electromyographic (EMG) measurements of fatigue are affected by unavoidable within-subject variations, but also by factors that are in principle controlled in the laboratory, as for example the electrode arrangement and the muscular force level during a test contraction. These factors inﬂuence the frequency characteristics of the EMG signal, and a knowledge of this inﬂuence is thus important for the reliability of the method. This knowl- edge is also of importance when comparing different studies. * Corresponding author. Tel.: +46-8-5858-3927: fax: +46-8-5858- 3905. E-mail address: britt.elfving@gym.ki.se (B. Elfving). 1050-6411/02/$ - see front matter  2002 Elsevier Science Ltd. All rights reserved PII:S1050-6411(02)00027-5 1.1. Interelectrode distance It is well known that the EMG power spectrum recorded with a bipolar conﬁguration is inﬂuenced by the interelectrode distance. In general, an increase in interelectrode distance shifts the power spectrum towards lower frequencies [1,17,33]. This is con- veniently expressed as a decrease in the median or the mean frequency. In his theory of the frequency spectrum of EMG signals, Lindstro ¨m [17] showed that, for a bipolar electrode conﬁguration parallell to muscle ﬁbers of ‘inﬁnite’ length and not located near innervation points, the effect of the interelectrode distance d on the power spectrum P( f ) is that the spectrum is simply proportional to a factor sin 2 (pf·d/v), where f is the fre- quency and v is the conduction velocity. (For con- venience, a short proof is given in Appendix A). This factor, known as the bipolar electrode ﬁlter factor, is per-