Torque and mechanomyogram correlations during muscle relaxation: Effects of fatigue and time-course of recovery Emiliano Cè a,b , Susanna Rampichini a,b , Eloisa Limonta a , Fabio Esposito a,⇑ a Department of Biomedical Sciences for Health, University of Milan, Via G. Colombo 71, 20133 Milan, Italy b Centre of Sport Medicine, Don Gnocchi Foundation, Via Capecelatro 66, 20148 Milan, Italy article info Article history: Received 23 May 2013 Received in revised form 30 August 2013 Accepted 26 September 2013 Keywords: MMG Force Torque Muscle relaxation Reliability Off kinetics abstract To assess the validity and reliability of the mechanomyogram (MMG) as a tool to investigate the fatigue- induced changes in the muscle during relaxation, the torque and MMG signals from the gastrocnemius medialis muscle of 23 participants were recorded during tetanic electrically-elicited contractions before and immediately after fatigue, as well as at min 2 and 7 of recovery. The peak torque (pT), contraction time (CT) and relaxation time (RT), and the acceleration of force development (d2RFD) and relaxation (d2RFR) were calculated. The slope and s of force relaxation were also determined. MMG peak-to-peak was assessed during contraction (MMG p–p) and relaxation (R-MMG p–p). After fatigue, pT, d2RFD, d2RFR, slope, MMG p–p and R-MMG p–p decreased significantly, while CT, RT and s increased (P < 0.05 for all comparisons), remaining altered throughout the entire recovery period. R-MMG p–p correlated with pT, MMG p–p, slope, s and d2RFR both before and after fatigue. Reliability measurements always ran- ged from high to very high. In conclusion, MMG may represent a valid and reliable index to monitor the fatigue-induced changes in muscle mechanical behavior, and could be therefore considered an effective alternative to the force signal, also during relaxation. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Skeletal muscle electromechanical properties are generally as- sessed by force and surface electromyogram (EMG) signal analysis (Cavanagh and Komi, 1979; Hopkins et al., 2007; Islam et al., 2013; Zhou et al., 1995). However, while several studies investigated the contraction phase of skeletal muscle activation, only few investiga- tions focused on muscle relaxation (Bichler, 2000; Cè et al., 2013b; Jaskolska et al., 2003), despite its important role in clinical, rehabil- itative and sport fields. A slowing in muscle relaxation is a common feature of several myotonic disorders. During clinical examination, though, muscle relaxation is routinely assessed on a subjective ba- sis. Indeed, the physician generally demonstrates the early clinical features of myotonic syndromes by asking the patient to strongly grip his/her hand and noting a slowing in muscle relaxation (Ho- grel, 2009). Beyond this subjective measure, the quantitative assessment of the myotonic phenomenon has been rarely at- tempted (Hogrel, 2009). Although the widely used EMG has attracted attention for dec- ades as a reliable tool for the assessment of neuromuscular activa- tion, some drawbacks exist on its usage, among which the sensitivity to external noise and interference that may limit its operating environment and range of application (Martin, 2009). In the last decades, the mechanomyogram (MMG) has been pro- posed as another tool to study muscle activation (Beck et al., 2007; Cé et al., 2013a; Esposito et al., 2011, 2009a,b, 2003; Malek and Coburn, 2012; Mamaghani et al., 2002; Orizio et al., 1999, 1989). MMG represents a reliable technique to assess the mechan- ical activity of a contracting muscle by using specific transducers, such as accelerometers, piezoelectric contact sensors and laser dis- tance sensors (Orizio and Gobbo, 2006). In particular, MMG records and quantifies the low frequency transverse oscillations of active muscle fibers which propagates to the skin surface (Barry, 1987; Frangioni et al., 1987), thus reflecting the mechanical counterpart of the motor unit electrical activity as measured by surface EMG (Gordon and Holbourn, 1948). During muscle activation, MMG is generated by three primary mechanisms: (i) gross transverse movements of the muscle as it moves toward, or away from, its line of pull during contraction and relaxation, respectively; (ii) smaller subsequent transverse oscillations of the muscle at its res- onant frequency; and (iii) dimensional changes of the active fibers (Barry, 1987; Beck et al., 2005; Orizio, 1993; Orizio et al., 2003). MMG can provide some notable advantages over EMG (Xie et al., 2009). MMG is a mechanical signal that is not influenced by changes in skin impedance due to sweating (Allen et al., 1995; Martin, 2009), by the motor point position and by skinfold thick- ness (Zuniga et al., 2011). 1050-6411/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jelekin.2013.09.007 ⇑ Corresponding author. Tel.: +39 02 5031 4649; fax: +39 02 5031 4630. E-mail address: fabio.esposito@unimi.it (F. Esposito). Journal of Electromyography and Kinesiology 23 (2013) 1295–1303 Contents lists available at ScienceDirect Journal of Electromyography and Kinesiology journal homepage: www.elsevier.com/locate/jelekin