Spectral properties of electromyographic and mechanomyographic signals during dynamic concentric and eccentric contractions of the human biceps brachii muscle Liping Qi a,c , James M. Wakeling b , Martin Ferguson-Pell c,⇑ a ASPIRE Centre for Disability Sciences, Institute of Orthopedics and Musculoskeletal Science, University College London, Brockley Hill, Stanmore, London HA7 4LP, UK b Department of Biomedical Physiology and Kinesiology, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6 c Faculty of Rehabilitation Medicine, University of Alberta, Edmonton, Alberta, Canada T6G 2G4 article info Article history: Received 10 December 2010 Received in revised form 23 August 2011 Accepted 24 August 2011 Keywords: Wavelet Principal component analysis Motor unit Elbow angle Mechanical demand Moment arm Recruitment Firing rate abstract The purpose of this study was to describe and examine the variations in recruitment patterns of motor units (MUs) in biceps brachii (BB) through a range of joint motion during dynamic eccentric and concen- tric contractions. Twelve healthy participants (6 females, 6 males, age = 30 ± 8.5 years) performed con- centric and eccentric contractions with constant external loading at different levels. Surface electromyography (EMG) and mechanomyography (MMG) were recorded from BB. The EMGs and MMGs were decomposed into their intensities in time–frequency space using a wavelet technique. The EMG and MMG spectra were then compared using principal component analysis. Variations in total intensity, first principal component (PCI), and the angle h formed by first component (PCI) and second component (PCII) loading scores were explained in terms of MU recruitment patterns and elbow angles. Elbow angle had a significant effect on dynamic concentric and eccentric contractions. The EMG total intensity was greater for concentric than for eccentric contractions in the present study. MMG total intensity, however, was lower during concentric than during eccentric contractions. In addition, there was no significant differ- ence in h between concentric and eccentric contractions for both EMG and MMG. Selective recruitment of fast MUs from BB muscle during eccentric muscle contractions was not found in the present study. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Electromyography (EMG) and mechanomyography (MMG) studies have described differences in MU control strategies during concentric and eccentric muscle actions. Greater EMG activation in concentric contractions compared to eccentric muscle actions has been reported in various muscles (Nakazawa et al., 1993; Linnamo et al., 2002, 2003). Moreover, it has been suggested that increases in muscle force were influenced more by motor unit (MU) recruit- ment than by changes in firing rate during concentric muscle con- tractions (Kossev and Christova, 1998; Coburn et al., 2005), whereas eccentric torque is primarily modulated through changes in motor unit firing rate of biceps brachill (Kossev and Christova, 1998) and vastus medialis (Coburn et al., 2006). Although it is con- troversial, some EMG studies examining MU recruitment during eccentric contractions have demonstrated a preferential recruit- ment of fast MUs over slow MUs (Nardone and Schieppati, 1988; Nardone et al., 1989). However, the effect of joint position during eccentric and concentric contractions was not clear. It has been shown that changes of joint angle associated with muscle length had a significant effect on the maximum muscle force production during isometric contractions (Kennedy and Cresswell, 2001; Mohamed et al., 2002). It is not clear how exactly surface EMG and MMG activities are altered by the joint angle during dynamic eccentric and concentric contractions. Measurement of muscle activation patterns during dynamic concentric and eccentric con- tractions in relation to the joint angle is important for understand- ing the basic mechanisms underlying motor control of limb movement, and very useful for constructing models of the neuro- muscular control system (Stein et al., 1995; Rosen et al., 1999). Therefore, the purpose of the present study was to describe and examine the variations in activation strategies of MUs in biceps bra- chii (BB) through a range of joint motions during eccentric and con- centric contractions by using surface EMG, MMG, and a combination of wavelet analysis and principal component analysis (PCA) of the EMG and MMG spectra. Wavelet analysis that is well-defined in time and frequency resolution, with the non-linear scaling adjusted to the physiological response time of the muscle, was used to decompose EMG and MMG signals from dynamic concentric and eccentric 1050-6411/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jelekin.2011.08.011 ⇑ Corresponding author. Address: Faculty of Rehabilitation Medicine, 3-48 Corbett Hall, University of Alberta, Edmonton AB, Canada T6G 2G4. Tel.: +1 780 492 5991, mobile:+1 780 938 4368; fax: +1 780 492 1626. E-mail address: martin.ferguson-pell@ualberta.ca (M. Ferguson-Pell). Journal of Electromyography and Kinesiology 21 (2011) 1056–1063 Contents lists available at SciVerse ScienceDirect Journal of Electromyography and Kinesiology journal homepage: www.elsevier.com/locate/jelekin